The pressure-compensated pump shown in Figure 8-22 can change outlet flow when pressure tries to go above a predetermined setting. This design pump only has outlet flow when there is a pressure drop due to circuit demand. (Most manufacturers offer an option to limit maximum flow when pressure drops to add versatility to the circuit.) The maximum-volume adjusting screw keeps the volume-destroking piston from retracting all the way even when pressure drops.

Pump operation is the same as previously explained for fixed-volume inline or axial piston pumps. The difference is that this design has a moveable swashplate that is held on stroke by the on-stroke spring. These pumps always produce full flow when pressure is below the compensator setting.

When this pump’s outlet flow meets resistance, pressure builds in the pump-pressure communicating port. This pressure pushes against the spring-loaded compensator spool. In its normal position, this spool allows fluid behind the volume-destroking piston to go to tank through the case drain. When pressure is high enough to force the compensator spool against its spring, the spool allows fluid to flow into the chamber behind the volume-destroking piston while it blocks flow to tank. Enough fluid enters the chamber behind the volume-destroking piston to push the swashplate against its spring and start destroking the pump. The swashplate moves to a position to stroke the pistons just enough to makeup for the bypass and control fluid used by itself and any fluid used in the circuit. This could be any amount of flow -- even zero. Because of this, the pump never sends fluid to tank across a high-pressure relief valve, so heat generation is minimal. Pump stroke varies anytime fluid is required -- from maximum to minimum depending on circuit use.

Two symbols can be used to show pressure-compensated pumps schematically. The complete symbol on the left shows all the functions, while the simplified symbol on the right omits the case drain and puts the compensating arrow inside the pump circle. Because most schematic drawings are done on CAD systems now, the simplified symbol is seldom used.

The inline pump design is subject two common problems:

  1. Operating the pumps at high vacuum inlet can quickly deteriorate the swaged connection between the piston and shoe (see Figure 8-23). When this joint is subjected to extra pulling and then pushing 12 to 1800 times per minute, it wears and comes apart quickly. When it does come apart, it wrecks the swashplate surface and the rest of the piston shoes. Most manufacturers recommend 1 psi or less vacuum at the inlet, and indicate longer life if the pump is supercharged by another pump at 5- to 30-psi inlet pressure.
  2. The shoe has hollowed out areas on its face that receive oil through an orifice in the piston as it forces fluid out. This bypass oil lubricates the shoe and causes it to float a few micrometers off the swashplate. This happens because the shoe’s area is greater than the area of the opposing piston. Because there is no metal-to-metal contact between these parts, the pump has long life expectancy. If contamination stops flow of pressurized fluid, the shoes will contact the swashplate at high force while there is minimum lubrication. The pump fails shortly thereafter. When possible, feed the pump with at least tank head pressure by mounting it alongside or under the tank. Also, keep fluid cleanliness level at least that specified by the pump manufacturer.

As noted in Figure 8-23, always fill the case of a new or repaired piston pump with fluid before startup. The pump needs lubrication and will have very little until bypass fills the case. Also, a filled case will seal clearances and make it easier for the pump to prime.

Fixed-volume bent-axis pumps

Another type piston pump is the bent-axis design shown in Figure 8-24. Like the radial piston pumps previously discussed, this is a very expensive pump and it is physically large when all its optional features are installed. Therefore, this design pump is not a common sight in industry.

Its main advantage over an inline piston pump is that it holds up much better when the inlet sees high vacuum. The piston connections are stronger and are not prone to separating from the drive.

This type pump is manufactured in fixed-volume, variable-volume, and pressure-compensated models, as well as with bi-directional flow and combinations these functions. It has an efficiency range from 95 to 98% and gives long service life when supplied with clean fluid. Most manufacturers make this pump in low- to high-volume sizes. Most are capable of 4000 psi and more.

The cutaway view of a fixed-volume bent-axis pump in Figure 8-24 shows that as the drive shaft turns, the cylinder block also turns at the same rate through the universal drive link. Because the cylinder block is at an angle to the drive shaft, the pistons reciprocate in their bores. The pistons draw in fluid during one half of each revolution and discharge fluid during the other half. Kidney-shaped openings in the valve plate direct the fluid in and out of the piston bores. Because the housing is a single piece, the angle and volume is fixed for a given rpm.