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Case Drain Flow?

Case Drain Flow?

Case Drain Flow?

Anyone know typical flows for case drain for a hydraulic motor?

Specifically, we have an end product with a hydraulic motor of unknown origin. The product maker is in the UK, and I am having trouble getting in touch.

The motor is bi-rotational, 20cc/rev, ~10 GPM max flow.  A port is #10, B port is #8, and the case is also #8.

I am designing a manifold for a series of these products, and space is a premium.  I do not have experience with CD flows.  I would like to the CD ports to be #4.

Any rules of thumb?


RE: Case Drain Flow?

You didn't mention what type motor this is-case drain will vary with the type motor, pressure and fluid viscosity.  Its been my experience in industry to set a base line for new equipment and monitor from that point-also I've never seen CD run thru a manifold-usually run thru a dedicated line back to tank unrestricted.  If I had to give a figue I would say 10 % CD for a piston motor, medium pressure and 150-200 SUS fluid would be an alarm point to think about a motor change out.  A lot would depend on the equipment and how downtime is figured.  Hope this helps,  Maytag

RE: Case Drain Flow?

Unless you can be sure that the pressure in the combined case drain lines will never rise high enough to rupture the motor shaft seal, all motor drain lines should be piped seperately.

Most standard shaft seals are only rated to about 5 BAR(73PSI).

Maytag is correct, there are many variables that can effect casing drain. Some manufacturers used to publish their drain figures. Most have stopped because they cannot cover all of the possibilites and it leaves them open to warranty issues.

I'm in the UK, who is the manufacturer and what type of motor is it. Obviously piston, radial or axial, bent axis possibly?



RE: Case Drain Flow?

I would recommend using plumbing that will handle 20% of the motor flow at 50 psi back pressure. The hose length and fitting back pressure are important things to consider.

Ed Danzer

RE: Case Drain Flow?

Also depends on the circuit:

-if this is an open loop application, then maybe design for case drain max 10% of flow. That would be a fairly sick motor, as there is much leakage port to port that doesn't end up in case.

-if hydrostatic, it depends on the charge pump of the pump, and the flushing/shuttle package. How big is the charge pump(s), how much of charge flow is taken out at pump vs. how much taken out at motor. Then, it's also more of a constant rate. Increasing leakage from wear means decreasing from flushing so it's more constant.


RE: Case Drain Flow?

Thanks all- great info, as I said, I've only worked with simple circuits, so I've got some learning to do!

These motors are bent axis.  the end products are thrusters (SubAtlantic is the maker)  for an ROV. System pressure is 3000psi.

Ours is a closed system.  Instead of a charge pump, we use spring charged bladder compensators (~15 psi) as our "tank".  the CD on the manifold is basically instead of a bunch of Tees at pump intake.  

We are filtering the return flow, before the compensators.  The CD is not filtered.  we are filtering the high-pressure output of the pump.



RE: Case Drain Flow?

This is a little off subject, but why do you filter the return and the pump outlet?

Are you expecting the system to generate or take in contaminants? If not, one of the filters, the one with the biggest micron rating, is not working?

I have looked at Sub-Atlantics web site and I see the application would suggest that one of the filters is to absorb water.

The images on the data sheet show motors that look like Parkers F2 motor, however Sub-Atlantic sell conversion kits to fit many standard motor flanges. That means that the motor could be one of many models.

The ports are,as you say #8(1/2" BSP). You cannot reduce the port size to #4(1/4" BSP). If you do, you will surely blow the shaft seals on the motors. The casing drain ports is sized acording to the amount of oil flow that the manufacturers expect to see. Reducing it will force the case pressure up. If you have have two motors, the port in the manifold needs to be #16(1" BSP). If you have 4 motors you will need #32 port(2" BSP).

You cannot cut corners, if you refer to any bent axis motor data sheet you will see that it says the casing drain must be piped seperately using the correct size piping. If you do anything less, it is only a matter of time before the shaft seals are blown out. Don't be guided by what people believe the flow may be, follow the size of the casing drain port on the motor. That way if there is a failure, there will be some warranty support.

If you give the motor manufacturer a get out, they will take it.

This is my experience from buying, designing, selling, servicing and maintaining marine hydraulic and industrial systems for 20 years.

Ultimately it is up to you to decide what to do, but if the motors start spilling oil, fingers will start pointing in your direction as you are the person who designed the manifold. From experience it is not nice when environmental investigators start asking difficult questions.

Good luck




RE: Case Drain Flow?

We are using a coarser filter on the return to protect the pump.  We are filtering the pressure side with a fine filter to protect the servo valves that control the thrusters.

We are using a filter cart with water removal filters on deck as a maintenance procedure.

Good point about the warranty issue.  My only caviot is that these thrusters will not be used in an industrial situation (dives will be for a few hours, a couple times a week).  As i understand , high CD flows occur as the motor wears.  I truly don't expect much wear on the motors. but that is indeed difficult to quantify.  I am planning on using larger ports as opposed to #4.

that said, flow is directly proportional to area, which is proportional to the square of the diameter.  to keep the same flow velocity using 3 #8 as input into the manifold, a #14 (hard to find) would be adequite.

thanks all!


RE: Case Drain Flow?

I understand with the filters. What is happening in your system to generate enough debris to create a need for a return line filter to protect the pump? I am just interested that is all.

When selecting the port size, bare in mind the pipe fittings. The holes through the middle of stud fittings can be considerably smaller than the threaded port.  Welded SAE flanges are sometimes an alternative.

All motors leak oil into the casing. Brand new motors leak less the old ones, they still leak. The higher the working pressure, the more oil you will have through the casing drain. The higher the viscosity, the higher the pressure drop accross the casing drain.

The duty cycle of the motor is not really relevant. If the casing pressure is spiked, the shaft seals are the weak point and they will rupture. Whether working for 2 minutes or 2 days, if the casing pressure goes too high you will have a mess.

I have done loads of work for US Navy and USCG applications. Whenever hydraulic oil was mentioned, the envioroentalists got really twitchy. I don't envy you with this one.

Good luck


RE: Case Drain Flow?

Its a pain when you dont have all the info made available to you when planning things but typically CD is around 10% of total displacement.Temperature is a great indicator of wear I would suggest a variation of more than 5 degrees on a component from a system would be an indicator of increasing bypass.Can you put magnetic switches in the case line to pick up wear metal and send a signal to an alarm?If you want to run a#6 CD and fit a 25 psi check valve at the manifold end of the hose,this back pressure should be adequate for your application as I have seen it done before.I read earlier that shaft seals are rated to 5 bar?The motors I have been involved with,had 7 bar seals and now I am led to believe industry standard is 10 bar max(I have worked in mining with hydraulics for 15 years.)It is the norm to run independant return penetrations back to your tank but you can have a drain manifold that is common then have that set through filters before returning to tank.     

RE: Case Drain Flow?

Shaft seals are not designed for high pressure applications and depending upon the manufacturer the maximum working pressure of standard seal is approx 5 BAR. High pressure versions are available with higher working pressures.

All that said, I once worked on an application on a winch on a Korean mine hunter. Someone from the Korean navy though it would be a good idea to put a shut off valve in the case drain line.

The casing pressure wss recorded at 350 BAR before the motor exploded. That was a good quality shaft seal.


RE: Case Drain Flow?

Another down side of high case pressure is reduced shaft seal life. Even if the seal doesn't push out, or roll the lip, it is gripping the shaft more tightly, thus more friction and wear and shorter life.

I rarely use filters on case drain lines, due to the pressure drop issues. Typically we run case drains into a manifold, (with a connection to tank) then draw from that with a pump larger than expected case flows. That pump pushes oil through the cooler and filter package. No back pressure on case drain, in fact it needs to be tested to make sure not going negative pressure. Say a charge pump is 10 gpm into drain, we may pull 15 or 20 gpm from that manifold and send to the cooler/filter loop. Then 5 to 10 is always coming from the tank line to the manifold. Ultimately, case leakage could go to the full 15 or 20 gpm without ever going backwards back up the suction line into the tank. Still gets filtered and cooled, but minimal back pressure on seals.


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