Hydraulic Bucket & Thumb Operation

[rScotty]

On the M59 TLB the standard operator's manual covers the tractor and loader but not the backhoe. I've heard that backhoe manuals are now available, does anyone know of a download site?

My question is about using the hydraulic thumb. To maintain grip on an object it seems to me that the bucket needs to overpower the thumb and force it backwards. Anyway, that's how I've used it and it's worked smoothly for years. The motion of the bucket+thumb was powerful with the bucket overpowering the thumb, and it didn't seem to differ if the foot pedal valve for the thumb was held open or not. Sometimes I use my foot to hold that valve open, sometimes not. But recently the motion as the bucket forces the thumb back has become a little jerky and as it jerks the thumb cylinder makes a little bit of honking noise as it is forced backwards....sort of like it is sticking. There isn't anything obvious and I was wondering if I had gotten some air into the bucket or thumb circuit. No, I still haven't reversed the foot pedal - even though that makes sense to do. Was going to do that and then this started.
rScotty

 

[wdchyd]

Put a straight edge on it to check for bent rod or bent/dented cyl barrel......

If questionable then remove cyl and pull out rod by hand and rotate it.....check mounting pins and ears for bends also.....

sounds like bent rod and/or internal damage to piston head/seals......I'd at least rule out cyl first.........

Hope this helps.....

 

[rScotty]

Put a straight edge on it to check for bent rod or bent/dented cyl barrel......
........
sounds like bent rod and/or internal damage to piston head/seals......I'd at least rule out cyl first.........
........

I have to admit that my first impulse was to say, "My parts aren't bent". But then I realized that is just foolish...if not arrogant. Truth is, your advice is spot on. I haven't gotten out my precision tools and slowly extended the rods while checking them - and I should have. Now, I still don't expect to find a bent rod is at fault, but it just makes sense to do a complete check of the rods and cylinder walls before looking for something more complicated.
And who knows what else might surface?
rScotty

 

[rScotty]

.......but it just makes sense to do a complete check of the rods and cylinder walls before looking for something more complicated. They
And who knows what else might surface?
rScotty

OK. I checked the rods and cylinders and they are perfect and undented. Then greased everything and cycled the bucket and then the thumb independently two dozen times in the down position and then another two dozen raisedd as vertical as the hoe will go - this in case the problem was an air bubble somewhere.

Darn!! It still makes the noise and is jerky when the bucket is forcing the thumb backwards.

It sounds to me like a dry bushing, but that's hard to imagine. Still, it might be because I remember when it was new there was a squawk from the swing motion on the BH that wouldn't go away with normal grease. I finally had to go to a moly high pressure grease. When I did that the swing noise went away. Maybe I'll have similar results with the bucket&thumb.
rScotty

 

[kennyd]

So, just so I understand...you are forcing the thumb cylinder closed (while the valve is neutralized) with the bucket? So you are forcing the PRV (pressure relief valve) of that valve (if it has one) to open?

 

[rScotty]

So, just so I understand...you are forcing the thumb cylinder closed (while the valve is neutralized) with the bucket? So you are forcing the PRV (pressure relief valve) of that valve (if it has one) to open?

Yes, exactly. I assume that is how they are designed to work. Otherwise there wouldn't be any way to maintain pressure with the thumb when curling the bucket.

I'm not completely comfortable with that and wish that Kubota put a little effort into explaining the thumb, but it has worked well for over 100 hours of backhoe work so this method seems to be OK. Most of the work so far has been cleaning out a rocky stream bed where I sit on the bank and lift buckets of stones out of a creek.

Yes, the shop manual shows a pressure relief valve right where you would expect there to be one. In the circuit picture it looks to be openable by pressure in the thumb line created by curling the bucket against the thumb. Released oil is dumped to the reservoir.
rScotty

 

[kennyd]

Yes, exactly. I assume that is how they are designed to work. Otherwise there wouldn't be any way to maintain pressure with the thumb when curling the bucket.

While I am not a "thumb" expert by any means...I think you will end up damaging your setup doing that. Hodling material against the thumb with a little pressure is OK, but "forcing" the thumb back is putting a lot of stress on the attachment points and the hydraulic.

 

[rScotty]

While I am not a "thumb" expert by any means...I think you will end up damaging your setup doing that. Hodling material against the thumb with a little pressure is OK, but "forcing" the thumb back is putting a lot of stress on the attachment points and the hydraulic.

I hope some more bucket and thumb users chime in here. Tractors are a pretty slow moving technology and thumbs are still new to everyone. Maybe we just don't know.

As an example of how slowly tractors change, my 1958 JD has every "modern" feature my 2008 tractor does except for HST....but then the new one doesn't have a sprung and hydraulically damped operator's seat either. They are 50 years apart and although the technology is quite different, the operating features are suprisingly similar. And neither one is yet a clear winner.
rScotty

 

[dfkrug]

I am curious how the factory hyd thumb is designed for the M59.
Does the thumb pivot on the same pin as the bucket does? I
suspect so.

If that is the case, then the greased area that takes most of the
forces of the bucket action is inside the dipper, while the thumb
pivots about the outer parts of the pivot pin. It is hard to provide
proper lubrication when 2 pivoting elements use the same pin. The
pin is stationary with respect to one or the other. This was
certainly a dilemma I had to cogitate over when designing my
hyd thumb. I opted to use separate pivots.

As for the work ports on the thumb valve, it would be great if
they had shock-RVs in both ports. That is the only way the
thumb could be forced back by the bucket. Although I matched
my thumb cyl size with my bkt cyl size, one or the other can still
exert more force depending on the geometry of the positioning of each
and where the load is placed.

The good thing about the bkt and thumb using the same pivot is that
when the thumb is forced back by the pressure of the bkt, the object
you are pinching is not being rolled between thumb and bkt.

BTW, I do NOT have shock-reliefs in my bkt/thumb work ports, and
neither element can force the other to retract.

This is an interesting problem....can you take a video?

 

[rScotty]

"I am curious how the factory hyd thumb is designed for the M59. Does the thumb pivot on the same pin as the bucket does?"

Yes, the factory hydraulic thumb and bucket both pivot on the same pin. BTW, thanks for all your insightful comments on the construction and geometry. You definitely spurred me to look deeper into the whole system.

"If that is the case, then the greased area that takes most of the forces of the bucket action is inside the dipper, while the thumb pivots about the outer parts of the pivot pin. It is hard to provide proper lubrication when 2 pivoting elements use the same pin. The pin is stationary with respect to one or the other."

I don't see a lube problem. On the Kubota the pin is fixed to the bucket and both then rotate together with the pin rotating inside of a large central bushing mounted in the end of of dipperstick. A zerk provides the bushing lubrication.
The thumb obviously needs to rotate independently of the bucket. And although it doesn't absolutely need to pivot on the same common axis of rotation as the bucket.....certainly the calculations for forces and geometry are greatly simplified if it does. In the case of the M59 both bucket and thumb pivot on the same pin with the thumb having ears that fit over the longer pin. That longer pin is the key to the design. Lubrication for the mounting ears of the thumb is provided through zerks.

"As for the work ports on the thumb valve, it would be great if they had shock-RVs in both ports. That is the only way the thumb could be forced back by the bucket."

Yes, it seems to me that some sort of pressure release valve (RV) with a return line to the reservoir is the easiest solution to allowing the bucket to force the thumb back without damage. Turns out that the M59 has a RV (relief valves) in both ports just as you say. Plus they have an interesting feature - an anticavitation valve that will draw some fluid in if it needs to. Your term "shock-RVs" is a new one to me.... I have never heard them refered to as "shock-RVs"...but you got me thinking.....and may have put me on the path of a fix. I'll save that for last, though.

"Although I matched my thumb cyl size with my bkt cyl size, one or the other can still exert more force depending on the geometry of the positioning of each
and where the load is placed. The good thing about the bkt and thumb using the same pivot is that
when the thumb is forced back by the pressure of the bkt, the object you are pinching is not being rolled between thumb and bkt."

I hadn't thought much about this before your comments....but then I've never designed a thumb myself. I agree yours would have been better off with a common pivot....Isn't hindsight great!?

"BTW, I do NOT have shock-reliefs in my bkt/thumb work ports, and neither element can force the other to retract."

This is my first experience with a BH having a thumb, but it seems to me that the abilty of one to force the other is very helpful. Plus it allows the whole thumb to be built a lot lighter. What about one pair of inline RVs with a line to a pressure reservoir to store the fluid? Then having a reservoir of oil under pressure all that is needed is one more additional set of RVs mounted in reverse to serve the anticavitation function....

This is an interesting problem....can you take a video?

Well, too late for a video. It is changed now. After reading through your thoughts I went out and spent a couple of hours convincing myself that Kubota indeed had the pivoting geometry and their lube scheme correct. In the end, I agree with how they built it...all except for one joint where they used a roll pin and that gives an escape route for lube pressure. But it isn't really in the problem path. I'll fix that someday...maybe..
Since I haven't found the root cause, I decided to reduce the relief valve activation pressure by about 25%. That helped the problem in that the jerking isn't so violent now. Before I was afraid that the violent hydraulic shock waves were going to break something.

HERE'S WHAT I'M THINKING TO TRY:
I'm now toying with the idea that either the relief valve or the spool valve valve have become slightly damaged internally because of hydrostatic shock from times when the thumb slipped or sqeezed so hard it broke something that it was holding. It was squeezing things hard enough to break them about the time when it all started getting jerky and noisy. Very sudden.
To add to this, a mechanic friend has told me he has heard rumors that Kubota now recommends lowering the trigger setting on the thumb's relief valve. That may help someone else but is a little late for me as my warranty expired last month. Still, if that is what happened then it may be easily fixed with new relief valves.
Enjoy! rScotty