Expert: Ray Wilkinson - 8/22/2006

Question
Hi Ray,

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
John

I don't know the design of DCV's so I can't be specific, but chatter is caused by an interaction between flow and pressure in the pipework around the valve, or some kind of pressure variation within the valve. The main areas I would consider would be some sort of venturi effect inside the valve - a partially open port may cause high-speed flow which causes the pressure to fall locally, pulling the valve close, stopping the flow, which allows the valve to open again, etc. However, this is mainly a design-related problem which should have been sorted before production, although it may be influenced by the attached pipework.

The most likely cause, though, is pressure fluctations in the pipes attached to the valve, and a resonant condition as you suggest. If the pipe length is just so, a returning pressure pulse will interrupt the flow, creating another pulse, and so on, leading to a standing-wave type condition. If you've modified the pipe lengths, or added branches, this might lead to a 'tuned' condition in the pipes. Also, any air in the pipes may exacerbate an existing condition by acting as a spring and reducing inherent damping, or a change of fluid viscosity might be to blame (but unlikely in your case, I would think)... the list goes on.

Nothing specific there, I'm afraid, but a few thoughts as to why it might happen. I hope this helps, but please let me know if you fix it, and what you did, as I'm curious.

Regards, Ray.