Bi-Directional Pumps Can be an Efficient Option Used to Control Motors and Cylinders.
RG Group Offers Several lines of Bi-Directional Pumps and can Assist with Specifying the Correct Pump for your Application.
Axial- and radial-piston pumps can output fluid from either port while rotating in one direction. Closed-loop circuits take advantage of this feature of piston pumps. A closed-loop pump circuit sends fluid to an actuator while fluid from the same device comes back to the pump’s inlet.
NOTE: Don’t confuse bi-directional pumps with bi-rotational pumps. Bi-rotational pumps can flow out of either port, but only when rotation reverses. A bi-rotational pump has one port hooked to tank and the other port piped to the circuit. Most bi-rotational pumps operate hydraulic circuits on off-road equipment because rotation of the pump-driving shaft is different from one piece of equipment to another.
Normally, bi-directional pumps do not have a port piped to tank. Both ports hook directly to the cylinder or motor ports. Many bi-directional circuits operate hydraulic motors, because they accept and return nearly the same amount of fluid. The most common closed-loop circuit is the hydrostatic drive — often used on off-road equipment.
Fig. 1. Symbol for bi-directional pump
Figure 1 shows the schematic symbol for a bi-directional pump. Notice that there are two energy triangles to show that fluid flows out of both ports. The pump only outputs from one port at a time while the opposite port is inlet. With one port hooked to a tank and the other port piped to a circuit, the pump serves as a variable-displacement uni-directional pump. Flow direction of a bi-directional pump hooked up this way depends on the position of the stroking control. By changing the stroking control position, either port can serve as the inlet or outlet.
Fig. 2. Bi-directional pump in closed-loop hydrostatic circuit
Figure 2 shows a hydrostatic transmission — a common bi-directional pump circuit. Small fixed-displacement pump A (called a charge pump) makes up for leakage in the main pump and motor while the circuit operates. Check valves B protect the charge pump and only allow oil into the return side of the closed loop. Charge relief valve C dumps excess charge flow to tank at 150 to 300 psi. Charge pump flow generates heat in hydrostatic systems. Many hydrostatic systems use charge pump fluid to operate pump controls and/or auxiliary circuits.
Fig. 3. Bi-directional closed-loop pump circuit
When return flow does not equal output flow, use the bi-directional pump schematic shown in Figure 3. With a single-rod end cylinder attached to a bi-directional pump, the volume of fluid going to the cap end when the cylinder extends is greater than the flow returning to the pump from the rod end. When cylinder direction changes, the opposite is true. Without a way to overcome flow inequalities, a bi-directional pump powering a single-rod cylinder wouldn’t work.
For single-rod cylinders, add check valve A, low-pressure relief valve B, and NC pilot-operated 2-way valve C to the closed-loop circuit. Check valve A allows the pump to take oil from the tank when the cylinder extends. Relief valve B and 2-way valve C provide a path for excess oil to go to a tank when the cylinder retracts.
Often large cylinders operating at high pressure and speed use bi-directional pumps with unequal flow capabilities. This circuit is very efficient and virtually eliminates hydraulic shock.