Low-pressure Hydraulic Motor

[sry110]

I have an application where I want to use a hydraulic motor to slowly turn a shaft forward until the clutch on the end of the shaft engages, and the hydraulic motor will then stall out. The torque to turn the shaft is only a couple of lb-ft. The available oil is about 20 psi. We don't need the motor to turn very fast....it's only for pre-engaging our clutch, not for achieving breakaway.

Anyone know where I can find a hydraulic motor that will turn (slowly..?) with only 20 psi inlet pressure?

 

[hydtools]

Need to know how much flow is available. Flow rate determines motor speed, pressure does not.

Once you know flow rate you can determine motor size to get the motor speed you want. How slow is slowly?

Once the motor stalls what do you want to do with the oil flow? Stall the pump? Bypass oil through a relief valve? Bypass oil at low pressure through a valve?

Ted

 

[Jackofalltrades2]

Not many motors will overcome internal mechanical resistance with 20 PSI most take 200 PSI or so to start with no load.

I am thinking an electric motor with a gear reduction might work better.

TN

Tom Nelson
Application Engineer (CFPS)
CATCO Parts and Service
St Paul,MN

 

[MikeHalloran]

Try backdriving a Jabsco (impeller) pump.



Mike Halloran
Pembroke Pines, FL, USA

 

[PNachtwey]

Need to know how much flow is available. Flow rate determines motor speed, pressure does not.

The motor will accelerate until the net torque is 0. At that point the angular acceleration will be 0 and the angular velocity will be constant. Flow only occurs because there is a pressure differences between the supply and the motor. Flow just attempts to equalize pressure.

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[hydtools]

So how fast will the motor run with 20psi?

Ted

 

[PNachtwey]

So how fast will the motor run with 20psi?

I can answer that when I know the motor size, the pressure at the exhausting port and the opposing torques as a function of angular velocity.

If the opposing torque is higher than T=PSID*VPR/(2*?) the motor isn't turning at all and there will be no flow except for leakage.

I think a constant flow source is required that will apply supply oil at what ever pressure is required to make the motor turn at the desired rate.

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[kcj]

Q think a constant flow source is required that will apply supply oil at what ever pressure is required to make the motor turn at the desired rate. Q



peter, you just gave the definition of the flow makes it go scenario that bugs you so much.

That is the situation with a flow limited circuit, a fixed, constant displacement pump driving against a steady state load, that has valves and loads and conductors such that it is not pressure limited. The motor steady state speed is defined by the flow rate, the pressure defined by load and pressure drops. Pressures are a measurement of load as long as the pump has sufficient input energy (including rotational inertias) to turn it. The dynamic affects are small compared to the overall picture. Yes, accel, decel, fluid capacitance are there always, but small compared to the larger picture.

During accel, decell, dynamics, or pressure limited situation (pressure limited does not mean constant pressure), then of course summation of forces, capacitance, valve restrictions become the determining factors. I agree with you 100% in that situation.

But FLOW and PRESSURE limited are two different limitations that could be applying to a circuit. It is not 'either or', pressure or flow. It is like arguing if voltage is current across resistance, or if current is voltage divided by resistance.......

You see pressure limited circuits and assume that 'flow makes it go' is totally wrong. There is another world of flow limited exactly as you described above.

Any circuit must meet summations of forces, AND summations of flows. The circuit could be limited by EITHER pressure or flow. Most of your world is pressure limited, as it should be. But it is not the only explanation of the total fluid power physics world.


Anyway, back to original poster:
-If this is similar to a gearbox engaging shifter (that we have had many problems with many versions) it is an interesting and difficult challenge for such a seemingly simple task.
-could you use some sort of magnetic coupling like a seal less pump drive?
-A spring loaded drive like on a Bendix starter drive?
-20 psi delta is pretty low for any sort of fluid power component.
-How do you know when it is engaged? Is there some position indication that signals the next step.
-What happens once the cluch is engaged? does this device get removed from the circuit, or does it have to free spin when the main clutch/drive begins turning at higher speeds?
-How often does it get used?

kcj

 

[kcj]

or how about a viscous coupling using the viscosity of the oil instead of the pressure? Temperature affects may be a fatal flaw. I used Linkbelt/Dodge/Reliance/others in a former job long back. kcj

 

[hydtools]

A little input from the original poster at this time would be good.

Ted

 

[PNachtwey]

peter, you just gave the definition of the flow makes it go scenario that bugs you so much.

I made it clear that no matter how much flow there is at 20psi, there will be no flow if the motor doesn't turn. Force/torque makes makes it go. Flow just equalizes pressure.

A current supply will supply the same amount of current regardless of the output voltage. What this application requires is a constant flow source but the pressure need to be what ever it takes to make the motor rotate. It is still force/torque that makes it go.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[EdDanzer]

A low speed high torque motor like Charlynn, White, Parker, Danfoss and several others may do the job. The difficulty is the low input pressure. It may not exceed the outflow backpressure from the motor. This backpressure can be created in the motor by exhaust flow pinching, valve cutoff pressure trapping and port design as well as fitting and hose friction, filtration restriction and tank pressure. The next problem at low speeds is internal leakage in the valve and mating components.

Both flow and pressure drop through a circuit are required to do work, flow volume will limit speed and pressure drop will limit output force.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com