Alternative 3rd function?

[orangetree]

for this thread, here is the relevant product type you want to solve this overpressure issue. Not normally included on budget grapples, but apparently common on some rentals, and larger/higher-end models

"double relief valve" == "cushion valve"
1/2 NPT 30 GPM 1500-3000 PSI Hydraulic Cushion Valve Prince Mfg DRV-4HH | Relief & Cushion Valves | Hydraulic Valves | Hydraulics | www.surpluscenter.com (this model is larger than needed; i haven't found a lower GPM model. It will work fine, though, just physically larger than necessary)

thanks for ericm979 on the thread about those

 

[orangetree]

That would be a cross-over relief valve

They make them, and they are common. But it will only protect the pressure surge IF and only IF the external forces are trying to extend the cylinder. You would have no protection in compression without a relief back to tank

see the "cushion valve" i linked above - I can't find a flow diagram, but my understanding is it works both ways. With no pressure on either control port (you're not using the lever; pressure drifts down to ~none), a high pressure on EITHER cylinder port will cross-over/short-circut, allowing the cyclinder to relax OR extend as needed to relieve the pressure.

 

[jigs_n_fixtures]

The problem I see with that valve is that as the load exerts a pressure on the hydraulics of the clamping side, the fluid bleeds to the other side of the circuit. So as the load wiggles and exerts pressure, it will gradually relax enough to drop the load. It doesn’t maintain the pressure.

Where an accumulator with a flow control valve, would dampen return flow to limit damage from the pressure spikes. But, would regain the clamping pressure, while allowing the pressure wave to be dissipated.

https://www.surpluscenter.com/_MoreSpecs/AS9-7960-4.pdf

 

[LD1]

see the "cushion valve" i linked above - I can't find a flow diagram, but my understanding is it works both ways. With no pressure on either control port (you're not using the lever; pressure drifts down to ~none), a high pressure on EITHER cylinder port will cross-over/short-circut, allowing the cyclinder to relax OR extend as needed to relieve the pressure.

Nope. Still doesnt work.

It will work extending. But not compressing the cylinder. The only way those DPCV's work is if there somewhere for the fluid to go when they reach their set pressure. When compressing a cylinder....there is no where for the fluid coming out of the base end of the cylinder to go.

 

[LD1]

Thanks for this - it's a great point. The grapple hydraulic circuit probably has it much worse, but the same leverage also works on the FEL.

Thankfully (or NOT) the FEL has a "relief valve" of sorts, even with the valves closed ....

tractor falls over if leverage(on FEL) is too high

although as you say, the impact/impulse into the system - on all parts bearing the load - is going to be much higher as you bounce around, when there is a cargo with a high leverage

Just for clarity: If the diverter is running the grapple, and sharing the function of the bucket hydraulics, the hydraulics to the grapple are isolated from the relief valve in the FEL valve when the diverter is closed. That is where the overpressure issue arises.

Most tractor loaders do not have any relief on the work ports when the loader valve is centered.

Sorry to burst your bubble. They just dont.

 

[orangetree]

Most tractor loaders do not have any relief on the work ports when the loader valve is centered.

Sorry to burst your bubble. They just dont.

Correct - i made a joke that the entire tractor tips over, as your 'relief' if overloaded

 

[orangetree]

Nope. Still doesnt work.

It will work extending. But not compressing the cylinder. The only way those DPCV's work is if there somewhere for the fluid to go when they reach their set pressure. When compressing a cylinder....there is no where for the fluid coming out of the base end of the cylinder to go.

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

 

[orangetree]

The problem I see with that valve is that as the load exerts a pressure on the hydraulics of the clamping side, the fluid bleeds to the other side of the circuit. So as the load wiggles and exerts pressure, it will gradually relax enough to drop the load. It doesn’t maintain the pressure.

Where an accumulator with a flow control valve, would dampen return flow to limit damage from the pressure spikes. But, would regain the clamping pressure, while allowing the pressure wave to be dissipated.

https://www.surpluscenter.com/_MoreSpecs/AS9-7960-4.pdf

Yes, the grapple would open(close) with the overpressure (assuming this concept even works, see my comment about area of top/bottom of cylinder). my assumption was this cushion valve would have a "high" breaking pressure, higher than the FEL OPfor instance

But you raise a good point that the accumulator could do the same, without that drawback. maybe the accumulator isnt as often used because two are required, and they arent cheap or small?

 

[jigs_n_fixtures]

You’re tying to relieve just the clamping pressure, I can’t really envision a circumstance where the grapple will want to close tighter in response to the load wiggling.

So you need one small accumulator, one valve and a Tee on the on the hydraulic line which extends the cylinder to clamp the grapple

If you have a 2-inch cylinder, you would move 3.14-in^3/in of travel. Which would be 0.05-liter/in of travel. So, on SCUTs and CUTs, you don’t really need a large accumulator. A 0.15 or 0.25-liter unit would be sufficient. I can find those for right around $150, on eBay or Amazon.

 

[npalen]

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

My understand that the cushion valve in question is designed for use on circuits such as for angling a snow plow blade. The two angling cylinders are one way cylinders with a hose plumbed only to a base port on each with the two hoses going to the same valve spool. The displacement on the base side of both cylinders is the same so oil can flow between the two cylinders when the valve is opened by over pressure as when striking an object with one end of the blade..