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scmn | A simple explanation. Without case drain the case pressure is equal to the return line pressure; with case drain, the case pressure is equal to case line restriction which is very low (typically under 100 psi).
If you are running a motor on an SCV with high flow (15gpm+) the return line pressure can easily exceed 500 psi (line head loss, quick-coupler head loss, return head loss in SCV, etc). If you have high return line pressure at the motor (i.e 500 psi) then the shaft seal on the motor needs to be capable of that pressure or a case drain needs to be installed to bleed off the case oil and keep the shaft seal on the motor below a safe level. Thus a case drain is needed when the return line pressure at the motor would risk blowing the shaft seal.
Some motors shaft seals can tolerate higher pressures and on others increasing the speed of the motor lowers the shaft seal rating, etc; when it is all said and done, the case drains purpose is to keep case pressure lower than shaft seal pressure rating. The decision to case drain a motor is typically financial decision (only case drain when necessary to keep costs down). If shaft speed is low and return line pressure is under 400 psi, most motors can get by without a case drain. Thus the need for case drain is application dependent.
FWIW I have seen circuits were two motors are plumbed in series and the first motor has a case drain line that tees into the return line on the second motor to keep the shaft seal of the first motor lower. In that example, return pressure on the first motor was around 1000 psi and by case draining to the second motor return line lowered the case pressure from 1000 psi to 250 psi; consequently the second motor did not use a case drain since the case pressure was only at 250 psi and thus low enough to not jeopardize the shaft seal. In this case had the return line pressure been above 400 psi, the motor manufacturer would have required a case drain line from both motors unrestricted back to the case drain line on the tractor.
Sometimes case drains are added to motors that do not need a case drain for the purpose of case flushing which lubricates and/or cools a motor/pump. | |
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Running a Hyd motor with a case drain - with no case drain
