Alternative 3rd function?

[LD1]

Doesnt this valve connect both sides of the circuit to each other? so fluid moves either direction?

However, thinking more about this, i realized that these cylinders have different areas on each end (due to the rod) - and thus different volume-per-inch-moved. meaning this wouldnt work in EITHER direction:

• compressing the cylinder would just overpressure the rod end, not enough space for fluid from bottom
• extending the cylinder would ~cavitate the bottom, too little volume sent to the bottom side

Now I'm confused because while i don't see how LD1 could be correct - it seems NEITHER way works on common cylinders... and yet that's the cushion valve's stated purpose

Yes, the different piston areas/cylinder volumes is why it doesnt work. The cylinder physicaly cannot compress without fluid leaving the system.

It can however still extend. Yes it will "cavitate". It will draw a vacuum on the cylinder, introduce air, etc.

Gasses (like air) are are not like a fluid. They can compress and cavitate with enough force. Fluid cannot. Thus the cylinder can still extend. But this would be no different than if the valve were NOT in the system

 

[npalen]

Yes, the different piston areas/cylinder volumes is why it doesnt work. The cylinder physicaly cannot compress without fluid leaving the system.

It can however still extend. Yes it will "cavitate". It will draw a vacuum on the cylinder, introduce air, etc.

Gasses (like air) are are not like a fluid. They can compress and cavitate with enough force. Fluid cannot. Thus the cylinder can still extend. But this would be no different than if the valve were NOT in the system

Unless there was no air on either side of the piston?

 

[ericm979]

I don't think you need to do anything other than not operate the grapple like an idiot, but if you do, a single relief valve on the extension side of the grapple cylinders would be the best solution. The accumulator would need to have more than the system pressure in it (which is possible though you might need someone to pump it to 3000 psi of nitrogen for you), and would need to be larger than the volume in the cylinders. Otherwise it will bottom out and you'll be back to the high pressure you're trying to avoid. Also its going to be kind of wierd if you pick up that badly off center load and the lid opens back up.

But really, you should be thinking about safe grapple operation rather than thinking about mechanical means to allow for unsafe operation.

 

[2manyrocks]

I've been wrestling with the same issue and holding off adding a third function to control the new grapple I recently bought. For me, if there is some additional relief valve or whatever protection I can add to the system, I'd be happy to spend another $100-$200 and possibly more if it would protect my tractor's hydraulic system at a relatively affordable price point.

The good thing about forks is you can build them so they bend before you damage the rest of your tractor.

So the other approach that comes to mind is to rebuild my grapple lid with some kind of shear pin that would break first. Or sell the grapple and build a set of forks.

I'd also rather learn from all of you before I break something instead of getting an expensive remedial education at the dealership repair shop.

 

[npalen]

I've been wrestling with the same issue and holding off adding a third function to control the new grapple I recently bought. For me, if there is some additional relief valve or whatever protection I can add to the system, I'd be happy to spend another $100-$200 and possibly more if it would protect my tractor's hydraulic system at a relatively affordable price point.

The good thing about forks is you can build them so they bend before you damage the rest of your tractor.

So the other approach that comes to mind is to rebuild my grapple lid with some kind of shear pin that would break first. Or sell the grapple and build a set of forks.

I'd also rather learn from all of you before I break something instead of getting an expensive remedial education at the dealership repair shop.

"Remedial education at the dealership" That a good one!

 

[LD1]

Unless there was no air on either side of the piston?

There doesnt need to be air. Infact a true vacuum is the absence of air. But it behaves much the same way as if there were air in the cylinder.
Trust me, the cylinder can still extend....crossover valve or not, with enough force. Not true for compression.

In the latest "theme" of this thread about over-pressurization from grapple mis-use......one instance comes to mind for me. With the grapple lid open but not open all the way....and bucket down-turned with the tines of the lid trying to pull on something (or even push).

Anytime you have external forces acting on a cylinder....you are imparting pressure in that circuit. Sometimes the tractive force of the tractor is greater than the hydraulics can handle. The cylinder then becomes like the pump on a bottle jack.....and pressurizes the fluid. When you go too far, you pop a hose or break something.

The grapple cylinders are usually pretty small, so its not hard to do if one is careless

 

[jigs_n_fixtures]

The really high loads occur from having the tractor moving, and then hitting a bump with the grapple locked on something which wants to wiggle, due to it’s momentum.

When it wiggles it tries to break free from the grapple. You have pressurized the cylinder(s) to the releif pressure, and then closed the valve. So, when the log you grabbed tries to wiggle loose, the grapple clamp side of things gets hit with the pressure wave. That pressure wave far exceeds the ”design” pressure and can damage things.. Most likely you blow a hose or line.

As I posted earlier, the cylinder isn’t really going to move much, so you would only need to absorb a small amount of fluid. In normal operation you don’t really want it to take in very much if any fluid. You want it to accept fluid only when the pressure waves are occurring.

As to having the accumulator pressurized: You can buy them pre-pressurized. But they are supposed be shipped ground. So it might take a while to get delivered.

 

[orangetree]

very interesting subject, these hydraulics are

Running some napkin calcs, I measure that despite the problem (mismatched blind/rod cylinder hydraulic flow), a simple cushion valve would offer some minor buffer against shocks ... but probably not much.

• assume the fluid compresses 0.5% per 1000psi increase
• a 2" cylinder has ~3.14cuin/inch in[out]flow (change in volume) per inch traveled. I have two cylinders in parallel, so that's ~6.3cuin/inch in[out]flow per inch, total between both cylinders
• and ~30cuin of total hydraulic fluid volume in the grapple itself.
• Say I want to prevent ~impulses (short pressure spikes) above say 5500psi (~arbitrary, chosen for clean #'s below) - judging by the videos it's easy to achieve this aggressive operation; perhaps too easy.
• that's 5500psi-2500psi = 3000psi pressure delta for the fluid == 1.5% compression of the fluid

30cuin compressed/decompressed an extra 3000psi changes in volume by 1.5% or 0.45cuin

0.45 cuin is the same as 0.075inof cylinder travel.

That's not very much. Might take the peaks off any seriously damaging impacts or loads .... but then again these devices aren't perfectly linear or fast to respond, so in all likelyhood a large pressure spike, the kind this cushion valve could theoretically help prevent, the pressure may be transmitted back to the valve anyway since the system can't respond fast enough.

So, this really does seem like an inadequate solution, unless i've done my math wrong here.

The accumulator seems like the way to go?

 

[npalen]

very interesting subject, these hydraulics are

Running some napkin calcs, I measure that despite the problem (mismatched blind/rod cylinder hydraulic flow), a simple cushion valve would offer some minor buffer against shocks ... but probably not much.

• assume the fluid compresses 0.5% per 1000psi increase
• a 2" cylinder has ~3.14cuin/inch in[out]flow (change in volume) per inch traveled. I have two cylinders in parallel, so that's ~6.3cuin/inch in[out]flow per inch, total between both cylinders
• and ~30cuin of total hydraulic fluid volume in the grapple itself.
• Say I want to prevent ~impulses (short pressure spikes) above say 5500psi (~arbitrary, chosen for clean #'s below) - judging by the videos it's easy to achieve this aggressive operation; perhaps too easy.
• that's 5500psi-2500psi = 3000psi pressure delta for the fluid == 1.5% compression of the fluid

30cuin compressed/decompressed an extra 3000psi changes in volume by 1.5% or 0.45cuin

0.45 cuin is the same as 0.075inof cylinder travel.

That's not very much. Might take the peaks off any seriously damaging impacts or loads .... but then again these devices aren't perfectly linear or fast to respond, so in all likelyhood a large pressure spike, the kind this cushion valve could theoretically help prevent, the pressure may be transmitted back to the valve anyway since the system can't respond fast enough.

So, this really does seem like an inadequate solution, unless i've done my math wrong here.

The accumulator seems like the way to go?

Nice to see someone putting a pencil to the issue!

 

[npalen]

There doesnt need to be air. Infact a true vacuum is the absence of air. But it behaves much the same way as if there were air in the cylinder.
Trust me, the cylinder can still extend....crossover valve or not, with enough force. Not true for compression.

In the latest "theme" of this thread about over-pressurization from grapple mis-use......one instance comes to mind for me. With the grapple lid open but not open all the way....and bucket down-turned with the tines of the lid trying to pull on something (or even push).

Anytime you have external forces acting on a cylinder....you are imparting pressure in that circuit. Sometimes the tractive force of the tractor is greater than the hydraulics can handle. The cylinder then becomes like the pump on a bottle jack.....and pressurizes the fluid. When you go too far, you pop a hose or break something.

The grapple cylinders are usually pretty small, so its not hard to do if one is careless

I've sometimes wondered how far a piston in an oil filled cylinder can travel when "cavitating". In other words, how far can it theoretically travel when perfectly sealed until a theoretically perfect vacuum is reached?
Anyone have a formula?