BH77 - has anyone considered converting mechanical thumb to hydraulic?

[dfkrug]

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

WDCHYD has it right: it is the induced loads caused by the cylinder that is extending (curl cyl or thumb
cyl) that could get you into trouble.

To avoid the need to add a work-port RV in the big side of either cylinder, you should:

> use a thumb cyl of the same diameter as the one on the bkt
> calculate the worst-case (greatest) force that would push on the thumb or bucket. This
occurs at the point of maximum leverage at the hoe's main RV pressure setting. What is the
maximum leverage? Where the force-action point is farthest from the moment arm pivot. So-
called mechanical advantage can result in internal pressures inside the stationary cyl that
could exceed the hoe's RV pressure.
> therefore, use a decent cylinder that is rated for at least 3000psi, not some crappy
tierod cyl, which often are limited to 2500psi. There is not a lot of math here; most of it is common
sense.
> finally, use rods and pins of at least 1" diameter to resist these forces. In my first mechanical
thumbs, I used only a 3/4" pins for the upper dipper connection. I bent them. This hoe, and
others in its class uses bkt cyl rods that are 1", or sometimes 1.25". That should tell you something.
> oh, yeah, don't forget to do a very accurate job welding your cross-tubes to the new thumb
cyl. Even small mis-alignments will increase the chance of buckling rods at full extension under
max loads. This assumes you want a thumb cylinder of just the right length for your application,
rather than compromising with an off-the-shelf size.

 

[jim_wilson]

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)

Ok - I think that is what I was thinking too. If I had a cylinder on the grapple that was smaller (less force ) than the cylinder on the bucket curl - then there could potentially be a problem because the bucket curl cylinder could overcome the grapple cylinder. (unless I assume you had the PRV on the grapple valve set at a pressure that would let the grapple cylinder release when it got back pressure from the bucket curl).

So - if the grapple cylinder was either (bore) sized in such a way that the bucket curl cylinder could not overcome it - or it was running a higher pressure (more force from the cylinder in either case than the bucket curl cylinder) - then should be no problem with the curl cylinder "breaking" the grapple cylinder. ??

Do I have that right?

I took some measurements of the grapple on my BH77 last night - and it looks like a cylinder at least as large as the curl cylinder - and maybe larger (in bore diameter) - can be installed on the grapple.

Is there some rule of thumb on how to figure out what the bore of a cylinder is from it's outside diameter? I'd like to see if I can figure out what the existing curly cylinder is so I can get an appropriately sized cylinder for the grapple.

 

[jim_wilson]

WDCHYD has it right: it is the induced loads caused by the cylinder that is extending (curl cyl or thumb
cyl) that could get you into trouble.

To avoid the need to add a work-port RV in the big side of either cylinder, you should:

> use a thumb cyl of the same diameter as the one on the bkt
> calculate the worst-case (greatest) force that would push on the thumb or bucket. This
occurs at the point of maximum leverage at the hoe's main RV pressure setting. What is the
maximum leverage? Where the force-action point is farthest from the moment arm pivot. So-
called mechanical advantage can result in internal pressures inside the stationary cyl that
could exceed the hoe's RV pressure.
> therefore, use a decent cylinder that is rated for at least 3000psi, not some crappy
tierod cyl, which often are limited to 2500psi. There is not a lot of math here; most of it is common
sense.
> finally, use rods and pins of at least 1" diameter to resist these forces. In my first mechanical
thumbs, I used only a 3/4" pins for the upper dipper connection. I bent them. This hoe, and
others in its class uses bkt cyl rods that are 1", or sometimes 1.25". That you tell you something.
> oh, yeah, don't forget to do a very accurate job welding your cross-tubes to the new thumb
cyl. Even small mis-alignments will increase the chance of buckling rods at full extension under
max loads. This assumes you want a thumb cylinder of just the right length for your application,
rather than compromising with an off-the-shelf size.

Thanks - very helpful response.

I've done some measurements of the existing manual thumb that is on the tractor now - and there is enough space in there to install a cylinder with the same outside diameter as the existing cylinder that does the bucket curl - I could get a cylinder with even a slightly larger OD to fit in there for the grapple cylinder I believe if that was a better option.

Is a grapple cylinder with a larger bore than the bucket curl cylinder a good - idea - to prevent any potential damage from the bucket curl forcing up against the grapple?

I'll have to see if I can figure out that force you referred to. I did notice something about the bucket/grapple interaction when I was look looking at the setup yesterday. With the grapple fully extended and locked in - and the bucket fully curled up against the grapple arm- the tip of the grapple and the bucket teeth do not touch - they are prevented from touching because the grapple and bucket come into contact much further "up" - if you're looking at the bucket from the open end - there is round cross tube across the top - near where the mounting arms on the bucket are. That tube actually contacts the grapple itself and that is where the bucket and grapple bottom out on each other - absent some thing being held in between the bucket and grapple that is.

All of the pins on the backhoe are 1". Both ends of the existing grapple are 1" diameter - so it appears I am good to go there. I have looked at a B26 before - and some of the pins on the hoe on the B26 were 1.25" diameter - if I remember correctly it was the bucket mount pin at the end of the dipperstick- all the rest were 1".

The rod diameter on the bucket curl cylinder is like 1 3/4" diameter if I remember correctly ( I wrote it all down yesterday but I don't have the paper in front of me right now).

I was planning on using a welded cylinder - not any of the tie rod cylinders. I've got a contact at the regional Parker supply house so I was going to call them and see what they do for custom cylinders - and I found a couple of places on the internet that advertise custom cylinders - and I have all the dimensions - so I should be able to fill out their spec sheets for what I want - get a cylinder that will fit exactly - and go from there.

I've looked at a bunch of the off-the-shelf cylinders - and I can't find anything that will really fit properly without some sort of modification - or compromise.

As an example - the fully retracted length is like 19 1/4 " if I remember correctly - and the fully extended length is like 28 1/2" - so to really make this work properly - with the grapple fully retracting enough to get it up against the dipperstick as far as it will go - and get it to fully extend out as far as it will go - it looks like you need a cylinder with a stroke if like 9 1/4 or so. I'm going by the locations that I can lock the grapple into using the existing manual arm.

 

[dfkrug]

Is a grapple cylinder with a larger bore than the bucket curl cylinder a good - idea - to
prevent any potential damage from the bucket curl forcing up against the grapple?

Then what happens when the thumb cyl pushes on the stationary curl cyl? You don't want to overpower
the curl cyl either. Same size for both is simplest approach. It would be close enought to
use same size ODs if you don't know the IDs. The range of wall thicknesses for 3000psi 2-in cyls are
usually betw 0.2-0.3", from what I have seen.

Remember if you make any calcs: maximum applied force about a pivot point (torque) is F x the
perpendiular distance to the pivot. The pivot point transfers this applied force to another
torque applied at a different point, at a different distance from the pivot. You might have to do
a little trigonometry if the max force applied to the load at this different point is not at a
convenient max perpendicular distance from the pivot point. Easier to see in drawings, for sure.

 

[dfkrug]

As an example - the fully retracted length is like 19 1/4 " if I remember correctly - and the fully extended length is like 28 1/2" - so to really make this work properly - with the grapple fully retracting enough to get it up against the dipperstick as far as it will go - and get it to fully extend out as far as it will go - it looks like you need a cylinder with a stroke if like 9 1/4 or so. I'm going by the locations that I can lock the grapple into using the existing manual arm.

Now that you are getting down to the nitty-gritty, you can see that the open-and-closed lengths of your new
thumb cyl is very important. No standard length cyl is going to be ideal. You can play with the pivot
point location on the thumb, and the dipper mounting point, but you must retract the thumb fully up
against the dipper. Hence, a custom-length cyl is the best way to go. This can be as simple as lopping off
the rod-end cross-tube and lenghtening/shortening it. Definitely do a mock-up first.

 

[jim_wilson]

Then what happens when the thumb cyl pushes on the stationary curl cyl? You don't want to overpower
the curl cyl either. Same size for both is simplest approach. It would be close enought to
use same size ODs if you don't know the IDs. The range of wall thicknesses for 3000psi 2-in cyls are
usually betw 0.2-0.3", from what I have seen.

Remember if you make any calcs: maximum applied force about a pivot point (torque) is F x the
perpendiular distance to the pivot. The pivot point transfers this applied force to another
torque applied at a different point, at a different distance from the pivot. You might have to do
a little trigonometry if the max force applied to the load at this different point is not at a
convenient max perpendicular distance from the pivot point. Easier to see in drawings, for sure.

How is having the thumb push against the bucket (against the curl cylinder) - going to be any different than using the bucket to dig - against potentially immoveable objects? Wouldn't the pressure relief on the bucket curl valve take care any back pressure against the bucket from the stronger grapple?

I understand the point of not needing any additional pressure relief on the grapple valve - IF the cylinder and so forth are configured properly - but I must be missing something because I'm thinking that the bucket curl overpowering issue is - or should be - already handled in the way the valve stack for the hoe is already setup to handle digging forces.

 

[jim_wilson]

Now that you are getting down to the nitty-gritty, you can see that the open-and-closed lengths of your new
thumb cyl is very important. No standard length cyl is going to be ideal. You can play with the pivot
point location on the thumb, and the dipper mounting point, but you must retract the thumb fully up
against the dipper. Hence, a custom-length cyl is the best way to go. This can be as simple as lopping off
the rod-end cross-tube and lenghtening/shortening it. Definitely do a mock-up first.

The way I'm looking at this is - I'm not looking to change any of the mounting locations either for the upper end of the cylinder - or on the thumb itself. I would like to make this work without any metal work or modification on the existing thumb. What I did to measure is put the thumb in it's fully retraced position (storage position) - and measured the distance between the centers of the upper and lower mounting pins for the thumb positioning arm. Then I put the thumb into it's fully extended position (last hole available on the manual positioning arm) and measured the distance between the centers of those pins again.

So - the only way I can see to make this work is to get a custom cylinder made. Not a big deal - I just have to figure out what I need. And you're right - the open and closed lengths of the cylinder are important because they will determine where the thumb gets positioned.

I also measured the clearance around that positioning arm to see what diameter cylinder could fit (with clearance) - with the thumb at all positions. The current setup (with the manual positioning arm) - appears to only let the thumb extend out to about a 90 degree angle to the bottom of the dipperstick. I've to take another look at this - because I was watching some Youtube videos this afternoon of hydraulic thumbs on excavators and backhoes - and it appears that most of the thumbs I saw will go past that 90 degree point. I'll have to take a closer look at this to see if this thumb will do that. And take that into account when putting down the cylinder info.

Once I think I have everything down - I'll have to see what I can think of as a way to mock-in a cylinder. Maybe some PVC pipe or something like that.

 

[dfkrug]

How is having the thumb push against the bucket (against the curl cylinder) - going to be any different than using the bucket to dig - against potentially immoveable objects? Wouldn't the pressure relief on the bucket curl valve take care any back pressure against the bucket from the stronger grapple?

Do you know for a fact that you have a work-port RV on the bkt cylinder valve? If so, they are
usually set to something greater than the hoe's main RV. This would protect against forces
from a too-strong thumb, yes.

Of course, that is different than curling against an immovable object; that effort would
open the hoe's main RV, not the shock-RV if you have one.

Yes, you should design your thumb to close more than 90-deg to the dipper.

 

[jim_wilson]

Do you know for a fact that you have a work-port RV on the bkt cylinder valve? If so, they are
usually set to something greater than the hoe's main RV. This would protect against forces
from a too-strong thumb, yes.

Of course, that is different than curling against an immovable object; that effort would
open the hoe's main RV, not the shock-RV if you have one.

Yes, you should design your thumb to close more than 90-deg to the dipper.

Good point - I don't know that for a fact. I also just realized I don't have the workshop manual for the BH77 - so I'm going to get a copy of that.

Which doesn't appear to be that easy - since I can't seem to find one listed anywhere that just pops up.

 

[CDN Farm Boy]

I've got the BH77 WSM in pdf that I can email to you

Is that easy enough for you??