Sizing a Backhoe Thumb Cylinder

[Dougster]

Based on your description, Woods must have designed for similar net forces applied by the thumb and bkt, then added a relief valve to each circuit so they would retract under opposing pressure. Otherwise opposing pressure would have exceeded the pressure limits of the cylinders. I find it a bit surprising that the thumb can push the bkt back. I wonder what pressures they are seeing... They also may have INTENDED the user to be able to push either back by the other so he could position his load. The problem I see here is only if a relief in the bkt circuit is set lower than max press, which limits your digging capability. A tradeoff.

I found that to be an interesting operational feature as well... and I found myself wondering if it would be a more helpful or not-so-helpful feature on my own machine. The notion of being able to adjust your load while under a near constant pinching force is kind of interesting... but at what price in terms of reduced pinching power? And yet it sounds from his description like this "feature" can be defeated, when necessary or desired, simply by applying pressure to both sides at the same time. Sure wish I had more info on this... but I don't think I should let it affect my cylinder sizing criteria... still looking like 2.75" based on all the factors previously discussed.

Dougster

 

[MadDogDriver]

I really have enjoyed my hyd thumb. Like you've already mentioned Dougster, it's so convenient not having to jump down and reposition the thumb when needed. I haven't grabbed anything heavy enough yet where I've needed to apply hyd pressure with both the thumb and bucket simultaneously, although I do normally grab items that way at first and then step off the thumb circuit and bucket simultaneously and it holds the load. Like you've mentioned, it is nice to reposition as needed.

I'm happy with it. I'm sure you will too when it's all said and done Doug.

 

[Dougster]

I really have enjoyed my hyd thumb. Like you've already mentioned Dougster, it's so convenient not having to jump down and reposition the thumb when needed. I haven't grabbed anything heavy enough yet where I've needed to apply hyd pressure with both the thumb and bucket simultaneously, although I do normally grab items that way at first and then step off the thumb circuit and bucket simultaneously and it holds the load. Like you've mentioned, it is nice to reposition as needed. I'm happy with it. I'm sure you will too when it's all said and done Doug.

Hi MadDog - Can you explain how they do that without affecting (i.e., reducing) bucket digging force? I certainly don't mind putting a carefully chosen relief circuit on the thumb side... might do that anyway... but I don't want to give up one single ounce of bucket digging force.

Dougster

 

[MadDogDriver]

I'm not sure. If you look back at an earlier post of mine that had the thumb install pdf, you'll see everything that I have available to me (as far as documentation from Woods). When I first had the BH it came with the mech thumb while I was waiting for the hyd kit to arrive. To my knowledge, all the dealer did was install the hyd kit in accordance with that pdf previously posted. The hyd power for the thumb came off the left stabilizer (left as sitting on the seat looking at the bucket).

I have not noticed any difference in digging power before or after the hyd kit install.

EDIT: Just called Woods Tech support and asked that question....they said it's designed that way and does not limit digging power of the bucket.

 

[Dougster]

Just called Woods Tech support and asked that question... they said it's designed that way and does not limit digging power of the bucket.

But I'll bet they wouldn't tell you how they do it!

Same thing with Bradco. They want you to dig deep and buy THEIR system! Not build your own.

Dougster

 

[MadDogDriver]

Nope, just said "It's designed that way".

 

[dfkrug]

Hey, MadDog:

Thx for posting your experiences with your Woods hyd thumb. Real
world first hand experience is very valuable to those of us designing
our own systems. I have only been able to try hyd thumbs on
excavators and neither thumb nor bkt could push the other back
on those.

What Woods may have done is use relief valves in every work port of
their stack valve. The action by the thumb against the stationary
bucket may be making use of a relief valve already in the bkt circuit.
That standard relief valve is prob set just above the max recommended
working pressure of the system, so it would not reduce your digging
force. On my hoe, some of my shock reliefs are set at 175 bar and some
are at 180 bar (2650psi).

 

[MadDogDriver]

No problem.......You guys crack me up (the ENGINEER in you comes out loud and clear). It'll drive you nuts until you know exactly how and why things operate the way they do.

All the more power to you two (and others). I'm at that stage in life where as long as it works okay I leave it alone. However, the eng genes still come through when I need to fix something then I'm in your camp trying to get all the info necessary.

Good Luck.

 

[Dougster]

No problem.......You guys crack me up (the ENGINEER in you comes out loud and clear). It'll drive you nuts until you know exactly how and why things operate the way they do. All the more power to you two (and others). I'm at that stage in life where as long as it works okay I leave it alone. However, the eng genes still come through when I need to fix something then I'm in your camp trying to get all the info necessary.
Good Luck.

It is "the curse" MadDog. We are well aware of it... but we can't control ourselves. Forgive us... and maybe even feel sorry for us. Most of all, be very glad you aren't us!

Dougster

 

[dfkrug]

It's not just about HOW something works, and it's not just about
taking things apart AND putting them back together. And it not
just about fixing something either. It's really about making something
BETTER, or designing & building something NEW. The curse.