Engineering or manufacturing issue w/backhoe thumb. Advice requested

[zmansmac]

I've had my WoodMaxx 6600 backhoe for 2 years now and have had a fair amount of use with it, mostly digging and lifting out stumps. I ordered the optional hydraulically operated thumb when I bought it. It has worked well for me and I've not had any real issues with it, until now.

Walking by it the other day while it's on a dolly I noticed that the pin that connects the cylinder to the thumb jaws is severely bent. The first picture is a view from the top and the second from the bottom.

The pin is a 3/4 inch rod about 9 inches long. The inside distance between the jaws is a little over 6 1/4 inches. There are two pieces of tubing that keep the cylinder end centered. Each tube is 1 1/2 inches long and they have a 3/8 inch wall.

If this is an engineering issue I would say that the pin diameter is too small for this design. But the pin diameter is probably sized for the size of the cylinder used.

So my other thought is that perhaps the tubing spacers should have been welded on the inside face of each jaw and maybe that was missed in production.

So that leads me to my current plan to correct this problem. I can weld the tubes to the inside face of each jaw to help prevent the pin from bending again, but is a 3/8 inch wall thick enough for that kind of support. I can also weld some small triangle pieces for additional support around each tube or I can get some thicker wall tube stock and use that. I'm looking for your valued opinions on the best way to fix this problem.

 

[downsizingnow48]

Here is a picture of a Kubota B76 factory mechanical thumb. The tubes are welded and gusseted as you intend to do. I converted it to hydraulic and have used for about 18 months with no issues. The pin diameter is 25mm and the tubes are 3/8" wall.

 

[ArlyA]

So you'll get a new and straight pin?? Then I'd clean up those spacers and weld them in holding them straight with some piece of 3/4 shaft. Not the new on you just got which I'll guess is a hard material. You'll need to take care so as to not warp them much as you weld them it.

 

[zzvyb6]

Those spacers do nothing but keep the rod centered. They can add no pull or push strength to stop the bending moments. Put a bigger diameter pin in there.

 

[1stDeuce]

Those spacers do nothing but keep the rod centered. They can add no pull or push strength to stop the bending moments. Put a bigger diameter pin in there.

I'm a mechanical engineer, and that guy is exactly right. If I understand correctly, those "spacer" tubes are NOT welded to the sides, but the outer ones obviously are... Putting in a bigger pin is not going to be easy, plus it's likely the cylinder won't take a larger pin anyway.

You can weld and brace the spacers, but you'll also need to add strength to the sides of the thumb where the spacers are welded, or you'll just bend the sides in.

Look at the Kubota pictures, and you'll note they've done all of this... and used a larger pin. They plated the outside of the thumb, and welded the tubes in with gussets. Honestly, the gussets don't do much, so you can skip them, IMO.

Alternately, you could also consider turning down the pressure to the thumb with an in-line pressure release valve... That would keep you from bending it again. Straighten the existing pin in the press and you're good to go... (Is there a relief in the valve for the thumb?)

BTW, how is the hydraulic thumb controlled on the Woodmaxx? Just curious.
Chris

 

[downsizingnow48]

Alternately, you could also consider turning down the pressure to the thumb with an in-line pressure release valve... That would keep you from bending it again. Chris

I used a 1200psi work port relief on my thumb. Down from system pressure of 2100psi. Plenty strong. Could get by with less.

 

[zmansmac]

Thanks guys. Obviously, Kubota has more design experience in this area. I do like that the pin is secured by bolts in the spacers, closer to the cylinder.

I can't use a larger, like the 1 inch Kubota, pin without a major rework to this area, including probably having to get a new cylinder. So I am not going to go that route.

I'm not a mechanical engineer but it seems to me that my spacers are the same wall thickness as the Kubota's and welding them securely means that there is only about 2 1/4 inches of unsupported rod in the middle which is where the cylinder end is. And I'm thinking that would be sufficient even with a 3/4 inch pin. So I will get a new pin and proceed that way. If it fails at least I'll have a plan B to go to.

There is a basic hydraulic schematic in the WoodMaxx manual but I don't see any reliefs. They may be built into the valves and just not shown. Working pressure on all of their backhoes is spec'd at 2320 PSI. Hydraulic power is supplied by a PTO operated pump.

To answer your question 1stDeuce, there is a lever on the right side of the console near the bottom. It operates a selector valve which is either the right stabilizer or the thumb.

 

[Stonehaller]

You probably have over powered the thumb with the bucket curl. Hydraulic thumbs should have circuits with port reliefs that will allow the bucket curl to collapse the thumb cylinder. As your thumb uses a stabilizer circuit you do not want port reliefs on the valve as you do not want the stabilizer cylinder moving when the valve is closed. You could add an inline relief to your thumb clyinder, but it may be more trouble than it is worth as you have to have a drain line on it as well. If you remember to position your bucket first and then close the thumb against the bucket you should be able to avoid damage in the future. If the thumb cylinder is strong enough to bend the pin, then the simplest way to reduce force may be to add a double relief valve in the lines to the thumb cylinder which requires no external drain. You would need new hoses or have your existing hoses cut and new fittings added to the cut ends to mate with the relief valve. The double relief would not prevent damage if you over power with the bucket curl.

1/2 NPT 3 GPM 15-3 PSI HYD CUSHION VALVE

Welding the spacers to the sides of the thumb jaws will increase the load capacity of the pin, but be sure to add multiple gussets to strengthen jaws as 1stDuce says.

 

[ray66v]

It's not hard to envision that pin bending, when you look at it, and imagine specifically were the forces are going to be applied.

Not an impressive design.

I have wonder if the pin is as hard, as the original design called for? It may not have be hardened properly, and that may be your biggest defect. I would contact the manufacturer, and see if they are willing to replace the pin. The may be aware of some of them not being up to par. And, willing to eat it.

After determining how much clearance is needed for the ram to pivot, I would add structure to that area. With at least one piece that goes all the way a crossed, welded to your bushings.