BH77 - has anyone considered converting mechanical thumb to hydraulic?

[jim_wilson]

I know what you are saying, my B3300 is the same. However, I"m not sure there is a difference between "Out" and "Return". I think the "out" of the BH has full pressure fitting because it runs the 3pt. To me, that just means that when the new valve "out" is T'd into the existing "out", full pressure fittings need to be used.

I could be way off, it does seem a bit too simple.

Here's another diagram that shows a similar setup to what would need to be done to add a grapple to the BH77:

http://www.princehyd.com/Portals/0/products/valves/SVInstS.pdf

see page 2 on the PDF

 

[CDN Farm Boy]

I think a case drain is only relevant to hyd motors, not valves and cylinders.

These are rather basic drawings, work ports have been omitted for clarity:



If I have to add an extra return line straight back to the tank for the new valve, I might as well just use longer hoses and plug them into one of my other sets of remotes. It would be much more simple (and cheaper) but I'd rather keep to only 2 hoses to have to mess with.

 

[jim_wilson]

I think a case drain is only relevant to hyd motors, not valves and cylinders.

These are rather basic drawings, work ports have been omitted for clarity:

View attachment 315345

If I have to add an extra return line straight back to the tank for the new valve, I might as well just use longer hoses and plug them into one of my other sets of remotes. It would be much more simple (and cheaper) but I'd rather keep to only 2 hoses to have to mess with.

If you look at the manual for the B3200 / B3300SU - there is a case drain coming from the FEL valve.

I believe a case drain is needed in some cases for valves controlling cylinders - so that the pressure relief can have somewhere to dump. In this case - where we would be adding another valve in series with the existing Kubota valve stack (and not adding a valve into that valve stack) - my current understanding is that a separate pressure relief is needed on this additional valve - to protect the Kubota valve stack - and that pressure relief will need somewhere to dump to.

I have the number of one of the sales engineers at the local hydraulics supply house - and I intend on calling him with some of these questions when I get a chance to see what he has to say - as well as see what kind of valve he would recommend using in this application.

 

[dfkrug]

I wonder if you could use a valve like the one I linked earlier mounted before the BH77 stack, use the PB port of the new valve to power the main stack as you suggest and T the 2 returns together?

No extra lines to mess with mounting/dismounting the hoe.

Yes, that is the only correct way to add a separate spool valve to the stack. If the BH77 had
a modular stack valve, then adding another stack section from the same maker would be ideal. In
that case the Return lines to the tank are all connected internally. No CUT backhoe attachment
that I have ever seen includes Power Beyond at the end of the stack; they are just 2-hose
setups that drain to the sump or thru the 3-pt valve. A separate valve before the stack would just use a
Tee for both return hoses as you say.

Of course, an electric spool valve can also be added, and it can be added to a work port instead
of the primary stack. That's what I did.

When I did my hydraulic thumb project, I found that the mechanical thumbs I had made before
that required complete re-designs rather than conversions. For one, the maximum forces needed
to be acommodated. That means the cylinder needed to be sized for stroke and diameter. Too
large a diameter, and excessive forces could require a relief valve. Pin sizes and rod sizes needed
to be at least 1 inch in my design, and pins needed to be fully greasable. Attachment brackets ended
up totally different from my mechanical thumbs.

 

[jim_wilson]

Yes, that is the only correct way to add a separate spool valve to the stack. If the BH77 had
a modular stack valve, then adding another stack section from the same maker would be ideal. In
that case the Return lines to the tank are all connected internally. No CUT backhoe attachment
that I have ever seen includes Power Beyond at the end of the stack; they are just 2-hose
setups that drain to the sump or thru the 3-pt valve. A separate valve before the stack would just use a
Tee for both return hoses as you say.

Of course, an electric spool valve can also be added, and it can be added to a work port instead
of the primary stack. That's what I did.

When I did my hydraulic thumb project, I found that the mechanical thumbs I had made before
that required complete re-designs rather than conversions. For one, the maximum forces needed
to be acommodated. That means the cylinder needed to be sized for stroke and diameter. Too
large a diameter, and excessive forces could require a relief valve. Pin sizes and rod sizes needed
to be at least 1 inch in my design, and pins needed to be fully greasable. Attachment brackets ended
up totally different from my mechanical thumbs.

I was under the impression that there is a difference between power beyond , "out" and " drain" . Power beyond is supplying full pressure. "Out" is or can be supplying either full pressure or something less - and "drain" would be at a much lower pressure.

The "drain" feed out of the FEL valve on my B3200 just uses a braided rubber hose (like you'd see on automotive cooling system) - with hose clamps. The "out" feed coming from the backhoe (and going back to the 3pt) - is a full high pressure hydraulic hose.

I suppose the way to really answer this question is to put a gauge on the backhoe "out" and see what kind of pressure it's running at.

From reading thru some of the earlier posts in this thread - as well as other posts here on TBN - my understanding is that any valve that is added for a hydraulic thumb should have a pressure relief of it's own anyway - and it should be set something lower than the backhoe control stack - to avoid damage to either the rod on the thumb cylinder - or to the backhoe stack itself.

 

[dfkrug]

I was under the impression that there is a difference between power beyond , "out" and " drain" . Power beyond is supplying full pressure. "Out" is or can be supplying either full pressure or something less - and "drain" would be at a much lower pressure.

The "drain" feed out of the FEL valve on my B3200 just uses a braided rubber hose (like you'd see on automotive cooling system) - with hose clamps. The "out" feed coming from the backhoe (and going back to the 3pt) - is a full high pressure hydraulic hose.

I suppose the way to really answer this question is to put a gauge on the backhoe "out" and see what kind of pressure it's running at.

From reading thru some of the earlier posts in this thread - as well as other posts here on TBN - my understanding is that any valve that is added for a hydraulic thumb should have a pressure relief of it's own anyway - and it should be set something lower than the backhoe control stack - to avoid damage to either the rod on the thumb cylinder - or to the backhoe stack itself.

Generally, an open-center (or tandem-center) valve will have an IN and an OUT, or an IN, OUT, and Power-Beyond. The
PB is the only pressurized out connection. The OUT is a low pressure return to tank, and is generally limited to 500psi max.

Hoe attachments often "cheat" the low pressure OUT thru the IN of the 3-pt valve because you can not use
the 3-point valve while the hoe is attached. If you did use the 3-pt valve and exceed 500 psi in that hookup, you would
blow out hoe valve(s).

If your hoe comes with 3 hoses, then you have PB. I have never seen a CUT hoe with 3 hoses. Not counting work
port hoses, of course.

My earlier point is that if you design a hyd thumb correctly, you do not need to have any additional relief valves.
Work port RVs on hoes are used as shock-reliefs in certain valves to limit forces in hi-momentum situations (usually
the boom).

 

[jim_wilson]

Not to poo-poo on anyones wishes to install one but realize I do cylinder rebuilding, and that includes damaged ones

Many bent rods (leading to scored barrels cuz the piston head cocked sideways internally) have been diagnosed back to the problem of induced load

Meaning a force pushing in to the thumb piston is greater than the hyd cylinder can survive from another source......the bucket curl.

The bucket curls larger displacement and leverage can put extreme forces while clamping down using the curl cyl, unless there is a relief setup to allow extra pressure to relax (ie:work port reliefs in the control valve) during curl cyl use

If the hyd thumb is properly installed, the thumb cyl should go back in when there is too much pressure.....not bend and self-destruct. Without the safety of the reliefs plumbed in to the base end of the thumb cyl, the pressure intensifies greater than the rated pressure of the cylinder (assuming the thumb cyl is smaller than the curl)

Ideally either work ports reliefs or sizing the cylinder large enough to match is my recommendation

I actually have no problem with hyd thumbs, just not on my machine, I don't make a living with it

So - in your opinion, what would be the "correct" way to set this up? Should a cylinder of equal bore size and rod diameter as the bucket curl cylinder be used for the grapple be used? - and then use a valve with work port reliefs built in - and set to a slightly lower pressure than the backhoe reliefs?

I'm doing the research on finding an appropriate cylinder. I took the measurements of the existing grapple last night so I think I know what will physically fit - I'm just trying to work out what size would actually be "correct"

 

[jim_wilson]

Generally, an open-center (or tandem-center) valve will have an IN and an OUT, or an IN, OUT, and Power-Beyond. The
PB is the only pressurized out connection. The OUT is a low pressure return to tank, and is generally limited to 500psi max.

Hoe attachments often "cheat" the low pressure OUT thru the IN of the 3-pt valve because you can not use
the 3-point valve while the hoe is attached. If you did use the 3-pt valve and exceed 500 psi in that hookup, you would
blow out hoe valve(s).

If your hoe comes with 3 hoses, then you have PB. I have never seen a CUT hoe with 3 hoses. Not counting work
port hoses, of course.

My earlier point is that if you design a hyd thumb correctly, you do not need to have any additional relief valves.
Work port RVs on hoes are used as shock-reliefs in certain valves to limit forces in hi-momentum situations (usually
the boom).

To your earlier point - my question is: What is "correctly"?

I can see how you could put a cylinder behind the thumb and not have to use a relief on the valve that controls that thumb - if that cylinder was large enough to hold pressure agains the force of the bucket curling against it. The valve stack for the hoe already has pressure relief - and when digging you will often run into situations where the bucket curl is stopped dead trying to dig an object out of the ground.

So is that the "correct" way to size out a cylinder - just make sure it's big enough to not be overcome by the force of the bucket curl cylinder?

 

[CDN Farm Boy]

So is that the "correct" way to size out a cylinder - just make sure it's big enough to not be overcome by the force of the bucket curl cylinder?

I'm guessing a whole bunch of complicated calculations depending on the geometry of the thumb & cyl & attaching points, also factoring in what angle the thumb/cyl is at while the bucket is pushing against it.

I'm planning on running a 2x8 cyl with 1.25" rod with 1" mounting pins. Mounting lengths of it fit quite well with the travel of the Kubota thumb

 

[wdchyd]

Jim,

the thumb work port relief can be set higher than the hoe relief....it's the induced load you need to worry about (from the larger cyl pushing against), not what the hoe PRV is set at

the work port relief can be set at the max of what that cyl can handle (it'll back off before it bends the rod)