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Problem - Hydraulic Cylinders vibrate when stopped

Problem - Hydraulic Cylinders vibrate when stopped

Problem - Hydraulic Cylinders vibrate when stopped

(OP)
Please help me to figure out hydraulic problem – I am not hydraulics expert. I got large ship steering system: tiller operated with two cylinders, controlled by directional valve. When the valve closes the rudder vibrates instead of stopping abruptly - makes several minute oscillations back-and-forth around the stop point which results in strong vibration propagating all over the ship. My questions:

Is it normal for hydraulic cylinder to vibrate when stopped?
What can cause such vibrations?
How to get rid of vibrations?

The directional valve used in the system - ATOS DPHI 3714 /FC 53 – two stage solenoid valve. Following some recommendations I installed between the stages Flow Restrictor Valve and adjusting controls in the Flow Restrictor Valve I can eliminate the vibration but this adjustment is so sensitive that I can not make it work continuously (I believe the adjustment is dependent on oil temperature).

Other system parameters: flow ~ 2 x 50 gallons per minute, working pressure 250 – 1500 psi depending on
Rudder load.
 

RE: Problem - Hydraulic Cylinders vibrate when stopped

Hi AndreyG

One possible cause for the vibration is entrapped air as the system been purged of air.

desertfox

RE: Problem - Hydraulic Cylinders vibrate when stopped

Other possible candidates for vibration :- cylinders binding on operation; sudden closure of a valve causing a pressure wave, operating at close to the natural frequency of the system.

desertfox

RE: Problem - Hydraulic Cylinders vibrate when stopped

Is this a new problem?  Has the system always vibrated?  Was a component changed and then the vibration appeared?

Ted

RE: Problem - Hydraulic Cylinders vibrate when stopped

(OP)
Hello desertfox and hydtools!

What means 'cylinders binding on operation'?


>> Is this a new problem?  Has the system always vibrated?

The system always vibrated


Is it any practical way to diagnose air in the system?

 

RE: Problem - Hydraulic Cylinders vibrate when stopped

Hi AndreyG

When I say binding I mean if the cylinder rods are subject to an out of plane load (ie not in plane of the cylinder), this would increase friction on one side of the cylinder making the cylinder rod shudder as it moves.
have you checked for air in the system as previously mentioned which might be caused by dieseling.

desertfox

RE: Problem - Hydraulic Cylinders vibrate when stopped

Flow dynamics over the tiller surfaces.

Hoses in the system allowing pressure oscillation.

Lack of sufficient mounting isolation to prevent vibration transmission to structure.

Ted

RE: Problem - Hydraulic Cylinders vibrate when stopped

The /FC options says there's a microswitch in the valve for detecting spool position.

I'm guessing that the switch engages when the spool is centered.
If it's wired into the control system, and inoperative, it might cause the control system to hunt continuously.

You probably need to find the schematic diagram of how the rudder position sensor interoperates with the solenoid valve...

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Problem - Hydraulic Cylinders vibrate when stopped

(OP)
Friends,

Thank you for all your comments!

I should say that the Control system is not the cause of the problem since I observe exactly the same vibration either I use control system or manually switch the valves on/off. (electronically manually, with a switch.)

>> have you checked for air in the system ...?

Please advice what is practical way to DIAGNOSE air in the system?


Cylinder motion: on the cylinder side the vibration is seen as back-and-force motion of the rode with amplitude ~ 1 mm. The cylinder size is some 5ft long by 10" dia OD. Would 1 mm play is considered acceptable 'play' in such cylinder at 250 - 400 psi?

Would this be considered normal 'cylinder play'

RE: Problem - Hydraulic Cylinders vibrate when stopped

"Please advice what is practical way to DIAGNOSE air in the system? "

Observe the reservoir for bubbles rising to the surface.

It sounds, based upon your original post, like the valve is closing too quickly, leading to a fluid "hammer" or shock.  I say this because you mention that you installed fluid restrictors (the "H" option for the valve mentioned in data sheet below?) in the pilot lines that were able to eliminate the vibration.  Does the vibration persist after the valve closes, or does it damp out fairly quickly?  

from this data sheet

http://www.tokimec.co.kr/hydraulics/upload_goods/E080.pdf

it looks like that valve can be ordered with a center spool piece that is shaped to reduce the closing rate, and reduce the shock.  See page 2, spool type 4/8.  Might be worth a try.  Alternatively, installing a low-rate valve in series with the main valve, and timed to open before, and close after, the larger valve, may also help to damp water hammer effects.

RE: Problem - Hydraulic Cylinders vibrate when stopped

This system may also benefit from counterbalance valves installed in a meter-out type configuration on the cylinders.  Although it entirely changes the power profile/energy use profile of the system, it would probably remove vibrations as it slows the movement of fluid due to physical resistance.  This would require a system redesign though, and needs careful consideration/engineering.

This problem may also be compounded by undersized components - e.g. lines and hoses causing high fluid velocity in the system.

Do you have a fluid schematic on how the system is plumbed?
 

RE: Problem - Hydraulic Cylinders vibrate when stopped

Have you checked for by-pass in the piston seals? If the load of the rudder pushes and pulls on the cylinder when it has stopped due to fast movement it may vibrate and cause the oscillating effect.

How big is the rudder ?

Did you meter in or meter out the flow restrictor that you installed?
 

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