Hydraulic motor case drain

[DRussell]

I have a question about hooking up a case drain on a hydraulic motor. If the system pressure is always well below the rated pressure of the motor is there any need for a case drain. For example, if the motor is rated at 2950 PSI and the tractor can produce only 2000 psi is a case drain needed?



Or am I thinking entirely incorrect about this? The case drain is there to drain any oil that leaks past the gears in the motor. If the oil leaking past the gears develops more pressure than the shaft seal can withstand it will leak past the seal. Is the shaft seal designed to seal at the same pressure as the motor is rated, or something less?

This is a gear motor, not a piston type.

 

[Ken Macfarlane]

Many closed centre systems normally the return line goes back to the pump not the sump reservoir, so case drains let leakage and debris get out of the pump/motor circuit and pass through the filter.

There is back pressure in the return line in all styles of systems, getting higher at high flows, having a very low nearly no pressure case drain lets you not pressurize shaft seals.

Probably are more reasons too.

 

[Steve@Advance]

It needs the case drain regardless of the operating pressure.

The seals are intended to handle only very low pressure and the case drain should go directly to the tank.

 

[Brendon-KS]

Your second sentence is correct - the shaft seal can only handle a fraction of the rated pressure of the motor. In theory the pressure never leaves the gear space within the motor but this physically isn't possible and oil migrates to places it would be damaging unless drained off.

Whether or not you need to hook up the drain line depends on how the motor is being used. If a single motor is being run by an open-circuit system and the return from the motor goes directly back to a return filter and reservoir a case drain line generally isn't needed. This is because the return line and the case drain would be at the same pressure so in effect the return line can also function as the drain. Having a drain certainly doesn't hurt anything but it typically isn't needed unless the motor manufacturer specifically requires it. (In my experience this requirement would be unusual, typically only for special applications such as a severe duty cycle or very high speeds where the shaft seal is under duress to begin with.) But, if two or more motors are linked in series all of them except for the last one in the line need to have case drains since their outlets are at an elevated pressure and cannot function as the drain.

If you're running a motor through tractor remotes one has to be a little careful in foregoing a drain since the backpressure will be higher than what a straight shot back to tank would be due to all the valving. Some tractors have a dedicated motor return line and if this is used then the case drain can be omitted. If the return is going back through the SCV it would be a very good idea to include a dedicated drain line.

 

[SVcummins]

If it has a case drain you need to hook it up or you will at least blow the seal and at most you’ll blow the seal and split the trust me on this

 

[DRussell]

This is a single motor system. It is running through the tractor remote, and the tractor does not have a dedicated motor return line. The motor is reversible and will be running in reverse at least some of the time. Because of that, hooking it up to the return line directly to the tractor would be problematic.

Do you have any ideas on how to add one? It will be run by various tractors including a Case 1030, Oliver 1750, Allis Chalmers 190 and Ford 5000.

What if I added a second remote valve on the machine itself and fed this second remote via the tractor hydraulics? The flow into this second remote would always be in one direction and I could tee the case drain line from the motor into the return line to the tractor after it leaves the second remote valve. This return line to the tractor should have minimal pressure.

 

[Brendon-KS]

However you do it the backpressure when returning back through the tractor's remote outlets will be higher than what you probably want to have the motor drain subjected to. When powering motors through the tractor's remotes the best thing would be to rig up a dedicated return line back to the reservoir. It doesn't need to be real big, 3/8" hose would be plenty.

 

[DRussell]

Thanks, you've been helpful. The case drain fitting on the motor is SAE 4 thread so the line would not have to be very big.

 

[buickanddeere]

Needs a drain line back to the lowest pressure region of the hydraulic system. Usually directly into the reservoir .