Alternative 3rd function?

2manyrocks · Apr 10, 2022

One of the problems that the Messick's video raises is grabbing a long piece of unbalanced material (tree trunk) in the grapple which then acts as a giant lever prying against the grapple cylinder. I'd venture a guess that too much stress can also be placed on tractor loader cylinders from trying to dig, push or grade with the front bucket instead of using the rear 3 point hitch. The loader arms are not in a straight line to the tractor frame like a dozer.

ovrszd · Apr 14, 2022

davedj1 said:
I have a question about the concerns of a grapple producing an alarming amount of back pressure on the hydraulic system. How is this different than the loader cylinders or bucket curl cylinders while digging, pushing or grading? Do each of those lines have check valves? I also don't understand why you would be pushing with the lid of a grapple. Respect the tool for what it is and it will last a long time.

A Grapple puts a tremendous amount of stress on the FEL hydraulics that mostly isn't possible with just a bucket. This added stress makes it very easy to overload the tractor hydraulics.

davedj1 · Apr 14, 2022

ovrszd said:
A Grapple puts a tremendous amount of stress on the FEL hydraulics that mostly isn't possible with just a bucket. This added stress makes it very easy to overload the tractor hydraulics.

How does it put any more stress than a bucket or forks on the hydraulics? I'm trying to rap my head around your line of thinking.

jigs_n_fixtures · Apr 14, 2022

Things like logs have high polar moments and inertia. You pick them up, and all the spools in the hydraulic valves are closed when you transport.

Every bump, turn, or change in speed, and it pushes against the grapple, lift arm and bucket cylinders. That push is translated into a hydraulic pressure surging in the closed hydraulics On your FEL and grapple.

dieselscout80 · Apr 15, 2022

Another possible way to mitigate these spike with an accumulator.

Guys moving lots of hay and are trying to go fast are using them and i makes a difference when stacking lots in a year my hay supplier does about 15k bales a year and he’s a small operation.

orangetree · Apr 23, 2022

davedj1 said:
How does it put any more stress than a bucket or forks on the hydraulics? I'm trying to rap my head around your line of thinking.

I'm here learning about hydraulics, TBC ... but I think this one makes sense:

with the other functions (FEL, etc), the load is "on" the action you're performing. As you lift, you are pushing against the load. Too high, and the OP valve will trigger, and the movement that's increasing the load stops.

now consider a grapple: you clamp with the tree on the ground. Cylinders are clamping with ~rated PSI. Now you LIFT. grapple cylinders are still clamped, but the load they are experiencing can increase dramatically as the tree exerts leverage. The FEL itself may be well within rated lift, and the OP valve never trips. But the grapple, with it's fluid path shut, sees an unknown amount of higher pressure.

orangetree · Apr 23, 2022

While the fact that the grapple runs the risk of unexpectedly high pressures makes sense as in the discussion above, and I can see how a check valve would at least isolate more of the system, I have two questions:

the pressure still [could] rise unacceptably in the grapple cyclinders & fittings. The check valve might even make this (slightly) worse do the smaller total elasticity in the (now tiny) isolated section of the system so exposed. Shouldn't this be an "over pressure" valve application that would allow the cylinder to retract when the pressure passes [some] threshold?
how does a check valve work in normal operation - fluid has to flow both ways ...
1. this is actually how I found this thread; struggling to find a good answer to this. A check valve is a one-way valve. But the flow needs to reverse when the implement opens. I'm missing something ....

2manyrocks · Apr 23, 2022

davedj1 said:
How does it put any more stress than a bucket or forks on the hydraulics? I'm trying to rap my head around your line of thinking.

When the engineers design the hydraulic system with a loader in mind, they've pretty well got the load and pressure confined to whatever kind of material fits in the bucket they spec for that tractor and loader. They can estimate that the loader is going to be lifting a certain number of pounds of material based on the cubic foot size of the bucket.

With a grapple, there are guys on youtube grabbing pretty good sized tree trunks both in weight and length. The leverage forces that the grapple is resisting can be much greater than material sitting in a bucket. Picking up a 15'-25' log, especially if it is off center and acting as a lever, exerts different force than scooping up a 5' bucket of gravel.

jigs_n_fixtures · Apr 24, 2022

orangetree said:
While the fact that the grapple runs the risk of unexpectedly high pressures makes sense as in the discussion above, and I can see how a check valve would at least isolate more of the system, I have two questions:

the pressure still [could] rise unacceptably in the grapple cyclinders & fittings. The check valve might even make this (slightly) worse do the smaller total elasticity in the (now tiny) isolated section of the system so exposed. Shouldn't this be an "over pressure" valve application that would allow the cylinder to retract when the pressure passes [some] threshold?

how does a check valve work in normal operation - fluid has to flow both ways ...

this is actually how I found this thread; struggling to find a good answer to this. A check valve is a one-way valve. But the flow needs to reverse when the implement opens. I'm missing something ....

The device to handle and disipate teh surge would be an accumulator, with restricted office to limit hours fast fluid could go in and out. Essentially a slightly larger version of a hammer arrestor in plumbing.

orangetree · Apr 24, 2022

jigs_n_fixtures said:
The device to handle and disipate teh surge would be an accumulator, with restricted office to limit hours fast fluid could go in and out. Essentially a slightly larger version of a hammer arrestor in plumbing.

Thanks for the name. Yes, that function sounds like it applies to the concern here - although in a ~month of reading about this I don't believe I've seen anyone else refer to it ... at a minimum it's highly uncommon on everyday TLB/grapple/etc (?)

@davedj1, @2manyrocks, @LouNY discuss "check valves" on several pages of this thread - and in the (few) other threads discussing this same overpressure risk, the phrase seems to be used the same way. I'm trying to understand what folks are proposing; those uses do not seem to be a reference to an "accumulator" type device.

Is this some sort of overpressure device that, rather than shunting the fluid to tank, just "short circuits" the cylinder, allowing it to open/close as necessary to avoid damage? As I am picturing it, if say the normal hydraulic OP valve is set for 2000psi, this "grapple saver" would be substantially higher, say 3000psi. Thus it would only ever open when the valve to the grapple circuit is closed and the grapple is isolated from the rest of the system.