Questions on rebuilding a valve pack

[Richard]

I'm sure I'm not using the right name.

My backhoe has two wobble sticks. Each stick works north/south & east/west as I'm sure most know.

Two of my boom cylinders have been rebuilt. The small bucket cylinder and the middle dipperstick cylinder.

Both of these were done by my local JCB dealer so I'm presuming they were done correctly.

My bucket will drift/sag pretty quickly. The dipperstick will sag, but not as quick. The boom (the largest cylinder) will also sag a bit.

I'm speculating that since the two cylinders are repacked, the problem is in the controls. (no external leaks).

I'm not yet doing anything but living with this. Right now I'm just trying to educate myself a bit. Especially since I was quoted a price for $400 on one of these parts.


Here's my question. I've attached a TIF to this to show the diagram that JCB sent to me. I don't know all the names of parts. I was thinking there would be a tapered valve in there that would be the main control of flow AND the main stopper.

If I'm right then this is part # 21 (or so I think)

What I'm wondering is, if my unit is sagging, which of these would be the first one to inspect the closest?

My philosophy would be to take the valve apart and replace anything that's worn. Actually, I was thinking I'd simply replace ALL the valves since this machine is from 1987 or so and as far as I know, this is the original valve pack.

Would this have EIGHT of these valves? (part #21) one for each position of the wobble stick?

This is the part that I was quoted $400.... and if there are eight of them, I'm beginning to think I can live with QUITE a bit of sagging before I just go replace eight of them for $3,200

That seems awfully proud. I've not yet called them to see if that was a typo but even at $40 each... that's kinda proud. Not peacock proud but still...


Then there is part #23 "Valve Check Load Hold" Is THIS the part that could be letting it sag since it might not be checking the load??

Again, if I decide to dig into this I'll fix anything that is worn so it's not really a concern which part is which... I'm just trying to get an idea of what does what.

If that pricing is accurate though, then I'll probably not fool with this. I might see if I can get some from a boneyard first.

 

[Whitey]

A few of years ago when I owned a JD TLB........I had the cylinders re-packed and I rebuilt my control spools. (JD 7 on a 770). I had a control issue with the operaton of the hoe. I bought rebuild kits from JD and they consisted of O-rings and flat washers. I took the entire spool sections apart and replaced all of the parts. The hoe controlled alot better than it ever did before and was quite smooth.........however it never did completely eliminate the drop in the bucket and dipper stick. I took pictures for this repair and posted them you could search in the repairs for my post to see it.

PS.....it was quite an easy project.......just make sure you mark the hoses!

 

[wdchyd]

Richard,

The problem with rebuilding the control valves is if the spools are leaking internally then the clearances between the spool and the housing (cast iron) is worn.......no orings will cure it...

In a perfect world, you'd have to have oversized spools and hone out housing to fit in order for this to work.....not going to happen.....you'll be buying new sections to fit your valve or a completely new valve bank to fix it...

If this was the case (worn out control valves) then you'd have your work cut out for you to clean up the contaminated hyd oil that did the damage....big job if needed....

Best to test out cyls and control valve with pressure guages to absolutely make sure your are going after the right problem....otherwise you'll be going into this blind......Maybe swapping hoses around would be the best option to rule-out cyls....

Control valves are NOT cheap, so make sure it wasn't a bad repair on the cyls..!!

Hope this helps...........Bill

 

[HydroTek]

#23 is indeed a load check, but it's function is to prevent oil from flowing back the inlet at the time when you first crack the valve spool ..it does not hold the cylinder in position.

#21 looks to be a relief valve, intended to protect each spool from excess pressures while digging. Say you hook the bucket on something that will not move and curl it inward ..the dipper stick will "let off" to protect the machine from breaking.

One of these valves may be not sealing, for the affected circuit. Looks like some Orings that may be blown ..an easy fix there.

The valve stack is either a unibody or section type, you can replace sections that are worn, but a unibody will be much more expensive.

 

[wdchyd]

One of these valves may be not sealing, for the affected circuit. Looks like some Orings that may be blown ..an easy fix there.

Orings will not fix a creeping valve spool.....the tolerances between the spools and valve housing is what's holding the load.......as a matter of fact, you could take OUT the orings at the valve spools and it will still hold a load (assuming the spools are NOT worn out) of course it would leak all over......:laughing:

Not a quick fix if it's the valve.......

 

[HydroTek]

wdchyd While what you say is true ..you took my quote out of context. What the sentence refers to is the circuit reliefs for individual functions, valve #21 in the diagram.

Spool to bore clearance is a factor in cylinder drift, and all hydraulic systems will have some drift, unless fitted with check valves ..most hoes only have these checks on the stabilizer "feet".
Damage from foreign materials, abuse, internal cracks can exaggerate the normal leakage caused by spool to bore clearance.

The OP stated: "My bucket will drift/sag pretty quickly. The dipperstick will sag, but not as quick. The boom (the largest cylinder) will also sag a bit"
Depends on how you qualify "pretty quickly", what unit of time are we talking about here?

Just because a cylinder has been repacked, that don't mean it wont leak ..the person that done the work may have cut a seal, or maybe the cylinder bore is damaged?
Testing is the only way to determine what part to fix. wdchyd suggested "swapping hoses around" ..maybe he means to swap your bucket hoses to another valve function that does not drift, to see if that stops the drift (an easy quick test :thumbsup: ).
Since the bucket seems to drift the fastest, I would suggest concentrating on that problem first, then look at the less severe problems.
.