Thumb with single action cylinder

Have been investigating the method to have a thumb on the Service Port hydraulics on a Kubota mini (U17). Service Port valve has no relief, no passage back to the sump when in neutral, no load check....just a valve that sends fluid to the load and allows a return flow to the sump. Works great for a hammer. Works OK for the tilt bucket cause the tilt bucket is built strong enough to tolerate the full system pressure.

Dont want a damaged/bent/cracked dipper, or, a bent thumb / bent ram when the thumb is overpowered by the bucket - and I learned a crossover relief isn't correct [as pointed out on this forum in other threads] 'cause the ram has a volume diff between the rod end and the cylinder end . That wisdom saved me from wasting time on that idea. Pilot operated relief is being invoked instead.

Also thinking about a pressure regulator to enable the thumb to have reduced clamping pressure to pick up wood posts and poles and not leave big marks. Regulator is pilot operated to allow adjustment from the seat.

The "hydraulic wizard engineer" has the drawing detailed and recently sent it to me with some hydraulic goodies etc but all the while we were thinking of a two way ram - 'cause that is what we see.

He is in NorCal and he sends me a pic of a medium sized JD steel track excavator with a single line going to the thumb's ram; our thinking is the bucket returns the ram to the stow position. He says this change from dual to single line ram will reduce the complexity of the hydraulics a great amount. He did mention the ram will be filled on both sides by the one hose and the ram force is equal to the rod area times the pressure. As opposed to the piston area. Likely this will make the motion quicker.

Any experience on or with this idea using a thumb such as this being described herein?

Good day
Attached a couple of documents that may help, if you need support with your specific machine please PM me.
Kind regards
Uffex

Hi Oceanobob,
I have the U25 which i think is the same hydraulics as yours.
I find i can tap the foot pedal to get less clamping force ok.
A huge issue is the service port is a motor spool and leak like a sieve when used on a cylinder.
I had to fit a pilot check valve in one side to stop dropping things all the time as the pressure would fall away quickly.
Cheers

Thanks for the assistance/comments.
The attached pdfs to be forwarded along - although the U17 valve for the Service Port is operated manually (and has no pilot)....
The U17 being similar to the U25 and the experience with the OEM service port valve allowing pressure to not maintain is indeed a issue....pilot check a good idea (did you need a bleed off feature on the pilot? ..... I recall being mentioned to assure repeatable seating of the check)

Any thoughts/experience on the thumb operation of the single action ram which will require the bucket to 'open' the thumb?

If your asking my opinion on single acting cylinder, I would not go there.
I don't know how the curl of your bucket is going to push the arm of the thumb far enough out of the way of normal digging.
You would have reduced holding force.
I think it would drive you around the bend pushing the thumb back with bucket and your foot on the peddle all the time.
Just use the Aux circuit (Service Port) the way it is on a double acting cylinder.
You have three issues to overcome in your design.
1/ The pressure falling away i used the valve in the pic which i think they call a pilot check valve.
You can get them bi directional but mine is a single with one cartridge in the block.
The way it works is the high pressure fluid on the cylinder side that is trying to clamp down is locked off by the cartridge so you get no bleed back.
When the opening side of the cylinder is pressurized it activates the pilot side of the cartridge to open and releases the fluid on the closing side of the ram.
The pressure that makes it release i think is a couple of hundred PSI but you can select different cartridges.
So the third line you see coming into the block is just the sense line from the opening side which has nill flow i think.
Mine is a bit messy but I chose to have the valve out with the cylinders so it left my machine in OEM state so i can attach other objects to Aux and have full flow.
2/ The curl of the bucket being stronger than the thumb.
I don't have that problem being im using 4way bucket but if you match the cylinder size to the bucket curl cylinder would help.
Fitting a over pressure valve might help.
I personally would just match the cylinders and make your arm so it had about the same mechanical advantage on both sides when in your typical clamping position. (Cylinders must be capable of 3100psi)
3/ the Aux circuit is fast as there is a lot of available fluid there and the cylinder can move fast.
At first i did not like this and messed around with restricts in the lines to slow the action down which worked ok.
As time has gone by I must have turned into an animal behind the controls now as I have removed them and like the action being fast now.

On a different note if it was me I would be using fast connections on your lines and making it so the thumb and cylinder could be removed and installed easily as required.
Hope this helps.

The attachment of the thumb is planned for removal installation and will utilize the quick connections to keep the weight down when the item is not needed.
I will have to investigate the shape of the thumb to verify if the bucket can place the thumb in the park position.

The ability of the stick to tolerate the clamping force from the bucket has a concern - maybe I have seen a couple bent thumb rams on others' machines or cracks/torn welds in the mounts or worse a cracked dipper...

Thus I am investigating a thumb ram pressure limitation and that thought seems like a simple request but upon the thinking of the different modes (thumb moves to bucket, or, bucket moves to the thumb) and the displacement issue of the two way ram plus the check valves .... this has lead to the less than optimum (perhaps) one way thumb ram.

The foot pedal would cause the thumb ram to move the thumb to the bucket but the bucket would move the thumb ram and the foot pedal would not be actuated since the pressure relief would actuate to allow the thumb to move in response to the bucket. Maybe the pressure relief could have a parallel dump valve by way of a solenoid.

Put bucket teeth under a (broken concrete or) boulder, but it starts to slip off the bucket.
With the foot, push on the [service port] thumb control on the floor and the thumb moves toward the boulder and the item is pinched. Agree that it may be necessary to hold the foot while moving the item to assure hydraulic is maintained in the event the pressure subsided for some reason.
Lift and position over the rubbish container, open the bucket, drop item. Move bucket to reposition thumb as swinging over the load, repeat. (Note the "wasted" movement of the bucket in the air is a Case backhoe trick to help dampen the hydraulic so as to smooth the operation).

More to follow...

Looking for the ability to relieve pressure in the thumb to not have damage but also to (1) intentionally allow the bucket to overpower the thumb for placing/positioning and (2) soften the clamp force to allow the handling of wood items.
Waiting on new sketch but a piloted check wont be part of the solution from what I gather in discussing this today.


With the lightness of the fabrication of this U17, this pressure relief (force limitation) will help preclude damage.

And along this line of pressure specs, I have been reading thumb kit installation instructions and many mfg's mandate maximum pressure on the thumb circuitry which my service port is not inclined to obey this idea LOL.

Thus the Pressure Relief Valve may be for me. I certainly appreciate the assistance which is why I post these inquiries.

Good day
Attached is an alternative way to control the thumb I guess if you send the file to your local Hydraulic valve supply they should be able to give you a price.Note we have used a U15 schematic in the absence of U17
Kind regards
Uffex