Md and cad ii notes

1 Machine Design and CAD-ii Notes, By: Muhammad Sheeraz

Keys and Coupling

Introduction

A key is a piece of mild steel inserted between the shaft and hub or boss of the pulley to connect

these together in order to prevent relative motion between them.

It is always inserted parallel to the axis of the shaft.

Keys are used as temporary fastenings and are subjected to considerable crushing and shearing

stresses.

A keyway is a slot or recess in a shaft and hub of the pulley to accommodate a key. Types of Keys

The following types of keys are important;

1. Sunk keys, 2. Saddle keys, 3. Tangent keys, 4. Round keys, and 5. Splines.

Sunk Keys

The sunk keys are provided half in the keyway of the shaft and half in the keyway of the hub or boss of

the pulley. The sunk keys are of the following types :

1. Rectangular sunk key. A rectangular sunk key is shown in Fig.

2. Square sunk key. The only difference between a rectangular sunk key and a square sunk key is

that its width and thickness are equal, i.e.

w = t = d / 4

3. Parallel sunk key. The parallel sunk keys may be of rectangular or square section uniform n width

and thickness throughout. It may be noted that a parallel key is a taper less and is used where the

pulley, gear or other mating piece is required to slide along the shaft.

4. Gib-head key. It is a rectangular sunk key with a head at one end known as gib head. It is usually

provided to facilitate the removal of key. A gib head key is shown in Fig. 13.2 (a) and its use in

shown in Fig. 13.2 (b).

5. Feather key. A key attached to one member of a pair and which permits relative axial movement is

known as feather key. It is a special type of parallel key which transmits a turning moment and also

permits axial movement. It is fastened either to the shaft or hub, the key being a sliding fit in

the key way of the moving piece.


6. Woodruff key. The woodruff key is an easily adjustable key. It is a piece from a cylindrical disc

having segmental cross-section in front view as shown in Fig. 13.4. A woodruff key is capable

of tilting in a recess milled out in the shaft by a cutter having the same curvature as the disc from

which the key is made. This key is largely used in machine tool and automobile construction.

The main advantages of a woodruff key are as follows:

1. It accommodates itself to any taper in the hub or boss of the mating piece.

2. It is useful on tapering shaft ends. Its extra depth in the shaft *prevents any

tendency to turnover in its keyway.

The disadvantages are:

1. The depth of the keyway weakens the shaft.

2. It cannot be used as a feather.

Saddle keys

The saddle keys are of the following two types :

1. Flat saddle key, and 2. Hollow saddle key.

A flat saddle key is a taper key which fits in a keyway in the hub and is flat on the shaft as

shown in Fig. 13.5. It is likely to slip round the shaft under load. Therefore it is used for

comparatively light loads.

A hollow saddle key is a taper key which fits in a keyway in the hub and the bottom of the key is

shaped to fit the curved surface of the shaft. Since hollow saddle keys hold on by friction, therefore

these are suitable for light loads. It is usually used as a temporary fastening in fixing and setting

eccentrics, cams etc.


Tangent Keys

The tangent keys are fitted in pair at right angles as shown in Fig. 13.6. Each key is to withstand torsion in

one direction only. These are used in large heavy duty shafts.

Round Keys

The round keys, as shown in Fig. 13.7(a), are circular in section and fit into holes drilled partly in the

shaft and partly in the hub. They have the advantage that their keyways may be drilled and reamed after

the mating parts have been assembled. Round keys are usually considered to be most appropriate for low

power drives.

Splines

Sometimes, keys are made integral with the shaft which fits in the keyways broached in the hub. Such

shafts are known as splined shafts as shown in Fig.13.8. These shafts usually have four, six, ten or sixt-

een splines. The splined shafts are relatively stronger than shafts having a single keyway. The splined

shafts are used when the force to be transmitted is large in proportion to the size of the shaft as in autom-

obile transmission and sliding gear transmissions. By using splined shafts, we obtain axial movement as

well as positive drive is obtained.

V-Belt and Rope Drives Introduction

V-belt is mostly used in factories and workshops where a great amount of power is to be

transmitted from one pulley to another when the two pulleys are very near to each other.

The V-belts are made of fabric and cords moulded in rubber and covered with fabric and

rubber.

The included angle for the V-belt is usually from 30° to 40°.

The power is transmitted by the wedging action between the belt and the V-groove in the

pulley or sheave.


Advantages and Disadvantages of V-belt Drive over Flat Belt Drive

Advantages

1. The V-belt drive gives compactness due to the small distance between centres of pulleys.

2. The drive is positive, because the slip between the belt and the pulley groove is negligible.

3. Since the V-belts are made endless and there is no joint trouble, therefore the drive is smooth.

4. It provides longer life, 3 to 5 years.

5. It can be easily installed and removed.

6. The operation of the belt and pulley is quiet.

7. The belts have the ability to cushion the shock when machines are started.

8. The high velocity ratio (maximum 10) may be obtained.

9. The wedging action of the belt in the groove gives high value of limiting ratio of tensions. Therefore

the power transmitted by V-belts is more than flat belts for the same coefficient of friction, arc of contact

and allowable tension in the belts.

10. The V-belt may be operated in either direction, with tight side of the belt at the top or bottom. The

center line may be horizontal, vertical or inclined.

Disadvantages

1. The V-belt drive cannot be used with large centre distances, because of larger weight per unit length.

2. The V-belts are not so durable as flat belts.

3. The construction of pulleys for V-belts is more complicated than pulleys of flat belts.

4. Since the V-belts are subjected to certain amount of creep, therefore these are not suitable for

constant speed applications such as synchronous machines and timing devices.

5. The belt life is greatly influenced with temperature changes, improper belt tension and

mismatching of belt lengths.

6. The centrifugal tension prevents the use of V-belts at speeds below 5 m/ s and above 50 m / s.

Rope Drives

The rope drives are widely used where a large amount of power is to be transmitted, from one

pulley to another, over a considerable distance. It may be noted that the use of flat belts is limited

for the transmission of moderate power from one pulley to another when the two pulleys are not

more than 8 metres apart. If large amounts of power are to be transmitted, by the flat belt, then it

would result in excessive belt cross-section.

The ropes drives use the following two types of ropes:

1. Fibre ropes, and 2. Wire ropes.

The fibre ropes operate successfully when the pulleys are about 60 metres apart, while the wire

ropes are used when the pulleys are upto 150 metres apart.

Fibre Ropes

The ropes for transmitting power are usually made from fibrous materials such as hemp, manila

and cotton.

Since the hemp and manila fibres are rough, therefore the ropes made from these fibres are not

very flexible and possesses poor mechanical properties.

The cotton ropes are very soft and smooth. The lubrication of cotton ropes is not necessary. But

if it is done, it reduces the external wear between the rope and the grooves of its sheaves.

Notes:

1)-The diameter of manila and cotton ropes usually ranges from 38 mm to 50mm. The size of the

rope is usually designated by its circumference or ‘girth’.


2)-The ultimate tensile breaking load of the fibre ropes varies greatly. For manila ropes, the

average value of the ultimate tensile breaking load may be taken as 500 𝑑2kN and for cotton

ropes, it may be taken as 350𝑑2 kN, where d is the diameter of rope in mm.

Advantages of Fibre Rope Drives The fibre rope drives have the following advantages:

1. They give smooth, steady and quiet service.

2. They are little affected by outdoor conditions.

3. The shafts may be out of strict alignment.

4. The power may be taken off in any direction and in fractional parts of the whole amount.

5. They give high mechanical efficiency.

Wire Ropes

When a large amount of power is to be transmitted over long distances from one pulley to

another (i.e. when the pulleys are up to 150 meters apart), then wire ropes are used.

The wire ropes are widely used in elevators, mine hoists, cranes, conveyors, hauling

devices and suspension bridges.

The wire ropes run on grooved pulleys but they rest on the bottom of the grooves and are

not wedged between the sides of the grooves.

The wire ropes are made from cold drawn wires in order to have increase in strength and

durability.

It may be noted that the strength of the wire rope increases as its size decreases.

The various materials used for wire ropes in order of increasing strength are wrought

iron, cast steel, extra strong cast steel, plough steel and alloy steel. For certain purposes,

the wire ropes may also be made of copper, bronze, aluminum alloys and stainless steels.

Advantages of Wire Ropes

The wire ropes have the following advantages as compared to fibre ropes.

These are lighter in weight,

These offer silent operation,

These can withstand shock loads,

These are more reliable,

These are more durable,

They do not fail suddenly,

The efficiency is high, and

The cost is low.

Classification of Wire Ropes

According to the direction of twist of the individual wires and that of strands, relative to each

other, the wire ropes may be classified as follows:

1. Cross or regular lay ropes. In these types of ropes, the direction of twist of wires in the

strands is opposite to the direction of twist of the stands, as shown in Fig. 20.8 (a). Such type of

ropes are most popular.

2. Parallel or lang lay ropes. In these type of ropes, the direction of twist of the wires in the

strands is same as that of strands in the rope, as shown in Fig. 20.8 (b). These ropes have better

bearing surface but is harder to splice and twists more easily when loaded. These ropes are more

flexible and resists wear more effectively. Since such ropes have the tendency to spin, therefore

these are used in lifts and hoists with guide ways and also as haulage ropes.


3. Composite or reverse laid ropes. In these types of ropes, the wires in the two adjacent

strands are twisted in the opposite direction, as shown in Fig. 20.8 (c).

Note: The direction of the lay of the ropes may be right handed or left handed, depending upon

whether the strands form right hand or left hand helixes, but the right hand lay ropes are most

commonly used.

Sliding Contact Bearings

Introduction

A bearing is a machine element which support another moving machine element (known

as journal).

It permits a relative motion between the contact surfaces of the members, while carrying

the load. Due to the relative motion between the contact surfaces, a certain amount of

power is wasted in overcoming frictional resistance and if the rubbing surfaces are in

direct contact, there will be rapid wear.

In order to reduce frictional resistance and wear and in some cases to carry away the heat

generated, a layer of fluid (known as lubricant) may be provided. The lubricant used to

separate the journal and bearing is usually a mineral oil refined from petroleum, but

vegetable oils, silicon oils, greases etc., may be used.

Classification of Bearings

1. Depending upon the direction of load to be supported. The bearings under this group are

classified as:

(a) Radial bearings, and (b) Thrust bearings.

a) In radial bearings, the load acts perpendicular to the direction of motion of the moving

element as shown in Fig. 26.1 (a) and (b).

b) In thrust bearings, the load acts along the axis of rotation as shown in Fig.26.1(c).


2. Depending upon the nature of contact. The bearings under this group are classified as:

(a) Sliding contact bearings, and (b) Rolling contact bearings.

a) In sliding contact bearings, as shown in Fig. 26.2 (a), the sliding takes place along the

surfaces of contact between the moving element and the fixed element. The sliding

contact bearings are also known as plain bearings.

b) In rolling contact bearings, as shown in Fig. 26.2 (b), the steel balls or rollers, are

interposed between the moving and fixed elements. The balls offer rolling friction at two

points for each ball or roller.

Types of Sliding Contact Bearings

The sliding contact bearings in which the sliding action is guided in a straight line and

carrying radial loads, as shown in Fig. 26.1 (a), may be called slipper or guide bearings. Such

type of bearings are usually found in cross-head of steam engines.

The sliding contact bearings in which the sliding action is along the circumference of a

circle or an arc of a circle and carrying radial loads are known as journal or sleeve

bearings.

When the angle of contact of the bearing with the journal is 360° as shown in Fig. 26.3

(a), then the bearing is called a full journal bearing. This type of bearing is commonly

used in industrial machinery to accommodate bearing loads in any radial direction.

When the angle of contact of the bearing with the journal is 120°, as shown in Fig. 26.3

(b), then the bearing is said to be partial journal bearing. This type of bearing has less

friction than full journal bearing, but it can be used only where the load is always in one


direction. The most common application of the partial journal bearings is found in rail

road car axles.

The full and partial journal bearings may be called as clearance bearings because the

diameter of the journal is less than that of bearing.

When a partial journal bearing has no clearance i.e. the diameters of the journal and

bearing are equal, then the bearing is called a fitted bearing, as shown in Fig. 26.3 (c).

The sliding contact bearings, according to the thickness of layer of the lubricant between

the bearing and the journal, may also be classified as follows :

1. Thick film bearings. The thick film bearings are those in which the working surfaces are

completely separated from each other by the lubricant. Such type of bearings are also called

as hydrodynamic lubricated bearings.

2. Thin film bearings. The thin film bearings are those in which, although lubricant is

present, the working surfaces partially contact each other atleast part of the time. Such type

of bearings are also called boundary lubricated bearings.

3. Zero film bearings. The zero film bearings are those which operate without any lubricant

present.

4. Hydrostatic or externally pressurized lubricated bearings. The hydrostatic bearings are

those which can support steady loads without any relative motion between the journal and the

bearing. This is achieved by forcing externally pressurized lubricant between the members.

Hydrodynamic Lubricated Bearings

In hydrodynamic lubricated bearings, there is a thick film of lubricant between the journal

and the bearing. When the bearing is supplied with sufficient lubricant, a pressure is build up

in the clearance space when the journal is rotating about an axis that is eccentric with the

bearing axis. The load can be supported by this fluid pressure without any actual contact

between the journal and bearing. The load carrying ability of a hydrodynamic bearing arises

simply because a viscous fluid resists being pushed around.

The load supporting pressure in hydrodynamic bearings arises from either

1. The flow of a viscous fluid in a converging channel (known as wedge film lubrication), or

2. The resistance of a viscous fluid to being squeezed out from between approaching surfaces

(known as squeeze film lubrication).

Assumptions in Hydrodynamic Lubricated Bearings

The following are the basic assumptions used in the theory of hydrodynamic lubricated

bearings:

1. The lubricant obeys Newton's law of viscous flow.

2. The pressure is assumed to be constant throughout the film thickness.

3. The lubricant is assumed to be incompressible.

4. The viscosity is assumed to be constant throughout the film.

5. The flow is one dimensional, i.e. the side leakage is neglected.


Rolling Contact Bearings

Introduction

In rolling contact bearings, the contact between the bearing surfaces is rolling instead of

sliding as in sliding contact bearings.

It is an outstanding advantage of a rolling contact bearing over a sliding bearing that it

has a low starting friction. Due to this low friction offered by rolling contact bearings,

these are called antifriction bearings.

Advantages and Disadvantages of Rolling Contact Bearings Over Sliding

Contact Bearings

Advantages

1. Low starting and running friction except at very high speeds.

2. Ability to withstand momentary shock loads.

3. Accuracy of shaft alignment.

4. Low cost of maintenance, as no lubrication is required while in service.

5. Small overall dimensions.

6. Reliability of service.

7. Easy to mount and erect.

8. Cleanliness.

Disadvantages

1. More noisy at very high speeds.

2. Low resistance to shock loading.

3. More initial cost.

4. Design of bearing housing complicated.

Types of Rolling Contact Bearings

Following are the two types of rolling contact bearings:

1. Ball bearings; and 2. Roller bearings.

The ball and roller bearings consist of an inner race which is mounted on the shaft or journal

and an outer race which is carried by the housing or casing. In between the inner and outer race,

there are balls or rollers.

Clutches Introduction

A clutch is a machine member used to connect a driving shaft to a driven shaft so that the

driven shaft may be started or stopped at will, without stopping the driving shaft.

The use of a clutch is mostly found in automobiles.

In order to change gears or to stop the vehicle, it is required that the driven shaft should

stop, but the engine should continue to run. It is, therefore, necessary that the driven shaft

should be disengaged from the driving shaft.

The engagement and disengagement of the shafts is obtained by means of a clutch which

is operated by a lever.


Types of Clutches

Following are the two main types of clutches commonly used in engineering practice:

1. Positive clutches, and 2. Friction clutches.

Positive Clutches

The positive clutches are used when a positive drive is required. The simplest type of a

positive clutch is a jaw or claw clutch. The jaw clutch permits one shaft to drive another

through a direct contact of interlocking jaws.

A square jaw type is used where engagement and disengagement in motion and under

load is not necessary. This type of clutch will transmit power in either direction of

rotation.

The spiral jaws may be left-hand or right-hand, because power transmitted by them is in

one direction only.

This type of clutch is occasionally used where the clutch must be engaged and

disengaged while in motion.

The use of jaw clutches are frequently applied to sprocket wheels, gears and pulleys. In

such a case, the non-sliding part is made integral with the hub.

Friction Clutches

A friction clutch has its principal application in the transmission of power of shafts and

machines which must be started and stopped frequently.

Its application is also found in cases in which power is to be delivered to machines

partially or fully loaded.

The force of friction is used to start the driven shaft from rest and gradually brings it up

to the proper speed without excessive slipping of the friction surfaces. In automobiles,

friction clutch is used to connect the engine to the drive shaft.

The proper alignment of the bearing must be maintained and it should be located as close

to the clutch as possible. It may be noted that:

The contact surfaces should develop a frictional force that may pick up and hold the load

with reasonably low pressure between the contact surfaces.

The heat of friction should be rapidly dissipated and tendency to grab should be at a

minimum.

The surfaces should be backed by a material stiff enough to ensure a reasonably uniform

distribution of pressure.

Material for Friction Surfaces The material used for lining of friction surfaces of a clutch should have the following

characteristics:

1. It should have a high and uniform coefficient of friction.

2. It should not be affected by moisture and oil.

3. It should have the ability to withstand high temperatures caused by slippage.

4. It should have high heat conductivity.

5. It should have high resistance to wear and scoring.

Md and cad ii notes

Engineering

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