Hydraulic and Pneumatic Actuating Systems

Fluid Power Systems Fluid power is nothing but utilization of liquids such as oil, water and gas as a media for transmission of energy. It is known as an efficient means for conversion from one form of energy to another. Continuous development of fluid power technology over the years has contributed to the applications of pneumatic and hydraulic systems in many areas such as manufacturing, process industries, transportation systems and utilities. The fluid power systems (pneumatic and hydraulic) may be used for the following:

1. Carrying out mechanical work: Fluid power systems are used for carrying out mechanical work using linear, swivel and rotary motion for plant equipment and machinery. This includes application for operations such as clamping, shifting and positioning, packaging, feeding, sorting, stamping, drilling, turning, milling, sawing, etc.

2. Controlling application: Pneumatic and hydraulic systems are used for controlling of plant, process and equipment. They are also used to sense operational status of process and feed this information to the controller element to take necessary corrective action.

3. 3- Measurement of process parameter: Pneumatic and hydraulic systems can be used to provide measurement of process parameters. These process measurements are then used to act on necessary output.

Hydraulic Systems In hydraulic systems, a liquid-based solution, such as oil, is used under pressure to carryout work. A simple hydraulic linear actuator is shown in Figure 1. The system essentially consists of a movable piston connected directly to the output shaft. The fluid is pumped into pipe M or TV to move the piston up or down. Note

Figure 1: Hydraulic actuator.

that some methods of retrieving fluid from the non-pressurized side of the piston must be incorporated. The pressure in the cylinder is given by P = W/A where "W* and "A" are the weight and cross-sectional area, respectively. It is such that a typical hydraulic pressure of 150 bar will lift a load of 2000 kg by 4.2 cm diameter piston.

In a hydraulic press, as shown in Figure 2, a piston of small surface area is connected to a larger piston. By applying Pascal's law

This means that a small force applied to the smaller piston can lift a large upward force of a larger piston by suitably selecting ratio of diameters:

Figure 2: Hydraulic press

Physical Components of a Hydraulic System The basic hydraulic system essentially consists of a pump and a cylinder. Figure 3 is a simple hydraulic system, where a hydraulic piston is lifted by a hydraulic pump driven by an electric motor. The system consists of a closed-loop piping network, with fluid transferred from a storage tank to one side of the piston and returned from the other side of the piston to the tank. The pump converts a mechanical force to hydraulic power and the fluid is drawn from the tank by a pump which produces pressure at the required amount.

Figure 3: Hydraulic system

A pressure regulation/relief valve protects the system from high pressure and is essential to spill excess fluid back to the tank, due to a dead end load, in order to prevent failure of a pipe or pump where fluid pressure rises.

The cylinder uses the moving oil to do work and the cylinder movement is controlled by a control valve. A control valve directs the oil to allow supply of oil from the pump to and from the cylinder. Port A is connected to the pressure line and port B to the tank for extending the cylinder. To reverse the motion, port B is connected to the pressure line and port A to the tank. Speed control of the piston is achieved by regulating the volume flow rate of oil to the cylinder.

The speed of piston is given by

Note that hydraulic fluid needs to be very clean; hence a filter is needed to remove dirt particles before the fluid passes from the tank to the pump.

Pneumatic System Systems using gas for power transmission are called pneumatic systems. When air is compressed and stored, it can be used as a medium for carrying out mechanical work, measurement and for controlling and operating equipment and machines. Figure 4 shows how extend and retract system can be implemented using pneumatics. A pneumatic system essentially consists of following: 1. Motor-driven compressor; 2. Air receiver; 3. Directional control valve (DCV); 4. Pneumatic cylinder.

Figure 4: Pneumatic system.

Figure 5: Elements of pneumatic systems.

A pneumatic system uses compressed air, and air is usually stored in a pressure vessel called an air receiver. Air is delivered to the air receiver by a motor-driven compressor. The compressor is controlled by a pressure switch in order to start/stop the compressor depending upon the demand.

The cylinder movement is controlled by a DCV which directs the air into various pressure ports and releases the air from the cylinder to the atmosphere. Main elements of a pneumatic system are shown in Figure 5.

Pneumatic systems require clean, dry air to remove dust and dirt. A primary air treatment consists of a cooler and a separator. The cooler is used to reduce the temperature of air after compression, whereas a separator removes water vapor in the air. The secondary air treatment provides oil mist to lubricate the system and also incorporates further filtration of water particles.

Comparison of Hydraulic and Pneumatic Systems It is important to consider various points for comparing pneumatic systems with hydraulic systems such as power level, noise level, cleanliness, speed, etc. Principal advantages and distinguishing characteristics of pneumatic and hydraulic systems are listed as follows.

1. Availability: Air is available everywhere in unlimited quantities. 2. Transport: Air and hydraulic fluid can be easily transported over large distances through

pipelines. 3. Storage: Compressed air can be stored in a reservoir and removed as required. Hydraulic

oil can be stored in accumulators. 4. Temperature: Compressed air is relatively sensitive to temperature fluctuations, whereas

hydraulic fluids are relatively insensitive. 5. Explosion proof: Compressed air offers minimal risk of explosion or fire. 6. Cleanliness: Any unlubricated air which escapes through leaking pipes or components

does not cause contamination. 7. Components: The operating components of pneumatic system are simple in construction

and they are relatively cheaper. 8. Speed: Compressed air and hydraulic fluids are very fast working mediums. But the

lightweight items can be operated faster with a pneumatic system. 9. Adjustable: With compressed air and hydraulic components, speed and force are infinite

variables. Table1: Comparison of the various systems

The disadvantages of using such systems are as follows:

1. Preparation: Compressed air and hydraulic fluid-require good preparation in order to remove the dirt and condensate.

2. Compressible: It is not possible to achieve uniform and constant piston speeds with pneumatic systems.

3. Force requirement: Compressed air is economical only when the working pressure of 6-7 bar is maintained.

4. Noise level: The exhaust air is noisy.

5. Costs: Compressed air and hydraulic fluid are relatively expensive means of conveying power. However, the operating cost of hydraulic system is less than a compressed air system with the same mechanical rower.

Table 1 gives some comparisons of the various systems.

System Structure and Signal Flow Fluid power systems can be broken down into a number of levels representing pneumatic/hydraulic elements and signal flow. The various levels of control paths along with signal flow from input to the output level are exhibited in Figure 6. There are four levels of signal flow in a pneumatic system:

1. Energy source: This includes compressor/pump and air service units. 2. Input element/switches/sensors: They act as primary switching devices comprising

pushbuttons, sensors, limit switches, proximity switches, etc. 3. Processing elements: They are essentially logic switches, pressure control valves and

flow control valves. 4. Actuators: They are linear or rotating actuators or could be some indictors or any other

output devices.

Figure 6: Signal flow in pneumatic systems.

Hydraulic Pumps and Pressure Regulation The pump is the most important part of the hydraulic system. It takes oil from the tank and delivers it to the rest of the hydraulic circuit. The pump is driven by a constant speed electric motor. The amount of oil coming out of the pump during one revolution of the drive shaft is very important for determination work. This is known as displacement, which is nothing but the volume of oil moved or displaced during each cycle. Accordingly, pumps can be classified as fixed displacement pumps or variable displacement pumps. A fixed displacement pump displaces the same volume during each cycle of the pump, whereas a variable displacement pump can change or vary the oil it moves with each cycle at the same speed. The main elements of a hydraulic pump are as follows;

1. An inlet port; 2. An outlet port; 3. A pumping chamber; 4. A mechanical means for activating the pumping chamber.

Figure 7: Symbols of a pump.

Figure 8 : Centrifugal pump

The symbol for a pump and motor is shown in Figure 7. There are two types of pump used as energy sources: 1. Hydrodynamic pump (e.g., centrifugal pump); 2. Positive displacement pump. Figure 8 shows the principles of a centrifugal pump, where fluid is drawn into the axis of the pump and (lung out to the periphery by a centrifugal force. These types of hydrodynamic pumps are used to lift fluid at low pressure.

Figure 9 shows a positive displacement (piston pump). The fluid is drawn into the cylinder when the piston is driven down as the inlet valve opens. The pump sends out the fluid, when the piston is driven up. During upward motion of the piston, the inlet valve is closed and the outlet valve is opened.

Pumps In a hydraulic system, the following positive displacement pumps are used:

1. Gear pumps: The simplest gear pump is shown in Figure 10. A gear pump has two rotating gear wheels enclosed in closely fitted housing. Note that direction of rotation of two meshing gears will always be opposite to each other. When the gear rotates, teeth come out of a mesh in the center and a partial vacuum is created which draws the fluid into an inlet chamber. The liquid is then trapped in the space between the teeth of the two revolving gears and the housing. The fluid is continuously drawn out from the inlet chamber to the outlet chamber. Note that close meshing of the gear teeth provides a seal between the inlet and the outlet ports. The fluid accumulated on the outlet port is forced out.

Figure 9: Reciprocating pump.

Figure 10: Gear pump.

There is a source of leakage in the gear pump arising from the small gaps between the teeth and also between the teeth and the pump housing. The output of the gear pump essentially depends upon the volume of fluid each gear displaces and the rotation per minute. Therefore, the displacement of the pump is constant at a given speed.

2. Vane pumps: A vane pump uses spring-loaded vanes slotted into a driven rotor as shown in Figure 1. here spring-loaded vane tips are held against the housing and the rotor is offset within the housing. The vanes are contained by cam rings as they cross inlet and outlet ports. The fluid is dragged from the inlet port to the outlet port when the vane rotates. As vane tips are held against the housing, there is little leakage.

3. Piston pumps: Piston pumps are essentially of two types as shown in Figures 12(a) and (b). Generally single cylinder piston pumps are used. A radial piston pump consists of a number of pistons arranged around stationary cylinder block. Each piston has spring-loaded inlet and outlet valves. Fluid is drawn from the inlet port to the outlet port and then transferred smoothly when the inner cam rotates.

Figure 11: Vane pump.

4. Axial piston pump: A schematic of an axial pump is shown in Figure 13 each piston itself rotates with cylinder barrel about drive shaft and exerts axial force on the swash plate when the supply pressure signal is provided via the inlet port. The pistons are contained in the cylinder block accommodating small holes in it and they can undergo reciprocating as well as rotating motions. As the chamber reaches the inlet port, the fluid is drawn due to the increasing volume by virtue of axial motion of the piston. This trapped fluid is transported ro the outlet with increase in pressure due to a decrease in the volume inside the chamber. The swash plate is able to tilt about diameter to adjust the stroke of the piston.

Power Required by a Pump The motor power required to drive a pump is given by

Since Force = Pressure x Area

therefore, power can also be written as

Figure 12: Piston pumps: (a) Single cylinder piston pump; (b) radial piston pump.

The power can be expressed in terms of flow rate also.

We know that flow rate is

Also power = pressure x flow rate.

Figure 13: Axial piston pump

If pressure is in bar and the flow rare is in l/min. then

Filters Normally, filters are used to prevent dirt or dust entering important elements of the hydraulic system such as valves, seats, etc. Filters are used to remove very fine particles and can be installed in three places as shown in Figure 14. The various types of filters are as follows:

1. Inlet line filters: Inlet tine fillers are placed in the system outside the reservoir and near to the pump in order to protect the pump. They can prevent large particles that may be in the reservoir from entering the pump.

2. Pressure line filter: The pressure line filter is at the pump outlet which can remove contaminants passing through or generated by the pump in order to protect the valves.

3. Return line filters: They have high pressure drops and are placed in the system return line to clean the hydraulic fluid before it enters the reservoir for protecting the pumps.

4. Mechanical filters: They arc the most commonly used and utilize fine wire mesh or disk/screen arrangements. A typical filter is shown in Figure 15. These filters are to be removed, cleaned and refitted frequently. The filter's element essentially consists of a cartridge with a fine wire mesh which prevents passing of dirt or dust through it.

Figure 14: Filter positions.

Figure 15: Mechanical filters.

Figure 16: Pressure regulation.

Pressure Regulation It is desirable to keep the pump running all the time. If valve V1 is opened and V2 is closed, as shown in Figure 16, fluid flows from the pump to the Cylinder. When valve V2 is opened and V1 is closed, the fluid is returned to the tank. This will lead to infinitely high pressure in the line which may lead to the failure of a pump or piping. Therefore, a pressure regulating valve is connected from the pump output back to the tank. The pressure regulating valve is normally closed (NC) when the pressure is below the preset level. When the pressure is above the preset level, the valve is opened, thereby bleeding fluid back to the tank.

Relief Valve Relief valves are used to protect hydraulic pumps from over pressure. Figure 17 shows the working of a relief valve. The valve is in closed position under normal conditions due to ball and spring elements. The spring force is adjustable. When the fluid pressure exceeds a certain value against the spring force, the valve opens, thereby letting the fluid out through the vent. Hence the pressure is maintained at constant value.

Figure 17: Relief valve.

Figure 18: Accumulator: (a) Construction; (b) symbol.

Accumulator An accumulator acts as a storage device for high pressure fluid and can store and release the hydraulic oil at a required system pressure. The system stores the oil during unloading and idle periods, and the oil is made available at a later time when the pump is shut down or supplements the oil in operation. The most common type of accumulator is shown in Figure 18 which is known as a bladder accumulator. Here a rubber member is used to separate gas from oil, known as a bladder. The gas is tilled inside the bladder and the oil surrounds the bladder. When the accumulator is empty, the gas is pre-charged to some pressure and the poppet valve at the bottom prevents any extrusion of bladder into the piping. When the oil is charged, the bladder reduces its size. As oil leaves the accumulator, the bladder conforms to the internal shape of the accumulator. The accumulator is either gas pressurized or spring-loaded. The symbols for the accumulator arc shown in Figure 18.

Air Compressors, Air Treatment and Pressure Regulation Air Compressor The type of air compressor selected depends upon quality of air, pressure and cleanliness. As a rule pneumatic devices such as cylinders and valves are designed for maximum operating pressure 6-15 bar. Practical experience has shown that approximately 6 bar should be used for economic operation. Various types of air compressors arc shown in Figure 19.

A reciprocating air compressor is very common and provides a wide range of pressure and delivery rates. The working of reciprocating air compressor is exactly similar to reciprocating hydraulic pump. For higher pressure, multi-stage compression is used with inter-cooling between each stage.

Figure 19: Classification of air compressor.

Air Receiver An air receiver is used to store high-pressure air and provide constant supply of air pressure in the pneumatic system regardless of varying and fluctuating consumption (Figure 20). Further, one of the functions of a receiver is the emergency supply of air to the system in case of power failure. Air coming from the compressor will be warm. The large surface area of the receiver dissipates this heat to the surrounding atmosphere.

Figure 20: Air receiver

Figure 21: Refrigerated dryer

Air Dryers Air humidity and dew point are raised by compression. The excess moisture has to be removed in order to safeguard pneumatic elements. With refrigerated drying, shown in Figure 21, the compressed air is passed through heat exchanger system. The refrigerant which, (lows through the refrigerator coil cools the moist air below dew point temperature when it passes through the heat exchanger. The aim is to reduce the temperature of the air to a dew point which ensures that the water in the air condenses and drops out easily.

Figure 22 shows a typical water trap and separator. Air (lows through the unit and undergoes a sudden reversal or direction and a deflector cone swirls the air.

Air Service Equipment The air service equipment shown in Figure 23 is a combination of the following elements: 1. Air filter; 2. Air regulator and gauge; 3. Air lubricator.

The task of a filter is to remove solid particles that are present in the air flowing through the filter. Further. ii also separates and collects water droplets. The pore size of the filter element determines the particle size which can he filtered out. When air passes through the filter from left to right and is fed through a baffle plate in the filter bowl, the baffle plate causes the air to rotate. Owing to this rotation, the heavier particles and water droplets arc spun by centrifugal force against the inner of the filter bowl.

Figure 22: Air filter and water trap

Figure 23: Air service equipment

The sliding parts in a pneumatic system require lubrication. An air lubricator provides an adequate quantity of lubricant and works on the principle of pressure drop between the oil reservoir and the upper pan of the lubricator due to air flow. The air generated by the compressor will have pressure fluctuation. The essential purpose of the regulator is to reduce this pressure fluctuation. The pressure is regulated by a diaphragm and the output pressure acts on one side of the diaphragm. The other side of the diaphragm is actuated by a spring force which can be adjusted by adjusting a screw. The pressure is regulated by the volume of air (lowing through the unit. The diaphragm moves against the spring force causing the outlet cross-sectional area at the valve seat to be reduced or closed entirely as and when the output pressure increases.

The symbols for air service unit are shown in Figure 24.

Figure 24 Symbols for air service equipment: (a) Lubricator; (b) Filter with water tap; (c) Pressure regulator; (d) Simplified air service unit.

Control Valves The primary function of a control valve is to direct and regulate the flow of fluid from an energy source to the various loading devices. Normally, control valves are used for the purpose of sensing, processing and controlling. Control valves cover following functions: 1. Allowing the passage of air/fluid and directing it to a loading line;

2. Cancel the signal by blocking its passage; 3. Alter or generate the signal; 4. Release the air atmosphere or return the fluid to the tank.

Valves arc divided into the following six categories: 1. DCVs; 2. Non-return valves; 3. Flow control valves; 4. Pressure control valves; 5. Combinational valves; 6. Solenoid valves.

DCVs The DCVs arc used for controlling the passage of a fluid signal. This is done by generating, canceling or redirecting signals. DCVs are essentially categorized into three groups: 1. Signaling elements (INPUT element); 2. Processing elements; 3. Power elements or final control elements.

Figure 25: Operations of DCVs: (a) Hydraulic system; (b) Pneumatic system.

Operations, Ports and Positions of DCVs Figure 25 shows valves in a pneumatic and hydraulic system. Note that there are four ports. Ports A and B are the signal output ports as load is connected to these ports. The pressure port is labeled as P and pressure signal is supplied to this port from a compressor or pump. In a hydraulic system the return port R ensures that fluids arc returned to the tank whereas in a pneumatic system the return port R is vented to the atmosphere. The internal operation of a DCV for the above application is illustrated with Figure 26.

In order to extend the piston forward, the pressure port P is connected to the signal output port B; thereby an air/oil pressure signal is directed to the cylinder from the left side via a signal output port B. The exhaust air from the right portion of a cylinder is expelled via exhaust port R through port A. To retract the piston, ports P and A arc connected to deliver the air to the left portion of the cylinder. The return air is expelled by connection of ports B and R.

In this case, the piston is controlled by actuating the DCV with two positions, extend or retract. Therefore, the valve has two control positions and hence is known as 4 ports and 2 position valve (4/2 valve). The position and symbol for this valve is shown in Figure 27. Figure 28 shows 4 ports and 3 position valve (4/3 valve). The three positions are retract, off and extend.

Construction of Control Valves I here are essentially three types of control valves: 1. Poppet valves:

Ball seat type;

Disk seat type.

Figure 26: Internal operation and control.

Figure 27: Position and symbol of 4/2 valve: (a) Two-position valve (b) Symbol for two-position valve (c) DCV symbol for 4/2 valve.

Figure 28: 4/3 DCV: (a) Three-position valve (b) valve action for 4/3 valve (c) Symbol of 4/3 valve.

2. Slide valves (spool valves): Longitudinal slide valve; Plate slide valve.

3. Rotary valves.

Poppet Valves The poppet is a movable member quite frequently used to accomplish the directional change or signal flow between various ports. The poppet configuration is available in two-way, three-way and four-way arrangements.

Figure 29: 2/2-way valve: (a) Construction; (b) symbol.

2/2-Way Valve Figure 29 shows the construction and symbol of a simple 2/2 valve which is used as an ON-OFF valve. Its function is to enable signal flow. By pressing the pushbutton, the spring-loaded ball is forced away from the seat which allows fluid to flow from port P to port A. However, a valve cannot release the air to the atmosphere or return line. The signal will be cancelled when the button is released.

3/2-NC Valve Figure 30 shows the construction and symbol of a disk-type 3/2-NC valve. The 3/2-way NC valve is a signal generating valve. It produces the output signal on actuation of pushbutton by connecting valve ports, P and A, and sealing port, R, as the valve disk is pushed down. When the pushbutton is released, the spring-loaded disk seals the pressure port (P) and connects output signal (A) to exhaust port (R).

4/2-Way Valve Figure 31 shows a 4/2 disk valve. When the pushbutton is pressed, port R is scaled and ports P and A are connected. In the unactuated initial position, ports A and R are connected via the hollow pushbutton stem and pressure port P is connected to the output signal port B.

Poppet valves arc simple to construct and manufacture, and are also relatively cheaper and robust. However, they require more force to operate and also complicated valve types cannot be constructed.

Spool Valves Spool valves utilize a horizontal moving spool within a valve body. A 3/2-way spool valve is shown in Figure 32.

In the initial position, the spring-loaded spool connects ports A and R; when the pushbutton is pressed against spring force, ports P and A are linked and port R is blocked.

5/2-Way Spool Valve A 5/2-way valve essentially consists of 5 ports and 2 positions. This valve uses a spring-loaded pilot spool for control movements as shown in Figure 33. This separates or connects the corresponding lines by means of longitudinal movements of a spool. Connecting ports can be distributed around the circumference of the spool housing. In the initial position, pressure port P and output signal port

Figure 30: A 3/2-NC valve: (a) Construction; (b) symbol.

B are connected to generate output signal. The return line AR is linked to release the fluid. By pressing the pushbutton the spool will move toward the left against a spring force and set a link between ports P and A for signal output. The return line is established by connecting B and S. Figure 34 shows the construction detail of a typical 4/2-way valve. The spool valves have an advantage of cost saving and can be used for different operations and functions.

The DCV as a Signaling Element The DCV is normally operated by a pushbutton or a roller or a lever. The roller-operated valve detects the piston rod position of a cylinder.

Figure 35 shows symbols for the various ways in which valves can be operated.

DCV as a Processing Element A processing element is normally known as logic valve. It generates or cancels or redirects signals depending on the desired control conditions.

Figure 31: 4/2-disk valve: (a) Construction; (b) symbol.

Figure 32: 3/2-way spool valve.

Figure 33: 5/2-way spool valve: (a) Construction; (b) symbol

Figure 34: 4/2-way spool valve: (a) Construction; (b) symbol.

Figure 35: Symbols for the various ways in which valves can be operated.

Figure 36: Symbol and truth table for OR function.

Logic OR Valve/Shuttle Valve A logic function requires at least one input device to be active in order to cause the output. The truth table for this function is shown in Figure 36. The pneumatic logic OR function is obtained by a shuttle valve as shown in Figure 37. This valve has two inlets and one outlet. If air signal is applied through B, the valve seat seals the opposing inlet (A) and output signal is generated by connecting B and C. If the air signal is reversed, the valve seat seals the inlet B and an output signal is generated by connecting A and C. It also

Figure 37: OR valve: (a) Construction; (b) symbol.

produces the output if either or both input signals are applied. Any one input simply breaks the flow of air to output port C.

Logic AND Function The logic AND function requires both input devices to be active for generating the output signal. The truth table for this function is shown in Figure 38.

Figure 38: Symbol and truth table for AND function

Figure 39: Construction of AND valve.

The logic function in a pneumatic circuit is obtained with a dual pressure valve as shown in Figure 39. The construction of this valve is such that air flows through valves only if signals arc applied to both inlet ports A and B.

DCV as a Power Element (Final Control Element) The final control element must deliver the required quantity of fluid to match the load in the actuator. Normally, these valves need to handle large volume flow rates and therefore appear in larger sizes. In such cases, a two-stage process called pilot operation is used. The operating force for actuating the valve is obtained by a pilot valve (Figure 40). A 5/2-way double pilot valve has the characteristic memory control. Here the spool valve is actuated by pilot air supply.

Figure 40: 5/2-way pilot operated valve: (a) Construction; (b) symbol.

Figure 41: Non-return valve: (a) Construction; (b) symbol.

Non-Return Valve or Check Valves The check valves allow the flow in one direction and in the other direction the flow is blocked. A simplest construction of a non-return valve with ball and seat arrangement is shown in Figure 41. Note that the flow of air is free in one direction.

Flow Control Valve The flow control valve restricts or throttles the fluid in a particular direction to influence the volumetric flow of the fluid. The flow control is very important to regulate the speed of hydraulic or pneumatic actuator. The construction of flow control valve is shown in Figure 42. It reduces the flow rate in both directions and is adjustable.

The one-way flow control valve is shown in Figure 43. It permits flow adjustment in one direction only. A check valve blocks the flow of air in the bypass leg. The air can flow only through regulated cross-section.

Quick Exhaust Valve The quick exhaust valve is used to vent a cylinder quickly by providing a shortcut for exhausting air. This is essential if rapid movement of the piston is required where resistance to flow in a normal air path through

Figure 42: Flow control valve - bi-directional: (a) Construction; (b) symbol.

Figure 43: One-way flow control valve

Figure 44: Quick exhaust valve: (a) Construction; (b) typical application.

DCV will slow down the movement of the actuator. The device is shown in Figure 44. The movable disk allows links between ports A and R or ports A and P. The quick exhaust valve is placed very near to the actuator.

Sequence Valves The sequence valve is used in a pneumatic circuit for switching operation depending upon a preset pressure. The sequence valve opens once its inlet pressure rises above a preset pressure. The signal output is generated only after the required operating pressure has been reached. The symbolic representation of a pressure sequence

Figure 45: Sequence valve.

valve is shown in Figure 45. The output signal is generated only if a preset pressure has been reached in the pilot line Z.

Time Delay Valve (Combined Valve) A pneumatic time delay valve is used to delay the switching operation. Referring to Figure 46, a time delay valve essentially consists of one-way flow control valve, a 3/2-way spool valve and reservoir.

Figure 46: Time delay valve: (a) Construction; (b) timing diagram; (c) symbol.

The pilot air supply is provided to a reservoir through the one-way How control valve. Depending on the setting or the throttling screw, a greater or lesser amount of air flows per

unit time into the reservoir. When the required pressure in the reservoir has built up, the pilot spool of the 3/2-way valve is moved downwards.

Now, the valve disk is lifted from its seat and ports P and A are linked to generate an output signal, whereas the air passage is blocked between A and R. The time delay between application of pilot pressure to port Z and the valve operation is exhibited by the timing diagram.

Actuator and Output Devices An actuator is an output device which performs useful work. The hydraulic and pneumatic systems are normally expected to produce work for moving, or gripping the objects by applying force. Based on motion there are two categories of actuators:

1. Linear motion:

Single-acting cylinders; Double-acting cylinders.

2. Rotary motion:

Air motors;

Rotary actuators.

Linear Actuator 1. Single-acting cylinders: A single-acting cylinder is the simplest type of linear actuator.

In single-acting cylinders, the fluid is applied on one side of the piston. Therefore, a cylinder can produce work in one direction. The other side of the piston is vented to the atmosphere. The restoring spring will assist the return movement of a piston. Figure 47 shows a typical construction of a single-acting cylinder.

2. Double-acting cylinder: In double-acting cylinders, the fluid pressure is used to extend and retract the piston. The construction of a double-acting cylinder is shown in figure 48. Double-acting cylinders do not have any return springs. There are two ports used alternately, one for supply and other for exhaust of air. The double-acting cylinder has an advantage that the cylinder is able to carry out work

Figure 47: Single-acting cylinder

Figure 48: Double-acting cylinder.

in both directions of motion. The pneumatic and hydraulic actuators are constructed in a similar manner; the major difference being the operating pressure. These arc five basic parts in a cylinder: Two end caps with port connection; A cylinder barrel; Piston; Piston rod; Seal.

It is not advisable for linear actuators to complete their working stroke at full speed due to the impact and wear. Therefore, the cylinders are cushioned to protect from the impact with a projecting sleeve on both sides of the piston. The projecting sleeve blocks the air path at the end of the stroke to leave a volume of curtained trapped air as shown in Figure 49 which provides air cushioning.

Figure 49: Adjustable cushioning

Rotary Actuator Rotary actuators are the hydraulic or pneumatic equivalent of electric motors which are used when a twisting or turning motion is required. Figure 50 shows the rotary actuator using rack

and pinion. This type of rotary actuator can provide up to one turn depending upon a linear stroke.

Figure 51 shows a rotary actuator based on the principle of a gear motor. Fluid enters at the top with high pressure, applying force on the gear faces resulting in rotation.

A typical vane motor is exhibited in Figure 52. The design and construction of a vane motor is similar to a vane pump. At the entry, a vane has high pressure on one side, whereas on the other side the pressure will be very low due to high pressure of fluid. This difference in pressure exerted on the vane will produce a torque that would result in rotation of vanes.

Figure 50: Rack and pinion type rotary actuator.

Figure 51: Gear motor.

Figure 52: Vane motor.

Electro-Pneumatics Nowadays, pneumatics systems are combined with electronics for the control and sequencing applications. In electro-pneumatics, the DCVs are controlled electrically, but other valves can also be electrical it needed. It is accepted that electrical control in a pneumatic system should not be used where there is possibility of hazards due to fire. An electro-pneumatic control offers many advantages. The signals are transmitted over great distances without any leakages and loss. Further time between signal transmission and signal reception is minimized with the use of electro-pneumatics. Essentially the electrical components will have two roles: (a) sensing the information and {b) processing information.

Components of Electro-Pneumatic System There are four levels of components used in an electro-pneumatic system: 1. Energy supply: This includes compressed air and AC or DC power supply. 2. Input elements: They are limit switches, contactors or switches, pushbuttons and

proximity sensors. 3. Processing elements: They are essentially logic elements, solenoid actuated DCVs,

pneumatic/electric converter and relays. 4. Actuators and final control elements: They are pneumatic cylinders, motors,

lamps/buzzers and solenoid-operated DCVs. We have seen working principles of many pneumatic elements in the previous section. We will now discuss contactors and switches, relay switch and solenoid-operated valves.

DC Power Supply There are many types of power supply systems with cither AC or DC outputs that can receive different local input supply voltages. Electro-pneumatic control system normally utilizes an external power supply. If a system uses AC power, then the use of a power supply will consist of a transformer only. When a DC power source is required, the AC power must be rectified, filtered and regulated to provide a constant DC output.

Figure 53: Contact configurations

Switches Switches arc normally distinguished by their contact configuration as 1. Normally open (NO); 2. Normally closed (NC); 3. Changeover contacts (CO).

If the switch is actuated, an NO contact enables current flow and an NC contact disables current flow. The CO contacts can be used as either NO or NC contacts. A range of actuation methods is available such as pushbutton, mechanical, electrical and pneumatic actuation. Figure 53 shows various contact configurations that will be used in electro-pneumatic circuits.

Relays Relays are used in electro-pneumatic circuits as processing and final control elements and may consist of a simple contact pair or have a larger number of NO, NC or CO contacts. The symbol tor electrical relays is shown in Figure 54.

Switches as a Building Block The switch and relay blocks perform tasks within circuits such as providing safety interlocks, manual overrides, holding of relay coils and toggling status indicator lamps ON and OFF. These blocks can be broadly grouped as follows: 1. Logic blocks; 2. Memory blocks.

Figure 54: Relay symbol.

Figure 55: AND block.

Logic Blocks The elementary logic functions that form the blocks are as follows:

1. AND function: The AND function uses the contacts connected in series as shown in Figure 55. Actuation of either S1 or S2 alone produces no output. An output can only be obtained by actuating S1 and S2.

2. OR function: The OR function uses contacts connected parallel to each other. If both switches are not actuated, there will be no output. If S1 or S2 is actuated, an output is produced (Figure 56).

3. NOT function: In NOT block, if the switch is not actuated, a signal is generated at the output; conversely, actuation of the switch removes the output signal (Figure 57).

Memory Blocks Relay and switch contacts can be used to hold solenoid valves, relays and output devices for extended periods without the need to apply additional signals. This type of function can be achieved using a memory to hold the device active.

Figure 56: OR block

Figure 57: NOT block.

Figure 58: Memory block.

A simple circuit for memory block is shown in Figure 58. Actuation of S1 energizes the relay coil K1. This will result in closing of K1 contacts. A latching current path is now established via K1 contacts as S2 is an NC contact. When S1 is released, K1 coil remains energized. Pressing S2 will break the current path to K1 coil and the circuit is unlatched.

Another contact configuration is depicted in Figure 59. The reset switch S2 is connected in series with S1. In this configuration the S2 switch is dominant over the set switch S1. Even if S1 is held actuated, opening S2 contacts will break the current path to K1 coil.

Electronic Sensors Certain applications require the use of electronic sensors employing the following principles: 1. Electrical induction; 2. Capacitance; 3. Infrared light.

These sensors can be used to sense the position of piston rod or used as switching devices. The symbols of these are shown in Figure 60. The electrical limit switch shown in Figure 61 is used to sense the position of piston rod. When the trip cam actuates the switch, the contacts will be closed with NO contacts.

Figure 59: Memory block.

Figure 60: Electrical sensors.

Figure 61: Limit switch (NO): (a) Unactuated; (b) actuated.

Solenoid-Operated Valve Actuation of SW in Figure 62 applies the current to the solenoid. The solenoid generates an electromagnetic force which moves the armature connected to the valve stem as shown in Figure 62(b). When the current is removed from the solenoid coil, the emf is dissipated allowing the internal spring to return the valve stem to the original position.

The electrical and mechanical representation of a solenoid-operated valve is shown in Figure 63. DCVs could be actuated by the solenoids either on one side or on both sides.

Figure 62: Solenoid-operated DCVs: (a) Unactuated; (b) actuated

Figure 63: Symbol for solenoid-operated valves: (a) Representation of mechanical parts; (b) representation of electrical parts.

Figure 64: Symbol of PE converter.

Pneumatic-Electric Converter (PE Converter) The PR converter SCUMS the air pressure of die air line and compares this with a preset value. The PE converter produces an electric signal when the preset limit is reached. This hybrid device combines a pneumatically actuated stem and an electric switch. PI converters are used in electro-pneumatic control systems where a specific pressure is required for switching operation. The symbol of PE converter is shown in Figure 64.

MECHANICAL ACTUATING SYSTEMS

Introduction Mechanical actuation systems mainly consist of mechanisms. If a number of rigid bodies are assembled in such a way that the motion of one causes constrained and predictable motions of the others, it is called a mechanism. Thus, a mechanism transmits and modifies a motion. Mechanisms can "be used for any of the following purposes:

1. They may transform linear motion into rotational motion as in the case of internal combustion (IC) engines where the reciprocating motion of the piston is transformed into rotary motion of the crankshaft.

2. They may transform a rotary motion into a translating motion as in the case of a cam and follower mechanism where rotational motion of the cam is converted into translational movement of the follower.

3. They may transform a motion in one direction into a motion in another direction at right angle to the first, as in the case of a bevel gear and worm gear drives.

4. They may increase or decrease and change the direction of rotational speed of one drive to another, as in the case of gear trains.

Various mechanical elements used in the mechanisms are linkages, cams, gear trains, rack and pinion, chains, belt drives, ratchet and pawl, bearings, etc. In the following topics, we will discuss these elements individually.

Nowadays, most of the actions (which were carried out by the mechanical elements in earlier days) are performed by the use of microprocessor-based electronic systems. For example, a floating ball and lever mechanism was previously used for water level control in an overhead tank in order to switch OFF the electric motor automatically when the tank was filled. Modern water level controllers in an overhead tank use a microprocessor-based touch sensor for sensing the water level in the tank and switch ON-OFF the electric motor accordingly. However, the mechanisms might still be used to provide the following functions:

1. Force amplifications by using mechanical elements such as levers; 2. Change magnitude and direction of speed by using gear trains; 3. Transfer axis of the rotation using a worm gear, bevel gear and/or timing belt.

In this section, the discussion focuses on only kinematic analysis of the mechanisms. Kinematics deals with relative motions of different parts of a mechanism without taking into consideration the forces producing the motions. Thus, it is the study, from a geometric point of view, to know the displacement, velocity and acceleration of a part of a mechanism.

Types of Motion An unconstrained rigid body moving in space can have a very complex motion which may be very difficult to describe. But the motion of a rigid body can be described by a combination of the following translational and rotational motions.

Translational Motions A translational motion is a linear motion along one or more of the following three axes as shown in Figure 65(a): 1. Movement along X-X axis; 2. Movement along Y-Y axis; 3. Movement along Z-Z axis.

Example: Motion of piston in an IC engine cylinder is a translational motion.

Rotational Motion A rotational motion can be considered as a rotation of a rigid body along one or more of the following three axes as shown in Figure 65 (b). 1. Rotation about X-X axis; 2. Rotation about Y-Y axis; 3. Rotation about Z-Z axis.

Figure 65: Types of motion: (a) Translational motion; (b) Rotational motion.

A large majority of mechanisms exhibit complex translational and rotational motions or a combination of these two. For example, the motion of pick and place when a robot picks a workpiece from one place and moves it to another, can be considered as a complex motion. This process involves a robot hand moving to A particular angle toward the workpiece, rotation of the hand and then the robot opening its ringers and moving them to the required position to grasp the workpiece. However, the complex motion can be broken down into a combination of translational and rotational motions so that it is very easy to design the mechanism for achieving the movements.

Degrees of Freedom As already explained in previous sections, a rigid body in space can have three independent translations and three independent rotations about X, Y and Z axes as given in Figures 65 (a) and (b). Therefore, any rigid body in a space can have six degrees of freedom. The minimum number of independent displacements required to specify the system completely is called degrees of freedom. If a link is constrained to move along a line as shown in Figure 66(a), then the number of degrees of freedom is 1. If a link is constrained to move on a plane as shown in Figure 66(b), then it has two degrees of freedom, that is, one translational and one rotational motion.

Constraints The number of degrees of freedom of any system can be reduced by constraints. A rigid body in a space has six degrees of freedom. These degrees of freedom can be reduced to one by

imposing five constrains. Therefore, a constraint is needed for each degree of freedom that is to be prevented from occurring. Hence, degrees of freedom are given by

Number of degrees of freedom = 6 - Number of constraints

If the system has any redundant constraint, then the number of degrees of freedom is given by

Number of degrees of freedom = 6 - Number of constraints -h Number of redundancies

Most of the design engineers follow the principle of least constraint while designing mechanisms.

Figure 66: (a) One degree of freedom; (b) two degrees of freedom.

Figure 67: Types of links: (a) Binary; (b) ternary; (c) quaternary.

Kinematic Chains A mechanism is made of a number of rigid bodies out of which some may have motions relative to the others. A member or a combination of members of a mechanism connecting other members and having a motion relative to them is called a link. A link may not be a rigid body but it should be a resistant body which is capable of transmitting motion and force from one clement to another with negligible deformation. A link is also called a kinematic link. Each link has two or more attachment points to attach with other links. These points are called nodes. Links can be classified into binary, ternary, quaternary, etc. depending upon the number of points of attachments. Figures 67(a)-(c) show three types of rigid links. There is no relative motion between the joints within the link. Links can be classified into the following three types depending on their type of connections:

1. Rigid link: If a link does not undergo any deformation while transmitting motion, then this type of link is called rigid link. For example, connecting rod of an IC engine, crank, cam, valve, etc.

2. Flexible link: Flexible link is a link which is partially deformed in a manner not to affect the transmission of motion. For example, belts, ropes, etc.

3. Fluid link: A fluid link is a link which is formed by having a fluid in a closed container and the motion is transmitting through the fluid by pressure only. For example, hydraulic systems such as brakes, jacks, press, etc.

A joint is a connection between two or more links at their nodes of attachment. A joint allows motion between the connected links. The two links or elements are joined together to form a pair. If the relative motion between them is completely or successfully constrained, the pair is known as kinematic pair. When the kinematic pairs are connected together in such a way that the last link is joined to the first link to transmit definite motion, it is called a kinematic chain. For kinematic chain to transmit motion, one link must be fixed. The other link will then produce predictable relative movement of various links.

For example, the crankshaft of an IC engine shown in Figure 68 forms a kinematic pair with bearings which are fixed in a pair; the connecting rod with crank forms a second kinematic pair. The piston with the connecting rod forms a third pair and the piston with the cylinder forms a fourth pair. The combination of all links forms a kinematic chain.

A mechanism is obtained by fixing one of the links in a kinematic chain and we can obtain as many mechanisms as the number of links in a kinematic chain by fixing the different links. This is known as an inversion of the mechanism.

The design of many mechanisms is based on two basic forms of kinematic chains: 1. four-bar chain or quadratic cyclic chain; 2. slider-crank chain.

Four-Bar Chain or Quadratic Cyclic Chain The simplest and most basic kinematic chain is a four-bar chain. It consists of four rigid links connected to form four joints about which turning can occur. A link that makes complete revolutions is the crank, the link

Figure 68: Kinematic chain.

opposite to the fixed link is the coupler and the fourth link is a lever or rocker if it oscillates or another crank if it rotates. A four-bar linkage has the following characteristics based on the lengths of its links: 1. It is impossible to have a four-bar linkage if the length of one of the links is greater than

the sum of the other three as shown in Figure 69(a). 2. If the sum of the lengths of the largest link and the shortest link is less than or equal to the

sum of the lengths of the other two links, then at least one link will be capable of making a full revolution with respect to the fixed link as shown in Figure 6.5(b). This condition is known as Grashof condition. Link a can only oscillate whereas link c can make a full rotation. This type of mechanism is called a crank-lever mechanism.

3. If the condition given in Figure 6.5(b) is not met then no link is capable of a complete rotation. This situation is shown in Figure 6.5(c). When link d is fixed and the relative lengths of links are such that links a and c can oscillate but not rotate, it is called as double-lever or double-rocker mechanism.

4. If in a four-bar chain [Figure 6.5(d)], two opposite links b and d are parallel and equal in length, then any of the links can be fixed. The two links a and c adjacent to the fixed link will always act as two cranks. In this type, both links a and c will make a complete rotation. This type of mechanism is called a double-crank mechanism.

Figure 69: Inversions of four-bar chain.

Figure 70: Coupled wheels of a locomotive.

The use of a double-crank mechanism is made in the coupled wheels or a locomotive in which the rotary motion of one wheel is transmitted to the other wheel. Figure 69 shows this type of mechanism. Links a and c (having equal lengths) act as cranks and are connected to the respective locomotive wheels. Link b acts as a coupling rod and link d is fixed in order to maintain a constant distance between the centers of the wheels.

The practical use of crank-lever mechanism is cine film advance mechanism which is shown in Figure 71. In this mechanism, link // rotates. So, the end of link b locks into a sprocket up and back into the next sprocket. This movement causes the film to advance for a particular distance.

A Watt's indicator mechanism is an example of double-lever mechanism. It consists or four links as shown in Figure 72. The four links are link a, link b, link c and link d (ternary link). Link a is fixed, and the links c and d act as levers. The displacement of link d is directly proportional to the pressure of gas or steam which acts on the indicator piston. On any small

displacement of the mechanism, the point 5 at the end of link c traces out approximately a straight line.

Slider-Crank Chain Slider-crank chain consists of a crank, a connecting rod and a slider. This type of mechanism is already described in Figure 68. In this engine mechanism, link a is fixed so that there is no relative movement between the center of rotation of the crank and the cylinder housing in which the piston slides. Link b is the crank that rotates, link c is the connecting rod and link d is the piston which moves relative to the fixed link. When the piston slides inside the cylinder or link a, the connecting rod makes the crank shaft rotate. Therefore, this mechanism converts the reciprocating movement into rotational movement. The various inversions of slider-crank mechanism as follows:

1. Pendulum pump: This mechanism is obtained by fixing a cylinder or link d as shown in Figure 73. When the crank or link b rotates, the connecting rod or link c oscillates about a pin pivoted to the fixed link d and the piston attached to the piston rod (link a) reciprocates.

2. Oscillating cylinder engine: Figure 74 shows the arrangement of oscillating cylinder engine mechanism. It is used to convert reciprocating motion into rotary motion. Link d is made in the form of a cylinder and a piston is fixed to the end of link a. When the crank (link b) rotates, the piston attached to link a reciprocates and the cylinder oscillates about a pin pivoted to the point A.

Figure 71: Cine film advance mechanism

Figure 72: Watt's indicator mechanism.

Figure 73: Pendulum pump.

Figure 74: Oscillating cylinder engine.

3. Rotary engine: In rotary engines, seven or nine cylinders are symmetrically placed at regular intervals in the same plane as shown in Figure 75- All cylinders revolve about same fixed center and form a balanced system, while the crank (link b) is fixed and the cylinder (link a) rotates about point O.

4. Crank and slotted lever mechanism: Crank and slotted lever mechanism is widely used in a quick return mechanism of reciprocating machine tools such as slotter, shaper, etc. A mechanism used in

Figure 75: Rotary engine.

a shaping machine is shown in Figure 76. In this mechanism, the ram is actuated by gear drive associated with electric motor. First, the electric motor drives the pinion gear. Next, the pinion gear drives the bull gear which rotates in the opposite direction due to external gear meshing. A radial slide is provided on the bull gear. A sliding block is assembled on this slide. The block can be positioned in radial direction by rotating the stroke adjustment screw.

The sliding block has a crank pin. A rocker arm is freely fitted to this crank pin. The rocker arm sliding block slides in the slot provided in the rocker arm called a slotted link. The bottom end of the rocker is pivoted and its upper end has a fork which is connected to the ram block by a pin.

When the pinion gear rotates along with the bull gear, the crank will also rotate. Owing to this, the rocker arm sliding block also rotates in the same circle. Simultaneously, the sliding block slides up and down in the slot. This movement is transmitted to the ram which reciprocates. Hence, the rotary

Figure 76: Crank and slotted lever mechanism.

Figure 77: Quick return motion.

motion is converted into reciprocating motion. From the Figure 77. A1 and A2 are rear and forward extreme positions of a link S1 and S2 are two extreme positions of a crank pin.

During a forward stroke, the link moves from Al to A2 as the sliding block moves from S1 to S2 in clockwise direction at an angle about crank pin C.

During return stroke, the sliding block moves from S2 to S1 in a clockwise direction through an angle about crank pin C, but the speed of the bull gear is constant throughout. Therefore, the time taken during these two strokes is directly proportional to angles and . However, angle is smaller than angle . So, the time taken by the return stroke will be reduced. Therefore the ratio of cutting time to return time is

5. Whitworth quick return mechanism: Referring to Figure 78, the shaft of an electric motor drives the pinion which rotates the bull gear. The bull gear has a crank pin. A sliding block slides over this crank pin and slides inside the slot of a crank plate. This crank plate is pivoted at point S eccentrically. A connecting rod connects the pin at P on one end and the ram at M on the other end. When the pinion rotates- the bull is also rotated along with the crank pin. At the same time, the sliding block slides on the slot provided on the crank plate. This makes the ram move up and down (reciprocating motion) by the connecting rod. The two important cases are discussed as follows:

When the pin A is at A", the ram is in forward stroke. At that time, the bull gear rotates in an anticlockwise direction at an angle a.

When the bull gear rotates further in the same direction from Y to X at an angle /3, the return stroke takes place. Here, angle f3 is less than angle a. So, the time taken for the return stroke is reduced. Therefore the ratio of cutting time to return time is

Figure 78: Whitworth quick return mechanism.

Cams A cam is a mechanical rotating machine element which is used for converting one motion into another. A cam has a curved or grooved surface which mates with a follower and imparts motion to it. In general, the cam may be rotating or oscillating whereas the follower may be rotating, reciprocating or oscillating. Complicated output motions can easily be produced with the help of cams. Cams are widely used in automatic machines, IC engines, machine tools, printing control mechanisms, spinning and weaving machineries, textile machineries, paper cutting machines and so on.

Types of Cams The most common types of cams according to cam shapes are as follows: 1. Wedge or flat cams: A wedge cam converts a translational motion of the wedge into the

reciprocating motion of the follower. The arrangement of the wedge cam and the follower is shown in Figure 79. A spring is used to maintain the constant contact between the cam and the follower.

2. Plate or disk cams: Plate or disk cams are also called radial cams. They arc made of a thick plate of the required shape and size. The follower moves radially from the center of rotation of the cam. Figure 80 shows this type of cam.

3. Cylindrical cams: Cylindrical cam contains a cylinder in which a circumferential contour is cut on the surface. The cylindrical cam rotates about its axis. The follower motion can be of two types: In the first type, the follower reciprocates in a direction parallel to the cam axis. The

follower rides in a groove at its cylindrical surface [refer Figure 81(a)]. In the second type, the follower oscillates in a direction parallel to the cam axis

[refer Figure 81(b)].

Figure 79: Wedge cam Figure 80: Disk or plate cam

Figure 81: Cylindrical cams: (a) Reciprocating; (b) oscillating.

4. Conical cams: In a conical cam, a cone which has a circumferential contour cut on the surface as shown in Figure 82 rotates about its axis. The follower is reciprocating in a direction parallel to the cone axis.

5. Globoidal cams: Globoidal cams may have convex or concave surfaces as shown in Figures 83(a) and (b). A circumferential contour is cut on the surface of rotation of the cam to impart motion to the follower. The follower has an oscillatory motion.

Types of Cam Followers The different types of cam followers are shown in Figure 84. 1. Knife-edge followers: They are quite simple in construction. However their use is

limited due to great wear of the cam surface at the point of contact. 2. Roller followers: They are essentially ball or roller bearings positioned at the place of

contact. Since the rolling motion takes place between the contacting surfaces, the rate of wear is greatly reduced. The roller followers are extensively used but they are very expensive. In case of steep rise, a roller follower jams and, therefore, is not preferred.

Gears and Gear Trains Gears are mechanical machine elements which transmit motion by means of successively engaging teeth. Gears arc used to transmit motion from one shaft to another or between a shaft and a slide. Rotary motion can be transferred from one shaft to another by a pair of plain cylinders or disks 1 and 2 as shown in Figure 87(a). Power transmitted between two shafts is small because there is a possibility of slip. In order to avoid slipping, a number of projections (called teeth), as shown in Figure 87(b), are provided on the periphery of wheel I, which fit into the corresponding recess on the periphery of wheel 2.

Assuming no slipping of the two wheels, the following relation exists for their linear velocity:

Figure 87: Working of gears: (a) Friction wheels; (b) gears.

where 1 is the angular velocity of wheel 1 in rad/s, N1 the speed of wheel 1 in rpm, r1 the radius of wheel 1 in rpm, 2 the angular ratio of wheel 2, N2 the speed of wheel 2 and r2 the radius of wheel 2. The relationship shows that the speed of the two gear wheels is inversely proportional to the radii of the wheels.

The ratio of the angular speeds of a pair of meshed gear wheels is called the gear ratio. The number of teeth on a wheel is proportional to its diameter. Hence the gear ratio is also given by

where T1 is the number of teeth on gear 1 and T2 is the number of teeth on gear 2.

Types of Gears Gears are classified as follows: 1. According to the relative positions of their axes: The axes of the two shafts between

which the motion is to be transmitted may be Parallel shafts: Depending upon the teeth of the equivalent cylinder, that is, straight

or helical, the following are the main types of gears to join parallel shafts. (a) Spur gears: Spur gears have straight teeth parallel to the axes of the wheel as

shown in Figure 87(b). (b) Helical gears: In helical gears (Figure 88(a)], the teeth are cuived and inclined to

the shaft axis. Two mating gears have the same helix angle, but have teeth of opposite hands. The engagement of a helical gear is very smooth due to continual gradual engagement of helical teeth.

(c) Herringbone gears: A double-helical gear is equivalent to a pair of helical gears attached together; one has a right-hand helix and other a left-hand helix. Such gears are known as herringbone gears and are exhibited in Figure 88(b). The teeth of the two rows arc separated by a groove used for tool run-out.

Intersecting shafts: Two non-parallel or intersecting shafts can be connected by means of bevel gears. The teeth of bevel gears are formed on the frustum of the cones. Two types of bevel gears are shown in Figure 89.

Figure 88: Gear to join parallel shafts: (a) Helical gear; (b) herringbone gear.

Figure 89 Intersections gears: (a) Straight bevel gear; (b) spiral bevel gear.

(a) Straight bevel gears: The teeth are straight, radial to the point of intersection of the shaft axes and vary in cross-section throughout their length. Usually, they are used to connect shafts at right angles. When two bevel gears of the same size connect two shafts at right angles, they are called mitre gears.

(b) Spiral bevel gears: When the teeth of bevel gears are inclined at an angle to the face of the bevel, they are known as spiral bevel gears. The engagements of these gears are very smooth and produce less noise during running.

Non-parallel and non-intersecting gears: The gears shown in Figure 90 are used to transmit motion between two non-parallel and non-intersecting shafts.

Figure 90: Non-parallel and non-intersection gears: (a) Spiral gear; (b) worm and worm gear; (c) hypoid gear

Figure 91 Types of gear based on contact between surfaces: (a) External gears; (b) internal gears; (c) rack and pinion.

(a) Spiral gears or crossed-hclical gears: The spiral gears have a helix cut on their periphery in such a manner that they have two non-parallel axes as shown in Figure 91(a).

(b) Worm gears: Worm gears consist of a worm and worm wheel, as shown in Figure 91(b). A worm and worm wheel can be visualized as a screw and nut pair. They are used to transmit motion between non-parallel and non-intersecting shafts.

(c) Hypoid gears: Hypoid gears are similar to spiral bevel gears but the pinion is larger and stronger than a spiral pinion. Figure 91(c) shows this type of gear.

2. According to the type of contact between surfaces of the gear: In such situations they can be differentiated into the following groups. External gears: When the gears of the two shafts mesh externally with each other, as

shown in Figure 92(a), they are called external gears. The larger of these two wheels is called a spur wheel and the smaller wheel is called a pinion. In external gearing, the

rotation of the two wheels is always opposite to each other, that is, if wheel 1 rotates in a clockwise direction, wheel 2 will rotate in an anti-clockwise direction.

Internal gears: When the gears of the two shafts mesh internally with each other, as shown in Figure 92(b), they are called internal gears. The larger of the two wheels is called annular wheel and the smaller wheel is called -A pinion. In internal gearing, the rotation of two gears is always in the same direction, that is, if wheel 1 rotates in a clockwise direction, wheel 2 will also rotate in the same clockwise direction.

Rack and pinion: A rack is a special case of spur gear; it is made of an indefinite diameter so that the pitch surface is a plane as shown in Figure 92(c). Therefore, a rack is a straight line gear whereas the pinion is a circular wheel similar to a spur gear. The rack and pinion arrangement converts rotary motion into translatary motion, or vice versa.

Gear Trains A gear train is a combination of two or more gears used to transmit motion from one shaft to another. A gear train may consist of spun bevel, helical or spiral gears. The gear train can be classified as follows depending upon the arrangement of wheels: 1. Simple gear train; 2. Compound gear train; 3. Reverted gear train; 4. Planetary gear train.

Simple Gear Train Figure 93 shows a simple gear train in which there is only one gear on each shaft. The gears are represented by their pitch circles. In a single gear train, a pair of mated external gears always moves in opposite directions. Since gear 1 drives gear 3 through gear 2, therefore gear 1 is called driver and gear 3 is called driven or follower. The intermediate gear 2 is called idle gear.

Consider a simple gear train consisting of gears 1, 2 and 3 as shown in Figure 93. Let 1, 2 and 3 be the angular velocities of driver, idler and driven, respectively, and T1, T2 and T3 be the number of teeth

Figure 93: Simple gear train

on a driver, idler and driven, respectively. Since the driver gear 1 is in mesh with idler gear 2, the gear ratio for these two gears is

Similarly, the idler gear 2 is in mesh with driven gear 3, the gear ratio for these two gears is

The overall gear ratio of a simple gear train is the product of gear ratio for each successive pair of gears:

Therefore

Thus, it is seen that the intermediate gears have no effect on the gear ratio and, therefore, they are called idlers. The effect of gear 2 is purely to change the direction of rotation of the driven gear. Also, the idlers arc used when the center to center distance between two shafts is large.

Compound Gear Train In compound gear trains, there are two or more gears on one shaft. Consider a compound gear train consisting of gears 1, 2. 3 and 4 as arranged in Figure 94. Gears 2 and 3 are mounted on the same shaft and hence known are as compound gears. In a compound gear train the intermediate gears are useful for increasing or decreasing the gear ratio of the arrangement. The gears arranged in the same shaft, that is, gears 2 and 3, will have the same angular velocity (2 = 3).

In the above arrangement, if gear 1 is the driver, the gear ratio of pair 1 and 2 will be

Figure 94: Compound gear train

The gear ratio of pair 3 and 4 is

The overall gear ratio is, therefore,

Since gears 2 and 3 are arranged in the same shaft, therefore 2 = 3 and so

That is

The major advantage of a compound gear train over a simple gear train is that a much larger gear ratio can be obtained with a smaller space.

Reverted Gear Train It the axis of the first and the last gears arc co-axial, it is called a reverted gear train. Figure 95 shows a reverted gear train in which gear 1 is a driver and gear 4 is driven. Both gears 1 and 4 are axial. This arrangement is used in clocks, industrial speed reducers and simple lathes.

With usual notations, the overall gear ratio of a reverted gear train is given by

Since the input and output shafts are arranged coaxially, the following condition should be satisfied:

where r is the pitch circle radius of a gear.

Figure 95: Reverted gear train

Planetary Gear Train Planetary gear trains, also referred to as epicyclic gear trains, are those in which one or more gears orbit about the central axis of the train. Thus, they differ from a simple or compound

gear train by having a moving axis or axes. Figure 97 shows a basic arrangement of an epicyclic gear train in which the axis of one of the gears also moves relative to the frame.

Consider two gears 1 and 2, the axes of which arc connected by an arm as shown in figure 6.33. When the arm is fixed, gears 1 and 2 rotate about their own axes and constitute a simple gear train. However, if gear 2 is fixed so that arm 3 can rotate about the axis of gear 2, then gear 1 would also move around gear 2. Hence, it is called epicyclic gear train. In the arrangement, gear 1 which roll around gear 2 is called an epicyclic gear or planet gear. Gear 2 which rotates in its own fixed center is called a sun gear. The arm is called planet carrier.

The gear ratio of the epicyclic gear train can be obtained by solving a set of equations. The number of equations depends upon the number of elements in the gear train. The common procedure to solve problem is explained next.

Figure 97: Epicyclic gear train.

Note that arm is fixed in the arrangement.

Speed of gear 1 relative to the arm = 1 - 3 Speed of gear 2 relative to the arm = 2 - 3

Here the speed of arm is 3

Since gears 1 and 2 mesh directly, they will revolve in an opposite direction. Therefore

If arm 3 is fixed, then 3 = 0, and so

If gear 1 is fixed, then 1 = 0, and so

The type of gear train used in a particular mechanism depends upon the overall gear ratio requirement, space availability, direction of rotation required, type of gear used in the gear train, the relative position of the axes of shafts etc. The overall gear ratio of a simple gear

train is normally limited to 10. Higher gear ratios can be obtained by incorporating compound gear trains.

Ratchet and Pawl A ratchet and pawl mechanism is a mechanical gearing used to transmit intermittent rotary motion. It permits a shaft which is attached to the ratchet to rotate in one direction, but locks it in the opposite direction. In this sense, it acts in the same way as a diode in an electrical circuit or a check valve in a water pipe. The mechanism consists of two elements such as ratchet and pawl (and hence the name) as shown in Figure 98. A wheel with saw-shaped angled teeth around its outer periphery is called a ratchet. The ratchet wheel usually engages with a tooth-shaped short lever called a pawl.

Figure 98: Ratchet and pawl. Figure 99: Weightlifting mechanism.

The pawl is pivoted and can move back and forth to engage the wheel. The pawl is normally spring loaded to ensure that it automatically engages with the ratchet. When the ratchet is rotated in one direction, the pawl is raised and moves smoothly between the angled teeth. When the rotation of the ratchet stops, the pawl rests between the "dip" of the teeth and arrests the rotation in the opposite direction. Ratchet and pawls are usually made of steel, stainless steel, cast iron, brass or other metal materials.

Applications for this mechanism include turnstiles, spanners, winding gear, wrenches, jacks, freewheel mechanism of bicycles, mechanical clocks, etc. One such mechanism used to arrest motion in the weight lifting mechanism is shown in Figure 99. A ratchet wheel is fixed to a shaft and winding drum. A rope is wound around the drum. In this mechanism, it is possible to wind up the rope by rotating the ratchet in an anti-clockwise direction. When the rotation is stopped, the load on the rope will try to unwind the rope. This might not be possible because of the action of the ratchet and pawl. Note that the pawl is held in position against the ratchet by a tension spring.

Belt Drive Belt drives are used to transmit power from one shaft to another by means of pulleys. A belt drive consists of three elements, viz. a driving pulley, a driven pulley and an endless belt which envelopes them, as shown in Figure 100. The pulleys are mounted on the two shafts by using keys. The belt is provided with a certain amount of initial tension to avoid slip. The

motion of the driving pulley is, generally, transferred to the driven pulley via the friction between the belt and the pulley. The amount of power transmitted between the pulleys depends upon the velocity of the belt, the tension under which the belt is placed on the pulleys and the arc of contact between the belt and the small pulley.

Here the driving pulley pulls the belt from one side (lower side) and delivers it to the other side (upper side). Thus, the tension in the lower side (also known as the tight side as the tension is more on this side) of the belt will be more than in the upper side (also known as the slack side) of the belt. The torque transmitted by a belt drive is due to the differences in tight and slack side tensions. Let T1 be the tight side tension and T2 be the slack side tension, then the torque transmitted by a belt drive is given by:

Figure 100: Belt drive Here rA and rB are the radii of pulleys A and B. respectively.

From the torque, we can obtain the power transmitted by the belt drive as follows:

where v is the belt speed in m/s and is the angular velocity of belt in rad/s.

If the ratio between the tight side tension and the slack side tension T1/T2 is more, the power transmission capacity of a belt drive increases. However, when the ratio T1/T2 reaches a certain value, known as the limiting ratio, the belt will slip on the pulley; therefore. T1/T2 depends upon the coefficient of friction () between the belt and pulley and the angle of lap () of the belt around the pulley. The limiting ratio of tension is given by

Figure 101: Types of belts: (a) Flat belt; (b) V belt; (c) circular belt; (d) timing belt.

Types of Belts Though there are many types of belts used now-a-days, the following are important types of belts: 1. Flat belt: Flat belt is rectangular in cross-section as shown in Figure 101(a). It is mostly

used in lactones and workshops, where .1 moderate amount ol power is to be transmitted from one pulley to another. Flat-belt drives have an efficiency of about 98%. They can transmit power when the two pulleys are not more than 8 m apart. In a flat-belt drive, the rim of the pulley is slightly crowned which helps to keep the belt running centrally on the pulley rim. Flat belts are made of leather, canvas, cotton and rubber.

2. V belt: A V belt is trapezoidal in cross-section as shown in Figure 101(b). The groove on the rim of the pulley of a V belt drive is made deeper to take advantage of the wedge action. The belt does not touch the bottom of the groove. V-belt drive has a better torque-transmitting capacity as compared to the flat-belt drive. Also, a multiple V-belt system, using more than one belt on a single pulley, can be used to increase the power-transmitting capacity. Generally, these are more suitable for short distances. V belts are made of rubber impregnated fabric with angle of V between 30 and 40.

3. Circular belt or rope: This belt is circular in cross-section as shown in Figure 101(c). It is used with grooved pulleys. The rope is gripped on its sides as it bends down in the groove, reducing the chances of slipping. Pulleys with several grooves can also be employed to increase the power-transmitting capacity. Rope drives are, usually, preferred when distances between the centers of shafts are long. The material used for circular belts is cotton, hemp, manila or wire.

4. Timing or synchronous belt: Timing belts have teeth cut on their inner surface as shown in Figure 101(d). They require toothed wheels like gears. The teeth cut on the belt fit into the grooves on the wheel. A timing belt enables positive drive because it does not stretch or slip and transmits power at a constant velocity ratio. It is extensively used in low power applications.

Chain Drive In a belt and rope drive, there is a chance of slipping and hence a constant velocity ratio cannot be obtained. A belt can be replaced by a chain whenever there is a need to have a constant velocity ratio or positive drive. There is no slipping in case of a chain drive. The chains are made up of rigid links which are hinged together in order to provide the necessary flexibility for wrapping around the driving and driven wheels. Like gears, chains are made of

metal and, therefore, occupy lesser space and give constant velocity ratios. Like belt they are used for long center distances.

The wheel has projecting teeth which fits into the corresponding recesses in the links of the chain, as shown in figure 102. This is known as a sprocket wheel ox simply sprocket. The wheels and the chains art-contained to move together without slipping and ensure constant velocity ratio due to the chain pass round the sprocket as a series of chordal links.

In general, the chains are used to transmit rotary motion and power from one shaft to another when the distance between the centers of the shafts is short. The chain drive is similar to the timing belt drive but can be used to transmit large torques. The drive mechanisms used with a bicycle or motor cycle are examples of a chain drive.

The disadvantages of chain drives are: 1. They are heavier compared to the belt drives 2. Due to gradual stretching (slipping) of chain drive, links have to be removed frequently.

Figure 102: Chain drive.

3. The production cost of the chains is relatively high. 4. The chain drive requires accurate mounting and careful maintenance.

Bearings A bearing is a stationary machine element which supports rotating shafts or axles and confines their motion. Naturally, a bearing will be required to offer minimum frictional resistance to moving parts so as to result in minimum loss of power. In order to reduce frictional resistance, a layer of fluid may be provided.

Classification of Bearing Bearings are mainly classified into the following types: 1. Based on the type of load acting on the shaft:

Radial bearings: In radial bearings, the load acts perpendicular to the direction of motion of moving parts (i.e. shaft), as shown in Figure 103.

Thrust bearings: In thrust bearings the pressure acts along or parallel to the axis of the shaft as shown in Figure 104.

2. Based on the nature of contact:

Sliding contact bearings: In sliding contact bearings, the shaft rotates in a bearing and there are no interposed elements between shaft and bearings. There is a direct contact between shaft and bearings as shown in Figure 105(a).

Rolling contact bearings or antifriction bearings: In rolling contact bearings, the steel balls or rollers are provided in between shaft and bearings to reduce friction as shown in Figure 105(b).

Figure 104: Thrust bearing,

Figure 105: (a) Sliding contact bearing; (b) rolling contact bearing.

Introduction to Journal Bearings A journal bearing is a sliding contact bearing which gives lateral support to the rotating shaft. It consists of two main parts:

1. A journal the part of the shah which runs in a sleeve or bushing.

103

2. A hollow cylinder (sleeve or bushing). In a journal bearing, the diameter of the journal is kept less than the diameter of the bearing to allow the flow of lubricant between the surfaces.

Types of Journal Bearing 1. Depending upon the nature of contact

Full journal bearing: In Kill journal bearings, the angle ol contact of journal with oil is 360 as shown in Figure 106. This bearing is capable of supporting a radial force in any direction.

Partial bearing: In partial bearing, the journal has less than 360" of contact with oil. Generally. the angle varies between 90 and 180 although 120 is the preferred angle of contact. It can support only the unidirectional load.

Fitted bearing: II in the partial bearing, the diameters of the journal and bearing are equal, it is called a fitted bearing. There is no clearance between journal and bearing.

2. Depending upon the nature of lubrication: Thick film type: Here the surfaces of the bearings are completely separated from

each other by the lubricant. Thin film type: Here although a lubricant is present, surfaces are partially in contact

with each other. It is also termed as boundary lubrication.

Figure 106: Types of journal bearing: (a) Full journal bearing; (b) partial bearing; (c) fitted bearing,

Hydrostatic bearings: Here the fluid film pressure is obtained by supplying the lubricant under high pressure such that the force exerted by the pressure supports the loaded shaft at all points.

Hydrodynamic bearings: In this type, the fluid film pressure is generated only by the rotation of the journal. The position of the journal gets adjusted in such a manner that a force supporting the journal load is produced due to film pressure.