directional valve overlap 1

[RobertHasty]

hello,

Does anybody know what kind of overlap do standard directional valves have if that is not specifically stated in a datasheet?

Thanks.

 

[Oldhydroman]

Hi Robert

Are you thinking about the transitional flow paths in three port and four port spool valves used in hydraulic systems?

There isn't really such a thing as a "standard" directional valve but there is such a thing as "sub-standard technical documentation". It would be fair to assume that "standard" spool type DCV's would not open up the P port and the T port at the same time (to prevent unloading the pump during the switching process) and the service line actuator which is going to connect to tank would be opened to tank before the pressure was applied to the other service line (to prevent intensification) - but these are pure guesses.

If it's critical, and the supplier can't [won't] help, then choose a different supplier.

DOL

 

[RobertHasty]

Hello,

I'm thinking about the overlap between the spool and the housing. There's a negative, positive and zero overlap. With negative overlap, for example, there are some leakages when valve is in central position. With positive overlap, valve shifts with a slight delay, etc. I was looking at a datasheet and couldn't find anything about overlap. That's why I'm asking whether there's a usual principle applied regarding this issue when nothing is stated explicitly.

It occured to me that there might be something about these mid-positions that are shown in some catalogues. Perhaps by looking at them some conclusions could be made.

 

[Compositepro]

There is no standard that I'm aware of, and I can't imagine how one could be established. It has to do with manufacturing tolerances and expected performance. A negative overlap might increase energy usage though leakage but (within limits) will not effect precision of control while a positive overlap will have a deadband. Positive overlap is used for on/off type of valves while a slight negative overlap is used for servo valves. Technically, there is no such thing as zero overlap.
The only relevant measurement would be leakage flow at null position.

 

[RobertHasty]

Hello Compositepro. Thanks for answering.

I'm aware that there is no "real" zero overlap because of tolerances. What I would like to know is what is with basic sizes (dimensions) of spool and housing cavities. Are they the same or they differ on "standard valves".

Maybe some conclusions could be made by looking at a valve schematics with in-between positions depicted.

http://files.engineering.com/getfil...3c-4eb8-86fa-aeb124a679ba&file=SpoolValve.JPG

 

[Compositepro]

Sorry, I don't get what you are asking.

 

[hydroman247]

They differ due to manufacturing tolerances. The figure is so small and the cavity's so small that measuring the flow would be near impossible.

 

[RobertHasty]

Hello, to understand what I mean by overlap, it is best to see attached photo.

http://files.engineering.com/getfil...-386e-47f3-96a5-1f855f87db44&file=Overlap.JPG

 

[hydroman247]

Oh, you mean the sliding spool port overlap. There is no standard, each application will have its own setting.

 

[RobertHasty]

Oh, you mean the sliding spool port overlap. There is no standard, each application will have its own setting.

Yes, that's what I meant. It would be nice if I could rename the title....