cargo lift, hyraulic cylinder

[tojojoseph]

i just commissioned a cargo lift with a 3 kw motor , 2400 rpm motor but while working there is a huge sound like a humming sound which seems to come from the cylinder. clueless

 

[HPost]

That will be the vibration coming from the pump and or motor. To shut it up, you'll need to attenuate the cylinder to stop it from vibrating.

It could also be that your motor speed is close to the natural frequency of the cylinder. In which case, you'll need to either change the motor speed, get a new cylinder with a different natural frequency or do something drastic to stiffen the cylinder up to move its natural frequency away from the motor speed.

Hpost

 

[tojojoseph]

dimension of the cylinder is thin which is 35(shaft)x80(cylinder)x 3.50 mts long in closed position.to change the natural frequency , how can i make my cylinder supplier understand it?

 

[HPost]

Natural frequency is equal to the square root of the stiffness over the mass. If you can't change the stiffness, change the mass.

The cylinder supplier probably wont help you much, it's not their fault that the cylinder's natural frequency is close to the motor speed.

That is a very slender cylinder, not surprising it's signing. Can you brace it at all in the middle at all?

Does the noise change as the cylinder extends...does it get any more noisy or less noisy?

 

[PNachtwey]

I would estimate the frequency of the humming and compare it with the calculated natural frequency of the cylinder and load to see if they could be the same but often they are not. Humming is often 60 or 50 cycle hum. Poorly designed hydraulics systems will have natural frequencies much lower than that.
The natural frequency of the system should be calculated.
On page 2/27 there is the formula for natural frequency.

http://deltamotion.com/peter/Mathcad/Mathcad - Hydraulic Cylinder.pdf

The pdf file shows how I can simulate the cylinder with changing load or disturbances and how the closed loop control will react.

My suspicions is that the mechanics are vibrating.



Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[tojojoseph]

the humming sound is more in a down position when the piston is contracting and in a particular position only and also when its extending when we lower the lift that sound is little less

 

[HPost]

Is the cylinder connected to the power unit with hoses or rigid tube?

 

[tojojoseph]

With hoses

 

[HPost]

That suggests that issue is from vibrations transmitted by the oil, rather than structure borne noise.

The pulsations or ripple from the pump are being transmitted to the cylinder via the oil. The frequency of the vibrations is, as stated, close to the resonant or natural frequency of the cylinder. As the issue is more about noise than actual movement, it suggests that it will not take much to reduce the noise.

I think that you best bet is to change the cylinder mass or stiffness. Maybe up, may be down...???

 

[tojojoseph]

so the vibrations are generated in the power pack , may be the kw for this much long cylinder is not enough? i had beforehand made arrangement for mounting of another pump to so shall i try by putting another pump with 3 kw motor?

 

[tojojoseph]

this the pic of cargo lift

 

[HPost]

It's not about power, it's the pressure pulsations in the pump that are causing the noise. If you can change the power unit, change the speed of the motor. That will change the frequency of the vibrations.

 

[tojojoseph]

So if I change the speed ,from 2800 rpm shall I decrease the speed?

 

[HPost]

Perhaps..?

Lowering the speed might push it towards the resonance point. Likewise, going up in speed could do the same. Without knowing the natural frequency of the cylinder, it's impossible to know which way to go.

Given the size and shape of the cylinder, my guess would be to go slower.

Slowing the motor down will decrease the flow. You'll need to get a bigger displacement pump to keep the cylinder speed the same.

 

[tojojoseph]

coming down to the pump there was a provision for one more pump of same size to be put there so , shall i try again to put one more pump with same specification and then reducing the flow of both of them will it work?
or shall i go ahead with changing the piston rod with a more thicker one like from 35 mm to 45 or 50 mm?

 

[HPost]

Seems easier to change the rod diameter. If you not concerned with the increase in speed caused by the bigger rod, go for it.

There is no guarantee of success here. you'll have to try it and see what you get.

 

[Deaks]

Is there a counterbalance valve in the circuit?

 

[tojojoseph]

finally the problem was solved when another pump was installed in parallel and there is no sound still i have the problem of piston swaying up and down

 

[HPost]

In what orientation is the cylinder installed? Horizontal or vertical?

 

[tojojoseph]

The cylinder is in vertical position.