Sizing a Backhoe Thumb Cylinder

[Dougster]

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?

Correct

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.

Also absolutely correct... or I'll need to provide relief.

And vis versa for thumb pushing against closed bucket circuit.

This shouldn't be an issue... but in theory you are correct.

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.

... and yet Bradco reduces that thumb-applied force by 20%... why?

Dougster

 

[Dougster]

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.

Some very good comments and suggestions here. I have been following your backhoe thumb project... and Ron's... almost from the beginning. I admire you both for your creative, innovative designs.

Because I have the Bradco fixed thumb already and am just converting it to hydraulic, there is not much to think about in terms of geometry (e.g., no funky retraction issues, etc.). Bradco has done all that for me. But you probably know that the Bradco kit includes a fully self-contained control system... whereas economics and sanity dictate my use of my new rear remote set. Some have said just to copy the Bradco hydraulic thumb cylinder exactly and be done with it. Unfortunately, being an engineer, I have to understand what I'm doing and possibly even improve a bit on the Bradco kit... but never at the cost of safety or possible equipment damage.

I see no big need to economize or compromise in my selection of a cylinder as no matter how you slice it this conversion is not going to be all that expensive since I am using the second remote set. If I can get a cylinder at a good discount, great! But so far I haven't seen anything anywhere that was close enough to my envisioned need.

The shock relief valve sounds like a good idea to me. I will look into it. Your idea on the solenoid valve is also very good, but unnecessary in my case. For me, simple is good.

Dougster

 

[Dougster]

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.

Thanks very much for the reference! Length and stroke are easy... I can measure them right off what I've already got. Plus they should match the Bradco kit cylinder anyway or I better understand why not! Cylinder diameter and rod size are slightly trickier. On design rating, at this point I think I should go with 3,000 lb unless someone gives me a good reason to do otherwise.

Dougster

 

[Dougster]

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.

With my "belt and suspenders" nature... the idea of building for maximum possible pressure PLUS having a system relief valve anyway... makes me very happy. That was the way it always had to be done back in my fluid systems design life!

Dougster

 

[MtnViewRanch]

Dougster, what are the dimensions for the cylinder that you need? I may have a source.

 

[Dougster]

Dougster, what are the dimensions for the cylinder that you need? I may have a source.

Hi Brian - I seem to have misplaced my "cheat sheet" (all the dimensions I took a couple weeks back) and everything is folded up tight right now in the garage. I'll remeasure and PM you later today.

Dougster

 

[Dougster]

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?

Hi Dave - Should have answered this yesterday. The 509 uses a 3" bore, 2" rod hydraulic cylinder for the bucket. Cylinder wall thickness is either 5/16" or 11/32" as best I can determine it. OD measures exactly 3.70", but that includes paint thickness. Either way, the cylinder design pressure is well above the 2,500 PSIG published design pressure for the backhoe.

As a practical matter, I'll probably be limited to using a 3,000 PSIG welded cylinder for the thumb.

Dougster

 

[dfkrug]

The 509 uses a 3" bore, 2" rod hydraulic cylinder for the bucket. Cylinder wall thickness is either 5/16" or 11/32" as best I can determine it. OD measures exactly 3.70", but that includes paint thickness. Either way, the cylinder design pressure is well above the 2,500 PSIG published design pressure for the backhoe.

As a practical matter, I'll probably be limited to using a 3,000 PSIG welded cylinder for the thumb.

Dougster

2" rod! That's huge.

Sizing the cyl for length was a tougher choice for me. A longer stroke
may allow the thumb to be retracted tighter against the dipper, while
still allowing useful range of motion. But longer stroke means a longer
cylinder so the pivot bracket on the dipper had to be farther up. Or...
I could use a shorter stroke and move the pivot position on the thumb
inboard. This gives less thumb leverage, yet the thumb has to be
stronger (longer moment arm). Lot of variables.

Here is a photo of a couple of the welded cyls I recently bought from
Surplus Cntr. I am quite happy with their quality. Photo is an unmodified
cyl and one that I shortened up for a tilt cyl for a T-n-T setup. I
substituted swivel eyes on each end.

 

[Dougster]

2" rod! That's huge. Sizing the cyl for length was a tougher choice for me. A longer stroke may allow the thumb to be retracted tighter against the dipper, while still allowing useful range of motion. But longer stroke means a longer cylinder so the pivot bracket on the dipper had to be farther up. Or... I could use a shorter stroke and move the pivot position on the thumb inboard. This gives less thumb leverage, yet the thumb has to be
stronger (longer moment arm). Lot of variables. Here is a photo of a couple of the welded cyls I recently bought from Surplus Cntr. I am quite happy with their quality. Photo is an unmodified cyl and one that I shortened up for a tilt cyl for a T-n-T setup. I substituted swivel eyes on each end.

And who says size doesn't matter! In terms of cylinder length, I must stick with what Bradco has already designed and left me. I sent these cylinder specs to Brian, but I may as well post them here as I am having difficulty finding a close match:

Bore: 2.75"
Minimum Rod Diameter: 1.50"
Minimum Pressure Rating: 3,000 PSIG
Maximum Closed Pin-to-Pin Length: 26.0"
Minimum Closed Pin-to-Pin Length: 24.5"
(Ideal Closed Pin-to-Pin Length: ~25.0 to ~25.5")
Base End Fitting: Tang, 1-1/4" pin, 1" wide
Rod End Fittiing: Crosstube, 1-1/4" Pin, 2" wide
Stroke: Whatever results from above!

Okay Folks... anyone know where I can buy this cylinder other than for megabucks through Bradco???

Dougster

 

[dfkrug]

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.