I have seen in many data sheets of Hydraulic Gear motor a spec called : "Minimum Speed = 450 RPM" ( 450 is an example)

here is a capture from the linked datasheet below : enter image description here

In your experience or theoretical knowledge what happens If I run the motor slightly below this speed (say 150 RPM ) by reducing the flow of Oil.

  1. Does it lose torque/or any other spec but stay running normally ?

  2. Does it stop moving smoothly? ( I have seen this "discontinuity effect" in real life when flow is too low the motor speed was around 10RPM)

note: what I mean by discontinuity is that the motor moves a push then stops as if it is gathering oil then move a push again.


  1. Torque will drop off with speed for the Brevini external gear motors in your datasheet (OT200 series). Unfortunately Brevini doesn't publish torque / speed curves below 1000 rpm. Here is a comparable graph from Vickers for their CR-04 internal gear motor:

enter image description here

Without providing a bunch of sources, external gear motor performance rapidly drops off with decreasing speed in pretty much every category - torque, heat, overall efficiency, motor life, etc.. Use a different type of motor if you need to maintain high torque at low speed - gerotor / piston / rol-vane. Or if you have only two specific speeds to run (one high speed, one low speed), there are tandem "2-speed" external gear motors for that purpose.

  1. Yes motor speed will become discontinuous at some point. The transition point is difficult to predict because it depends on oil viscosity, pressure, load static friction, and other factors.
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    $\begingroup$ thank you very much for the detailed answer. 1) small follow up , gerotor = orbital motor ? ... 2) i found piston motor with 150 rpm lowest speed which will suit me but it is 5x the cost and with extremely high torque that I donot need. So from your experience which is better controllable ( for precision positioning ) : a. External gearmotor , b. Internal gearmotor . c.Orbital motor . d. axial piston motor ? $\endgroup$
    – Eng Sam
    Jul 1 '20 at 20:01
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    $\begingroup$ 1) Correct, gerotor = orbital motor. 2) Without knowing about the application, an orbital motor is usually best. I've been able to position them smoothly below 50 rpm when load friction is stable. External gear motors can overheat when run at full pressure below 300 rpm. Internal gear motors are better than external gear, but still prone to overheating at low speed. Axial piston motors start to grind the surfaces off their shoes at low rpm. The best option for high torque / low speed is a radial piston motor, but cost is similar to axial piston. $\endgroup$ Jul 1 '20 at 22:46
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    $\begingroup$ Last thing to watch out for on the orbital motor is maximum speed. They can't run as fast as external / internal gear units. $\endgroup$ Jul 1 '20 at 22:50

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