Fluid Power Journal

Hydrostatic Transmission Components

HYDROSTATIC TRANSMISSION COMPONENTS today are of such high quality and efficiency they don’t allow enough exchange of oil in the loop to keep temperatures at an acceptable level. This is especially true if the transmission is heavily loaded during its work cycles. The oil simply goes from the pump to the motor or motors and back to the pump without getting a chance to return to the reservoir to cool and relax. Using a hot oil shuttle or loop-flushing valve can help remedy this situation.

One common mistake to look for when troubleshooting a circuit that has one of these flushing devices but still seems to be running over temperature is the improper setting of the shuttle valve relief cartridge.

When the transmission is in neutral, the connection to the low-pressure shuttle relief is blocked. The flushing valve at this point has no function, and charge pressure is controlled by the charge relief at the pump. When the transmission is operated either in forward or reverse, the low-pressure relief in the flushing block should now see the low side of the circuit, and if properly set, 30-45 psi (2-3 bar) below the charge pump relief will direct oil from the closed loop back to the heat exchanger and tank.

*Make sure to always verify the setting value of the shuttle relief. This will still provide sufficient design pressure to satisfy critical machine function needs such as brake release, two-speed shifting, etc.

Mark Perry, CFPHS, Fitzsimmons Hydraulics, Inc.


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