Hanson Hydraulics Ltd. Blog

Hydraulic Transmission Case Flushing

Posted by Doug Hanson - 24/05/10 at 05:05 pm

Hydraulic Transmission Case Flushing

Case flushing in hydrostatic transmissions is almost essential in extending the life of the transmission. Flushing will substantially reduce pump and motor operating temperatures. The arrangement depends on whether the Hot Oil Shuttle is integral to the motor or is an external block. If it is integral to the motor, the case drain line from the top of the motor should run over to the bottom case drain port on the pump. The upper case drain port on the pump should then run through a cooler or heat exchanger if equipped. Depending on the make of pump, it may be possible to run the case drain line through a designated case return filter, however with some shaft seal arrangements, the seal will not tolerate the back pressure created from pushing oil through the filter, especially cold oil. Please keep in mind that any back pressure in the pumps case drain line will now be additive to the back pressure on the motors case drain line.

If the Hot Oil Shuttle is external to the motor, plumb the relief flow from the Hot Oil Shuttle to the bottom case drain port on the motor, then continue as stated above. The draw back to such an arrangement is that contamination created by the drive motor may end up contaminating the case of the pump, however if that is the case, chances are you already have damage within the pump as well.

This arrangement makes a huge difference on Low Speed High Torque motors that have extremely low case drain flow rates. With these motors, the stagnant case drain oil sits in the case, becomes extremely hot, and looses viscosity and lubricating properties. In a Staffa or KYB motor, the bronze surfacing of the connecting rod then smears on the crank shaft, as well as the connecting rod ball-end and piston gall up. Life on these motors is extended greatly by case flushing.

Caveat – do not exceed maximum recommended case pressures!

Older designs such as Dynapower units had charge pumps that did not relieve into the case of the pump; instead they discharged their charge pump flow across to the suction side of the charge pump. These charge pumps can be modified to discharge relief oil externally, which can then be plumbed into one of the ports in the trunion caps. This makes a huge difference on mobile equipment such as pavers or rollers that spend a lot of time idle over hot tarmac. A picture of such an arrangement is shown here. If you have any interest in this particular modification, please email me and I will provide more details.

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This entry was posted on Monday, May 24th, 2010 at 5:56 pm UTC and is filed under Pump and Motor Repair. The entry is tagged under You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.