Thumb Hydraulic question

[TimT]

I have had two E27B New Holland "Kobelco" Mini's. I installed the Thumbs on both the old mchine and my new one. They are USA attachment thumbs. On the first machine, I put a gauge on the Auxiliary valve lines used for "Nibbler/Breaker" attachments, and adjusted the Aux valve block relief pressure to 3,000 lbs... the rating of the cylinder. The instructions say to lower the Aux valve pressure to 80% of the thumb cylinder rating. I adjusted the valve so the thumb would just barely push the bucket back.
On my new machine, the valve for the Auxillary is a variable flow valve with a toggle instead of a full flow push button style. I never bothered to lower the pressures on anything. The piston rod on the thumb is bigger than my bucket cylinder rod and it WILL push the bucket back if I'm not careful with it. I figure that the relief opens on the bucket cylinder circuit and will hurt nothing if I don't make a habit of it. The system relief pressure of the E27B is 3336 psi. I am I asking for trouble?... I have not broken anything yet and have used it quite a bit. I was reading a thread here about having to install all kinds of external relief valves etc on a Kubota machine. I had a guy that should know tell me that unless I relay pound the thumb into the bucket all the time it should be fine, he said those small machines can't relay blow a thumb apart. I realy like having the variable control so I can squeeze right to the point of pushing the bucket back. What do you think?

 

[Bls repair]

With no photos can't see how thumb is mounted . I have seen on some machines where the thumb mount ripped off, some times tearing up the boom .

 

[John C.]

The bucket is supposed to be able to push the thumb back. I've always set the thumb circuit reliefs at 2,100 PSI.

 

[TimT]

Its a weld on thumb. Believe me its not coming off. I tried that setting to 2100 psi on the older machine and I hated it. Could not pinch anything. Now on the new one one with variable flow, I can be slower and more careful. I guess I will be ok with it. No problems so far. I just can not picture the bucket cylinder being able to wreck anything. Its a very heavy built thumb. Big machines..I'm sure they could. These little ones?? I just can't see it.

 

[Shimmy1]

I can't really tell from the pics, are the thumb mount and cylinder mounting pads welded to a separate plate, which in turn is welded to the stick? If not, your failure point (a likely one) will be beer-canning the stick where the thumb cylinder is attached.

 

[TimT]

The thumb comes complete with cyl and pin mounts welded to a plate that is in turn welded to the stick. This is the "Old Machine"... But the 2015 machine is the same mount. BUT the new one was not welded on by me, it was done in a pro welding shop that I have high regard for. Once again, I think if a bucket cylinder or thumb cylinder on this size machine can produce enough force to break off the stick, you are likely in trouble long before you put a thumb on it. Trust me they don't have the power to tear apart the stick. I have run Cat 385's and other big machines that could but not these 6,000 lb minis with normal use, not abuse. In fact Minis pound for pound are a tougher machine to break than a big one. I suppose it can happen, but I have never seen a mini break off a stick or boom using there own power, but I sure have seen machines as big as a PC 1250 do it.

 

[007]

The design of your thumb is one of the better ones which has the load on the ram clevis pin very high on the stick which makes it very strong.
Some have this load coming on the stick much further down where the stick is weaker and the thumb has a higher mechanical advantage.
I think I was reading the same thread as you where the guy was messing around with relief valves.
I have a new four way bucket coming for mine in a few weeks and the rams on it are rated for 3000psi.
My kubota U25-3 is like yours i think with 3300psi so im wondering if i have to loose some pressure?
Maybe some one knows my pressures and if i have to adjust?

 

[Ronsii]

Hi Tim, Glad to hear you got the thumb working the way you like it

In the above picture I see you have your work lead connected to the pin at the end of the thumb cylinder while welding... just a heads up to you and anyone else that sees this, while it may seem like a good place to clamp onto being it's bare metal without paint it is actually a really bad place to pick as the welding current will travel through the cylinder to the rod whilst arcing between the piston/bore and any other slip fit areas!!! this can cause lots of damage!!! I usually keep a magnetic ground block around for boom and stick work where there isn't a good place to clamp also it's good practice to protect any exposed rods near the weld spatter.

 

[TimT]

Thanks Ronsii, I am no real welder, I can get by. I never would have thought of that... although I did cover the rod with a couple rags later for spatter protection. "I do know that much" but as far as grounding on the cylinder...never thought of it. My welding held up, but on the new machine I had a real pro do it at his shop.

 

[Smokin Joe67]

I had a customer bring in his little excavator and had me do some welding to it. It failed and started leaking. Come to find out his son had his friend try to weld it first and he did weld the piston to the barrel. On the hydraulic thumbs that mount on with the stick bucket pin you must decrease the pressure with installing adjustable valves to around 2300psi. If you don't you will bend the U-shape out ward and bend the pin on the stick. I know because I've done it. If your running your machine you should be ok but if you have a bunch of monkeys running then in time you'll probably start tearing things up.