Thumb on 2.5t excavator

[The Learner]

hey guys long time away but I’m back

At home I have a Cat 302.5c
It currently has a homemade hydraulic thumb on it, I think the thumb is far to heavy
It’s made of 25mm plate, (1inch)
I’m redesigning it’s shape and profile and wondering what thickness plate I should have it cut from
I’m thinking 16mm (5/8)
As that is slightly thicker than the side profile of a bucket 10mm(3/8)

Who else has a thumb on a 20-30 class machine
What thickness has bent and what thickness works good before I get this cut in the wrong thickness

Thanks guys

 

[oceanobob]

T1 plate or A36? The latter will need to be quite substantial compared to the former.

 

[The Learner]

I was thinking mild steel Bob
And then having a rib along most of its length
I believe the thumb I already have is mild steel I’m just trying to reduce its weight

 

[007]

Thats not a simple thing people can comment on as there is so many variations in designs on home built thumbs.
A picture of the shape you were considering and existing thumb and ram would give members a better idea to comment.
At the end of the day the aux hydraulic pressure is usually adjusted to prevent ram and arm damage .
I personally like them narrow as less twist and less to get in the way when trenching.
Cheers

 

[The Learner]

007 I understand exactly where you are coming from
The geometry is fine and my new thumb is going to have the same length
The main difference is it’s thickness

It is a hitch pivot style
And the hydraulic cylinder in use is a bucket cylinder from the same machine
The thumb has approximately 160 degrees of travel

I’ll post some photos up shortly

 

[The Learner]

The new thumb will be a straight version of this without the “step” as the step fowls on the quick hitch

 

[007]

yeah 1/2 or 5/8 sounds good.
5/8 prob ideal IMO.
If you went 1/2 you would have to have a gusset closer to the end.
I have the U2.5-s and that would be ok i think for mine.

Cheers

 

[Batkom]

I think if you use something like hardox steel or T1 you can definitely go to 1/2 inch.
The thumb on my PC 75 (8.5 tons)
Is 1/2 material vertical plates/ teeth with a middle rib/tooth that matches the profile of the side teeth.
These are tied together with 1/4 inch plate and a tube at about the midpoint.
Looks homemade, and welds look crude.
I do have a relief valve on it set at 2000 psi.
It shows no signs of fatigue and looks like it has been on there a long time!
When I bought it there was no relief on the thumb cylinder and they had bent the cylinder rod severely-unusable!
I put a new cylinder on it tweaked the shape more to my liking and needs, and it is still working great.
From the work I did to it I quickly realized that it was definitely NOT mild steel! It is some really tough material.
If it was mild steel I am certain it would be trashed.
The shape of your current thumb I personally would find almost unusable for most of the logs and rocks I pick with mine.
I had a thumb of that shape on my hitachi 50 and hated it!
Have fun! Always wanted to build my idea of a perfect thumb,but just never have the time to get er done!

 

[The Learner]

I was thinking of just using mild steel as it is easy to work with and make the thumb.
And I can hard face the wear points.

The shape that it is currently is fine and I have no trouble picking up trees and rocks.
It just fowls on the quick hitch and you can never “close” it fully to the bucket.
Also when at full reach with the thumb retracted, the step in the thumb hits the ground before the bucket. it’s frustrating when trying to pull material up a bank.

 

[JD8875]

I have a home brew thumb on my IHI 30NX. It's 1/2" plate on the sides with a 1/4" plate welded down the center that holds the cylinder pivot. My tines extend 4" beyond the center plate and have never been. My thumb is 11" wide so itnfits in a ditch with my 12" trench bucket. The tines on my thumb line up with the teeth of my 12", 16", and 24" bucket. It's a slight arc shape with 1" radius serrations to bites logs and rock. Never bent the thumb in 5 years but I've toasted 2 thumb cylinders. I'd be lost without my thumbs.

John

 

[The Learner]

My new thumb will be an arc shape
And will have some “blunt” teeth incorporated along the jaw
The current thumb is as wide as a 300 bucket (12inch) line up in the inner edge of 450 (18) teeth
I find it works best with my 600 (24) as it is half way between the outer 2 teeth

Interesting that you have destroyed 2 cylinders
Is there something a little off in the geometry of your thumb?

 

[JD8875]

The relief in that circuit has not been set, and I was running some salvage cylinders that had rods which were way too small. It was built out of salvage scrap in the junk pile.

John

 

[The Learner]

Oh ok that makes a lot more sense

Luckily when I purchased the machine I was given a spare bucket cylinder so I’ve utilised that as my thumb cylinder

 

[PeterG]

I'm not sure making your own thumb is a good idea unless the machine is very old with a lot of hours, and it will be used just for work around your personal property. If you're working on a job site, and with an employee nearby, you definitely don't want a thumb failure, which would be that the thumb design does not work well to grasp the lift capacity of the machine, the weld or material fails, or the hydraulics fail. If you're pretty much a professional welder with the right equipment and work on heavy equipment for a living, then building your own might be the way to go especially if you want to design additional features that you can't get on a standard thumb. Why not just buy a Cat thumb and weld it on yourself? We just bought a thumb kit for our Takeuchi TB153FR and had the dealer weld it on. I wasn't happy with the quality of the welding, and made them go over it again. I have a Takeuchi TB016 which we added a thumb to it by a local company. I had it for just a few months when one of the thumb fingers bent. They said we didn't know how to operate correctly, but we suspect they used scrap steel. In any event, to retain the resale value of the machine, you might want to remember that the buyer may not want a home made looking thumb.

 

[The Learner]

Thanks for your concern peter
The machine is purely a farm machine it’s not likely to go back to work on any sort of site
It already has a home made thumb I’m just looking to refine it and make it lighter
Thumbs aren’t common here and for that same reason they aren’t cheap

 

[Simon C]

Thanks for your concern peter
The machine is purely a farm machine it’s not likely to go back to work on any sort of site
It already has a home made thumb I’m just looking to refine it and make it lighter
Thumbs aren’t common here and for that same reason they aren’t cheap

Thanks for your concern peter
The machine is purely a farm machine it’s not likely to go back to work on any sort of site
It already has a home made thumb I’m just looking to refine it and make it lighter
Thumbs aren’t common here and for that same reason they aren’t cheap

Here are 4 Pictures of a Bobcat 442 Thumb Cylinder that I built from scratch. It had a rod that was longer than needed so Cut about 6 inches off . Cut the good ends off of the Bent rod Cylinder, and carefully tacked them on to the cylinder base and cut rod end perfectly straight with Arc-Tec 223 Welding rod. The rod end was cut perfectly straight on a 14 inch chop saw and chamfered 1/2 inch deep all around for maximum weld strength. The rod was pulled out all the way before welding the base end lug to prevent packing weld damage. The rod was covered with burlap for spatter protection. Cooled down the seal are with wet rag after every pass. Costed 1/4 of new one and is still working. Put 2 blocks on stick to prevent over retracting. Set pressure was less than 2000psi. Cylinder $190 Can. plus 7 hours to do a good job first time around. Simon