B21 Backhoe Thumb

[dfkrug]

6mm holes are drilled where needed for flush fit drive-in type grease fittings.

I tried those with my CADDigger project years ago, and I found that the pressure from the grease gun would push
them out. Now I just tap and use the threaded ones.

 

[downsizingnow48]

Yeah I wondered about that. Same thing might happen here too.

 

[downsizingnow48]

This weekend made the cylinder base end bracket out of a piece of 4" square tubing.

 

[downsizingnow48]

Finished the progressive linkage today.

Photo 1 - I used a 1" bolt and spacers to keep the bushings aligned while welding.
Photo 2 - As planned the linkage allows the thumb to lay right up along the stick.
Photo 3 - The linkage won't win the TBN welder-of-the-month award but it will never break.

The cylinder is from Surplus Center. It is an exact replacement for the B21 stabilizer cylinders. For the thumb application, it is correct for closed length, stroke, location of fittings, rod end diameter, and color too! For $79. It is way stronger than needed.

Next steps to fabricate two 25mm shafts for the linkage, complete the rod base bracket, and order hoses.

 

[lhansman]

Very nice build downsizing. I can't wait to see a few action shots.

Regards,

Lauren

 

[downsizingnow48]

Last night I ordered the hoses so now I can provide the cost breakdown. Bottom line is $1500. That could be brought down almost 30 percent, into the range of $1000, using a single valve instead of a double valve, and by fabricating the thumb and shafts instead of buying the Kubota items.

 

[downsizingnow48]

Lauren thanks for the note! I am totally ready to stop whittling on metal and start using the thing.

 

[downsizingnow48]

Finishing up work on brackets.
Photo 1 - this shows the bracket holding two disconnects for the two work ports (above) and the disconnect for the high pressure in (power beyond) from the loader valve.
Photo 2 - Kubota has a junction at the top of the boom where the bucket hoses change from plastic hose guard to steel wire hose guard. I piggybacked on that bracket and will do the thumb hoses the same way.
Photo 3 - final test fit for the cylinder base bracket prior to welding.

 

[downsizingnow48]

I finally got around to making the two custom pins for the progressive thumb linkage. There are five pins total. Two are Kubota factory pins (the shared bucket/thumb pin and the thumb pin). The third is a Kubota BT751 bucket linkage pin that happens to be the correct size and configuration to fit the cylinder base bracket. The remaining two pins had to be made from 1.250 round bar.

One of them is the cylinder rod end pin. The cylinder rod end, the yoke, and the two progressive links rotate on this pin. It has a drilled grease passage that takes grease from the end of the pin to the middle where it provides grease to the cylinder rod end bushing. (The two yoke bushings and the two progressive links have their own separate grease fittings.) On the grease fitting end it has a machined shoulder 1.22" OD. This holds in place a 1.0" ID x 1.5" OD ring (3/16" thick) that retains the thumb link on one end of the pin. On the other end of the pin, to retain the other thumb link, there is a similar ring except 1" thick. This is held in place by a 3/8" through bolt.

The second pin is identical except it has an additional through bolt hole. This is to lock onto the bucket linkage. In the stock configuration, the bucket linkage locks onto the stock Kubota pin, hence does not rotate, and therefore has no direct grease provision. I had to make this pin the same way. So this pin rotates in the stick bushing, the thumb links rotate on it, and it carries, but does not rotate in, the bucket links.

As I write this it sounds a bit complicated, and in fact it did take me a while to come up with a bulletproof way to retain the progressive thumb links.

 

[downsizingnow48]

Finally got the last of the mechanical parts assembled.

Photo 1. All along I have been planning to weld the cylinder base bracket to the bottom of the stick, using a doubler plate. But I've also had a nagging thought that the designers of this backhoe had no intent of a thumb operating force of any kind being applied to the bottom of the stick. And, the point where the bracket would have to be welded is also the point where the stick reinforcement plate ends. Welding a bottom bracket that bears 1/2 on the reinforced part of the stick, and 1/2 on the un-reinforced part, bothered me.

So instead, I decided to make a girdle out of 1/4" steel. It clamps on to the stick with twelve 5/16" bolts Grade 8. The girdle jams up tightly against the taper top and bottom, and has spacers welded inside that tighten against the sides of the stick as the bolts are tightened up. As a result the thumb operating force will be distributed somewhat, compared to being concentrated in one segment of the bottom of the stick.

The cylinder base bracket attaches to the girdle with six 1/2" bolts Grade 8.

Photo 2,3. The progressive linkage works exactly as planned. When closed it tucks tightly against the stick, and opens about 180 degrees. In most progressive linkages that I looked at, the links have to be curved to get clearance from the bucket boss when fully open. After considerable experimentation (in plywood, including some curved links) I decided on these straight 6.75" links. A shorter link would have interfered with the bucket boss when the thumb is open all the way. When the thumb is closed all the way, a longer link would have moved the hydraulic cylinder too far up the stick. This would cause interference with the boom when the stick is tucked in. Needless to say, the straight links were much easier to make than curved links would have been.

Photo 4,5. Testing the movement of the thumb hoses with the stick tucked in and then fully up. A retaining loop made from 1/4" rod keeps the hoses up close to the stick. Using the Kubota stabilizer cylinder, with fittings on the side, and a tubing loop to reverse direction of the bottom fitting, makes for a tight installation. Throughout this project I have had in mind to maintain the bulletproof B21 design.

Photo 6. Now that I know the hoses are correct I will install the steel wire guard. The first task waiting is to demolish two ratty old sheds on the property, it will be a lot easier with the thumb.

The last remaining part of this project is to run the hoses and make the connections. No more fabrication. I am glad I went for the progressive linkage, but it is a lot more work than a straight cylinder-to-thumb linkage would have been.