Relief valve shims

[RickB]

Johnny, Andy's tractor has a hydrostatic transmission. All the fluid for the transmission and the main hydraulic pump is pulled through the same hydraulic filter. If the hydraulic filter was restricted enough to cause a hydraulic system pressure problem, Andy would also be experiencing erratic transmission problems. Since he is not reporting any problems that sound like a cavitating pump, I think his filter is fine. I have never had any problems with my filter even when I went slightly over the recommended hours before changing fluids.

You suggestion of checking the filter is a good one, but I just don't think the symptoms point to it in this case.

I respectfully disagree. Before I will consider making adjustments to a main relief valve, I want to be sure there are no simple, curable limiting factors to hydraulic performance. This means changing the suction filter, or being sure one way or another that it does not need changing. A filter restriction most often, but not always limits flow rather than pressure. But a filter restriction and a strong hydrostatic charge pump can steal performance from the auxiliary circuits in unpredictable ways.

 

[jinman]

I respectfully disagree. Before I will consider making adjustments to a main relief valve, I want to be sure there are no simple, curable limiting factors to hydraulic performance. This means changing the suction filter, or being sure one way or another that it does not need changing. A filter restriction most often, but not always limits flow rather than pressure. But a filter restriction and a strong hydrostatic charge pump can steal performance from the auxiliary circuits in unpredictable ways.

I see your disagreement and respectfully raise you one.

I think most of us check hydraulic pressure with the tractor sitting firmly parked and the engine running at 2000 rpm. Sure, the charge pump is running, but is it pumping as much fluid sitting still as it would while the tractor is moving? My guess is that the charge pump is at maximum pressure and minimum flow while the tractor is sitting still. When the HST pedal is depressed to get the HST pump going, charge pump pressure drops and flow increases. Wouldn't a tractor sitting still without the 3PH, loader, or HST operating have the least amount of hydraulic flow demand of anytime except when the tractor is off?

If I was near the service interval on the filter, I'd change it, but I'd do nothing more than glance in its direction with this problem. I think if AndyG has been using his tractor for many years and always thought his hydraulics a little weak, he is the best judge of whether they are acting normal. If he suddenly had a drop in performance, then I would not suspect the shimmed relief valve is his problem. If it's always been an under performer, then you have to suspect the valve shim count. I applaud him for actually measuring the pressure before making adjustments. I have had numerous PMs from people wanting to "overpressure" their systems without knowing what the current pressure is. I always strongly discourage this as I'm sure you would too.

 

[AndyG]

Thanks for the tips and ideas. The idea about a clogged filter could be valid but I would also be experiencing very slow hydraulic action which I was not. I checked with the dealer last week and he did not have a shim set on hand. Luckily there was a washer just the right size and thickness in my desk at work. I put it in and got the required pressure. I haven't had a chance to lift the heavy load yet but I'm sure to notice a difference.

 

[schmism]

I respectfully disagree. Before I will consider making adjustments to a main relief valve, I want to be sure there are no simple, curable limiting factors to hydraulic performance. This means changing the suction filter, or being sure one way or another that it does not need changing. A filter restriction most often, but not always limits flow rather than pressure. But a filter restriction and a strong hydrostatic charge pump can steal performance from the auxiliary circuits in unpredictable ways.

The suction filiter? isnt that just a mesh screen on the suction tube on in the hydraulic reservoir? The screw on "oil filter" isnt on the suction side, there are a number of posts about blowing off (or bursting) the filters in cold weather = not on suction side.

 

[RickB]

The suction filiter? isnt that just a mesh screen on the suction tube on in the hydraulic reservoir? The screw on "oil filter" isnt on the suction side, there are a number of posts about blowing off (or bursting) the filters in cold weather = not on suction side.

The main hydraulic filter inboard of the right rear wheel is indeed a suction filter. The hydrostatic charge pressure filter is, as described, a charge pressure filter. Oil supplied to both the hydrostatic transmission and hydraulic system flows through the suction filter directly after leaving the reservoir. The suction filter and its plumbing, along with the reservoir (rear axle housing, etc.) are the only components shared by the hydrostatic and hydraulic systems.

 

[texasjohn]

AndyG.... great to hear that you found a washer, dubbed it a shim, inserted it and are now pressurized... I know you will feel the difference.

Things I am sure you know... but are worth repeating.

If you need more lifting power and don't need the bucket, drop it and simply use chains to pick up stuff... increases your lift payload.

Don't forget additional rear weight... and care when lifting at max capacity...a low carry is gold when lifting right at the edge of the bypass valve pressure.