Hydraulic motor case flushing

Hydraulic motor case flushing

Hydraulic motor case temperature

At the latest my project I used hydraulic motor Rexroth A2FLM710 (710cc). The motor works at 1400 rpm and provides 590 HP to the consumer. For safe motor work, I always try to keep the case temperature below 80*C. The easiest way to do this is a flushing flow adjustment.

In addition to the flow, you need to keep your eyes on the motor case pressure and try balancing to prevent overpressure in the motor case (check in the motor manufacturer’s catalog the max available case pressure to make longer life of motor shaft seals).

The values I have gotten: 21 GPM at 30 psi case pressure and in the worst-case scenario (max motor load, warm hydraulic oil) max case temperature was around 80*C

Hydraulic motor case flushing

Hydraulic motor case flushing flow and case pressure

There are no prescriptions or recommendations for valve or orifice size in motor catalogs for flushing flow, so the selection of flushing valves is a challenge.

Of course, you can find orifices (with different diameters) provided by the manufacturer with the motor in the motor’s catalog. But the flow and result case temperature will be different from application to application and the selection of the correct orifice is an engineering responsibility without any help or advice from the motor manufacturer.

Moreover, the manufacturer can’t provide all range of orifice diameters so the selection in the catalog is usually limited. And as you can see, sometimes values of flushing flow can be really huge and the only experience helps me to select the right flushing valve size at the beginning of the project.

I still believe, manufacturers can provide some diagrams/charts with correlation “power -> flushing flow” for approximate/preliminary estimation of the flushing valve size. Because I do not think everyone has a chance to make long tests during production and play with valves sizes…

What do you think?


  1. Armando H

    Hello, check the table in the training 4 by Rexroth, pg 61, I will send you by email,
    regards from Mexico,

    1. Dzyanis (Post author)

      Hello Armando. Thanks for your post. The photo of the book page you sent me by email is an orifice size selection at the pressure-flow chart like I have already posted in the article https://fluidpower.pro/orifice-flow-calculation/. With this chart, you can select propper orifice to get the required flow at a known pressure. The problem I try to illuminate in the post above is different. The problem is in a preliminary determination of required case flushing flow (using a motor size and a power provided by the motor to the consumer) to prevent overheating the motor. As soon as you know at least approximate value of required flow you can play with orifices (or flow control, or pressure flushing valves) to find the best size – using the chart you provided.
      I can’t find the way how to calculate the required flushing flow at the beginning of the new project to get the theoretical value of max safe motor case temperature. But I’m sure, manufacturers can provide some charts to help engineers to find a required flow as a start point.

  2. Grant Green

    Great bit of work you did here testing and verifying temperatures in the actual circuit. I think the problem for manufacturers like Bosch-Rexroth is that in this case the motor you referred to could be used anywhere from 100 up to nearly 600 HP and beyond. The speed, operating pressure, cyclic loading, duty cycles, oil type, oil viscosity, axial bearing loads, and radial bearing loads all should be considered when determining flushing rates.

    In many cases for applications as a manufacturer we are not supplied all the details of the application, in some other cases clients choose to pair our pump or motor up with another manfuacturer’s product to get maximum or specific benefits.

    From strictly a technical datasheet and ‘catch all’ standpoint when it comes to motor flushing, it is easier to state ‘don’t exceed this case pressure value’ and ‘don’t exceed this temperature’ value and state maximum values allowable values here.

    Any flushing flow charts that Bosch-Rexroth or another manufacturer could supply for their motor product I think would be deeply flawed and inaccurate with the myriad of details, variables, and the specific conditions of the machine.

    While it might not have been the easiest or quickest task to do the temperature testing and adjust the flushing flow rates, I think it was absolutely the right thing to do. I wish other customers were always so diligent to take the time to do this necessary testing before the motor and machine goes out to the field to work.

    Kudos to you and your team to get this done right.

    1. Dzyanis (Post author)

      Thanks, Grant! I appreciate your help and support in this project!


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