The circuit in Figure 8-32 is a typical hydrostatic-transmission setup. It uses a variable volume, bi-directional pump to drive a hydraulic motor at infinitely variable speed. Hydrostatic drives are normally used to drive vehicles but can be used in industrial applications where smooth acceleration, deceleration, and reversing are required. These circuits usually incorporate an inline or axial-piston pump coupled to a variety of hydraulic motors. As a closed-loop circuit, all pump flow goes to the motor and all motor flow returns to the pump. With 100% efficient parts, the circuit could run with the same oil its whole life. In the real world however, the hydraulic motor and bi-directional pump have internal bypass so a fixed-volume charge pump is placed in the circuit to make up for leaks. The charge pump can also supply fluid to control circuits and accessory devices.

The charge pump inlet draws fluid from a reservoir through a low-micron filter and sends it to the inlets of the charge check valves. When the hydraulic motor is not turning, any fluid not used by the closed loop goes through the charge-pump relief valve, then back to tank through the pump case and a heat exchanger. When the hydraulic motor is turning, all charge flow goes to the low-pressure side of the loop through one of the charge check valves, the hot-oil bypass valve, and the hot-oil bypass relief valve at a lower pressure. This action makes sure the closed loop receives cooled, filtered oil that can carry away heat and contamination. It also sends cool, clean oil through the motor and pump case to flush contamination and dissipate heat.

Small hydrostatic pumps can be controlled manually, hydraulically, or electro hydraulically. Larger systems cannot be controlled manually, due to the high force required to move the swashplate.

The system-relief valves protect the hydraulic motor and bi-directional pump from excess pressure when the motor is powered. When the pump center’s motor outlet flow is blocked, the motor may be driven by external forces and cannot stop immediately. At this time, the system-relief valves allow fluid from the motor -- now acting like a pump -- to bypass at high pressure to the opposite motor port. This allows the motor to stop smoothly even when an operator tries to stop it abruptly. (Other options to protect the circuit from bypassing through the system-relief valves during deceleration are available from most suppliers.)

Bi-rotational pumps

Unirotational pumps can only move fluid when rotating in one direction. These pumps usually have a larger inlet port in relation to the outlet port size. They are limited as to inlet-outlet function because internal bypass is always ported to the housing on the inlet side. This means all internal bypass goes to the case and then back to the pump inlet. Because all pumps have a shaft sticking out of the housing, there must be a seal to stop fluid leak when the pump is at rest and vacuum leak when it is running. A unirotational pump could move fluid when turning either way but the shaft seal would blow above 25- to 50-psi outlet pressure in reverse flow.

Bi-rotational pumps can move fluid while turning in either direction of rotation if they are piped correctly. Both ports on these pumps are usually sized as inlets. Figure 8-33 shows a cutaway view of a bi-rotational pump with internal check valves that allow bypass to go to the inlet side of the pump. Bi-rotational pumps are mainly used on mobile equipment where the prime mover cannot easily change direction of rotation. This means right- and left-hand rotation pumps would have to be kept to satisfy different pieces of equipment.

In industrial applications where a 3-phase motor’s direction of rotation can be easily changed, pump rotation direction is not important. There is only one instance where a left-hand rotation pump must be specified. This is the case where a double-shafted electric motor drives a pump at both shafts. One of the pumps in this application must be setup for left-hand rotation.

Pump horsepower

Two formulas often used to figure hydraulic pump horsepower are:
hp = (psi)(gpm)/1714 -- (to calculate pure horsepower), and
hp = (psi)(gpm)/1714(actual pump efficiency).
Normally efficiency is assumed to be 85% because most new industrial pumps are at or above this figure.

The first formula is for a known pump volume -- figured from its cubic inches/revolution times the number of revolutions per minute. Say this displacement at 1200 rpm came to 12 gpm, but a flow meter at the pump outlet only shows 10.6 gpm at 1000 psi. The pump is still moving 12 gpm as far as its horsepower requirement is concerned, but the speed of the driven device will be that produced by 10.6 gpm. The 12-gpm pump was picked because the actual flow required was at least 10 gpm.

The second formula is applied when pump efficiency is known and horsepower is being figured for the actual 10-gpm requirement. Now pump efficiency must be considered because theoretical flow is greater than 10 gpm, and the electric motor must be able to pump the extra fluid even though it does not get to the actuator.

These two formulas can be simplified to:
hp = 0.000583 (gpm)(psi) -- (for pure horsepower), and
hp = 0.0007 (gpm)(psi) -- (for an 85% efficiency pump).
A common rule of thumb is: 1 gpm at 1500 psi = 1 hp.

Most suppliers’ catalogs show the horsepower required to drive a given pump at different pressures. These figures are usually conservative so designers can use them with confidence. Also, most electric motors can operate continuously at 110% of nameplate rating (and up to 140% for short bursts). Remember too that the only time a fixed-volume pump will be at full flow and full pressure is when the device it is driving has stalled. A pressure-compensated pump draws the highest horsepower just before it starts reducing flow slightly below its pressure setting. That event usually is not of long duration.

Many formula-data books have horsepower charts that make picking an electric motor simple. These charts are usually based on the 85% efficiency formula.