Backhoe Building a thumb for a model 48 Backhoe

[DanMc77]

There is one geometric detail that I think will require an RV on the thumb extend line. There is an intermediate link between the bucket cylinder and the bucket that helps to extend the buckets range of motion. This also helps to maintain a pretty consistent radius of pivot for the bucket cylinder. On the thumb, I don't need that degree of motion so I'll have the cylinder tied directly to the thumb. Because of this, when the thumb is fully extended, the effective pivot radius on the thumb will be reduced to just a couple inches. In this configuration, the bucket cylinder will have a tremendous mechanical advantage over the thumb cylinder, so without a relief in the thumb extend line it would lead to some tremendous pressures to the thumb cylinder. I hope that makes sense.

So the thumb will necessarily be quite weak when it's fully extended. When the thumb is at a right angle to the dipper, then the bucket and thumb will be fairly well matched and the thumb will be at its greatest strength.

I dug through some of the earlier posts and went to the link for the cylinder that I bought. I tried to get the thumb cylinder to be matched with the bucket cylinder. I have to figure out what the cylinder bore is. I also called JD to get the bore on the bucket cyl.
Welded Hydraulic Cylinder Tang 3500 PSI.
Applications: Welded Double Acting Hydraulic Cylinder.
Tube: High tensile honed cold drawn tube.
Rod: High tensile ground & polished hard chrome plated rod.
Gland: Precision threaded gland with wear rings for long life.
Piston: High tensile steel for heavy duty applications.
Base ends: Welded steel tang mounts.
Piston seals: Top quality five component compact seals for excellent performance.
Rod seals: Standard U-cup polyurethane rod seal with inner O-ring, POM-C wear rings and polyurethane wiper.
Warranty: 2 year limited warranty.
Pressure: 3500 PSI

Here's an earlier comment that I posted: "I dropped by the local JD dealer. They were able to look up the bucket cylinder specs. It's a 2 1/4" bore. My thumb cylinder is a 2" bore." 2.0x14x1.25 side ports. Posting this stuff is serving as a nice documentation repository of the technical details, and having more experienced people reviewing things is invaluable.

 

[texasranger556]

Like I said at the beginning of your thread, I wouldn't worry too much about your cylinder and perfectly matching pressures. As a simple example, your loader has lift cylinders and curl cylinders. If you get under something that you can't lift, what do you do? Curl the bucket. Breakout force is often double or triple the loader lifting capacity, and there are is no relief on your lift cylinder work ports to relieve pressure on the hoses and cylinders, but they are seeing a lot more more pressure than your 2500 psi system relief valve is set to. Similar instances on your backhoe when the bucket cylinder force is amplified to your dipper and boom cylinder, not to mention the outriggers cylinders.

The thing I would be most concerned with is bending something. For example the bucket/thumb pivot pin since half of your force vector is several inches out from the end of the cross tube in the dipper, and both the bucket and thumb cylinders are working in the same direction trying to shear it off or bend it.

 

[downsizingnow48]

These are pins that I made for the linkage on my hydraulic thumb. Does not take long on the lathe.

 

[downsizingnow48]

My B21 has reliefs on both bucket cylinder and thumb. I have had the bucket push the thumb back and the thumb push the bucket back, depending on their relative positions and which has the leverage advantage. Unless the system is carefully matched, as dfkrug suggests, you will need reliefs to keep from bending something.

 

[downsizingnow48]

duplicate post deleted

 

[downsizingnow48]

The thing I would be most concerned with is bending something. For example the bucket/thumb pivot pin since half of your force vector is several inches out from the end of the cross tube in the dipper, and both the bucket and thumb cylinders are working in the same direction trying to shear it off or bend it.

This is a definite concern. Depending on how long your shared pin is, how thick it is, how many holes and grooves it has, what it is made of, and your system pressure.

 

[ovrszd]

My B21 has reliefs on both bucket cylinder and thumb. I have had the bucket push the thumb back and the thumb push the bucket back, depending on their relative positions and which has the leverage advantage. Unless the system is carefully matched, as dfkrug suggests, you will need reliefs to keep from bending something.

Yep. That was why I asked.

 

[dfkrug]

On the thumb, I don't need that degree of motion so I'll have the cylinder tied directly to the thumb. Because of this, when the thumb is fully extended, the effective pivot radius on the thumb will be reduced to just a couple inches. In this configuration, the bucket cylinder will have a tremendous mechanical advantage over the thumb cylinder, so without a relief in the thumb extend line it would lead to some tremendous pressures to the thumb cylinder.

Yes, I see what your are saying. I say that you can easily avoid that scenario.

As I said above, design the thumb with a cyl-thumb pivot that is not too close to the dipper-thumb pivot. How
close is that? I think mine is something like 6" or so. Then design your thumb cyl stroke to allow for
full retraction AND not over-extension. An over-extended thumb will cause you to be subjected to too much leverage (torque multiplication) from the bkt pushing against it.

I do not think you need a thumb that extends much beyond the point at which it is perpendicular to the dipper. Beyond that point is the danger zone and a port-RV becomes necessary.

 

[DanMc77]

I do not think you need a thumb that extends much beyond the point at which it is perpendicular to the dipper. Beyond that point is the danger zone and a port-RV becomes necessary.

Good advise. I'll see how much I can move the cyl pivot out.

Downsizing: Thanks for the pics of your pins. Looks like you turned them down and left a boss on one end. Is that the end you put the zerk in? I've seen some pins with end zerks where the counter bore and set the zerk inside for a bit of protection. That would be important, especially for a pin that's getting into the dirt.

 

[downsizingnow48]

Yes the zerk goes in the boss end. The other end is a collar and through bolt. Rather than counterbore, I used flat face zerks. They have been fine. They shed dirt rather than have it get packed into the counterbore.