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?

 

[orangetree]

Perfect vacuum? Not possible since you have finite space + pre-existing particles

Since there is obviously no such thing as "negative pressure", the cavitating (not sure if this is specifically a correct usage btw, but seems close enough) space will just ~approach zero ~ and probably not very closely.

How far will it travel? How it moves is simply sum of forces. assume hard zero on the cavitating end. add the force on the rod, and the force from pressure on the other end. Assume a non accelerating reference system for good measure! piston moves opposite from the greater force.

 

[npalen]

Perfect vacuum? Not possible since you have finite space + pre-existing particles

Since there is obviously no such thing as "negative pressure", the cavitating (not sure if this is specifically a correct usage btw, but seems close enough) space will just ~approach zero ~ and probably not very closely.

How far will it travel? How it moves is simply sum of forces. assume hard zero on the cavitating end. add the force on the rod, and the force from pressure on the other end. Assume a non accelerating reference system for good measure! piston moves opposite from the greater force.

So lets assume a 2" diameter piston for simplicity so the area is 3.14 sqin.
How much force would be required to pull the piston 1" creating "negative pressure"?

Edit: Would the force required eventually be theoretically infinite or would the maximum force be 46.182 pounds and never exceeded regardless of how far the piston was "pulled"?

 

[ovrszd]

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).

Yep. And this added load can be transmitted to the curl cylinders of the FEL. If those cylinders are in an extended position it's very possible to bend the rams. I did this on a previous 45hp tractor/FEL.

A Grapple, Tree Puller or any 3rd function attachment that has the ability to "grip" something changes the dynamics of the load on the hydraulics and super structure of the implement and tractor involved.

 

[orangetree]

So lets assume a 2" diameter piston for simplicity so the area is 3.14 sqin.
How much force would be required to pull the piston 1" creating "negative pressure"?

Edit: Would the force required eventually be theoretically infinite or would the maximum force be 46.182 pounds and never exceeded regardless of how far the piston was "pulled"?

Yes, the lowest the pressure on the 'cavitation' side could have is zero. std 1atm is ~14.7psi. Ignoring friction, ambient pressure on the rod area itself, asymmetries, etc, it's only the fluid pressure on the opposing side exerting force on the plunger. At a minimum that fluid is vented to air (eg, at 14.7psi absolute), so that's the only other force on the piston. acceleration = force / mass, so the piston accelerates (eg moves) in the direction (net) force is. and that force is, at a minimum, 14psi exerted on 3.14sqin ~= 45lbs.

in real life, I'd expect also quite a bit of friction (these seals can withstand thousands of psi, after all), so the actual force required to "try this" by plugging one 2" cylinder port in your hand is likely much higher But if you have kids, grab a pneumatic lego cylinder and try it. Not much force required to "pull a vacuum" in that little cylinder.

note that in everyday life we typically talk about pressures relative to atmosphere - ie "gauge pressure" - because it's the most relevant. This discussion is talking about vacuum, so it makes more sense to talk absolute pressure. Just keep in mind that's a different reference than the way you presumably are used to thinking about it.

 

[npalen]

Yes, the lowest the pressure on the 'cavitation' side could have is zero. std 1atm is ~14.7psi. Ignoring friction, ambient pressure on the rod area itself, asymmetries, etc, it's only the fluid pressure on the opposing side exerting force on the plunger. At a minimum that fluid is vented to air (eg, at 14.7psi absolute), so that's the only other force on the piston. acceleration = force / mass, so the piston accelerates (eg moves) in the direction (net) force is. and that force is, at a minimum, 14psi exerted on 3.14sqin ~= 45lbs.

in real life, I'd expect also quite a bit of friction (these seals can withstand thousands of psi, after all), so the actual force required to "try this" by plugging one 2" cylinder port in your hand is likely much higher But if you have kids, grab a pneumatic lego cylinder and try it. Not much force required to "pull a vacuum" in that little cylinder.

note that in everyday life we typically talk about pressures relative to atmosphere - ie "gauge pressure" - because it's the most relevant. This discussion is talking about vacuum, so it makes more sense to talk absolute pressure. Just keep in mind that's a different reference than the way you presumably are used to thinking about it.

So are we saying that the 45lbs stays constant regardless of how far the piston is "pulled"? Also, how far does the piston have to move before it generates the 45lbs force requirement and zero PSI absolute on the blind (sealed) side of the piston? Would be interesting to see this on a graph.

 

[LD1]

So are we saying that the 45lbs stays constant regardless of how far the piston is "pulled"? Also, how far does the piston have to move before it generates the 45lbs force requirement and zero PSI absolute on the blind (sealed) side of the piston? Would be interesting to see this on a graph.

Disregarding seal friction.....the fact that it takes FAR FAR more than 45lbs to extend a cylinder is because of the piston seals.

Under ideal circumstances.....the ram CANNOT move either direction....because the fluid is trapped on either side of the piston and blocked off by the spool valve.

 

[npalen]

Hydraulic accumulators have been mentioned and are also widely used in industrial applications where continuous clamping of components is needed when machining parts, for example.

It would be interesting to experiment with a long coil of hose plumbed to the base side of the grapple clamping cylinder(s) to act as an accumulator. The hose would expand when pressure is applied to the cylinder and releasing the valve spool would trap the oil in the expanded hose holding clamping force on the grapple.

Leakage by the valve spool would start reducing the force but that is true with any accumulator device.

We used to have issues with hose expansion and contraction on large tillage equipment with master/slave type cylinder lift systems. The problem came when multiple cylinders that were synchronised perfectly would soon be out of phase due to expanded hoses pushing oil back past a leaky valve spool.

Edit: A single braid larger diameter hose would work best as an accumulator in the above grapple clamping application. A 3/4" diameter single braid hydraulic hose, for example, would have much more "accumulation" than, say, a 3/8" diameter double braid hose.
Would need to check the hose pressure rating, however.

 

[orangetree]

Disregarding seal friction.....the fact that it takes FAR FAR more than 45lbs to extend a cylinder is because of the piston seals.

Under ideal circumstances.....the ram CANNOT move either direction....because the fluid is trapped on either side of the piston and blocked off by the spool valve.

AFAICT this is just a thought exercise, trying to add some intuitive understanding if one hasn't taken a statics or fluids course.

Agreed, in the real world there is fluid on the other side of that cylinder, and that fluid is constrained by a closed valve ... you are not going to move that cylinder by hand

 

[orangetree]

It would be interesting to experiment with a long coil of hose plumbed to the base side of the grapple clamping cylinder(s) to act as an accumulator. The hose would expand when pressure is applied to the cylinder and releasing the valve spool would trap the oil in the expanded hose holding clamping force on the grapple.

I couldn't find estimates of the elasticity of the container(s) here, so i just took an estimate of fluid compressibility. Do you have any educated-WAGs at expansion of the hoses? estimating strain in the cylinder itself would be straightforward enough just looking at hoop stress, if I knew the wall thickness ... but that's all beyond the scope of this I think A working estimate from a shop that can test it would be fantastic, though. We're really just looking at Fermi estimation here as there are so many variables, and (at least speaking for myself) not an engineer in this field.

Leakage by the valve spool would start reducing the force but that is true with any accumulator device.

I'd rely on this as a feature, not a bug. IMHO I don't think I want my CUT/hobbyist tractor storing residual clamping force. For my tractor, I feel "what you see is what you get" is the safest - I don't want it "rebounding"; that sounds like a recipe for disaster. EG if i overload the grapple and it opens: great, stay open. I have a nifty lever that can re-close it.

So that's a problem with accumulators; but they seem be the only solution here that doesn't involve a complex, valved reservoir: to drain the excess when force-compressing the blind end, or to makeup the missing fluid when force extending to the rod end.

 

[orangetree]

So are we saying that the 45lbs stays constant regardless of how far the piston is "pulled"? Also, how far does the piston have to move before it generates the 45lbs force requirement and zero PSI absolute on the blind (sealed) side of the piston? Would be interesting to see this on a graph.

~45lbs is simply atmospheric pressure, applied to a to a ~3inch diameter circle. Yes, that stays ~constant. We *are* starting to get to spherical cows, and TBC that's not my area

How far does it have to move? That depends entirely on the assumptions we have to apply to simplify the problem enough to post these ~qualitative answers
> Pressure = APysicsConstant * Temperature_Kelvin / Volume
So if you double the volume, you half the pressure. This is "ideal gas law" and it's very accurate estimate for "everyday" temps & pressures.

So the 45lbs exists no matter what on the end we're assuming is fluid "free" to flow in and out (eg NOT up against a valve like LD1 points out). And lets say the cylinder is bottomed-out blind, with "negligible" fluid on the blind end. That fluid on the blind end is also starting atmosphere (if the rod is motionless and our above assumption hold).

So you start to pull. The blind end cavitates. You assumed it has "negligible" contents, so we moved from "zero" volume to "more than zero" - an infinite increase. The force would basically jump up immediately to that ~45psi.

Ignoring static & dynamic friction, and lots of other stuff. But yes, with respect to the pressures acting on the 3sqin of piston, it's that simple.