hydraulic shims

buckrrt · Jun 19, 2004

i have read that you can increase pressure by adding a shim pack. where do you put this in at? and how?

OkieG · Jun 20, 2004

The hydraulic pressure relief valve opens and goes into bypass mode when spring pressure is overcome. Shimming the spring means more system pressure is tolerated before the spring yields and the valve opens to bypass.

The Technical manual will give you the wheres and hows. The Parts catalog will show a nice exploded view of all the parts you'll encounter. Your dealer might photocopy the appropriate pages for you.

Please do your research re: a reasonable target pressure to shoot for, and get a pressure gauge before adding shims.

OkieG

glalonde · Jun 21, 2004

For the JD790, I've measure the PSI before and after adding one pack of shims. It added 400 PSI (from 2000 to 2400 at idle, from 2100 to 2500 at full throttle). I did not want to do this blindly. Based on the JD online parts, the pump used on the 790 is the same as the one used on the 4110. The default psi on the 4110 is ~2400. Therefore in theory this modification should not damage the pump. The hoses are rated at 3000 psi. I do not know about the SVC rating, the front axle support capability and the loader structural strenths but so far I had no issues and so does other people on this site which had the mod done longer than me. This is a modest increase of 20% i.e. if I was lifting max 800 lbs, I would now be lifting ~160 lbs more (i.e. 960 lbs).
This allows the loader to break more easily into a pile of stuff.
The long term effects are not known. Perhaps there will be metal fatigue somewhere along the line ? Perhaps nothing will ever happen ?

However, this does not mean that you can do this for all tractors. I've read somewhere that someone has added ~300psi more (I believe those were the numbers) from 2500 psi to 2800psi (a 12% increase only on another model of tractor) and the pump got damaged within a very short period of time.

tomrscott · Jan 23, 2005

While we are on this thread, I found a comment in the JD technical manual that is interesting:

"The purpose of the system relief valve is to:
1. Protect the hydraulic system components from damage caused by too high of pressure.

2. Prevent the operator from lifting more than the tractor is stable enough to handle. " TM1470: 270-10-20, 15MAR99

The first reason goes without saying, and any hydraulic system should have that, but the second confirms what I had been told by the dealer, especially considering (as I have been told) that they make that judgement with a completely unballasted tractor. Now if I only want to lift an object 12" with a pallet fork and move it around, I should be able to do that, or if I am willing to add a 1000 pounds of balast and want to lift a loader full of gravel into a truck, this tractor should be capable of that.

Just burns me to have a lawyer decide what I need or don't need. /forums/images/graemlins/mad.gif

I just don't need or want JD lawyers to make my tractor "safe". I am happy to take responsibility for my actions.

tomrscott · Jan 24, 2005

Add one more experience with the shim kit on the 790. Did the job today with some quick test results to report:

1) I very carefully tested the peak pressure with a special gauge setup to capture and hold peak pressure (like a resettable tire gauge). Before adding shims, doing a slow lift at full RPM I could get about 2050 psi. I rigged the peak holding fixture because I wanted to see what happens when you jerk it a bit and play with the fork tilt on a heavy load. in those conditions, levering a load up with the "bucket tilt" on the pallet forks, and then pulling the forks back up I could get a peak of about 2200 psi on the loader arms. I've got a big chunk of steel, about 1500+ pounds, that is way to heavy to pick up outright, so it gives me a good test load.

Before the shim kit I could just barely budge the sheet out of the ground an inch or two. No way to lift it clear either with the lift arms or tilting the pallet-forks.

2) With one shim kit, the slow lift dead load pressure went up to about 2500 psi like others have indicated, I still couldn't easily lift the steel plate mentioned above, but when I put the pallet forks under it and then pryed them up with the "bucket tip" control, I could lever it a foot off the ground to get it unstuck. Then I could tip the forks back up and hold the steel off the ground a foot or so with the loader arms. Very worst case pressure when pushing it this way pretty hard came up to almost 3000 psi peak with no leaks or evident problems.

Without the peak reading pressure gauge, I wouldn't probably ever have seen this 3000psi. This is the peak when you're trying to jerk something free, or rocking the fork tilt. The pressure relief valve is acting at 2500 psi, but sometimes you can apply more load to your cylinders than the valve sees.

Another example of this that is maybe easier to visualize is just scraping the ground with your bucket and hitting a rock. The valve is doing no work, the pressure relief valve isn't active, but the loader arm cylinder can see tremendous peaks of pressure. This seems to be often how lines and seals get damaged. A word to the wise, take it easy when you don't know what might be in the ground.

I'm not planning to run the loader this hard as a general practice, but I wanted to see what limits the system might experience if I did push it hard. Now I have a better idea what happens when I'm pushing it hard. I think the 2500 psi is plenty safe, even 3000 psi should be within the rating of the system components, but I wouldn't want to run it there as a general rule. Now I can understand better what things might cause the system to see a peak load like that.

Time will tell if this will cause any problems long term, but personally, I'm not worried.

YMMV /forums/images/graemlins/wink.gif

- ts