B21 Backhoe Thumb

[downsizingnow48]

After 10 years I finally decided to install a hydraulic thumb on the B21. I was getting ready to make the thumb, and looked at Kubota parts diagrams to get some ideas. I learned that the BT751 backhoe and the BH77 backhoe use the same bucket pin. Meaning the buckets and also the mechanical thumb for the BH77 have to fit the BT751. The thumb cost $405 at the dealer, not a give-away, but not bad either considering steel prices and the work involved in making one from scratch.

So today I did the easy part, installed the thumb. First picture shows the pin going in. I used a line-up pin with a tapered nose to get thumb, bucket, and stick in alignment, then chased that out with the hardened Kubota pin. It took about 3 minutes. The second picture shows the thumb installed.

It will be a while before I figure out exactly what the next step will be. Probably have to paint the old bucket, it didn't look that bad until I put the thumb on.

I am a bit concerned about the 25mm bucket/thumb pin, which is a Kubota factory item for the BH77, but looks a bit long and thin. The middle part of the pin is not supported (it is in a grease cavity). The bucket will work the same as ever, no reason to hold back on that, but I believe I might take it easy with the thumb. Pressing thumb and bucket hard together would have the effect of flexing the pin in the middle.

 

[dfkrug]

I am a bit concerned about the 25mm bucket/thumb pin, which is a Kubota factory item for the BH77, but looks a
bit long and thin. The middle part of the pin is not supported (it is in a grease cavity).

You are right to be concerned. Based on my experience with about a dozen hoe attachments and 6 thumbs,
a shared pin of this type should be at least 30mm. 1.5" even better.

That and the greasing problem eliminated pin-sharing in my homemade thumb design.

BTW, I also had a B21 and they do operate at lower pressure (2000-2100psi), but I still think that
that design is problematic.

 

[downsizingnow48]

Thanks for the note. I'm glad to know someone else sees that as a weak design. A larger pin would be better for sure, I see the B26 has 1 1/4" pins in that location. I did a couple web searches for broken BH77 bucket/thumb pins, didn't find anything, but that doesn't mean much. As a preventive measure (better safe than sorry) I can set the work port relief low, I don't need much pressure for that thumb. On grease, that is also a good point. The BH77 thumb was designed as mechanical so not expected to rotate. My plan there is to drill/tap the thumb pin bosses for flush-fit grease fittings.

 

[downsizingnow48]

I decided to use a progressive linkage, to obtain more range of movement. It adds two more pins to share loads somewhat. This linkage also allows placement of the cylinder base further up the stick, reducing bending force.

 

[dfkrug]

A larger pin would be better for sure, I see the B26 has 1 1/4" pins in that
location.

They did do a number of improvements with the B26. The only non-improvements were the loss of the hood
armor and the higher price. Still a great machine.

When I speak of the problem with greasing a shared pin, I am referring to the fact that 3 or 4 places must get
grease, and this requires either 3/4 zerks on the bosses, or a single zerk on the pin's end with 3/4 holes. You
need only 3 places to grease if the pin is locked to the bucket bosses, which is usually the case.

A mechanical thumb setup does not accommodate greasing of the dipper stick pivot, either, which has
to be upgraded for hyd.

Since you are planning a multi-link system, it is good that you are doing some wood mockups. I did
that first with my articulating mech thumb.

I posted a thread about making a hyd thumb here:
http://www.tractorbynet.com/forums/...ustom-hydraulic-backhoe-thumb.html?highlight=

When you complete your design and fab, show it off here. I always like to see what folks come up with.

 

[downsizingnow48]

Many thanks for referencing your build. Very detailed and useful.

 

[downsizingnow48]

Last weekend I installed the control valve. Rather than have one valve dedicated to the thumb, I decided to install two valves, in a position where they can be used for other things as well.

The first photo shows the valve installed. It fits (barely) in the space between the left fender and the seat.

The second photo shows the valve in relation to the seat, forward facing. Both levers are easily to hand on the left, behind the transmission selector lever.

The third photo shows the valve in relation to the seat, backward (backhoe) facing. Both levers are still easily to hand, now on the right. It took a while to get the position figured out.

The fourth photo shows valve levers before and after bending, using a piece of pipe. The levers as supplied were not suited to this installation. So I bent them 90 degrees up and about 15 degrees to each side. This provides for easy operation.

The fifth photo show the fabricated main bracket, which carries the valve and also the quick couplers leading back to the thumb. It is a bit complicated because it bolts to the forward fender mount, the middle fender mount, and the ROPS mount, at different angles and elevations. It holds the heavy valve solidly in the correct position with no trace of movement.

 

[dfkrug]

It fits (barely) in the space between the left fender and the seat.

Wow, that's a tight fit. Do the levers clear the seat?

I guess that is why the OEM AUX valves from Kubota (actually made by Gannon for Kub) are
mounted to the FOPS. The photo shows the triple valve.

 

[downsizingnow48]

There isn't any room to spare, for sure! But it all works OK. The levers, bent to the correct angle, clear the fender on push and the seat on pull. The seat rotates freely, and it has full travel on the slide adjustment. The fly in the ointment was the bulky inertial reel of the seat belt, which occupied about 100 cubic inches of space right in the way of everything. I'm still thinking about how to re-mount the seat belt, or maybe replace the stock item with a more compact alternative. It can be done one way or the other, I just haven't thought it through yet. On valve location, I considered the FOPS. I also looked at the factory setup, which is pretty nice. In the end I decided in favor of a valve location that would be equally accessible facing forwards or backwards.

 

[downsizingnow48]

The photo shows the progressive linkage (yoke, links, shafts, bushings). After experimenting with the wooden mockup to test pivot point lengths from 6" to 8", I decided on 6.75" center to center. This is the final test assembly before welding up the yoke. The links are made of 3/4" x 2" mild steel bar, with 1.125" bronze bushings. The links don't have to be that heavy but that was the easiest way to get decent size (3/4") bearing surfaces for the shafts. The yoke is made of 3/16" mild steel rectangular tubing (1 piece 1.5" x 2.5", 1 piece 2" x 3") The tubing is sized to fit the thumb boss on one side (1.5"), and the cylinder rod end on the other (2.25"). The bushings for the yoke are chrome-moly tube, 1" ID x 1.5" OD. At the cylinder rod end there are two 1" bushings, and at the thumb boss end there is one 1.5" bushing. 6mm holes are drilled where needed for flush fit drive-in type grease fittings.

 

[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.