Power Beyond Hydraulic valves

[kbrothers]

Trying to understand power beyond capability on a valve Do you just plug it off if not in use? Do you need the sleeve for the power beyond when you buy the valve or can you buy it later? One cataloog shows two options -one power beyond and one non Is the valve un useable without one of the options? Cofused Ken in Pa Thanks

[mmzullo]

Ken,
I'm trying to make a log lift for my log splitter. I'm trying to figure out the hydraulics and was on a couple hydraulics web sites. But I think power beyond is when fluid runs through the lift spool valve and powers a second spool valve at pump pressure . It should be able to be capped and used later. I think I'm going with double spool valve. But this is just a guess on my part.

[DaveKamp]

Power Beyond is a pretty simple thing.

When you design a fluid power system, you have 'open center' systems, and 'closed center' systems. If you're running a 'variable displacement' pump (like a vane pump), you'll use a 'closed center' system, and when using a 'fixed displacement' pump (a gear or georotor, or piston, or whatever), you'll need to run 'open center'.

Here's the difference:

In a fixed displacement pump, the pump MUST be able to move fluid. If you have ANY circumstance where the pump is prevented from moving fluid, the pump will either stop the prime mover, or blow hydraulic lines, or fragment the pump... because one turn of pump rotation MUST move liquids... they're not compressible.

In a fixed-displacement situation, you must ALWAYS have a pressure-relief valve capable of carrying MORE than 100% of the pump's total flow... if you don't, you'll have one of the three situations above. What you DON'T want to do, though, is put the pump and prime mover in a situation where there is no other option BUT flow through the relief... by doing so, you'll put the pump under full load, just to maintain the relief's pressure setpoint. What you REALLY want to do, is dump whatever the pump draws, right back to the tank. This means the pump's output volume is unrestricted, but output pressure is zero, and engine load is thus, nothing.

An open-center or tandem-center valve does exactly this... when in the center position, all fluid flowing in from the pump, is dumped back to the tank. When a valve spool is moved, fluid is directed to the cylinder (or motor, or whatever), as demanded (by lever position). Once the lever is released, the pump's flow is dumped to tank.

In a variable-displacement situation, the pump (typically a vane pump) is set up so that the vanes extend and move fluid, but only when fluid is allowed to flow. They're built so that once a certain pressure-point is reached, the vanes retract, and cause the pump to be it's own 'pressure relief'. In the case of a variable-displacement setup, hydraulic valves controlling cylinders will NOT dump fluid back to tank under any circumstances... they instead block off incoming fluid, and force the pump's vanes to retract. This arrangement is called 'closed center', because when in a centered position, the incoming flow is simply blocked off.

In typical hydraulic valve assemblies, you have 'sections' that can be purchased and assembled into a group to meet whatever needs you have. On the ends, you place 'endplates' that have fittings to connect to your tank and pump. You can include a section for a pressure relief valve, too. The way the valve is set up, if you pull ONE lever to operate one cylinder, and then you pull a SECOND lever to operate a second, there's one of two things that happen- either it will have to SPLIT fluid from the pump, and send part to one, and part to the other, or it will have to route fluid into one circuit, through the cylinder, and the RETURN from the cylinder will be routed to the NEXT segment, through the NEXT cylinder, and then out... etc... and at the last one, return the displaced result to tank... and if an over-pressure condition occurs, anything the pump pushes which exceeds the safety valve, will be dumped to tank.

In a 'power beyond' situation, what you're essentially doing, is adding more sections to the valve, but placing them in some other location.

In a common modular valve, there's two 'galleries' which fluid flows... one is the pressure side (from pump) and the other is return to tank. Fluid flows in the pressure side, and every valve can either take it (pulled), or leave it (center). After the last port, the fluid needs to go somewhere, so the end is just a U-shaped casting, and anything left there goes to the return gallery. In a power-beyond capable endplate, there'll be a hole in that endplate, into which you could install a hose to the next valve assembly. If you're not using the power-beyond feature, there'll be a plug blocking off the beyond port, and anything flowing through (when a handle isn't pulled) dumps to the tank return gallery.

IF you employ the power-beyond feature, you'll remove that plug, and slip in a 'sleeve' that fluid can flow through, but fluid WON'T be able to pass down to the return gallery. THEN you'll install a hydraulic line going to the second valve assembly. On THAT valve, you'll be connecting the hose to the valve's INLET (just as if it's a pump), and you'll connect the RETURN line either back to a port on the RETURN gallery of the first valve, or more often, just a line returning to the tank.

SO... there are some valves that simply don't have an endplate or gallery access ports suitable for use in a power-beyond application. Most valve systems have the power-beyond capacity built into the endplate, or they offer a plate that IS... and then, you decide wether to fit it with a plug (so that it has the capability, but you're not using it), or a sleeve and hose (at which point, you MUST have some other downstream valve installed). Keep in mind that most of the time, if you anticipate need for the ability to use a second valve assembly, you won't need power-beyond features in the LAST valve... but the FIRST valve set WILL need it... and of course, if you're running gobs of circuits, there's nothing that says you can't run six power-beyond valvesets, and the last one as a non-power-beyond... with them all daisy-chained together. Most manufacturers, though, would spec a power-beyond valveset, and then have plugs and sleeves on hand so that they could set 'em up for however they wanted per needs.

Is that clear as mud?

Here's another description that isn't half bad:
http://www.insidersecretstohydraulics.c ... eyond.html

[junderwood]

Yep, Dave is back. Wow that post brings back memories of the old forum. You sure haven't lost any of your knack for long thorough explanations Dave. You need after that.

It's good to have you back.

John