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.

 

[Dougster]

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. 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.

Okay! Glad we are back on the same page.

Now... where the heck do I find this silly cylinder???

Dougster

 

[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.

My point exactly my megabucks rich, left coast dwelling, 7520 driving, land baron friend! Just because I am dirt poor and living in Taxachusetts doesn't mean I want to make my 509's new fixed thumb into a useless toy! The day you buy a 2015 is the day you can tell me to back off on the performance capabilities of my beloved red (even though not a 7520) machine!

New Holland 100 HP... heh? You must really LOVE the color blue!!!

Dougster

 

[MtnViewRanch]

My point exactly my megabucks rich, left coast dwelling, 7520 driving, land baron friend! Just because I am dirt poor and living in Taxachusetts doesn't mean I want to make my 509's new fixed thumb into a useless toy! The day you buy a 2015 is the day you can tell me to back off on the performance capabilities of my beloved red (even though not a 7520) machine!

New Holland 100 HP... heh? You must really LOVE the color blue!!!

Dougster

Hey, what year is that Chevy truck that you drive, me I'm running around in a 1988 F250 with 276,000 miles and the ac quit working and it's in the 90's out here.

Anyway, I still don't understand why you need that much pressure on the thumb. Are you trying to crush stuff? I thought that a thumb was to hold stuff to be able to get it out of the way.(which I could use digging out the 100's of scrub oaks that need to be removed) What am I missing out on? Could I be in need of one of these for my 580? It's already plumbed for aux hydraulics.

 

[Dougster]

Hey, what year is that Chevy truck that you drive, me I'm running around in a 1988 F250 with 276,000 miles and the ac quit working and it's in the 90's out here. Anyway, I still don't understand why you need that much pressure on the thumb. Are you trying to crush stuff? I thought that a thumb was to hold stuff to be able to get it out of the way.(which I could use digging out the 100's of scrub oaks that need to be removed) What am I missing out on? Could I be in need of one of these for my 580? It's already plumbed for aux hydraulics.

You know that I am so close to broke I can taste it. One can of soup a day, one tea bag and one or two stale slices of bread. You could never live like this... but anytime you feel like exchanging equipment, bank accounts and real estate... I am soooooooooo ready!

All I can say is THUMBS ROCK!!! And hydraulic thumbs are the cream of the crop... provided, of course, you aren't giving up too much performance. And performance for a thumb is lifting and gripping power!!!

Of course you should have a thumb on your 580. I'm surprised that you don't have one already. Sell that new Hummer and get thee a thumb!!!

Dougster

 

[MtnViewRanch]

You know that I am so close to broke I can taste it. One can of soup a day, one tea bag and one or two stale slices of bread. You could never live like this... but anytime you feel like exchanging equipment, bank accounts and real estate... I am soooooooooo ready!

All I can say is THUMBS ROCK!!! And hydraulic thumbs are the cream of the crop... provided, of course, you aren't giving up too much performance. And performance for a thumb is lifting and gripping power!!!

Of course you should have a thumb on your 580. I'm surprised that you don't have one already. Sell that new Hummer and get thee a thumb!!!

Dougster

A Hummer, no Hummer here, I wish that I could afford one, not that I want one, just wish that I could afford one.

Back to the thumb, I guess that I'm worn out today or something, what good would it do if you had a thumb that put a kabillion lbs of pressure on that tree trunk that is stuck in the ground. The hoe is not going to be able to lift it. It may not let go, but it still won't lift it. If you want to be able to hold something with 5000lbs pressure that requires 5000lbs of pressure to hold it, will your hoe still move it, or is that something now to heavy for you to be able to move? Does any of this make any sense?

 

[Dougster]

Back to the thumb, I guess that I'm worn out today or something, what good would it do if you had a thumb that put a kabillion lbs of pressure on that tree trunk that is stuck in the ground. The hoe is not going to be able to lift it. It may not let go, but it still won't lift it. If you want to be able to hold something with 5000lbs pressure that requires 5000lbs of pressure to hold it, will your hoe still move it, or is that something now to heavy for you to be able to move? Does any of this make any sense?

Ahhh... no. Not to me anyway. The thumb is there to grip things... for whatever reason you want... it grips things. Sometimes it crushes and splits things apart, but mostly it grips. And the harder it can grip those funky things you want to grip, the better. Whether pulling trees or moving rocks or picking up construction debris or just scaring neighborhood kids away... more gripping power is simply better. It does not increase or decrease what you can lift. It just grips. Why would I want less gripping force than I've already got with the fixed arrangement (other than for the added convenience)? Makes no sense.

That's the best I can explain it at this time of night.

Dougster

 

[MadDogDriver]

Dougster,

If I have my thumb extended and close my bucket I can push the thumb back, likewise if I extend the thumb into the bucket I can push the bucket back........and if I want to try to push the noodle with both simutaneously these just turn that mean ol noodle into spaghetti without either side gaining ground. I surmise that's what you're trying to emulate.

 

[Dougster]

Dougster, If I have my thumb extended and close my bucket I can push the thumb back, likewise if I extend the thumb into the bucket I can push the bucket back........and if I want to try to push the noodle with both simutaneously these just turn that mean ol noodle into spaghetti without either side gaining ground. I surmise that's what you're trying to emulate.

Good photo... very similar in geometry to the Bradco!

I'm just trying to convert my fixed thumb to a hydraulic so that a) It will rattle less, b) No getting off the seat to set it up, and c) because hydraulic thumbs are the cat's meow!

I just don't want to lose excessive gripping force while doing so. Simple as that.

Dougster

 

[MtnViewRanch]

Ahhh... no. Not to me anyway. The thumb is there to grip things... for whatever reason you want... it grips things. Sometimes it crushes and splits things apart, but mostly it grips. And the harder it can grip those funky things you want to grip, the better. Whether pulling trees or moving rocks or picking up construction debris or just scaring neighborhood kids away... more gripping power is simply better. It does not increase or decrease what you can lift. It just grips. Why would I want less gripping force than I've already got with the fixed arrangement (other than for the added convenience)? Makes no sense.

That's the best I can explain it at this time of night.

Dougster

I understand that you want to be able to hold things tight, I guess that what I don't understand is why hold something with 5000lbs of force that only needs to be held with 2000lbs of force. Your hoe isn't going to move a 2000lb object is it? So what is the purpose of getting that 5000lb clamping force? I guess my point might be, why not use the 2 1/2" cylinder with the 4000lb gripping power and get the convenience of the hydraulic thumb. That in itself most likely exceeds the capacity of your hoe.

I'm tired and may not be thinking clearly, so I will go away and be quite now.

 

[dfkrug]

Dougster,

If I have my thumb extended and close my bucket I can push the thumb back, likewise if I extend the thumb into the bucket I can push the bucket back........

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.

Based on what I think Dougster is trying to do, I think he should
get a 2.75" cyl so he can maximize forces without needing a relief
valve.