Lecture Five

Pumps and CompressorsPumps and compressors are primary sources of flow in fluid power systems. Maximum system horsepower is controlled by the size of these components along with system flow.

The following hydraulic formula illustrates a relationship between Horsepower, Pressure, and Flow.

Hydraulic Horsepower = GPM x PSI x .000583

The three basic types of pumps are the Gear, Piston, and Vane designs.

HYDRAULIC PUMPS AND POSITIVE DISPLACEMENT

In its most basic sense, positive displacement means what you take in you put out. In other words, for each revolution of a hydraulic pump of this type, a specific quantity of fluid is produced relating to the displacement of the pump.

The Rule of 1500The rule of 1500 is a engineering reference to a predictable relationship between Horsepower, Flow, and Pressure. This is used when sizing an electric motor for a particular hydraulic system.

In any hydraulic system operating at a pressure of 1500 psi, every GPM of flow produced by the pump will require at least one horsepower to drive it. This relationship is linear – in other words, at a pressure of 750 psi, each GPM of flow would require ½ horsepower. At 3000 psi, each GPM would need 2 horsepower, and so on.

In pneumatic systems there is a similar relationship. However, since the relationship is not linear (because air compresses), the math required to calculate it is beyond the scope of this course.

Pumps and CompressorsBefore we go any further it should be pointed out that no matter what design of pump or compressor is being discussed, all pumps produce flow the same way.

Pumps and compressors produce flow by creating a “pressure differential.” Fluids always flow from a higher pressure to a lower pressure. By using the increasing pump volume at the inlet, the pump (or compressor) creates a region of decreasing inlet pressure (vacuum). This causes the higher reservoir pressure (atmospheric) to push the fluid to the lower pressure inlet area. An example would be a person drinking water through a straw. One sucks, creating a lower pressure (vacuum) inside the mouth, and the higher (atmospheric) pressure on the liquid surface pushes it up the straw.

The fluid is trapped inside the pump, and it is carried to the outlet where the volume is decreased and pressure increased. Fluid will take the path of least resistance, which is out the device outlet.

Pumps and Compressors

Hydraulic Pump SymbolPneumatic Compressor Symbol

Its important to note that the above symbols do not indicate a specific design type, just function. As you can see, the only difference is the triangle. Remember, pumps and compressors only produce FLOW. Pressure is created by resistance to that flow. If there is no resistance, flow moves at atmospheric pressure, and there is no (gage) pressure.

Vane Pumps

In the above illustration, only the internal parts are shown. Normally, one port would be connected to the “increasing” volume side and another port would be connected to the “decreasing” volume side. The outer piece (called the “cam ring”) does not move. The center piece (the “rotor”) rotates and is off center. The dark lines are the “vanes”. They move in and out because of centrifugal force to maintain a seal against the cam ring.

Vane Pumps

To understand pump operation, first imagine that the area in green is attached to an inlet port and is under low pressure (vacuum). Fluid, as influenced by the atmospheric pressure in the reservoir, rushes in to fill the voids as the assembly rotates and the volume increases causing a pressure decrease (Boyle’s Law).

Vane Pumps

As the fluid passes from left to right it becomes “trapped” between the rotating group and the pump housing. It is carried around to the outlet port. There, the volume containing the fluid decreases, causing the pressure to increase. Fluids always take the path of least resistance (as does electricity), so out to the system it goes. All pumps operate in this manner regardless of design or configuration.

Balanced Vane Design

The previous slides showed normal vane pump. In normal operation vane and gear pumps are “loaded” to one side because of pressure at the outlet port. This “loading” has an adverse effect on bearing life. A balanced vane pump has its ports located in four distinct locations around its shaft (2 inlets and 2 outlets, each 180 ° apart) to balance the forces on the shaft. The result is extended bearing service life.

Cartridge Assembly

A lot of manufacturers of fixed displacement vane pumps have incorporated the rotating group into a removable assembly that can be replaced independently of the housing, so piping can remain connected. This assembly, called a “cartridge”, consists of the rotor, vanes, cam ring and port plates. It minimizes down time to rebuild a pump, and reduces piping leaks in the system. Cartridge assemblies are rarely used in unbalanced vane pumps.

Double Pump

Although vane pumps are sometimes put together in pairs to form a “double pump,” any design could be made a double pump. All this means is that you have two pumps driven by one motor which may have their flows put together or separated. Typically, one of the pumps is several times larger than the other, so the larger pump will produce much more flow. An application for this type of pump will be discussed in Unit 11.

Schematic symbol for a double pump

Variable Volume Vane Pump

“Variable volume” means that the “amount” of oil which is displaced by a pump each revolution can change whereas in other ”fixed” displacement models it cannot. What controls the “total amount” of oil (flow - GPM) produced by a fixed displacement pump?

Speed and Displacement

Variable Volume Vane Pump Operation

The key to understanding this illustration is knowing that amount of displacement depends on the offset that exists between the rotor and cam ring. The more the offset, the more the displacement, and the less the offset, the less the displacement. If the rotor becomes centered, there is NO displacement. Flow STOPS, but outlet pressure is maintained. If the rotor travels past the cam ring centerline from one side to the other, flow reverses ports.

Volumetric Output of a Pump

Theoretical Pump Flow = Speed x Displacement

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What this means is that if there is no internal mechanical mechanism that changes the displacement volume causing flow rate change, the only two things that control flow from a pump are the physical size of the pump and how fast you run it.

Pressure Compensated Variable Volume Vane Pump Operation

As pressure builds in the system it is felt everywhere including the pump outlet. As outlet pressure rises, the cam ring will push away from the pressure direction toward the path of least resistance which is the spring. When the pressure of the system is equal to the tension of the spring, the rotor will be in the center of the cam ring, Then flow will stop while pressure in the system is maintained as determined by the spring adjustment.

Pressure Compensated Variable Volume Vane Case

Drain

All pumps experience internal leakage but it is worst in the models illustrated here. To alleviate the internal pressure caused by this leakage, a case drain is provided. This relieves the internal pressure and prevents the front seal on the pump shaft from blowing out,

Gear Pumps

In a gear pump, an increasing volume (therefore, decreasing pressure) is generated as teeth un-mesh or move away from each other. The fluid drawn into the pump inlet from the reservoir is forced around the teeth toward the pump outlet, not through the middle. As the teeth move toward each other, the volume decreases, and the increasing pressure forces the fluid out the outlet port.

Piston Pumps

There are two major categories of pumps: Axial and Radial.

Axial(swash plate)piston pump

Piston Pumps

Illustrated above are examples of Radial piston pumps. Their pistons meet the drive shaft axis at 90 degrees . They are more compact yielding space savings and can be ran at a wide speed range. As motors, they are very popular in cabling systems.

Piston Pumps

Piston pumps operate under the same controlling principles as all other pumps. With this design, a piston moves back and forth in a barrel. As the piston moves back, a larger volume is created that provides a vacuum. As the piston moves forward, the volume is decreased (pressure increases) and fluid is forced out. Axial piston pumps have pistons that move in parallel to the drive shaft axis. Radial piston pumps have pistons that move at 90 degrees to the drive shaft axis. Axial piston pumps are, by far, the most common. Either type can be made variable volume by adjusting the amount of stroke the piston travels in the cylinder bore.

Pressure Compensated Axial Piston Pump

Low pressure-full stroke conditionCompensator fires at pressure setting- no flow

In the axial pump above, a pressure build up causes the compensator rod to push against the swash plate which in turn decreases the stroke of the pistons. This reduces amount of flow to zero when the pressure completely balances the tension of the spring, in the same manner as the variable volume pressure compensated vane pump. The symbols for the pressure compensated vane and piston pumps are the same.

Over-center (reversible) Axial Piston Pumps

As in the vane pump, reverse flow in the piston pump is accomplished by moving the rotating group beyond a “center” point. In the piston pump the swash plate is the member that moves to + or – 0 degrees to achieve this feature. These types of pumps are often found in hydrostatic transmissions. The symbol above is could indicate any the piston and vane pump with reverse flow capability. The description of the component on the system schematic would tell the technician the details of the pump used.

Compressors

Compressors operate by drawing in air at lower than atmospheric conditions and then trapping, and compressing it. Once compressed, the air is allowed to escape to the path of least resistance, usually into the receiver tank. Compressors create flow using the same differential pressure principles as pumps do, but the fluid is a gas. Pump and Compressor symbols are identical, except for the flow arrow. In both cases, the arrow points “out” of the device, indicating flow is produced. For pumps, the arrow is solid – for compressors, transparent. For a quick determination if a schematic is hydraulic or pneumatic, find the flow producer symbol. Solid is hydraulic – transparent is pneumatic.

Compressors

Compressors fall into one of two main categories; Dynamic and Displacement.

Dynamic Compressors

Dynamic compressors are not positive displacement. They move air by adding kinetic energy to it or in other words they “throw” the air. Examples of dynamic compressors would include a leaf blower, hair dryer, and common fan. Dynamic compressors are primarily used for low pressures but high volumes of air. Dynamic compressors (fans) are very common at low pressures. When typical pneumatic system pressures are required, their high cost limits applications to oil free air requirements, such as in the food and pharmaceutical industries. A jet engine is another example of a dynamic compressor.

Displacement Compressors

Standard displacement compressors can be single stage where the air is compressed once or multi-stage where the air is compressed two or more times to achieve higher efficiency. In operation, air is drawn in as the piston moves down. When the piston moves up air is compressed and then released to the receiver tank.

Multi-stage Compressors

In multi-stage compressors, air is compressed twice in order to get it to the receiver tank at a higher pressure but lower temperature. The hot compressed air is cooled after the first stage in an “intercooler” to reduce the air temperature entering the next stage. Since the entering air temperature is lower, the outlet air temperature is also lower. Single stage compression is less efficient because more heat has to be given up in the receiver which translates into lost pressure.

Screw Compressors

Dry and Oil flooded

Screw Compressors

Oil Separation

1. Explain the purpose of the pump.

2. List three different types of pumps.

3. Assuming fixed displacement, what two things determine the flow rate of a pump?

4. What is the relationship between flow and input horsepower?

5. What is the rule of 1500?

6. Name the major internal components of a vane pump assembly.

7. What type of vane pump is balanced?

8. What is pressure compensation?

9. What is the purpose of a case drain in a hydraulic pump?

10. What is the difference between a axial piston pump and a radial piston pump?

11. What compressor type is the most common?

12. What is multi-staging?

13. Why is multi-staging more efficient?

14. What is a dynamic compressor?

15. What is the most common use for dynamic compressors?

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