Do backhoe controls have their own releif valves? What happens when thumb closes against bucket?

[mmurray70]

Hey guys wondering if kubota backhoe controls have their own releif valves? Im thinking of adding a DIY thumb to my B26 and just wondering what happens when the thumb closes on bucket? If my thumb cylinder is too strong and overpowers bucket does that fluid just bypass in the control valve? Or do I break something?

Opposite situation would be thumb cylinder being on small side and bucket overpowering it. Im assuming a basic diverter valve would have no releif in it and totally closed off?? So this would probably cause a huge spike in that cylinder/line. Probably safer to keep the thumb cylinder on the bigger side?

Anybody have any advice or suggestions to keep me out of trouble here? Anybody know the bore/stroke of the factory thumb option on the B26?

 

[Dave_eng]

You are realizing why hydraulic thumbs are not common except on machines specifically designed to dealing with the pressures when one cylinder is pushing against another.

Read through the thread with link below and when in that thread in my post there is another link I provided on how Yanmar deals with hydraulic thumbs.
Hydraulic thumb thread

Look for my post #14 in the second thread

Dave

 

[mmurray70]

Did a fair bit of reading on this past couple days. Seems like best setup is a thumb with reliefs that are overpowered by bucket, like in the link above.

But I might have been over thinking this a little. I was really worried about the induced pressure from the other cylinder. But in reality we are dealing with induced pressure all the time in our tractors. When we dig with the bucket there are huge forces induced in the dipper and boom cylinders and they do just fine. Cylinders and hoses are designed with a little extra strength to deal with this to a certain extent.

So I think im going to just try and match forces of bucket/thumb as close as i can and hope for the best with a simple diverter valve or possibly even just an on/off valve with thumb paralleled to bucket cylinder. Seen a few guys on here running that setup and like how simple it is.

 

[motionclone]

Heres what happens to an excavator when the relief valve for the thumb isnt set lower than the bucket curl circuit. When the bucket is fully curled its cylinder is fully extended and easier to bend.

I had the machine at the dealership putting in a new pump and setting relief valves specifically to prevent this from happening, well they forgot. I got a new cylinder out of it though.

 

[kenrevo]

You definitely want a hydraulic valve with port relief and possibly anti-cavitation. On my thumb set up I' researched the valve block on the backhoe and just bought another section like how the OEM option works. It was cheap, the section was only ~$150 but is a 15 week lead time. It was a Bucher Hydraulics valve, HDS11 series; they also sell mono blocks not just sectional valves.

I should be able to get you dimension of the cylinder next week.

 

[Jchonline]

Hey guys wondering if kubota backhoe controls have their own releif valves? Im thinking of adding a DIY thumb to my B26 and just wondering what happens when the thumb closes on bucket? If my thumb cylinder is too strong and overpowers bucket does that fluid just bypass in the control valve? Or do I break something?

Opposite situation would be thumb cylinder being on small side and bucket overpowering it. Im assuming a basic diverter valve would have no releif in it and totally closed off?? So this would probably cause a huge spike in that cylinder/line. Probably safer to keep the thumb cylinder on the bigger side?

Anybody have any advice or suggestions to keep me out of trouble here? Anybody know the bore/stroke of the factory thumb option on the B26?

Mine goes into relief.

 

[mmurray70]

You definitely want a hydraulic valve with port relief and possibly anti-cavitation. On my thumb set up I' researched the valve block on the backhoe and just bought another section like how the OEM option works. It was cheap, the section was only ~$150 but is a 15 week lead time. It was a Bucher Hydraulics valve, HDS11 series; they also sell mono blocks not just sectional valves.

I should be able to get you dimension of the cylinder next week.

A factory valve for $150 would be awesome. I have no idea what valves are used on my machine or how to track that down. Any ideas where i could find out?

Looks like the boom, dipper and factory thumb valves do have releif valves on my B26. Im guessing they only limit pressure while moving that circuit but im not sure. Does anybody know for sure if the releif is working all the time or just while moving that ciruit? Maybe they build the cylinders strong enough for the induced load, then limit pressure a little lower to get the force they need for that function.

 

[mmurray70]

Mine goes into relief.

What machine? Will the bucket overpower the thumb and close it or does it just stop when it hits it?

 

[mmurray70]

I got some good news on the thumb cylinder! Kubota doesnt list diameters on the factory thumb option. But I did a search on the piston part number and its the same P/N used on a BX25/BT601 bucket cylinder. So its a 50mm bore.

And even better news... I happened to have a spare bucket cylinder here from my old BX25! Almost threw it in when i sold it but thought it might come in hand someday. Its a little too long but I can easily shorten it.

 

[kenrevo]

A factory valve for $150 would be awesome. I have no idea what valves are used on my machine or how to track that down. Any ideas where i could find out?

Looks like the boom, dipper and factory thumb valves do have releif valves on my B26. Im guessing they only limit pressure while moving that circuit but im not sure. Does anybody know for sure if the releif is working all the time or just while moving that ciruit? Maybe they build the cylinders strong enough for the induced load, then limit pressure a little lower to get the force they need for that function.

I opened up the access panel for the valve block and got the manufacture and part number off of the name plate on it. I reached out to a distributor and work with them for what I wanted. It can be difficult if you get someone in customer service who isn't experienced.

There are multiple relief valves; there will be a main relief valve that controls the pressure from the feed source and then specific port will have relief valves (called port relief). The port relief is after the directional valve and is always "monitoring" the pressure in the cylinder/ hoses and will relieve even if the valve is not actuated. The port relief can be set higher or lower than the main relief and is specific for the function (swing is usually set lower as it is one of the weaker parts of a backhoe). Port relief is specific to the port and you can have a section that only one port has relief and other doesn't. Most of the thumb setups I've see have relief on both ports.

 

[Jchonline]

What machine? Will the bucket overpower the thumb and close it or does it just stop when it hits it?

M62

Stops when it hits it.