Figure 8-15 shows a cutaway view of a basic radial-piston pump that can function as fixed volume, variable volume, pressure compensated, and bidirectional flow, or a combination of these functions. The pump in Figure 8-15 is variable volume only. As a fixed-volume pump it would have the reaction ring offset as shown in the right hand cutaway view, with no method of changing that condition. (This is one configuration that will probably never be used with this design pump.)

As the cylinder block and pistons rotate, centrifugal force pushes the pistons against the reaction ring. When the pump is in the on-flow condition (as in the right-hand cutaway view), the pistons are moving out of their bores in the lower half of the picture and forming a vacuum. Fluid is forced into the inlet and fills these voids. As the pistons pass left center, they stop extending and begin to be pushed back into their bores. During the top half of their travel, the pistons force the trapped fluid through the outlet to the circuit. Moving the reaction ring’s centerline closer to the cylinder block’s centerline reduces flow

Pressure-compensated, radial-piston pumps

The radial-piston pump in Figure 8-16 is pressure compensated. This pump produces flow when the outlet pressure falls below the level set by the pressure-adjusting screw. When pressure in the pilot line increases enough to compress the compensator spool’s spring, pilot flow is connected to the compensator piston, and its drain to the case is blocked. Pilot flow to the compensator piston forces the reaction ring to move against the return spring and reduce outlet flow. The reaction ring never reaches center because the circuit, pilot control, and internal leakage must be overcome to hold pressure.

Two symbols can be used to show pressure-compensated pumps schematically. The complete symbol at the lower right of Figure 8-16 shows all the functions, while the simplified symbol above it omits the case drain and places the compensating arrow inside the pump circle. Again, because most schematic drawings are done on CAD systems now, the simplified symbol is seldom used.

A radial-piston pump can also produce bi-directional flow. It can take in or force out fluid from either port while turning the same direction. This design pump is used in closed-loop circuits where all outlet flow goes to an actuator and return flow from the actuator goes back to the pump inlet. A common circuit of this type is a hydrostatic drive. Fluid from a bi-directional pump goes to a bi-directional motor to give infinitely variable output speed and force in either direction of rotation without requiring a directional control valve.

Bi-directional, radial-piston pumps

The pump in Figure 8-17 has a small opposing piston that pushes continuously against a larger control piston on the opposite side of the reaction ring. The control piston can be pressurized or exhausted by a 3-way servovalve, thus infinitely varying the reaction ring position to either side of center. Input signals to the servovalve can come from manual, mechanical, or electronic controllers. A common circuit produces four manually variable flows and directions, using four single-solenoid directional control valves.










A charge pump, driven off the main pump shaft, supplies pilot oil to maintain pressure on the opposing piston. It also supplies oil to the mechanical-feedback servovalve that pressurizes or exhausts the control piston. The charge and pilot circuits usually run at 250 to 400 psi. Notice that the “A” and “B” ports are only connected to the actuator -- not to tank -- when using a hydraulic motor or double rod-end cylinder. (The pump must have added tank ports to operate a single rod-end cylinder circuit.)

Figure 8-18 shows a cutaway view and schematic drawing of a bi-directional pump driving a single rod-end cylinder. Because there is less volume in the rod end of a single rod-end cylinder, flow to and from that end is less in relation to the cap end. This poses a problem when using a closed-loop circuit.












The pump cutaway and schematic show how adding suction check valves, a shuttle valve, and a bypass relief valve allow the pump to bypass excess flow from the cap end and take in added flow for the rod end. This is a common circuit for this type pump. With this circuit, cylinder speed is infinitely variable and direction change requires no directional control valve. Direction change is very smooth because flow must go to zero in one direction before it can reverse. Because of this, the actuator rapidly and smoothly decelerates to a stop condition. When flow reverses, it increases steadily to full flow in the opposite direction without system shock.