How Does a Hydrostatic Transmission Work

How does a hydrostatic transmission work? Hydrostatic transmissions are the way to go in lawn tractors and farm equipment. It is a simple system that has been used for many years. A hydrostatic transmission consists of a variable pressure pump and a set of control line pressure pistons, a relief valve and a hydraulic motor. The pump turns by the v-belt from the engine.

Hydrostatic transmissions operate in the 150 to 375 psi range. The pump provides a constant pressure while the position of the pressure-controlling handle regulates the position of the pressure regulating valves. As the control handle is moved forward to increase speed, the pressure valve moves to increase hydraulic flow to the hydraulic motor. This increases the speed of the motor. All excess hydraulic flow passes the relief valve and returns to the reservoir. This creates heat, which is dissipated by the fan located under the v-belt pulley on the top of the transmission.

The nice thing about these transmissions is that the engine can be operated at its best operating rpm while the speed of the vehicle can be regulated through the transmission. The torque will be more consistent at varying engine speeds than other transmissions under load.

It is recommended to change the oil every year or 500 hours, whichever comes first.

Always keep the transmission washed off when finished with it so that the cooling fans are not obstructed. This is a potential killer for this transmission. It must remain cool in operation.

Never tow or push this type of transmission or damage will occur. Remember that the hydraulic motor turns with the wheel, but the pump does not. This causes damaging back pressure through the system that could damage seals. Some tractors have a disengagement rod in the back of the tractor. This rod must be pulled out and slid into a groove in order to push the tractor.

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