What kind of hydraulic flow restrictors do I need?

RickB · Dec 19, 2004

We are getting pretty far out here aren't we? The reason a same size restrictor will result in a faster speed if used on the rod side hose is as follows. All other factors equal, the rod moves farther with the addition or removal of the same volume of oil to the rod end of the cylinder than if that same volume was added or removed from the base end. Since the flow rate through the restrictor is assumed to be constant for this discussion, the cylinder will move faster with the restrictor on the rod end hose than if it were in the base end circuit. The percentage of speed difference is equal to the percentage of section area the rod takes up.

beenthere · Dec 19, 2004

Yes.........Go for it. Report back how it works for you, please.

PineRidge · Dec 19, 2004

In two words YES, YES

 The reason why a cylinder will move faster in the retract direction than in the extend direction is because there is more volumn on the side that does not have the cylinder rod inside it, taking up space. For a given flow to a double acting cylinder, that cylinder will retract faster than it will extend. 

Henro and Jerry, I never thought of it in that perspective but the explanation makes good sense. I'll say uncle here.... /forums/images/graemlins/grin.gif

Banjo · Dec 19, 2004

I only speed read through this thread, but I don't think I saw this point ...

In the text for the flow restricter: "Adjustable flow control valve provides precise control of oil flow and shut-off in one direction and full flow the opposite direction ".

Most (all?) flow restricters have a valve in them so you get full flow in the reverse direction. Thus for the case of slowing down the grapple in both directions you'd need two, *AND* you'd have to make sure you put them in the right way round so that the restriction would be on the "return" side (to stop the cavitation and the like referred to above).

HTH. Appologies in advace if I missed this. Andrew

Ooops, missed the switch to pinhole ones. I'll leave this anyway. I think the concern with the pinhole, bi-directional ones, is that you could have cavitation problems when it's used on the supply side. Having them on the return side solves this problem (which is why they're made this way I would assume). For 25 bucks each I would think putting in two, adjustable, "one way" ones would be the way to go.

Henro · Dec 19, 2004

I think the concern with the pinhole, bi-directional ones, is that you could have cavitation problems when it's used on the supply side. Having them on the return side solves this problem 

Banjo,

Could you explain why this would be the case?

Now let me say that I just want to understand and am by no means an expert on hydraulic systems.

First thought that came to my mind was that if cavitation did occur, what would it matter anyway in a simple single cylinder circuit? I know cavitation is a major concern in a pump, but we are talking cylinders well beyond the pump...

My second question concerns the difference between having a flow restrictor on the high side of the cylinder as compared to the low pressure side. Would there really be much difference? When the cylinder is unloaded almost the same pressure would be developed across the pinhole restrictor, regardless of which side of the cylinder it was on. As the cylinder takes more and more load, the cylinder would take more pressure drop up to the point when the cylinder stalls and full pressure is felt by the cylinder and the system pressure relief valve trips. So to this maybe simple mind, it appears that it should matter little which side of the cylinder a pinhole-type restrictor is on.

Just to be clear, I am asking why cavitation would be more of an issue when a pin-hole restrictor is placed on the high side of the cylinder as compared to the low pressure side.

Also, why is cavitation a concern anyway, in a simple two-way cylinder application that has a very low duty cycle?

MMM · Dec 19, 2004

I've never used them, but heard they can create a jerky cylinder movement when heavily loaded. Cavitation? Is this true, pin hole users?

jinman · Dec 20, 2004

Tom, here at TBN we end up discussing the merits of "brain surgery" just to describe how to pull a burr out of our toe. /forums/images/graemlins/crazy.gif /forums/images/graemlins/smile.gif For around $5, I'd go get a pinhole flow restrictor and put it in my grapple circuit in the easiest spot. If it doesn't give you the response you need, then go to the adjustable flow restrictor. Orifice type restrictors are used all over aircraft for limiting and timing applications. Even there, it ain't really "brain surgery." /forums/images/graemlins/wink.gif

PineRidge · Dec 20, 2004

If you're needing some for your own use we spotted them at TSC with different configurations. They were rather inexpensive to boot.

adlertom · Dec 20, 2004

( Tom, here at TBN we end up discussing the merits of "brain surgery" just to describe how to pull a burr out of our toe. /forums/images/graemlins/crazy.gif /forums/images/graemlins/smile.gif For around $5, I'd go get a pinhole flow restrictor and put it in my grapple circuit in the easiest spot. If it doesn't give you the response you need, then go to the adjustable flow restrictor. )

Yes, I had already decided to do just that. I picked up a pinhole restrictor at my dealer the other day and will install it to see what happens. I may not get around to it for a few days, but will let you all know what happens. I spent $6 on it. If it doesn't work I'll just go try the more expensive scenario of two one-way restrictors.

My main concern, of course, is not wanting to damage something. I can't think of how installing this restrictor would cause a problem, however I'm not an expert so I wanted to check with you guys first.

Other than the mention of possible cavitation, no damage issues have been raised by those participating in this thread. Again, I'm not a hydraulics expert, but I don't think that installing this restrictor will cause cavitation.

My understanding of cavitation is that it occurs when a pump "runs away" from its incoming fluid. In other words, the outflow of the pump starts to exceed the inflow. Since the inflow to the hydraulic pump is from the sump and not the restricted grapple line, there should still be plenty of fluid to supply the pump.

It's been an interesting discussion anyway, even if it's not brain surgery. /forums/images/graemlins/smile.gif Thanks to all who offered info and advice.

Tom

Banjo · Dec 20, 2004

Henro, et.al.,

I think it turns out that this might be a bit of a case of the fingers being faster than the brain /forums/images/graemlins/blush.gif.

You can get cavitation in the supply line if you have a flow restricter there, AND the cylinder is being driven by an external load (as would happen in lowering a loader with a load on it for example). In this case the fluid can exit the system faster than it can be supplied. It's hard to figure out how this situation would occur for a set of grapples though.

A similar situation can occur with hydraulic motors if they end up running as a pump due to an external load (e.g. flywheel loads, braking with wheel motors).

Managing the return flow addresses these issues. FWIW, I think of it as making the cylinder "look" like it's under a heavier load.

I found a number of references to "smoother" control with the "one way" type of valves in the return path. This would be consistent with the above information, where the system might work like it had "air" in it.

The "brain surgery" comment is well placed too. The "right" solution take a lot of factors into account. It'll be interesting to hear how the simple solution works out in practice.

I should also mention, that I haven't had the need (chance) to use any of these valves in the hydraulics that I've done to date (loader and backhoe), so I was just going from (recollected ... always hazardous /forums/images/graemlins/grin.gif) experience from others, and from my efforts to "figure out" hydraulics. I'm no expert, and sometimes having an engineering background can work against you /forums/images/graemlins/wink.gif.

HTH, and sorry for any mis-information. Andrew