In your last hydraulics bulletin, I explained the pitfalls to look out for when assessing the condition of a hydrostatic transmission by measuring its case drain flow.

There’s another component present in most hydrostatic transmissions that complicate this issue further: the flushing valve. A closed-circuit flushing valve (also called a transmission valve or replenishing valve or purge valve) usually comprises a pilot-operated directional valve and a low-pressure relief valve. When the hydrostatic transmission is in neutral, the directional valve is centered and the gallery to the low-pressure relief valve is blocked.

When the transmission is operated in either 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 relief valve gallery.  In a closed circuit, fluid from the motor outlet flows directly to the pump inlet. This means that apart from losses through internal leakage, which is made up of the charge pump, 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.

You can watch a two-minute simulation video that explains how it does this, here. When the hydrostatic transmission is in neutral, the flushing valve has no function and charge pressure is controlled by the charge relief valve in the transmission pump.  When the transmission is operated in either forward or reverse, the flushing valve operates so that charge pressure in the low-pressure side of the loop is controlled by the relief valve incorporated in the flushing valve. This relief valve is set around 30 psi lower than the charge pump relief valve located in the transmission pump.

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, which is not required to maintain charge pressure in the low-pressure side of the loop, vents across the flushing valve relief and back to the tank, often via the motor and/or pump case.

So if a flushing valve is fitted to transmission, it acts as the charge pump relief valve once the transmission is operated in forward or reverse. So if the flushing valve 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. If the flushing valve 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.

This reinforces the point that using case drain flows to determine the condition of the components of a hydrostatic transmission, without a thorough understanding of the circuit in question, can result in incorrect conclusions and the costly change-out of serviceable components. In your next hydraulics bulletin, I’ll explain another important concept to keep in mind when testing hydrostatic transmissions.

Yours for better hydraulics knowledge,
Brendan Casey