# Unit Seven: Pumps and Compressors

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Unit Seven: Pumps and Compressors
Pumps 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 three basic types of pumps are the Gear, Piston, and Vane designs. The following hydraulic formula illustrates a relationship between Horsepower, Pressure, and Flow. Hydraulic Horsepower = GPM x PSI x

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 1500 The 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 1500psi, every gallon of flow produced by the pump will require at least one horsepower to drive it. In pneumatic systems there is a similar relationship.

Pumps and Compressors Before we go any further it should be pointed out that no matter what design of pump or compressor is being discussed they all produce flow the same way. Pumps and compressors produce flow by creating a “pressure differential.” An example would be a person drinking water through a straw.

Pumps and Compressors Hydraulic Pump Symbol Pneumatic 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.

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 does not move. The center piece rotates and is off center. The dark lines are the vanes and they move in and out.

Vane Pumps To understand pump operation, first imagine that the area in green is attached to a port and is under low pressure. Fluid, as influenced by atmospheric pressure, rushes in to fill the voids as the assembly rotates.

Vane Pumps As the fluid passes from left to right it becomes “trapped” between the rotating group and the outlet port. 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 In normal operation most pumps are “loaded” to one side because of pressure at the outlet port. This has an effect on bearing life. A balanced vane pump has its ports located in four distinct locations around its shaft to offset this effect and extend service life.

Cartridge Assembly A lot of vane pump manufacturers have incorporated the rotating group into a removable assembly that can be replaced independently of the housing.

Double Pump Schematic symbol for a 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.

Variable Volume Vane Pump Speed and Displacement
“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 “amount” of oil displaced by fixed displacement pumps? Speed and Displacement

Variable Volume Vane Pump Operation
The key to understanding this illustration is knowing that displacement depends on the amount of 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. If the rotor travels from one side to the next, flow reverses ports.

Volumetric Output of a Pump
Theoretical Pump Flow = Speed x Displacement 231 What this means is that other than an internal mechanical mechanism that changes flow rate the only two thins that control flow 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. 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 and flow will stop while pressure is maintained.

Pressure Compensated Variable Volume Vane Case Drain
All pumps experience internal leakage but it is worst in the models illustrated here. To alleviate this pressure, and thus prevent the front seal from blowing out, a case drain is provided.

Gear Pumps In a gear pump, an increasing volume is generated as teeth un-mesh or move away from each other. The fluid drawn in is forced around the teeth, not through the middle. As the teeth move toward each other, fluid is forced from the outlet port.

Axial(swash plate)piston pump
Piston Pumps There are two major categories of pumps: Axial and Radial. Axial(swash plate)piston pump

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 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. 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 condition Compensator 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 amount of flow to zero when the tension of the spring has been reached.

Overcenter 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.

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 the receiver tank. All compressors operate under the same principles as pumps but the fluid is a gas.

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 known for low pressures but high volumes of air. 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. Single stage compression is less efficient because so much heat is given up in the receiver which translate into lost pressure.