question about tractor hydraulics

[karmakanic]

I have a question about relief valve placement in a typical tractor hydraulic system, like as used on a FEL.

Say I have a loader that will lift 2k lbs. What would happen if I lifted an empty bucket 3 feet off the ground and then dropped a 4k weight in it. Would the loader hold it, if a line or seal didn't blow? Or would it slowly sink to the ground as a relief valve opened up and bled off the pressure. Would it make any difference if the tractor was running or not? And with the tractor running, would it make any difference if the operator attempted to raise the bucket as the 4k weight was applied or if he left the control in the neutral position?

Just wondering about this. I understand hydraulic systems in general, but wondering how the specific design works on something like a FEL that's subject to induced loads in combination with external shock loads. I would think the system would be plumbed so the relief valves are active under all conceivable conditions, but would like to hear yay or nay or any other thoughts on the matter.

Thanks,
Dave

 

[DUMBDOG]

I assume (which is more times than not a mistake) that we are speaking hypothetically.

 

[rwmeyer]

Most, but probably not all, relief valves are installed in the pressure line/side of the system they are controlling. If your bucket, or whatever, is raised and your control lever is in "neutral", the pressure applied to your cylinder(s) from the external load is isolated from the relief valve. The weakest point in the valve, cylinder(s), and lines will allow the bucket to (maybe) slowly drop. The fluid will bypass the cylinder seals, the valve spool or, especially if it is a large shock load, blow out a line! Hope this helps, BUT PLEASE don't try it to see (or feel!) what happens. Enjoy (with all fingers arms, eyes, etc.) Bob

 

[Hoeman00]

I'll second this set-up.
The relief valve is before the spool in control valve.
Over load it and U WILL find the weak link.

 

[jinman]

Dave, I should start off with a disclaimer that different tractors may have different plumbing, but my comments will apply to New Holland Class III Boomers (TC35, 40, 45).

The primary function of the "system" relief valve is to protect the pump and piping from excessive pressure. It works for the FEL, rear remote valves, and 3PH hydraulic lift.

In the case of the 3PH, the hydraulics have additional safety relief valves built into the control valve and a relief valve actually built into the lift cylinder to protect from shock loads (like your dropping a 4k lb load onto the 3PH).

Essentially, the design of the a rear remote valve could move to relieve pressure from the neutral positon if the pressure became high enough, but I'm not sure you wouldn't exceed the seals/hoses/hydraulic cylinder maximums first. Looking at my diagram of New Holland's remote valve, it does look like the valve could shift to the opposite direction from internal pressure, but if you have your hand on the handle, holding the the valve in neutral position, there is no way for any excessive pressure to be released.

In the case of the loader's remote control valve, I don't see any relief whatsoever beyond the system relief valve I mentioned earlier. If you drop in your 4k lb and the rear wheels stay on the ground, you may be in for a surprise if you try to raise or lower the load. As soon as you crack open the loader remote valve the system relief valve would probably release the pressure and drop the load if you haven't ruptured a hydraulic line or seal before this happens.

I may be wrong on some of these points, but if my combination of experience (blown hydraulic hose) and reading my Repair Manual's description of the systems is accurate, this is how my New Holland system works. The 3PH is the safest system of the three./w3tcompact/icons/cool.gif