HYDRAULIC MOTORS CASE DRAIN REQUIRMENT WHEN IN SERIES

[kresseng]

I have a vendor design for driving cooling fans with hydraulic motors in series. The motors have internal case drains. Is this a problem? Could the first motor in series see a detrimental back pressure from the second motor?

 

[MikeHalloran]

Both motors must be rated for full supply pressure.
First motor _should_ see a backpressure of roughly half the supply pressure unless the second motor has somehow failed open.
Both case drains should go to the tank.

... Ah. Now I understand your question.

If the case and shaft seal of the first motor are not rated for full system pressure, there might be a problem. See the motor cut sheets.



Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

Yes. The first motor shaft seal will see the operating pressure of the second motor. Motors with internal case/seal drains expect the motor outlet port to be connected to low, return line pressure.

Ted

 

[kresseng]

I would have to agree ... the ramification here being? Elevated case pressure? Reduced motor life?

Any estimates? We are working with 2500 PSI system with each motor/fan at ~ 17HP.

We also have the fans controlled by proportional relief valve. Worse when max power to fans, yes?

 

[hydtools]

Reduced shaft seal life. No quantitative guess as to how much. If shaft seals are desiged for 100psi, max, and are operated at 1250psi, maybe seal life will be reduced by ~90%. What is the original design life?

Control flow, not pressure, to control fan speed.
Do you know that the motors will, in fact, demand full system pressure capacity?

I'd recommend a parallel motor arrangement with individual flow controls to control motor speed. Maybe some thermal feedback to control motor speed.

Ted

 

[kresseng]

Thanks for the expert direction. We are making arrangement for dedicated drains.

 

[hydromech]

As a general rule, shaft seals should not be exposed to any more than 0.5 - 3.0 BAR, depending on the type, manufacturer and application. To avoid possibility of seal failure, it is always recommended that casing drains are returned to the reservoir with the minimum of restriction.

Although I have personally seen an application where the shaft seal contained the casing pressure up to 300 BAR. The cast iron case of the motor was the weak link, it cracked.

There are always exceptions, but to be sure, they really need to be separate.

Adrian

 

[IceStationZebra]

Not to mention you could also just blow the seals out! ISZ

 

[heads1983]

Yes series motors should have drain ports connected direct to Tank

 

[chief]

fit check valves on each casing drain to prevent inter communication ,or alternatively pipe the drains separately to the reservoir.

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