Custom hydraulic backhoe thumb

[dfkrug]

I'm still a little worried about the 3/4" pins given the forces involved.

You should be. Somewhere in this thread, I posted about this issue when I talked
about converting from mechanical to hydraulic.

On a couple of my mechanical thumbs, I bent the 3/4" pins I used. Of course, my
Deeres and Kiotis ran at 2500psi, while my Kubs were down at 2000. I think your
B is also limited to 2000psi. Check that. 1" pins should be used, IMO.

To belabor another point I made, converting an existing mechanical thumb will
force you into getting exactly the correct hydraulic cylinder stroke and length. This
is cuz your pivot point on the thumb is already fixed. I suggest you use mockups
of your cylinder to get the length and stroke just right. Even a 1/2" off and your
rest position against the dipper will be sub-optimal.

 

[Ron50]

On the small Kubota excavator the bucket curl will overpower the hyrdaulic thumb and move it back, Relief valve maybe? I'm not sure but either a Cat 320 or a Komatsu 200 did the same thing. Built in protection.
On my Branson BH76 I got down to almost getting the correct valve to add to the stack to operate the thumb. I just need to verify which one is correct. Ansung should be able to just sell a replacement valve, but they never call back.

 

[bumperm]

I'm not a mechanical engineer, but it would depend on the steel and heat treating on the pin. You are correct that the unsupported span between the cylinder boss and the dipper bracket would be the problem, as that could impart a lot of bending force. If you tried it and it bent, it would be unlikely to cause damage, but could be difficult to remove as you would no longer be able to drive the pin out, and trust me, you don't want to try and saw it. An abrasive wheel on an angle grinder would be the way to go.

Going the other way, boring the cylinder ends to 1", might leave the walls of the cylinder boss thin? I'd first look some more for a more suitable cylinder.

My cylinder to thumb to dipper fit is perfect. On retraction, the thumb comes to within 1/8" of the cylinder, on extension the rod comes to within 1/8 of the dipper bottom. You could get different strokes, but none longer! The cylinder I chose, IIRC, was just a little longer than the mechanical linkage, but since the mechanical linkage was by design a little too short, all worked well. Being able to extend the thumb all the way is good to have, as it allows you to directly pick up objects or pluck long tap root weeds, in a larger arc further away from the tractor.

I bought a Bro-tek ripper that has a curved cutting edge. Works great but doesn't mate with the thumb, leaving a crescent shaped opening that is frustrating when trying to rip brush out by pulling or pushing. So, I'm making a convex anvil to bolt to the thumb (when I get the time!).

I agree, the two inch cylinder might be a better match. With equal geometry, the issue becomes when a larger cylinder is driving into a stationary smaller cylinder, the pressure in the smaller cylinder will go higher than the source pressure driving the larger cylinder and the cylinder will not be protected by the backhoe relief valve.

 

[bumperm]

On the small Kubota excavator the bucket curl will overpower the hyrdaulic thumb and move it back, Relief valve maybe? I'm not sure but either a Cat 320 or a Komatsu 200 did the same thing. Built in protection.
On my Branson BH76 I got down to almost getting the correct valve to add to the stack to operate the thumb. I just need to verify which one is correct. Ansung should be able to just sell a replacement valve, but they never call back.

That doesn't happen on my BH77. With the solenoid valve on the cylinder side of the thumb cylinder, with the solenoid open so both thumb and curl cylinders are extending, they happily meet in the middle (I wouldn't want to be in the meeting). With solenoid closed, fixing thumb, the curl drives the bucket into the thumb and stops there solid.

I've picked up a large rock, one that' not cradled but more held just by pressure, and it holds fine for several minutes anyway - haven't had one drop while doing that.

 

[Racerwannabe]

You should be. Somewhere in this thread, I posted about this issue when I talked
about converting from mechanical to hydraulic.

On a couple of my mechanical thumbs, I bent the 3/4" pins I used. Of course, my
Deeres and Kiotis ran at 2500psi, while my Kubs were down at 2000. I think your
B is also limited to 2000psi. Check that. 1" pins should be used, IMO.

To belabor another point I made, converting an existing mechanical thumb will
force you into getting exactly the correct hydraulic cylinder stroke and length. This
is cuz your pivot point on the thumb is already fixed. I suggest you use mockups
of your cylinder to get the length and stroke just right. Even a 1/2" off and your
rest position against the dipper will be sub-optimal.

You're correct. My max pressure is 1991 PSI per Kubota WSM.

Sounds like I gotta nail the retracted length @ 20 1/2" for sure - no compromises. I could get there with the original 12" stroke cylinder that I was looking at by hacking both ends off and converting to 1" DOM and adding a 1/2 spacer on the cylinder end. That would get me to an "effective" extended length of 12 1/2" vs. the 13" extended length of the mechanical linkage. Any of the longer stroke cylinders that I looked at from Bailey's have a retracted length that's too long.

The good news is that my existing thumb linkage is linear; Channel that slides within another channel. So it mimics the operation of a cylinder and makes taking measurements pretty simple.

I'll search on on here for other sources of cylinders. Suggestions welcome! I imagine a true custom cylinder will cost an arm and a leg.

 

[bumperm]

While I agree with dfkrug, 1" is better than 3/4", Bolts and pins in shear are incredibly strong. A 3/4" Grade 8 bolt is over 40,000 pounds. Of course this is measuring pure shear, not the fasteners ability to span a gap and resist flexing. This is why, when you inspect a piece of hydraulic equipment, the gaps or spaces between cylinder clevis and their mounts will be minimal, typically a thick boss or bushing will be welded to the structure and will mate up to the cylinder clevis with 1/8" or similar gap.

On my B3350 tractor, the WSM says 2450 psi for the relief valve setting. Backhoe may be less, haven't checked, but the machinery operating pressure will be whatever the relief valve with the lowest bypass setting in the circuit is. BTW, it seems small tractors often have their hydraulic relief valves set low for some reason, and more power can be had by adjusting them to spec.

If you have the machinery to cut and weld, then starting with a cylinder that is too long, it's possible to custom make whatever length you need, within reason. Best to take the cylinder apart, before cutting and welding, but not required. If the cylinder needs to be shortened, the ram can be extended and the cylinder end cut off, shortened and re-welded. Wrap the barrel with wet rags and extend the ram fully to weld it back together without disassembly. Same idea for shortening the piston rod end, or welding in a spacer between cylinder and mounting clevis to lengthen the retracted dimension.

 

[dfkrug]

I'll search on on here for other sources of cylinders. Suggestions welcome! I imagine a true custom
cylinder will cost an arm and a leg.

If you paid a weldor to customize a cyl, then it may cost $100-200. Customizing one yourself is
not hard. Or changing the pivot point on your thumb: cut out the old pivot and weld in a new
piece of DOM. You must match the thumb cyl ID to the dipper cyl ID, unless you want to mess
around with work-port relief valves.

 

[Racerwannabe]

I'm obviously a total newb when it comes to hydraulics, but am learning that these cylinders are much lower-tech than I thought. Thank you all very much for the lessons!

Best to take the cylinder apart, before cutting and welding, but not required. If the cylinder needs to be shortened, the ram can be extended and the cylinder end cut off, shortened and re-welded.

So in the absence of the "driven attachment" stopping the travel of the ram (like the thumb grasping a rock), what limits the travel of the ram? In extension, it's gotta be the piston hitting the inside end of the barrel since Bailey's told me I can limit the extension by putting a bushing on the rod. Is it the same in retraction? If so, then shortening a cylinder will cause loss of stroke travel, right? Or is there some other type of mechanical stop inside that results in squish (like in engine combustion chamber) where you could shorten the barrel and whatever stop is in there to reduce the squish, but not compromise the overall stroke travel?

You must match the thumb cyl ID to the dipper cyl ID, unless you want to mess around with work-port relief valves.

Not messing around with those! Wanna keep this as simple as possible. Less stuff for me to break.

BTW, it seems small tractors often have their hydraulic relief valves set low for some reason, and more power can be had by adjusting them to spec.

I've learned that as well. This is a popular adjustment with the guys that own the little Kubota BX hoes. Of course I learned this AFTER I had the hoe installed on my tractor, otherwise I would've had the dealer check and shim the relief valve back to spec. But I must say, this is a pretty strong machine as is...

Update: I may have stumbled on a cylinder that will work with only minor trimming (3/16") to the base cross tube:dance1:

20.5" retracted length, 1" pins, and 13.25" stroke. Any reason NOT to consider a 2500 psi cylinder when my relief valve is supposed to be < 2000psi? The hoe cylinders are 3000 psi. I guess the only issue would be if I ever sell the hoe I gotta tell the next guy that it's only a 2500 psi cylinder if he's putting it on a strong tractor

 

[dfkrug]

Bailey's told me I can limit the extension by putting a bushing on the rod.
Is it the same in retraction? If so, then shortening a cylinder will cause loss of stroke travel, right?

You certainly can use some mechanical means of stroke control, internal to the cylinder, but I don't
think you need to here. Stroke control bushings are commonly sold, and I suppose you can limit the
contraction, too, with some kind of blockage inside the base end. I have not seen those.

You can shorten a cyl by cutting the rod cross-tube and welding it back on after trimming the rod
a bit. If the modified cyl's new rod-end cross-tube now hits the gland on retraction, you have
some stroke limitation. You can prob cut as much as an inch out of the rod before that happens.
Depends on the cyl.

Cutting the base end and moving the fitting will reduce the stroke, too.

 

[dfkrug]

Bailey's told me I can limit the extension by putting a bushing on the rod.
Is it the same in retraction? If so, then shortening a cylinder will cause loss of stroke travel, right?

You certainly can use some mechanical means of stroke control, internal to the cylinder, but I don't
think you need to here. Stroke control bushings are commonly sold, and I suppose you can limit the
contraction, too, with some kind of blockage inside the base end. I have not seen those.

You can shorten a cyl by cutting the rod cross-tube and welding it back on after trimming the rod
a bit. If the modified cyl's new rod-end cross-tube now hits the gland on retraction, you have
some stroke limitation. You can prob cut as much as an inch out of the rod before that happens.
Depends on the cyl.

Cutting the base end and moving the fitting will reduce the stroke, too.