B21 Backhoe Thumb

[dfkrug]

Photo 2,3. The progressive linkage works exactly as planned. When closed it tucks tightly against the stick, and
opens about 180 degrees.

In your 3rd photo above, it looks like the force exerted by the bkt on the thumb in the 180-deg position
will be quite high. I hope you do not over-pressure the thumb cyl.

 

[downsizingnow48]

Thanks I appreciate your comments very much, it always helps to have another perspective on this stuff.

Here is how I have been thinking about the problem. I am not sure I am thinking about it right so welcome the chance to lay it out and get corrected if needed.

Let's assume the bucket is pressing down on the end of the thumb with 2000 lbs force. The length of the thumb is about 16 inches and the length between pin centers (bucket pin to thumb pin) is about 5 inches. So the leverage is about 3:1. That gives 6000 lbs force on the yoke. From there some of that force pushes on the cylinder, and some of it is pulling on the progressive links. The distribution of force between links and cylinder depends on the angles of yoke to links and yoke to cylinder. Fully extended as in the picture, the yoke-linkage angle is about 95 degrees and the yoke to cylinder angle is about 125 degrees. If the two angles were the same, the force would be split in half. So lets say about 2/3 of the force is on the cylinder, that is 4000 lbs pushing on the 1.375" cylinder rod in a 2.5" bore. The cylinder has a area of 4.9 sq in. So the 4000 lbs works out to about 800 psi. The cylinder is rated at 3000 psi. I think that is all right but could be completely wrong.

In practice I am taking a different approach to the problem rather than rely on my calculations. The operating valve has a work port relief which is factory set at 1200 psi. I just got a hydraulic pressure gauge and will use it to re-set the initial operating relief considerably lower, at 500 psi. I want to start out very conservative, and anyway I suspect that will generate all the force I need to do the work I intend. A low relief pressure will keep forces low not only on the cylinder but everything else. In this case, I believe the cylinder would not even see the 800psi because the valve relief will open before that.

Again thanks for the comment and let me know what you think about this or anything else that looks questionable to you.

 

[dfkrug]

From there some of that force pushes on the cylinder, and some of it is pulling
on the progressive links.

Pat, a freely rotating pivot will transfer any applied torque fully to the resisting element.

Draw a diagram showing the forces and pivots (a free-body diagram).

You can always avoid problems with excessive leverage forces with a work port RV in the
base end of the thumb cylinder. If you are doing that, do not worry about the leverage
from the bkt at the 180-deg position.

 

[downsizingnow48]

Thanks for straightening me out on that! Agreed the work port relief is the answer to the problem of overpressure.

 

[dfkrug]

Agreed the work port relief is the answer to the problem of overpressure.

But I still think that a work-port relief is a bandaid to a less-than-optimal design. You can design with a
cylinder and geometry that does not result in any overpressure conditions. I do not use one in my
hyd thumb, with no problems and hundreds of hours of use.

 

[downsizingnow48]

Time to wrap up this thread. The thumb works great, exactly as planned. Everybody always shows their new thumb picking up some huge thing. See photo 1. The hydraulics are smooth, it is easy to pick up a can of beer without even denting it.

Before going further, I want to give a big tip of the hat to JJ and others on the hydraulics forum. JJ I have studied all your stuff for a year or more and couldn't have done this without the knowledge you have shared. Many thanks. Another hat tip to jfkrug who has been there and done that and I appreciate the comments received.

From the start I wanted an installation that was as unobtrusive as possible. The valve fits in the space between fender and seat, and all the hose connections fit under the valve or under the seat. Photo 2: main bracket up against the fender. Photo 3: valve installed. Facing forward, the front lever is ready to the left hand for controlling the grapple. Facing backward, the rear lever is ready to the right hand for controlling the thumb.

The rear-facing disconnects are for the thumb or any other rear-mounted attachment, see Photo 4. The front-facing disconnects are for a grapple, that is my next project. See Photos 5,6.

The hoses for the thumb follow the same path as the stick and bucket hoses. See Photos 7,8.

The new valve tank return port tees into the loader valve tank return, see Photo 9.

The hoses that go under the transmission and along the frame rails have steel wrap, see Photo 10. All other hoses have plastic wrap except the short hoses to the thumb cylinder that are steel wire wrap the same as Kubota uses for the bucket hoses.

I left the workport reliefs set at 1200psi. The thumb is strong enough to put one-inch dents in old yellow pine timbers. And delicate enough to pick up a beer can as above. I tested the reliefs by closing the bucket on the thumb, the thumb goes backward smoothly.

 

[dfkrug]

I am sure you will find the new thumb very useful, Pat. A lot of folks own B21s and an example
of a working hyd thumb should be inspiring.

Now, to go pluck some more rocks out of my latest mudslide.....

 

[downsizingnow48]

I've been using this hydraulic thumb for a year now. It seems a good time for an update on real world use.

Pros:
1. The progressive thumb linkage, which provides 180 degree motion, was a lot of extra work, but worthwhile. I can pick stuff up over a wide area and it is very useful for demolition. I position stuff with the thumb first, then clamp with the bucket.
2. The cylinder with ports on the side. I would probably have scraped off the fittings a dozen times by now, if they were located on the bottom. This also keeps the hoses completely out of the way.
3. The 1200psi work port relief. I constantly run up against the relief when grabbing stuff hard, there is no doubt I would have broken something without the relief.

Cons:
1. The cylinder is 2.5in bore, more powerful than it needs to be. The problem is, that makes it slow to fill, and I find myself waiting on the thumb. A 2in cylinder would have been perfect. But I couldn't find a cheap one with side ports.
2. The weight. The additional weight of the thumb mechanism doesn't affect digging as such, but when doing fast backhoe cycles from side to side, it throws the tractor around more than before.
3. Not really much of a consideration, but the thumb linkage adds 9 grease points to the previous 37.

All in all the progressive hydraulic thumb adds a lot of practical utility to the basic backhoe and I am glad I finally did it.

 

[100 td]

Great job!
I have been looking/thinking about mech/hyd thumbs, not sure if I want one yet, but just in case you want to share some drawings and specs, I'd certainly be happy to squirrel them away for future use!

 

[downsizingnow48]

I would be happy to answer any questions. I made some drawings at the time but don't have them now.
The basics:
Cylinder - this is the exact same as on the B21 stabilizers. Perfect for retracted length and stroke.
Thumb - this is the part used on the Kubota BH77 mechanical thumb. It is easily converted to hydraulic.
Links - these are made from 3/4"x2" flat bar. The holes are 6.75" OC.