Reduce pressure to thumb (bends pivot pin)

[Sodo]

My Kubota KX41 has a user-built thumb added. It pushes hard enough to bend the pivot pin (other thread). I would like to limit the pressure to the big end of the thumb cylinder. I've attached some pics of the hydraulics. It seems like I could just put a relief valve across the hoses to the cylinder, and set it for the thumb grip pressure that I want. Currently the thumb can back-drive the bucket, I thought I might just reduce pressure until the bucket can backdrive the thumb then find the middle.

- Is this a good plan?
- Could a Kubota mini-ex be already equipped with a pressure adjustment somewhere for the thumb pedal?
- Is there a better way to reduce the pressure to the big end of the thumb cylinder??

 

[GreatWhitehunter]

I can't say for sure as I'm not that familiar with Kubota but my Deere 110 TLB has a adjustable relief on the aux hydraulic circuit to not allow the thumb to over power the bucket and cause the issues you're seeing. I'd contact your dealer to see if your circuit has a separate relief.

Matt

 

[Sodo]

Now we're talkin'!

Heres a pic of the control valves. It looks like some valves have adjustments. But the one controlling the thumb does not have that component. Do you think I can simply remove the plug and add the same adjuster that's on the other valves?

 

[dickfoster]

It seems to me that you could simply reduce the size of the bore for the thumb cylinder. You can always use the cylinder you have for something else. Pi R Sq X PSI = total force applied. Don't forget to apply any mechanical leverage into the final mix. From the pic it looks like you have equal force for both the thumb and the bucket, maybe more since the lines going to the thumb cylinder are larger, they may have some restriction in the bucket cylinder that you're not counting on.

My Kubota KX41 has a user-built thumb added. It pushes hard enough to bend the pivot pin (other thread). I would like to limit the pressure to the big end of the thumb cylinder. I've attached some pics of the hydraulics. It seems like I could just put a relief valve across the hoses to the cylinder, and set it for the thumb grip pressure that I want. Currently the thumb can back-drive the bucket, I thought I might just reduce pressure until the bucket can backdrive the thumb then find the middle.

- Is this a good plan?
- Could a Kubota mini-ex be already equipped with a pressure adjustment somewhere for the thumb pedal?
- Is there a better way to reduce the pressure to the big end of the thumb cylinder??

View attachment 463081

View attachment 463080

 

[Sodo]

It seems to me that you could simply reduce the size of the bore for the thumb cylinder. You can always use the cylinder you have for something else. Pi R Sq X PSI = total force applied. Don't forget to apply any mechanical leverage into the final mix. From the pic it looks like you have equal force for both the thumb and the bucket, maybe more since the lines going to the thumb cylinder are larger, they may have some restriction in the bucket cylinder that you're not counting on.

Looking at the bucket curl linkage it appears that the thumb (with equal sized cylinder) would be able to backdrive the bucket (only?) when the bucket is at max curl and the bucket cyl is getting close to the pivot pin (all things being equal of course).

Simple goal is to prevent the thumb from having enough power to bend the bucket pivot pin; reduce the power some. Not looking forward to reducing the size of the cylinder. I can imagine welding 16 inches down in the bottom of a pipe for hours.... not a job for me. Just kidding. Prefer to solve it by turning some wrenches, I think there must be a simple way.

 

[dickfoster]

Looking at the bucket curl linkage it appears that the thumb (with equal sized cylinder) would be able to backdrive the bucket (only?) when the bucket is at max curl and the bucket cyl is getting close to the pivot pin (all things being equal of course).

Simple goal is to prevent the thumb from having enough power to bend the bucket pivot pin; reduce the power some. Not looking forward to reducing the size of the cylinder. I can imagine welding 16 inches down in the bottom of a pipe for hours.... not a job for me. Just kidding. Prefer to solve it by turning some wrenches, I think there must be a simple way.

I didn't say to reduce the length, just the bore. A smaller ID cylinder will put out less force with the same pressure applied but the stroke can be equal to what you have now. Nothing changes but the force on the thumb. Simply pull the pins, swap cylinders and swap the hydraulic lines.

 

[dfkrug]

Heres a pic of the control valves. It looks like some valves have adjustments. But the one controlling
the thumb does not have that component. Do you think I can simply remove the plug and add the same
adjuster that's on the other valves?

Those look like work-port relief valves. They are usually not adjustable, but maybe yours are.

Anyway, when you say the thumb can push the bkt back, that means you have a work-port RV in
the bkt circuit. Where you note "plug", you may be able to add one of the same to the thumb
circuit.

If you remove one of the RVs, look closely to see if it has the pressure marked on the outside.
You will be able to see if it is adjustable, too. Your excavator may be running at as high as
3500psi, and WP RVs would be necessary, IMO. You could also tee in a pressure gauge at the
bkt cyl to see what the RV setting is.

The design of your thumb pivot with the shared pin is limiting since it is so long.

 

[Sodo]

Here's a closeup pic. It has a hex socket on top. Kinda looks like an adjuster with a jam-nut. But I suppose I don't need to adjust it, simply need to "add" it.

Big problem for this project is I called Kubota and the dealer wants $550. Lesser prices on the internet but still too much. But it would be a clean solution,,,,,,and my wallet would be clean after that too. And the service guy said he didn't know if it would work. Typical - nobody knows anything anymore. Gotta think how badly I want to do this.

 

[kevin37b]

Can you put an orifice or restrictor in the fitting on the big end of cylinder ?

 

[dickfoster]

Can you put an orifice or restrictor in the fitting on the big end of cylinder ?

I think an orifice restriction simply slows things down but does nothing to reduce pressure. Pressure is pressure regardless of flow but restricting the flow will slow things down.
Like volts and amps with electricity. A resistor (an orifice restrictor) will control current (flow) but not voltage (pressure).