Sizing a Backhoe Thumb Cylinder

[Dougster]

Dougster: Try Item# 9-7336 from Surplus Center. $99.95 + shipping. You will have to shorten it, but that is not bad. You can do the
cutting and take it to a welder and maybe get away for $100 for the welds. SC has other choices, too. 2.75" bore will limit your choices somewhat. I never find exactly the size of cyl I want. I either design around what I buy or modify it to suit.

The price is certainly right, but I was hoping to get a little closer than that. Are base ends easy to change? I know they are on some... but on all? No problem on the rod end.

If it means going to a 3" cylinder, I've have to recheck clearances very, very carefully and get a relief valve for sure. On the other hand, 2.5" just doesn't have enough force. It would be a joke.

Anyway, thanks very much for checking!

Dougster

 

[dfkrug]

The price is certainly right, but I was hoping to get a little closer than that. Are base ends easy to change? I know they are on some... but on all? No problem on the rod end.

If it means going to a 3" cylinder, I've have to recheck clearances very, very carefully and get a relief valve for sure. On the other hand, 2.5" just doesn't have enough force. It would be a joke.

Anyway, thanks very much for checking!

Dougster

12,300lb is not enough force? That's at the rod end for 2500psi press.

Tie-rod cyls are very easy to change....no welding of the cyl or
port bosses.

Welded cyl sshould generally be similar. The photo I posted shows the
cut to reduce length by 2". I needed different ends anyway.

 

[Dougster]

12,300lb is not enough force? That's at the rod end for 2500psi press.

Sounds like a lot... until you divide by ~3.1-3.2 (thumb leverage ratio in hydraulic configuration) and see how that stacks up against a 5,700 lb bucket force. You've just reduced my thumb pinching force from 5,700 lbs (fixed thumb configuration = full bucket force) to under 4,000 lbs. :-( In reality, something less for both of these numbers since my tractor system runs at 2,400 PSIG max.

Tie-rod cyls are very easy to change....no welding of the cyl or port bosses. Welded cyl sshould generally be similar. The photo I posted shows the cut to reduce length by 2". I needed different ends anyway.

Tie-rod cylinders are fine for many applications... maybe even most applications... but not for a serious backhoe thumb. Far too easily caught and/or damaged. This backhoe was bought to do some very serious paying work... can't afford for this to fail on a paying job.

Dougster

 

[MtnViewRanch]

Sounds like a lot... until you divide by ~3.1-3.2 (thumb leverage ratio in hydraulic configuration) and see how that stacks up against a 5,700 lb bucket force. You've just reduced my thumb pinching force from 5,700 lbs (fixed thumb configuration = full bucket force) to under 4,000 lbs. :-( In reality, something less for both of these numbers since my tractor system runs at 2,400 PSIG max.
Dougster

I guess that I'm missing something. 4000lbs isn't enough clamping pressure for your size machine? Just what is it that you need to hold on to that tight?

 

[dfkrug]

Tie-rod cylinders are fine for many applications... maybe even most applications... but not for a serious backhoe thumb. Far too easily caught and/or damaged. This backhoe was bought to do some very serious paying work... can't afford for this to fail on a paying job.

I would not use them in a thumb app either. I have bought and used
several tie rod cyls in the past, but I do not see buying any more if I can find
welded cyls for about the same price. The cheap tie rod cyls I bought
were only rated 2500 psi, too. All of the welded ones, 3000.

 

[Dougster]

I guess that I'm missing something. 4000lbs isn't enough clamping pressure for your size machine? Just what is it that you need to hold on to that tight?

Hi Brian - I think you're missing the point. The fixed thumb is capable of sustaining full bucket force... 5,700 lbs of pinching force at 2,500 PSIG. Enough to pull and crunch up small trees and hold stumps, heavy stones and other objects very securely. Bradco uses a 2.75" cylinder for its hydraulic option, although this reduces pinching force by 15-20% from the fixed arrangement, in part due to a change in geometry (brace point comes way in from tip of thumb... attachment point on stick goes back to allow longer cylinder/longer stroke). All things being equal, why wouldn't I want to duplicate the Bradco design seeing as how it appears to be the biggest, strongest diameter cylinder I can fit... with the least derating effect?

Believe me... I'm not putting on this hydraulic cylinder to end up with a worthless toy here. I want the smallest derate possible or I will stick with the fixed thumb arrangement and have no derate at all.

Look at it this way: Why did you buy a 7520 when maybe all you really needed was a 2015??? Gotcha!!!

Dougster

 

[dfkrug]

Dougster, I see nothing wrong with getting the widest cyl you can, and
therefore maximizing the force your thumb design will allow. Don't
forget that since you are using 2 separate hyd circuits for the thumb
and bkt, either element will be able to put its full force to bear on
the object at hand. If the net force (due to geometry) delivered by
the thumb is say 4000 lb, and the net force delivered by the bkt is
say 5000 lb, the max force you can pinch an object with will be 5000
lb. That is due to the bkt acting against the thumb, which will have a
valve with load checks. I am sure you are aware of that.

In my design where the bkt and thumb will be on the same ckt, I need to
make the thumb and bkt deliver similar forces because the lower force
will be the resultant. Unless or until the thumb is fully retracted to its
stop. More like a grapple.

 

[Dougster]

Dougster, I see nothing wrong with getting the widest cyl you can, and therefore maximizing the force your thumb design will allow. Don't forget that since you are using 2 separate hyd circuits for the thumb
and bkt, either element will be able to put its full force to bear on the object at hand. If the net force (due to geometry) delivered by the thumb is say 4000 lb, and the net force delivered by the bkt is say 5000 lb, the max force you can pinch an object with will be 5000 lb. That is due to the bkt acting against the thumb, which will have a valve with load checks. I am sure you are aware of that.

I believe we are finally getting back to the heart of my original post. But the fact is that I don't completely see what you are saying. Always remember that I am old and fuzzy!

Let's take that case of that proposed alternative 2.5" diameter cylinder. Again, at 2,500 PSIG, the bucket can impose a force of ~5,700 lbs. The 2.5" cylinder can impose a force of ~4,000 lbs. I believe you are saying that I could position the thumb first... let's say against on a log... and then use the bucket circuit to apply a force of 5,700 lbs to the log resting against the thumb?

To me... assuming the thumb won't overstress and give out mechanically at anything over 80% of 5,700 lbs plus 1 pound (per Bradco's hydraulic thumb kit spec)... we are now feeding back about 18,000 lbs of force to the 2.5" cylinder and cylinder circuit. That equates to a 2.5" cylinder circuit pressure of a little over 3,700 PSIG... well over the system's 3,000 PSIG design pressure.

In actual fact, I would have installed relief at some lower pressure... but all that would do is allow the thumb cylinder to back off instead of blowing it (or hoses, fittings, etc.) apart.

Is my logic wrong? Seems to me that the weaker link would always set the maximum usable pinching force.

Dougster

 

[dfkrug]

Let's take that case of that proposed alternative 2.5" diameter cylinder. Again, at 2,500 PSIG, the bucket can impose a force of ~5,700 lbs. The 2.5" cylinder can impose a force of ~4,000 lbs. I believe you are saying that I could position the thumb first... let's say against on a log... and then use the bucket circuit to apply a force of 5,700 lbs to the log resting against the thumb?

OK, I am with you here. Regardless of whether you close the bkt against
the object touching the thumb or close the thumb against the object
touching the bkt, the max force will be what the bkt circuit can deliver
since it is greater than the thumb's force. The actual force will be
that delivered from the last circuit operated; 4000 if from the thumb,
5700 if from the bkt.

To me... assuming the thumb won't overstress and give out mechanically at anything over 80% of 5,700 lbs plus 1 pound (per Bradco's hydraulic thumb kit spec)... we are now feeding back about 18,000 lbs of force to the 2.5" cylinder and cylinder circuit. That equates to a 2.5" cylinder circuit pressure of a little over 3,700 PSIG... well over the system's 3,000 PSIG design pressure.

Yes, the max resultant force on the cyl should be kept to 3000 if that is
the spec for the cyl you buy. If you are getting that amount of force
from the bkt, you should just do what you planned, go for a bigger
ID (2.75). I am with you.

 

[MtnViewRanch]

Look at it this way: Why did you buy a 7520 when maybe all you really needed was a 2015??? Gotcha!!!

Dougster

TIME. I suppose that I could have just used my 16hp Sears garden tractor. Heck, I have a MMM, a 36" disk, 36" rear blade, even a 36" rollover Gannon for it. I guess that I could get stuff done on my 80 acres and my brothers 40 next to mine in no time at all. Shoot, might only take my lifetime. Just think of all that seat time.I actually wanted a bigger New Holland,(100hp) but could not afford it.