Expert: Paul Soderman - 8/22/2006

Question
Hi Paul,

What causes chatter in directional control hydraulic valves?

I have a four way, three position DCV that is spring biased to the middle position (off or blocked) and is manually shuttled to the left or right position.  When the operator pushes the valve mid-position where fluid is just starting to pass through the valve, the valve chatters.  I understand the relative pressure differences across the small opening when the valve just begins to shuttle and how this changes with flow.  But what's a more technical explanation for this. Is this resonance?  I realize holding the valve in the "barely open" position will cause this and firmly shutteling the valve to it's respective positions but this is a newly occurring problem in an application where it's never been observed before.  What are potential causes of this and how do you eliminate it?  Stiffer spring?  

Thanks,

John

Answer
Hi John
Without details on your valve geometry and flow conditions, I can't be sure of what is happening.  Here is a possibility.  As the flow enters the small opening it accelerates and the pressure drops, but as it exits it sees an abrupt area increase.  This results in an abrupt velocity change and a corresponding abrupt pressure increase.  The pressure increase radiates as a wave and impedes the flow entering the opening momentarily.  Or maybe it reflects off something.  Once the wave passes, the original flow conditions restarts and the cycle continues.  This is chatter.  It is not resonance so much as a hysterisis condition.  The flow is oscillating about a steady state condition it keeps overshooting.  In some systems, the phenomenon would appear as water hammer.  In your case it sounds like chatter and causes both vibration and noise.  With larger valve openings, the pressure changes are smaller and less likely to cause chatter.

What to do ?  Here are some things I found on line for relief valves which may or may not be applicable to your case.

       •    Oil viscosity is too high.
  •     Poppet or seat is faulty or worn.
  •     Line pressure has excessive return.
  •     Pressure setting is too close to that of  another valve in the circuit.
  •     An improper spring is used behind the  valve.

Hope this is right.
Paul