Hydraulic motors in series Problem!

[Jan1235]

Hello,

So I have 4 hydrauliek OMR 100 motors in series. The motors each drive a fairly heavy disc/rotor (around 25cm wide and around 20kg each). The disc/rotors rotate freely and do not drive anything (yet). Each motor drive’s in opposite direction.

The oil is deliverd by a LS pump and the motors are controller by a double working proportional valve (no clue if this makes sense? I’m not english). The valve delivers 35L/pm max.

Here comes my problem:

If I engage the motors, it goes well until I reach about 60% and then the motors shock a bit and the return hose’s will bounce. Anything higher then 60%, the worse it gets.

If this happens, the return pressure will dance between 10/20 bar. However, if I put a control valve in the return line and increase the return pressure to arround 50bar, it stops an they will turn normal.

So probably, when the rotordiscs will get a load (dirt) I will not have this problem. But they will turn without a load quite often too, wich I dont think my motors will like very much.

I opted for somekind of counterbalance valve at my hydraulic-suplier, but they say that that wont help.

Long story, hope it all makes sense, and I hope someone might have an idea

Thanks!

 

[FluidPowerUser]

Yes, it does make sense.

The inertia in the discs is trying to drive the motors, so the system pulses as the work required to drive the motors goes up and down.

If you dampen the affect of the inertia by sticking the discs in the ground or by giving them some work to do, they will settle down.

Adding a restriction in the outlet of the motor will change the pressure drop across the motor and it will stall if the inlet pressure is not high enough. Adding a counterbalance valve will not fix it, it will only help to slow the motors when the power is removed.

 

[Jan1235]

Yes kind of figured that the motors are starting to act like pumps, specially the last one in line cause it has the least return pressure.

But.. is there anything to fix this?

Most of the time they will be under load, but they will turn without any for a few minutes quite often too..

 

[FluidPowerUser]

You can put an orifice in the outlet of the last motor to increase the decrease the pressure drop over the motors so that is barely enough torque to keep them turning. Then, when the motors need to do work, you can bypass the orifice with a valve, either manual or solenoid controlled.

The total pressure drop across all 4 motors needs to be just below the setting of relief valve that controls the system pressure. That will keep them turning, but there won't be enough torque to accelerate them to the point where the inertia takes over.

 

[Jan1235]

Oke too bad. Really hoped there was some kind of “mechanical” solution.

Its al part of a existing harvester, so the whole machine turns on with one button. Putting an extra signal in for a solonoid is a real pain..

Really thanks for the help though!

 

[LittleInch]

Could you add a little bit of load all the time? Say 10% of your full load?

An electric brake or something?

Then either release it when fully working or accept this extra load as a way to get the system up to 100% speed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jan1235]

I dont really have room something like that, couldn’t even figure something out to test/replicate a load without putting it in the ground.

For the work it really doesnt matter that it does this. If they are not under load, they dont have to do anything. But im affraid it will destroy my motors and even worse, my pump.

 

[BEMPE16524]

The motors each drive a fairly heavy disc/rotor (around 25cm wide and around 20kg each). The disc/rotors rotate freely and do not drive anything (yet). Each motor drive’s in opposite direction.

The oil is deliverd by a LS pump and the motors are controller by a double working proportional valve

if you use proportional valve, you can control the start of the motor once you touch the load.

What you experiencing is normal especially when you put the motor in series and the disc that you fix on the motor.

just try to remove the possible trapped air in the line and see how it works.

also why did you arrange the motor in series? any particular reason?
Note: OMR100 has max 175 Bar continuous working pressure (60 lpm output).

 

[Jan1235]

The motors/rotors are build on an existing (potato)harvester, the proportional valves are original on the machine and used to drive something different. So yes, I can change the speed but it wouldnt be very convinient to constantly changes that in the field. I asked the manufacture if they would help me out in there software, but yeah.. no.. so I have to work around it without having to push to much buttons.

I needed 8 motors/rotors and had the two proportional valves free, since the rotors replace what the valves use to control. The 35L (per valve), so 70L, is all that I have left over from the pump. So I choose to put 2x 4 motors in series to reach the desired speed/power

 

[Jacc]

You could possibly put a sequence valve with external pilot in parallell with the return needle valve and have that sequence valve open from the load pressure.
This Parker valve is a candidate for the job:

https://www.partsgopher.com/product/svh102s30/svh102-sequence-valve

 

[Jan1235]

I really dont see how this would work?

 

[73lafuite]

Hello, to have a correct life of your shaft seals it is desirable to put an external drain on the first 3 motors. Maybe this will prevent having a pressure wave which produces a small variable torque on the shafts or even a detachment of the distribution glass.

 

[Jan1235]

Thanks! All the motors have there external drain hose connected.

 

[BEMPE16524]

yeah, drain line is common. the manufacturer should already connected all the lines.
here what may be the cause:
1. When the motors run under light or no load, the hydraulic fluid may not be under enough pressure to fully fill the motors’ chambers, leading to cavitation. Cavitation happens when there's insufficient back pressure or the flow isn’t smooth, causing small bubbles that implode, which can lead to jerky motor behavior.
2. Running multiple motors in series can introduce resonance issues. Because the motors are in series, any fluctuation or instability in one motor can propagate to the others, amplifying the effect.

The fact that the system stabilizes when you add back pressure via a control valve suggests that the motors need a minimum return pressure to function smoothly at higher speeds.
Why this happens: Low back pressure could allow the hydraulic fluid to return too quickly, causing flow irregularities and even cavitation, especially with no load on the rotors. Adding back pressure helps stabilize the flow and keeps the motors from cavitating or "running away" with too little resistance.

Maybe you should Install a Back Pressure Valve:

Why it could help: A back pressure valve in the return line would maintain consistent pressure in the return circuit without the need to manually adjust a control valve. This would help stabilize the motors when running without load by preventing the motors from "freewheeling" too easily.
How it works: The valve maintains a minimum pressure in the return line, preventing the motors from going too fast in low-load conditions.

that's just my recommendation base on my research on the web.

R.Efendy

 

[Jan1235]

Yes I understand a backpressure valve will help. But when they are under load (wich they are 98% of the time) I dont think that back pressure is really desireable..

They do “softstart” with the proportional valve

 

[3DDave]

I would add a check valve at each inlet to prevent oil from coming back. This should prevent oscillation.

 

[Jan1235]

Thought about that a few times too! But I figured that might be foolish. Haha..

But then the rotor can still “overrun” the motor and will give an even bigger chance of cavitation, or not?

 

[73lafuite]

If a back pressure at the outlet of the 4th motor solves the pulsation problem: then it is enough to put a sequence valve with internal drin and external piloting. And connect the piloting to the upstream inlet of the 1st motor. Thus optimal efficiency during work and only losses when there is no torque to the 4 motors.

 

[73lafuite]

sequence valve with internal drain and external piloting

 

[73lafuite]

Try putting a check valve set to 5 bar on the collector of your 4 drains? Could this re-enter through the internal valves of the 4 motors? Without having a big influence on the life of the shaft seals.