Grapple cylinder is too strong

Knight · Oct 13, 2009

When it closes, It ends up ripping the grapple frame apart if I hold the valve too long. It's a 2 1/2" cylinder and I need a pressure relief valve.
Put I only want pressure relief on this cylinder only. How do I do it?

kennyd · Oct 13, 2009

You can buy a small PRV, but you will also need to run another hose back to the sump for the return.

HERE is just one example on one.

PILOON · Oct 13, 2009

You could also insert a limiter 'tube' inside of the offending cylinder.

Example; if the stroke is 2" too long, add a 2" tube so that the piston bottoms out 2" short.
For a 2 1/2" cyl probably a suitable length of 1 1/2" water pipe would do the trick.

I have a piece of equipment that uses that method, probably because the mfg wanted to only keep 1 size of cylinder in stock.

RickB · Oct 14, 2009

kennyd said:
You can buy a small PRV, but you will also need to run another hose back to the sump for the return.

HERE is just one example on one.

Or tee the pressure bypass into the other side of the cylinder circuit.

kennyd · Oct 14, 2009

RickB said:
Or tee the pressure bypass into the other side of the cylinder circuit.

Good idea...but when that line gets pressurized by operating the cylinder the other way will it hurt the PRV? Not sure if it can handle pressure on it's OUT port

J_J · Oct 14, 2009

If you put the relief valve in the extended line, that side will be protected, On the retract, not much pressure, it wouldn't matter unless he held the levers back to much, Maybe a double relief valve, to be safe.

Maybe rethink the grapple geometry, It doesn't look right to me. Maybe beef up the metal somewhat.

Ductape · Oct 14, 2009

RickB said:
Or tee the pressure bypass into the other side of the cylinder circuit.

Someone educate me. Wouldn't this be (more or less) the same as putting one of the cushion valves in like you would put on a snowplow? When the pressure is too great (like when you hit something with the end of the plow) it just bypasses.

I'm trying to pay attention here, since eventually I intend to add hydraulics to my Woods thumb and want it slight weaker than my bucket curl (don't I??).

MtnViewRanch · Oct 14, 2009

If you want to fix the problem, you should rebuild the mount. It looks like you have used metal that is way to thin for what it is being used for. If you use the relief valve, by the time that you get the pressure down low enough that you won't have any damage, the thing is not going to work how it should.

Just my opinion from looking at the picture.

J_J · Oct 14, 2009

Ductape said:
Someone educate me. Wouldn't this be (more or less) the same as putting one of the cushion valves in like you would put on a snowplow? When the pressure is too great (like when you hit something with the end of the plow) it just bypasses.

I'm trying to pay attention here, since eventually I intend to add hydraulics to my Woods thumb and want it slight weaker than my bucket curl (don't I??).

I believe he does not has the steel sized for the force of the cylinder, and apparently he has damaged it at one time. He is wanting to reduce the force, so it will not bend/break anything. Even if he puts a relief valve in there, that reduces the pressure form 3000 to 1500 psi, the force will only be halved. If that cyl was a 2 in cyl, the pressure at 3000 psi the force would be over 9,000 lbs, and at 1500 psi, below 5,000 lbs.

bucmeister · Oct 14, 2009

I'm with J_J on the geometry of the design. The way most grapples are designed they self limit the ultimate force applied, but with that tower up like it is the force at full extension of the cylinder is going to be huge and I would guess the limiter inside the cylinder is going to be the only way to stop the breakages. Even at that, if you grab a stump, large log, rock, or anything that the teeth are tight on a bit before the limiter it hit then the teeth are going to get broken again if you do not get off of the valve quickly enough.

Considering all of that may very well point you back to a pressure bypass as the best option because due to the design this inherent weakness is going to manifest throughout the range of travel of the jaws due to the leverage. If you were to beef the jaws up enough to take the pressure without breaking then next weakest part is going to give anyway.

If you want to see the kind of geometry that is typical/classic for grapples take a look at the thread by GuglioLS in the Build-it-Yourself forum titled FEL Grapple from Scratch W/Lots of Pictures. Here

That grapple is the cats meow.