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!

 

[LittleInch]

what is the coupling between all these items?

Could you hard pipe it, or some of it to reduce the flexibility of the hoses?

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

 

[3DDave]

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

You have a problem with oscillation CAUSING the overrun. This stops the oscillation so overrun cannot happen.

Since the check valve allows fluid INTO the motor, the motor cannot cavitate.

 

[Jan1235]

“You have a problem with oscillation CAUSING the overrun. This stops the oscillation so overrun cannot happen.

Since the check valve allows fluid INTO the motor, the motor cannot cavitate.”

Well yeah… but if the rotor still tries to go faster then the motor and I would “restrict” the intlet with a checkvalve, oil can go in but he cant suck it in… or not?

Cannot hard pipe everything. Rotors can move a little..

 

[Jan1235]

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.


I am sorry to ask, but do you maybe want to explain this a bit more? Im not very familiar with sequence valve’s….

 

[3DDave]

Ok. You have all the answers, apparently. Obviously you would select a check valve to intentionally to have too high a restriction, unlike the restriction by friction in the hoses connecting the motors, which cannot under any circumstance be under consideration as a cause.

Why would you select a restrictive check valve?

 

[Jan1235]

Oke did I offend you or something? I dont have the answers at all…

With restriction I meant that (theoraticly in my head!) if there is a checkvalve in the inlet and the rotor out spins the motor, the motor will try to suck more oil wich it cant because of the checkvalve. Or am I that wrong?

As for the checkvalve in the last return line; the return pressure does not drop to zero when it is pulsating, so would a checkvalve even get the chance to help at all?

I am just trying to fully understand all your solutions and I am very thankfull for everybody’s help!

With the “hard pipe” I awnserd LittleInch

 

[3DDave]

" if there is a checkvalve in the inlet and the rotor out spins the motor, the motor will try to suck more oil wich it cant because of the checkvalve. Or am I that wrong?"

If you don't know how check valve works that is a problem I cannot solve for you. Spend some time learning about check valves and then you will be able to make an informed comment about what they would do.

 

[Jan1235]

You okay? Want to talk about it?

Like I said earlier, my english isnt that great. If I understood something wrong, or if I explained my thoughts incorrectly you do not have to belittle me.

And yes, I know what a check valve does, and yes, I was thinking wrong… glad you probably never have that.

Thanks for your help and I wish you a very nice day. Please do not waste any more of your precious time on an dumbo like me.

 

[73lafuite]

Hello, Externally piloted sequence valves are not available from all suppliers. But a counterbalance valve would do the job. It is available everywhere and at a fair price. Take a low ratio if possible.

 

[73lafuite]

.....

 

[LittleInch]

Could some small accumulators work to smooth out the pressure fluctuations?


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

 

[3DDave]

It's acting like a spring-mass system. Accumulators lower the spring rate; that should lower the frequency and increase the amplitude.

It needs either a ratchet/diodes/check valves to prevent the oscillation, or some dampers to remove energy from the system.

The problem with dampers, such as an orifice, is they normally work in opposition to motion, something that is the opposite of the requirement to make motors move.

One could add harmonic balancers to each of the output discs to maintain a smooth rotation, but those would be difficult to properly tune. In car engines they are to resist/restore torque impulses that are at a much higher frequency than the desired crankshaft frequency. In this case it is likely the oscillation speed is similar to the operating speed, so there would be little difference to leverage for dampening.

 

[Jan1235]

Oke so I added 2 checkvalves to the last two motors (only had two left). It improved significantly. Ordered a few more. still cant wrap my head around it why this helps though.

Only downside this way I loose the reverse option. But I dont think I will need it.

If I have some time left, I will try to fiddle around with it some more. For now, it has to go to the field.

Thank you all so much for the help!

 

[FMJalink]

Hi Jan1235,
I think the following (simplified) happened with your original set up. One flow stream entering and leaving the system with 4 hydrmotors in series. With no real constant load on the rotors, the load on each motor comes from the acceleration of the coupled disc. If I did understand correctly, the turning of discs if not mechanically coupled somehow, leaving the load on each dics independent from each other. With high flowrate (more opened valve) a motor has more than enough torque (and flow) to accelerate a disc. When the speeds of the dics is beyond that of the downstream disc, it will pull hydraulic liquid from that upstream disc. Reversely happens with the downstream disc. This makes it a highly dynamic system. A fast closing check valve, e.g. requiring an opening pressure (not allowing minor reverse back flow) stops that train of vibrating discs.
Most likely everything preventing or reducing this process will help.
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