Sizing a Backhoe Thumb Cylinder

[Dougster]

I've got a Bradco 509 BH with a fixed thumb that I want to convert to hydraulic. Buying the fully integrated Bradco conversion kit is out of the question due to cost and complexity. I've added a second rear remote set for that purpose and for other needs... but not being integrated, there is some question about how to size and select the new thumb cylinder for: a) correct applied force and b) maximum physical strength and prevention of system damage or breakage.

Because the Bradco system is fully integrated into the BH hydraulics subsystem, proper design and controls prevent the hydraulic thumb hydraulics from overpowering and damaging the bucket hydraulics and vice versa. But with a completely separate control valve running the hydraulic thumb, this leaves some questions that even Bradco could not (or would not) answer about how to properly size and select the new cylinder.

Any thoughts on how you might approach this question are most appreciated.

Dougster

 

[RonMar]

Bunch of questions:

1. System max operating pressure(for available energy)?
2. Size of existing bucket curl cylinder(for available applied cylinder force)?
3. Distance from bucket pivot to cylinder link attachment(for mechanical
leverage advantage to the bucket?
4. Distance from bucket pivot to bucket lip/rippers(for mechanical leverage
loss from the bucket)?
5. Size of dipper tube, dimension and wall thickness(for determining dipper
strength)?

By multiplying the piston area (Radius squared X PI) by the system max pressure, you can determine the force applied by the ram.

Determining the mechanical loss in the bucket(distance from rippers to pivot divided by distance from bucket pivot to cylinder link attachment) and divide the applied force from the ram by this number to determine the force applied to the ripper edge of the bucket.

You can do the same thing with the thumb arm, figure the mechanical loss based on attachment point of the hydraulic ram then size the thumb cylinder diameter so the thumb arm can't apply more force out where it meets the bucket lip than the bucket can apply.

The dipper and structure dimensions are just to get an idea of how strong the structure is. The dipper structure is mostly in tension(strongest mode) when the bucket is curling, especially if it is pushing against/pushed by a thumb.

 

[Farmerford]

Dougster:

As an ME you are probably way ahead of me on this, but here are my thoughts.

I presume Bradco sized the bucket curl cylinder and set the system relief valve so that when the bucket is deadheaded against the thumb (or against something solid between it and the thumb) the force on the bucket does not exceed some design limit. If that is true, then don't you have to just make sure the movable thumb can't exceed that force on the bucket?

Can't that limit be achieved hydraulically by plumbing a relief valve in the curl cylinder base circuit (the one that closes the bucket) that dumps overpressure into the curl cylinder rod circuit through a check valve and (since the base circuit will produce more fluid than the rod circuit can accept when the piston moves) and also dumps into the reservoir? When the relief valve setting is reached, fluid will flow out of the base end of the curl cylinder to two places: the rod end of the cylinder through the check valve to the extent needed to fill the rod cavity, and balance to the tank.

But isn't the mechanical solution just to calculate the maximum force on the bucket cutting edge when it contacts the fixed thumb (or perhaps when the cylinder rod, measured on the perpendicular, is furthest from the bucket pivot, which will be the point of maximum torque) and then size the thumb cylinder (using the system relief setting that applies to the source of fluid for the thumb)to limit the force produced by the thumb on the bucket to the same amount or perhaps a bit less?

 

[Dougster]

Bunch of questions:

1. System max operating pressure(for available energy)?
2. Size of existing bucket curl cylinder(for available applied cylinder force)?
3. Distance from bucket pivot to cylinder link attachment(for mechanical
leverage advantage to the bucket?
4. Distance from bucket pivot to bucket lip/rippers(for mechanical leverage
loss from the bucket)?
5. Size of dipper tube, dimension and wall thickness(for determining dipper
strength)?

By multiplying the piston area (Radius squared X PI) by the system max pressure, you can determine the force applied by the ram.

Determining the mechanical loss in the bucket(distance from rippers to pivot divided by distance from bucket pivot to cylinder link attachment) and divide the applied force from the ram by this number to determine the force applied to the ripper edge of the bucket.

You can do the same thing with the thumb arm, figure the mechanical loss based on attachment point of the hydraulic ram then size the thumb cylinder diameter so the thumb arm can't apply more force out where it meets the bucket lip than the bucket can apply.

The dipper and structure dimensions are just to get an idea of how strong the structure is. The dipper structure is mostly in tension(strongest mode) when the bucket is curling, especially if it is pushing against/pushed by a thumb.

Thanks Ron... Excellent thought process. You have done this before... yes? Just a couple questions:

1) I believe you are recommended that I match the force that can be applied at the worst case joining point, assumed to be the bucket lip for now... correct? One thing that bothers me about this is that Bradco allows more force to be applied to/by the fixed thumb than to/by the hydraulic thumb... 80% to be exact... hydraulic vs. fixed. The reason for this is not entirely clear. It could be strength of components or it could simply be the smaller diameter cylinder they put on the hydraulic. One thing for sure: The thumb geometry is considerably different on the hydraulic vs. the fixed arrangement. To be safe, should I use the same 80% criteria as Bradco? Or does that risk physical damage to the thumb hydraulic components in this separated (i.e., non-integrated) control scheme?

2) I understand the importance and implications of cylinder diameter (ID)... but what about cylinder design pressure rating and piston rod size? Any thoughts there? My tractor hydraulic system operates at ~2,000 PSIG (relief crack setpoint) to ~2,300 PSIG (full relief valve setpoint)... both values approximate and unverified. Published design pressure of the Bradco subsystem is 2,500 PSIG. Would you buy a higher rated thumb cylinder given the accidental risk of overpressurization by excessive bucket force against a closed thumb subsystem? Or should I keep it the same and take the hit there lest I inadvertantly damage other, more costly components?

Or is the real answer some sort of supplementary relief system?

Dougster

 

[Dougster]

Dougster: As an ME you are probably way ahead of me on this, but here are my thoughts.

I am never too proud or overconfident NOT to ask the fine, experienced people here to confirm, assist or supplement my own thought process. That sort of thinking comes with age.

I presume Bradco sized the bucket curl cylinder and set the system relief valve so that when the bucket is deadheaded against the thumb (or against something solid between it and the thumb) the force on the bucket does not exceed some design limit. If that is true, then don't you have to just make sure the movable thumb can't exceed that force on the bucket?

That much is clear. There is no way we want the thumb cylinder to be able to place more force on the bucket lip than the bucket cylinder.

Can't that limit be achieved hydraulically by plumbing a relief valve in the curl cylinder base circuit (the one that closes the bucket) that dumps overpressure into the curl cylinder rod circuit through a check valve and (since the base circuit will produce more fluid than the rod circuit can accept when the piston moves) and also dumps into the reservoir? When the relief valve setting is reached, fluid will flow out of the base end of the curl cylinder to two places: the rod end of the cylinder through the check valve to the extent needed to fill the rod cavity, and balance to the tank.

Unless necessary (and maybe it is), I would prefer not to add any further relief capability into the bucket circuit. From my discussions with Bradco... and the apparent need for me to keep the thumb-imposed force (at lip) below the bucket-imposed force... I think I am covered.

But isn't the mechanical solution just to calculate the maximum force on the bucket cutting edge when it contacts the fixed thumb (or perhaps when the cylinder rod, measured on the perpendicular, is furthest from the bucket pivot, which will be the point of maximum torque) and then size the thumb cylinder (using the system relief setting that applies to the source of fluid for the thumb)to limit the force produced by the thumb on the bucket to the same amount or perhaps a bit less?

Exactly correct. Great minds think alike. The basic design criteria in that regard is fairly clear... but the "trick" of it (for lack of a word that can't be published) is all in the component selection (and added relief?) details.

Dougster

 

[RonMar]

. Would you buy a higher rated thumb cylinder given the accidental risk of overpressurization by excessive bucket force against a closed thumb subsystem? Or should I keep it the same and take the hit there lest I inadvertantly damage other, more costly components?

Or is the real answer some sort of supplementary relief system?

Dougster

That is a very good question. My thought process hadn't rolled far enough along to include that scenario. For the sake of remaining simple, I would avoid any additional relief system if possible.

The force applied to a closed thumb system would come from the bucket system which is limited by the system relief pressure setting. A fixed thumb can withstand the bucket curl force and stops the bucket travel and opens the relief right? You will need a heavy enough hydraulic thumb structure and cylinder to withstand the potential closed circuit pressure from the force applied by the bucket without exceeding safe rated component pressure in that closed circuit. And vis versa for thumb pushing against closed bucket circuit.

I guess it would be best to match the thumbs maximum applied force to that of the buckets maximum applied force. That way the closed system on either side should be able to withstand the force/pressure applied by the driven system without exceeding pressure limits.

 

[dfkrug]

I am just finishing up my 3rd Generation backhoe thumb, which
will be an hydraulic version of my fixed or articulated thumb. Some
comments come to mind as I read this thread:

> don't get into an "analysis paralysis" situation when doing your
design. It is not that complicated.

> the forces exerted by the thumb cylinder will vary as the thumb goes
thru its arc. Your thumb needs to be strong enough but also
retract far enough to get out of the way, while not hitting the
dipper. These are conflicting parameters.

> I like welded cyls for this application and Surplus Center had some
3000psi 2x8 cyls for sale for only $45. I bought several. This is a
slightly larger ID than my curl cyl, but the bkt curl will have more leverage
thru most of its arc. I do not know the max pressure specs for my
curl cyl. It is prob not stated for the Bradco either. I doubt they are
the cheapo 2500psi cyls since the Bradco and my Prairie Dogs handle
2500psi wp by design. What is the 509's curl cyl OD?

> if you are concerned about overpressure damage to your thumb
cyl, consider using a shock relief valve on the retract side of the thumb
valve you will be installing. I don't think it is necessary myself.

> In my design, I am using a poppet style solenoid valve connected to
the curl cyl ckt so the thumb cyl operates in parallel with the curl cyl.
One advantage that offers is both jaws close at the same time.

 

[MadDogDriver]

Dougster,

I realize that your Bradco 509 isn't the same as my Woods BH90-X, but for reference, my hyd thumb cylinder dimensions are 2.5 x 1.5 x 16.75.

Also, you can refer to Woods' hyd thumb installation manual to get an idea of how they've plumbed it into the BH, http://www.woodsequipment.com/Manuals/manuals/SubFrames/MAN0540.pdf.

Hope this may help give you an idea of how they've done it.

 

[Farmerford]

dfkrug:

That's an interesting twist to use poppet valves to put the thumb and bucket cylinders in parallel. I have wanted to try that with the backhoe on my John Deere 455D, but I was hesitant to spend the time (and money) modifying the existing setup without knowing how it would work. Are the cylinders on the bucket and thumb about the same size? How do the bucket and thumb respond? That is, do both move until one strikes the target and then the other one closes up to complete the grasp? When you release do both open at about the same speed or does the thumb, being lighter, move first? If you want to retract the thumb fully, I assume you open the valves and retract all the way; then close the valves to hold the thumb in the retracted position? Finally, if you extend the stick and boom so that the "claw" is horizontal (or close), do the bucket and thumb swing down with the weight of the load? Or do you close the poppet valves to prevent that?

I agree with our advice to Dougster to avoid overanalyzing the deal. The only place I might disagree is regarding the relief valve in the thumb circuit: it would be easy (since you are plumbing the thumb anyway) to put an adjustable relief valve across the thumb work ports. That way you could oversize the thumb cylinder a bit and then control thumb pressure with the relief valve rather than relying on precise calculations for the moment arms, etc.

 

[dfkrug]

That's an interesting twist to use poppet valves to put the thumb and bucket cylinders in parallel. I have wanted to try that with the backhoe on my John Deere 455D, but I was hesitant to spend the time (and money) modifying the existing setup without knowing how it would work. Are the cylinders on the bucket and thumb about the same size? How do the bucket and thumb respond? That is, do both move until one strikes the target and then the other one closes up to complete the grasp? When you release do both open at about the same speed or does the thumb, being lighter, move first? If you want to retract the thumb fully, I assume you open the valves and retract all the way; then close the valves to hold the thumb in the retracted position? Finally, if you extend the stick and boom so that the "claw" is horizontal (or close), do the bucket and thumb swing down with the weight of the load? Or do you close the poppet valves to prevent that?

My design is done and I have the cylinder ready, but I have not received
my solenoid valve yet, so stay tuned. It IS an experiment, so I have not
verified all my intended results yet. My cyls are very close in size at 2" ID.
My plan is to use ONE poppet valve (rather than a solenoid spool valve) on
only one side of the thumb cyl. A toggle switch on my console will switch
betw a fixed thumb (retracted or not), and a thumb that moves with the
bucket. Note that poppet style solenoid valves allow some leakage under
pressure in one direction. I am planning for that by installing it in the
proper direction so it will hold in the fully retracted position.

As for how it would operate, I anticipate that with nothing to grab, the
bkt would close first most of the time due to gravity. The neat thing
is you are able to put more even pressure on what you are grasping. As
soon as the bkt touches the object, the thumb starts moving, or vice
versa.

I would have gone with a separate manual spool valve on the console
but I do not like hoses on the outside of boom and dipper and I don't see
a very good way to mount and operate the valve. (I would like to try
the Woods valve, but I have not had the oppy.)

I agree with our advice to Dougster to avoid overanalyzing the deal. The only place I might disagree is regarding the relief valve in the thumb circuit: it would be easy (since you are plumbing the thumb anyway) to put an adjustable relief valve across the thumb work ports. That way you could oversize the thumb cylinder a bit and then control thumb pressure with the relief valve rather than relying on precise calculations for the moment arms, etc.

I agree you can adjust the thumb force with a relief across the
work ports. I don't think is it all that valuable as you should make the
thumb robust enough for full system pressure. Note that hoe manufacturers
do not seem to offer this option for the curl or thumb cyls.