Hydraulic pumps - part 2

Fig. 8. Axial-piston pump varies displacement by changing angle of swashplate.

Fig. 9. Radial piston pump.

Fig. 10. Pressure-flow curve of fixed-displacement hydraulic pump.

Fig. 11. Pressure flow curve of variable-displacement hydraulic pump with ideal flow and pressure compensation.

Fig. 12. Schematic of typical proportional pump pressure compensator control.

Fig. 13. Pressure-flow curve of variable-displacement hydraulic pump equipped with pressure compensation.

Fig. 14. Schematic of pump two-stage compensator control.

Piston pumps

The piston pump is a rotary unit which uses the principle of the reciprocating pump to produce fluid flow. Instead of using a single piston, these pumps have many piston-cylinder combinations. Part of the pump mechanism rotates about a drive shaft to generate the reciprocating motions, which draw fluid into each cylinder and then expels it, producing flow. There are two basic types, axial and radial piston; both area available as fixed and variable displacement pumps. The second variety often is capable of variable reversible (overcenter) displacement.

Most axial and radial piston pumps lend themselves to variable as well as fixed displacement designs. Variable displacement pumps tend to be somewhat larger and heavier, because they have added internal controls, such as handwheel, electric motor, hydraulic cylinder, servo, and mechanical stem.

Axial piston pumps

The pistons in an axial piston pump reciprocate parallel to the centerline of the drive shaft of the piston block. That is, rotary shaft motion is converted into axial reciprocating motion. Most axial piston pumps are multi-piston and use check valves or port plates to direct liquid flow from inlet to discharge.

Inline piston pumps

The simplest type of axial piston pump is the swashplate design in which a cylinder block is turned by the drive shaft. Pistons fitted to bores in the cylinder block are connected through piston shoes and a retracting ring, so that the shoes bear against an angled swashplate.

As the block turns, Figure 8, the piston shoes follow the swashplate, causing the pistons to reciprocate. The ports are arranged in the valve plate so that the pistons pass the inlet as they are pulled out and the outlet as they are forced back in. In these pumps, displacement is determined by the size and number of pistons as well as their stroke length, which varies with the swashplate angle.

In variable displacement models of the inline pump, the swashplate swings in a movable yoke. Pivoting the yoke on a pintle changes the swashplate angle to increase or decrease the piston stroke. The yoke can be positioned with a variety of controls, i.e., manual, servo, compensator, handwheel, etc.

Bent axis pumps

This pump consists of a drive shaft which rotates the pistons, a cylinder block, and a stationary valving surface facing the cylinder block bores which ports the inlet and outlet flow. The drive shaft axis is angular in relation to the cylinder block axis. Rotation of the drive shaft causes rotation of the pistons and the cylinder block.

Because the plane of rotation of the pistons is at an angle to the valving surface plane, the distance between any one of the pistons and the valving surface continually changes during rotation. Each individual piston moves away from the valving surface during one-half of the shaft revolution and toward the valving surface during the other half.

The valving surface is so ported that its inlet passage is open to the cylinder bores in that part of the revolution where the pistons move away. Its outlet passage is open to the cylinder bores in the part of the revolution where the pistons move toward the valving surface. Therefore, during pump rotation the pistons draw liquid into their respective cylinder bores through the inlet chamber and force it out through the outlet chamber. Bent axis pumps come in fixed and variable displacement configurations, but cannot be reversed.

Radial-piston pumps

In these pumps, the pistons are arranged radially in a cylinder block; they move perpendicularly to the shaft centerline. Two basic types are available: one uses cylindrically shaped pistons, the other ball pistons. They may also be classified according to the porting arrangement: check valve or pintle valve. They are available in fixed and variable displacement, and variable reversible (over-center) displacement.

In pintle-ported radial piston pump, Figure 9, the cylinder block rotates on a stationary pintle and inside a circular reacting ring or rotor. As the block rotates, centrifugal force, charging pressure, or some form of mechanical action causes the pistons to follow the inner surface of the ring, which is offset from the centerline of the cylinder block. As the pistons reciprocate in their bores, porting in the pintle permits them to take in fluid as they move outward and discharge it as they move in.

The size and number of pistons and the length of their stroke determine pump displacement. Displacement can be varied by moving the reaction ring to increase or decrease piston travel, varying eccentricity. Several controls are available for this purpose.

Plunger pumps

These reciprocating pumps are somewhat similar to rotary piston types, in that pumping is the result of pistons reciprocating in cylinder bores. However, the cylinders are fixed in these pumps; they do not rotate around the drive shaft. Pistons may be reciprocated by a crankshaft, by eccentrics on a shaft, or by a wobble plate. When eccentrics are used, return stroke is by springs. Because valving cannot be supplied by covering and uncovering ports as rotation occurs, inlet and outlet check valves may be used in these pumps.

Because of their construction, these pumps offer two features other pumps do not have: one has a more positive sealing between inlet and outlet, permitting higher pressures without excessive leakage of slip. The other is that in many pumps, lubrication of moving parts other than the piston and cylindrical bore may be independent of the liquid being pumped. Therefore, liquids with poor lubricating properties can be pumped. Volumetric and overall efficiencies are close to those of axial and radial piston pumps.

Measuring pump performance

Volume of fluid pumped per revolution is calculated from the geometry of the oil-carrying chambers. A pump never quite delivers the calculated, or theoretical, amount of fluid. How close it comes is called volumetric efficiency. Volumetric efficiency is found by comparing the calculated delivery with actual delivery. Volumetric efficiency varies with speed, pressure, and the construction of the pump.

A pump's mechanical efficiency is also less than perfect, because some of the input energy is wasted in friction. Overall efficiency of a hydraulic pump is the product of its volumetric efficiency and the mechanical efficiency.

Pumps are generally rated by their maximum operating pressure capability and their output, in gpm, at a given drive speed, in rpm.

Matching pump power with the load

Pressure compensation and load sensing are terms often used to describe pump features that improve the efficiency of pump operation. Sometimes these terms are used interchangeably, a misconception that is cleared up once you understand the differences in how the two enhancements operate.

To investigate these differences, consider a simple circuit using a fixed-displacement pump running at constant speed. This circuit is efficient only when the load demands maximum power because the pump puts out full pressure and flow regardless of load demand. A relief valve prevents excessive pressure buildup by routing high-pressure fluid to tank when the system reaches the relief setting. As Figure 10 shows, power is wasted whenever the load requires less than full flow or full pressure. The unused fluid energy produced by the pump becomes heat that must be dissipated. Overall system efficiency may be 25% or lower.

Variable displacement pumps, equipped with displacement controls, Figure 11, can save most of this wasted hydraulic horsepower when moving a single load. Control variations include hand wheel, lever, cylinder, stem servo, and electrohydraulic servo controls. Examples of displacement control applications are the lever-controlled hydrostatic transmissions used to propel windrowers, skid-steer loaders, and road rollers.

While matching the exact flow and pressure needs of a single load, these controls have no inherent pressure or power-limiting capabilities. And so, other provisions must be made to limit maximum system pressure, and the prime mover still must have corner horsepower capability. Moreover, when a pump supplies a circuit with multiple loads, the flow and pressure-matching characteristics are compromised.

A design approach to the system in which one pump powers multiple loads is to use a pump equipped with a proportional pressure compensator, Figure 12. A yoke spring biases the pump swashplate toward full displacement. When load pressure exceeds the compensator setting, pressure force acts on the compensator spool to overcome the force exerted by the spring.

The spool then shifts toward the compensator-spring chamber, ports pump output fluid to the stroking piston, and decreases pump displacement. The compensator spool returns to neutral when pump pressure matches the compensator spring setting. If a load blocks the actuators, pump flow drops to zero.

Using a variable-displacement, pressure-compensated pump rather than a fixed-displacement pump reduces circuit horsepower requirements dramatically, Figure 13. Output flow of this type of pump varies according to a predetermined discharge pressure as sensed by an orifice in the pump's compensator. Because the compensator itself operates from pressurized fluid, the discharge pressure must be set higher - say, 200 psi higher - than the maximum load-pressure setting. So if the load-pressure setting of a pressure-compensated pump is 1,100 psi, the pump will increase or decrease its displacement (and output flow) based on a 1,300-psi discharge pressure.

A two-stage pressure-compensator control, Figure 14, uses pilot flow at load pressure across an orifice in the main stage compensator spool to create a pressure drop of 300 psi. This pressure drop generates a force on the spool which is opposed by the main spool spring. Pilot fluid flows to tank through a small relief valve. A spring chamber pressure of 4,700 psi provides a compensator control setting of 5,000 psi. An increase in pressure over the compensator setting shifts the main stage spool to the right, porting pump output fluid to the stroking piston, which overcomes bias piston force and reduces pump displacement to match load requirements.

The earlier stated misconception stems from an observation that output pressure from a pressure-compensated pump can fall below the compensator setting while an actuator is moving. This does not happen because the pump is sensing the load, it happens because the pump is undersized for the application. Pressure drops because the pump cannot generate enough flow to keep up with the load. When properly sized, a pressure-compensated pump should always force enough fluid through the compensator orifice to operate the compensator.