When a pump or motor is worn or damaged, internal leakage increases and therefore the flow available to do useful work decreases. This means that the condition of a pump or motor can be determined by measuring the flow from its case drain line (internal leakage) and expressing it as a percentage of its theoretical or design flow. However, using case drain flows to determine the condition of a hydrostatic transmission, without a thorough understanding of closed circuits, can lead to incorrect conclusions and the costly change-out of serviceable components.

What is a Hydrostatic Transmission?
A hydrostatic transmission consists of a variable-displacement pump and a fixed or variable displacement motor, operating together in a closed circuit. In a closed circuit, fluid from the motor outlet flows directly to the pump inlet, without returning to the tank (Figure 1).


Figure 1. Basic Closed Circuit Comprising Variable
Displacement Pump (PV)
and Fixed Displacement Motor (MF)

As well as being variable, the output of the transmission pump can be reversed, so that both the direction and speed of motor rotation are controlled from within the pump. This eliminates the need for directional and flow (speed) control valves in the circuit.

Because the pump and motor leak internally, which allows fluid to escape from the loop and drain back to the tank, a fixed-displacement pump called a charge pump is used to ensure that the loop remains full of fluid during normal operation.

In practice, the charge pump not only keeps the loop full of fluid, but it also pressurizes the loop to between 110 and 360 PSI, depending on the transmission manufacturer. A simple charge pressure circuit comprises the charge pump, a relief valve and two check valves, through which the charge pump can replenish the transmission loop (Figure 2).


Figure 2. Charge Pump Circuit

Once the loop is charged to the pressure setting of the relief valve, the flow from the charge pump passes over the charge relief valve, through the case of the pump or motor or both, and back to the tank.

A variation to this arrangement is where the transmission is fitted with a flushing valve (also called a transmission valve or replenishing valve). Because the fluid in a closed circuit flows directly from the motor outlet to the pump inlet, it means that apart from losses through internal leakage, the same fluid circulates continuously between pump and motor. If the transmission is heavily loaded, the fluid circulating in the loop can overheat. The function of the flushing valve is to positively exchange the fluid in the loop with that in the reservoir.

A closed circuit flushing valve usually comprises a pilot-operated directional valve and a low-pressure relief valve (Figure 3).


Figure 3. Simple Closed Circuit
Showing Flushing Valve Arrangement

When the transmission is in neutral, the directional valve is centered and the gallery to the low-pressure relief valve is blocked. In this state, the flushing valve has no function and the charge relief valve in the transmission pump (Figure 2) regulates charge pressure. When the transmission is operated in forward or reverse, the high-pressure side of the loop pilots the directional valve. This opens the low-pressure side of the loop to the flushing relief valve gallery. This relief valve is set around 30 PSI lower than the charge relief valve and therefore it regulates charge pressure when the transmission is operating in forward or reverse.

A flushing valve is most effective when it is located at the motor, assuming the charge check valves (Figure 2) are located in the transmission pump, as is the norm. The effect of this is that cool fluid drawn from the reservoir by the charge pump, charges the low-pressure side of the loop through the check valve located close to the transmission pump inlet. The volume of hot fluid leaving the motor outlet that is not required to maintain charge pressure in the low-pressure side of the loop, vents across the flushing valve relief into the case of the motor and back to the tank, usually via the pump case.

When using case drain flow to determine the condition of a hydrostatic transmission, charge pump flow must be taken into account. Consider an example where charge pump flow is 10 gallons per minute (GPM), of which 4 GPM are leaking out of the loop through the motor’s internals (case drain) and 2 GPM are leaking out of the loop through the pump’s internals. The balance of 4 GPM must therefore be going over either the charge or flushing valve relief - but still ends up in the pump or motor case, depending on the location of this valve.

Before any meaningful conclusions can be drawn, the case in which the charge or flushing valve relief is venting (motor or pump) must be determined and if connected, the two case drain lines must be isolated from each other. If the charge or flushing valve relief vents into the case of the pump, then it is possible to determine the condition of the motor by measuring its case drain flow, but not the pump. If the charge or flushing valve relief vents into the case of the motor, then it is possible to determine the condition of the pump by measuring its case drain flow, but not the motor.

It is not possible to determine the condition of the component that has the charge or flushing valve relief venting into it because there is no way to determine what proportion of the total case drain flow is due to internal leakage - unless the relief valve can be vented externally while the test is conducted.

When conducting these tests, it is important to understand that the volume of internal leakage from a hydrostatic transmission cannot exceed the flow rate of its charge pump. Consider a transmission that has a volumetric efficiency of 100 percent, that is, the pump and motor have no internal leakage. The transmission loop has a total volume of two gallons and is full of fluid. Because there is no internal leakage there is no need for a charge pump.

The pump is stroked to maximum displacement, which circulates the two gallons of fluid in the loop at a rate of 50 GPM. Because it’s a closed loop, with no leakage, the flow from pump to motor is 50 GPM and the flow from motor to pump is 50 GPM.

Now introduce internal leakage of 0.5 GPM in both pump and motor. The result is that with no charge pump, after one minute only one gallon of fluid will remain in the loop (the other gallon will have leaked back to the tank). However, within a second of the transmission starting to leak, the transmission pump will start to cavitate and the severity of this cavitation will increase with each passing second until the transmission destroys itself.

If a charge pump with a flow rate of 1 GPM is installed in the circuit, the problem is solved, at least temporarily. With 1 GPM leaking out of the loop and 1 GPM being replenished by the charge pump, the status quo is maintained until wear causes the internal leakage of the transmission to exceed 1 GPM.

As you can see, it’s not possible for the internal leakage of a hydrostatic transmission to exceed the flow rate of its charge pump. Charge pump flow rate is typically 20 percent of transmission pump flow rate. This means that volumetric efficiency can drop to 80 percent before the transmission will cavitate and destroy itself. The trick is to overhaul the transmission before this point is reached.