mini ex thumb

[bcantell]

Newbie here. I'm a building contractor and do some limited landscaping. I bought a really sweet Volvo ec25 mini excavator this spring and she has been excellent. I put a TAG thumb on her and hooked up the hydraulic lines (one metal line was extended by the factory to the end of the boom, and the return metal line I had made locally and installed. I had soft hoses made also.) It works fine but the thumb won't stay in place when retracted. It worked perfectly for about three weeks, then started moving down a little bit and now won't stay in place at all. The company that sold me the TAG tells me it's most likely not the hydraulic cylinder but the valve spring in the valve body (???) I checked out the valve body and the valve that runs the thumb is different in that it does not have a small hose at the top of it. I'm wondering if that is some kind of check valve system and Volvo doesn't include it in the auxiliary line. One buddy told me to change the fluid and filter first. Any ideas??

 

[kennyd]

Welcome to TBN

Sounds like the seals are leaking/bypassing internally in the cylinder. Try disconnecting the cylinder and see if it holds position.

 

[J_J]

The valve that controls the aux is probably a motor spool valve. So in the neutral position, the cyl will try and equalize or move due to the weight of the grapple.

Maybe aux spool leaking.

Did you install new or used thumb cylinder?

What do you mean about the small hole at the top of the valve?

How about a picture of your thumb installation.

 

[kennyd]

The valve that controls the aux is probably a motor spool valve. So in the neutral position, the cyl will try and equalize or move due to the weight of the grapple.

Then explain this;

It worked perfectly for about three weeks, then started moving down a little bit and now won't stay in place at all.

 

[J_J]

Welcome to TBN

Sounds like the seals are leaking/bypassing internally in the cylinder. Try disconnecting the cylinder and see if it holds position.

Do you mean disconnect the cyl from the thumb, or disconnect the hoses from the cyl. What do you hope to get from that.

I would extend the BH and let it rest on the thumb to see if the cyl is leaking internally. If the cyl is leaking, it will try and equalize. If the spool is leaking, then the thumb will collapse all the way.

If you are not satisfied with the leak rate, then put a pin in the thumb to keep it up, or repair cyl or valve as necessary.

 

[bcantell]

JJ & Kennyd, thanks for the posts. The Tag thumb was new about two months ago. The mechanic at the shop (not a Volvo shop) were I bought the machine and thumb, believes it's in the valve. I installed the thumb. The machine is out working this weekend but first chance I get, I'll post some pics of the valve body. The valve is different from all the others so I haven't swiched the hoses to test the cylinder because of that reason. I called the local Vovlo service and they really were not much help, basically saying "bring it in"

 

[J_J]

Your hook on the bucket is a good idea.

Is there a name or number on the valve?

Can you push or pull on the thumb with the power off, and if so, what happens. The cyl may be to tight, but try and see if you can move it.

 

[bcantell]

JJ
Yeah, the hook on the bucket is very handy. The pic is of my valve body. the valve that runs the thumb is the one to the right of the single yellow capped one. (don't know what the caps are) It's run by a cable. The two to the right of it are for the tracks, also run by cables. Do you know what the tubes coming out of the tops of the other four do? This is all new to me but it's cool to try and figure it out. Thanks for your help

Bill

 

[Duff]

The lines coming on top of your valve body are pilot lines to push the valves open or close. These would be coming from your joystick.
The valves with the yellow caps, Im pretty sure are Secondary Relief valves or circuit relief valves they may also be called.

The Secondary relief valves are used on some functions ( especially diggers ) as they come under some extreme forces. A circuit or function that is not being used at the time, ( even only for a slight bit of time ) is not open back to the Main relief valve. Therefore when that circuit comes under some extreme forces, and the hydraulic pressure spikes up, then secondary relief valve will open up to dump some fluid back to tank.

The seconday relief valves is set lower ( not by much ) than your main relief valve.
As your Thumb circuit does not have this Seconday relief valve, then I would not worry about it with it falling down. However if it did, then I would be checking this out.



Now to the Thumb falling down:
I really am suspecting your seals in the Thumb cylinder have had it,

however 1st of all just put your Thumb as far back as possible, turn the machine off and go to your valve body and just listen to see if you can hear any fluid bypassing through the valve. If it is, then check out the spool, bore and centralising spring.

If not ( as I suspect ) then lets start the machine up, put the Thumb all the way back, and lower the boom down so that the bucket and Thumb rest on the ground.

Turn off machine and take off the piston side ( not rod side ) hose off. Blank the hose ( I think its a -12 JIC male plug, but please check ).

Now start the machine, leave bucket and Thumb on the ground. Now try and and pull the Thumb back with the Hydraulics. Now there should be NO oil coming out of the piston side of the cylinder. If there is, your cylinder seals have had it. Go buy a new set of seals and install in cylinder.


Ok so your probably asking why have my seals gone if its only 3 weeks old?

Some reasons why I think so.

1/ Cheap cylinder with cheap seals installed.
2/ Thumb does not have a mechancial stop on it, so basically the piston is bottoming out all the time its pulled back. When pulling it back all the way, ensure the operators do not slam it back, make sure they do it smoothly or stop pulling it back before it bottoms out.
Or put in a mechanical stop, to stop the piston bottoming out on the cylinder.

3/ Make sure that when digging, the Thumb is way out of the way. ( It has no secondary relief valve to protect it ) Even to a point of lowering the bucket on the ground, make sure the bucket is on the ground 1st before, you come down with the Thumb.
I can see some paint has rubbed off at the bottom of the Thumb and it looks like it has been dragged through the ground a bit. Not designed for this, even just a little bit.

Ok I hope some of this helps, good luck !

Duff:thumbsup:

 

[J_J]

Could be detent, float, and/or motor spools.