Hydraulic Motor & Constant Torque

  • #1

Main Question or Discussion Point

Hi, I'm a beginner on hydraulics and have been reading up alot of literature. But I'm not sure if my assumptions are accurate and I hope I can discuss with other forumers here! My apologies in advance if my questions seem silly!

Background: My goal is to drive a hydraulic motor at constant pressure & constant RPM so that I can achieve constant output torque.

1. Hydraulic pump is a positive displacement pump. (the RPM is variable)

2. I would like the hydraulic motor to produce constant output torque. Is it correct to assume constant pressure & fixed RPM = constant output torque. Would a hydraulic accumulator be able to perform this function - give out constant pressure & fixed output/discharge flow (which would fix the RPM at a specific motor displacement) using a flow controller?

3. How do I design the circuit to make sure all fluid from the pump goes into the accumulator first, before passing into the hydraulic motor? Are there hydraulic accumulators with 2 separate ports (charge & discharge)? Can charge & discharge happen simultaneously at different rates?

I'm learning to use Automation Studio to simulate the circuit (please let me know if you can recommend better software for beginners!)

Many thanks!!


Answers and Replies

  • #2
Science Advisor
Gold Member
Output torque from a hydraulic motor is determined by the design details of the motor and the input pressure .

The driven load actually determines what the output torque needs to be and hence what the inlet pressure needs to be .

Running speed is determined by the volume flow of oil going through the motor . Volume flow of oil is set to give running speed suitable for driven load requirements .

Output torque and running speed together determine output power .

You don't normally need a hydraulic accumulator in a simple hydraulic pump / motor drive system .
Last edited:
  • #3
Thanks Nidum!

The reason for thinking of using the accumulator is so that the flow can be regulated (something like a capacitor bank). The accumulator charge rate would be more than the discharge rate to ensure that I have a constant input flow into the motor.

The flow controller will take care of the input flow into the motor. But I don't know how to fix the pressure. Do I need to use a pressurized reservoir to have constant pressure in the circuit? From what I've been reading, it seems that accumulator can be used to maintain system pressure, but I don't understand what that really means.

And I'm trying to figure out how to make sure all the fluid goes into the accumulator first before discharging back into the circuit to drive the motor. It seems like the fluid may bypass the accumulator and go directly to the motor. If that's the case, then it would not function as a flow regulator as I would like it to be. Using the analogy of a hydro dam where water gets collected and then discharged at a slower rate via a smaller outlet, I'm trying to emulate a hydro dam with an accumulator.

If possible, I'm hoping it can be simultaneously charged & discharged so that the circuit is constantly flowing.

I look forward to more discussions!
  • #4
Last edited:
  • #5
Gold Member
Something is out of place here for me.

Most of the hydraulic systems I work with on injection moulding machines have variable displacement pumps. Displacement is adjusted according to feedback from a control system. Usually comprising of a transducer and a control circuit.

Feedback is usually in the form of pressure or speed. The control works out the displacement required to achieve the required pressure or speed. Such systems are well understood and have been used for many years.

Maintaining a constant torque might be required (to maintain a constant pressure for example) but torque is normally only secondary to the control input.

My feeling on your post is that torque is not your primary input or requirement. Something more basic (and therefore easier to measure) like speed or pressure is what you need I think. Simplifying your problem (or explaining it fully for more precise help) may solve your problem

I may be wrong. If torque is your primary requirement then finding a way to transduce the torque on the output of your system to a voltage and feeding that back through the control to your pump will give you closed loop control.

Accumulators are only typically used to provide large volumes at a given pressure for quick movements. As long as your torque requirements do not exceed the flow and pressure capacity of the pump you will not need one.
  • Like
Reactions: OCR
  • #6
Science Advisor
2019 Award
For a fixed displacement pump you will get constant torque by running at a constant pressure. You will get a constant RPM by running at a fixed flow rate. You cannot have it both ways, always.

The way to achieve both, most of the time, would be to run an electric motor driving a pump that sets the flow rate. A fixed displacement pump will then be driven at a fixed speed. By placing a pressure relief valve across the circuit you will limit the torque to a maximum.

Any attempt to draw more power from the hydraulic motor output than power = ( RPM * torque ), will see a drop in either RPM or torque from the specified settings.
  • #7
Science Advisor
Gold Member
I don't think it is typical at all to use an accumulator in a hydraulic motor circuit. Hydrostatic drive systems consist of a variable displacement pump and a hydraulic motor that is sometimes a variable displacement unit. Skid steer loaders that have 2 speeds will sometimes switch to a smaller displacement by changing the position of the swash plate in the motors. But that is pretty much it, at least in terms of main components.
In your case, you can't have constant pressure and constant speed unless the load is fixed. If you want a fixed speed out of the motor you need a constant specific flow. If the load changes, the pressure will change. You cannot really have it any other way and I don't know why you would need to in a hydraulic motor circuit. Think of it this way: Replace the hydraulic motor with an imaginary orifice in which you can vary the size. Small orifice represents large mechanical load on the motor. Large orifice represents a small mechanical load on the hydraulic motor. Hook this to a positive displacement pump turning at a specific RPM. What do you think will happen?
  • #8
Thanks guys for all the replies! I'm digesting all the information and explanation, they are definitely very helpful!

Related Threads for: Hydraulic Motor & Constant Torque

  • Last Post
  • Last Post
  • Last Post
  • Last Post
  • Last Post
  • Last Post
  • Last Post