Wobble-plate piston pump

The wobble-plate piston pump design shown in Figure 8-19 is one type of inline or axial-piston pump. As the wobble plate turns, the spring-loaded pistons reciprocate -- drawing in fluid as they spring return and discharging it as they are forced to extend. Direction of rotation is not important for this pump because flow is the same when it turns either way.

Many pumps of this design operate at very high pressure and can flow high volume as well. Another feature is the ability to isolate the outlet of one or more pistons to give more than one flow volume to a circuit. This allows a single pump to function like other double or triple pumps in hi-lo circuits or to operate different actuators at various flows and pressures. This design pump can also be made variable volume and/or pressure compensated. Some designs use a restricted inlet to accomplish both functions because the spring-loaded pistons will not fill as far if their inlet is restricted.

Inline or axial-piston, fixed-volume pumps

Figure 8-20 shows a more common design for piston pumps. This design is seldom used as a fixed-volume pump because it can be made pressure compensated -- which many circuits require. This design can be fixed-volume, variable volume, pressure compensated and bi-directional flow, the same as the radial-piston design. The main reasons for its popularity are its compact design and its lower price. A radial-piston pump of the same flow will normally cost four to six times as much as the inline design and be three to four times larger physically.

An inline piston pump like the one in Figure 8-20 is similar in design to the wobble-plate pump. The main difference is in the way the pistons move and stroke. An inline pump uses a fixed-angle swashplate instead of a wobble plate. The pistons are not spring loaded but are held against the swashplate by piston shoes and a shoe plate. The pistons are pulled out of and pushed into their bores mechanically.

The pistons are fitted in the cylinder block, which is splined to the drive shaft, and they turn along with the shoes and the shoe plate. As the pistons slide down the swashplate, they are pulled out of their bores and create a vacuum at the inlet port. Atmospheric pressure forces fluid to fill the piston bores until the pistons reach the bottom of the swashplate angle. Fluid enters through the kidney-shaped openings half way around one revolution. As the cylinder block continues to turn, the pistons are forced back into their bores and fluid discharges through the outlet. A kidney-shaped opening on the other half of the valve plate allows fluid to flow until the pistons are fully returned. Inline pumps always have an odd number of pistons, so one never can be directly across from another at the transition from being pulled out to being pushed in.

Inline piston pumps require a case drain to send bypass and/or control oil back to tank. The drain line should be unrestricted at all times and should terminate below the fluid level in the tank. If the drain line terminates above fluid level, the pump housing can be vacuumed dry, causing damage to the pump.

It is good practice to install a flow meter in the drain line. The flow meter indicates when to change out the pump before it loses efficiency or is worn beyond repair. A flow meter with an integral limit switch can be set to give a warning when case drain flow goes above a specified volume. Usually a pump should be changed when case flow is greater than 7 to 10% of maximum rated flow.

Inline piston pump efficiency runs in the 95 to 98% range. They, are very versatile, have many control options, and would work well on any type circuit. They are more expensive than gear and vane pumps so they lose out when price is the deciding factor.

Variable-volume inline or axial piston pumps

Most inline pumps have some way to change the angle of the swashplate. This makes the pump capable of variable volume, pressure compensation, and bi-directional flow. Figure 8-21 shows a variable-volume setup with a manual control. Low-flow pumps (those under 20 gpm) can use manual controls. Higher-flow pumps need hydraulically powered pistons to move against the higher forces in the pump.

The basic operation of this pump is the same as a fixed-volume inline piston pump. The difference here is the angle of the swashplate can be changed manually to allow longer or shorter piston strokes for more or less volume while the pump turns at the same speed. This feature can conserve energy when an actuator needs variable speeds. It replaces a flow control that limits flow and either sends excess fluid across a relief valve or forces a pressure-compensated pump to go to high pressure and reduced flow. Other controls include manual servo, manual handwheel, and electronic servo, to name a few.

If this pump is in an open-loop circuit, make sure the control cannot go past center -- or no flow -- condition. If the lever is moved left of perpendicular, flow reverses and the pump tries to take fluid from the circuit and send it to tank. Very soon the pump will run dry and be damaged due to lack of lubrication. (Later in this text, a bi-directional pump circuit is shown with all the necessary additions to make the pump work properly in a bi-directional mode.)

Notice that the symbol in Figure 8-21 duplicates the standard pump symbol with a sloping arrow added to it. This indicates a pump with variable or adjustable flow.