Basic Hydraulics - Pumps
CHAPTER 2 - Pumps
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| Figure 2.1 - a gear pump |
GEAR PUMP
Pumps are fluid power components that transform mechanical energy transmitted by a prime mover
into fluid power energy. Gear pumps are compact, relatively inexpensive, and have few moving parts. External gear pumps consist of two gears, usually equal in size, that mesh with each other inside a housing. The driving gear is an extension of the drive shaft. As it rotates, it drives the second gear. As both gears rotate, fluid is drawn in through the inlet. This fluid is trapped between the housing and the rotating teeth of the gears where it travels around the housing and is pushed through the outlet port. The pump creates flow at a given pressure, which transfers energy from the mechanical input source to a fluid power actuator.
UNBALANCED VANE PUMP
The rotating portion of the pump, or rotor, is positioned off center of the cam ring, or housing. The rotor is connected to a prime mover by means of a shaft. As the rotor is turned, the vanes are thrown out by centrifugal force and contact the ring, or housing, forming a positive seal.
Fluid enters the pump and fills the large volume area formed by the offset rotor. As the vanes push the fluid around the cam the volume decreases, and the fluid is pushed out of the outlet port.
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| Figure 2.2 - a unbalanced vane pump |
BALANCED VANE PUMP
In the unbalanced vane pump, which has been previously illustrated, one half of the pumping mechanism is at less than atmospheric pressure. The other half is subjected to full system pressure. This results in side loading the shaft while under high-pressure conditions. To compensate for this, the ring in a balanced vane pump is changed from circular to cam-shaped. With this arrangement, the two pressure quadrants oppose each other. Two ports intake fluid and two pump fluid out. The two intake ports and the two outlets ports are connected inside the housing. Because they are on opposite sides of the housing, excessive force or pressure buildup on one side is canceled out by equal but opposite forces on the other side. With the forces acting on the shaft balanced, the shaft side load is eliminated.
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| Figure 2.3 - a balanced vane pump |
Flow is created in the same manner as was illustrated in the unbalanced vane pump, the only difference being that there are two discharge and two suction cavities rather than one. It is notable that constant volume, positive displacement vane pumps used in industrial systems are generally of the balanced design.
PISTON PUMP
Axial piston pumps convert rotary motion of an input shaft to an axial reciprocating motion, occurring at the pistons. This is accomplished by a swashplate that is either fixed or variable in its degree of angle. As the piston barrel assembly rotates, the pistons rotate around the shaft with the piston slippers in contact with and sliding along the swashplate surface. With the swashplate vertical, no displacement occurs because there is no reciprocating motion. As the swashplate increases in angle, the pistons move in and out of the barrel as it follows the angle of the swashplate surface. During one half of the circle of rotation, the piston moves out of the cylinder barrel and generates an increasing volume. In the other half of the rotation the piston moves into the cylinder barrel and generates a decreasing volume. This reciprocating motion draws fluid in and pumps it out.
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| Figure 2.4 - a piston pump |
FIXED VS VARIABLE
There are two types of positive displacement hydraulic pumps: a fixed pump, which produces a fixed flow (gpm) based on the rpm of the prime mover or electric motor, and a variable pump, which can vary its rate of flow (gpm) while the input speed (rpm) remains constant. Although displacement is typically measured in volume displaced per revolution, output is measured in gpm.
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| Figure 2.5 - | gear pump and output device |
In Figure 2. 5, a motor turning at 1200 rpm is driving a fixed displacement gear pump producing
5 gpm flow. The flow (gpm) can be changed if the speed (rpm) of the motor changes. When a variable displacement pump is used in the system, the flow (gpm) can be varied in two ways. As with fixed displacement pumps, the flow (gpm) will be changed if the speed (rpm) of the motor is changed. The second way is to vary the displacement of the pump. For example, the displacement of an axial piston pump is determined by the distance the pistons are pulled in and pushed out of the cylinder barrel.
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| Figure 2.6 - | displacement pump and output device |
Since the swashplate angle controls this distance in an axial piston pump, we need only to change the angle of the swashplate to alter the piston stroke and pump volume. Several means of varying the swashplate angles are used. They may include hand levers, mechanical stops, or more sophisticated, hydraulically positioned devices. If the pump produces 5-gpm flow with 1200 rpms and maximum displacement, the flow (gpm) can be varied by moving the swashplate in the upright position or de-stroking the pump. This will vary the flow from 0 – 5 gpm. The gallon per minute discharge of fixed displacement pumps can only be changed by increasing or decreasing the speed of the electric motor.
PRESSURE COMPENSATED
Variable volume pumps can also be pressure compensated. A pressure compensated piston pump de-strokes, or moves to zero output, at a predetermined pressure. This is accomplished by hydraulically positioning the pumping chambers to zero output while maintaining compensator pressure at the outlet of the pump. Figure 2.7 illustrates a pressure compensated piston pump. It is helpful to understand the functionality of a piston pump. As the pistons rotate around the shaft and follow the angle of the swashplate, they are pumping fluid out the outlet, which provides pressure to move a component such as a cylinder. When the cylinder reaches the end of its stroke, pressure rises at the outlet of the pump as the fluid’s flow path is blocked.
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| Figure 2.7 - a pressure compensated piston pump |
This pressure forces the compensating spool up, allowing the pressurized fluid to energize the de-stroking piston and push against the swashplate, forcing it to a vertical position. With the swashplate vertical, the pump is now de-stroked and the pressure at the outlet board is maintained at a constant level. A very slight amount of flow is produced to maintain de-stroke pressure. This flow is bypassed into the case and carried back to the reservoir through the pump case drain outlet.
Of the three types of hydraulic pumps discussed (gear, vane and axial piston), only the vane and piston may be pressure compensated.
SUMMARY
- A pump’s purpose is to provide flow for the hydraulic system. Pumps are fluid power components that transform mechanical energy transmitted by a prime mover into fluid power energy.
- External gear pumps consist of two gears, usually equal in size, that mesh with each other inside a housing.
- In the unbalanced vane pump one half of the pumping mechanism is at less than atmospheric pressure.
- The balanced vane pump has two pressure quadrants which cancel each other out because they are on opposing sides of a cam-shaped cylinder.
- Axial piston pumps convert rotary motion of an input shaft to an axial reciprocating motion occurring at the pistons.
- A pressure compensated piston pump de-strokes, or moves to zero output, at a predetermined pressure.
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