Mini excavator thumb - relief valve question

[Duff]

Your relief valve in your circuit is working correctly.
When you hear your relief valve working, when you put your thumb against the bucket, 40+ Litres/min ( from the pump ) is flowing through your relief valve.
When you put the bucket up against the thumb, and the cylinder comes in, your relief valve has worked correctly, except you haven't got 40 Litres/Min going through it, just 50ml ( as a guess )has relieved back to tank.

Do you need a relief valve on the rod side of the cylinder? Depends
If you are going to use the thumb as intended, then NO.

However, if you are going to use it to lift, or strap it to the bucket and then unintentionally rotate the bucket or put Quick disconnects or flow control valves on the rod side, then I would say YES, due to Pressure Intensification.

If you had quick disconnects or a flow control valve on the rod side and it was not connected properly or the rod side oil was blocked from going back to tank, then pressure intensification will happen.

If you have a 2:1 ratio cylinder and you put 3000psi on the piston side and the rod side oil could not return to tank, then pressure will increase to 6000psi ( 3000 x 2 = 6000). If your hose is rated for 4000 psi and you now have 6000 psi on it, the hose can burst and if someone is hit by high velocity oil, then an oil injection can incur.

All the best
Duff

 

[Tarnold]

Question again, new to this so need a bit more info. In the ops attached pic there are 3 hoses. I’m guessing that the 3rd hose is to route excessive oil/pressure back to the tank? But how to plumb this back into the system? As posted above, I’ve added a thumb to my kubota x61 2 and I realize I have excessive pressure while over operating the bucket/thumb. So far I’ve just been gingerly using the thumb, which is just a seat of the pants feel to avoid too much pressure. Currently that thumb will open/close very quickly if not better controlled.

 

[Duff]

Question again, new to this so need a bit more info. In the ops attached pic there are 3 hoses. I’m guessing that the 3rd hose is to route excessive oil/pressure back to the tank? But how to plumb this back into the system? As posted above, I’ve added a thumb to my kubota x61 2 and I realize I have excessive pressure while over operating the bucket/thumb. So far I’ve just been gingerly using the thumb, which is just a seat of the pants feel to avoid too much pressure. Currently that thumb will open/close very quickly if not better controlled.

The 3rd hose is to send oil back to the tank, once the set pressure of the relief valve has opened.

"But how to plumb this back into the system?"
You have plumbed it into the system, its going back to tank.
You could add a crossover relief valve block, and this would dump oil from the piston side to the rod side and vice versa, but in the end the oil is still going back to tank.

"Currently that thumb will open/close very quickly if not better controlled."
You can add flow control valves to both and open and close circuits to slow it down.

 

[ROcHE1234]

Your relief valve in your circuit is working correctly.
When you hear your relief valve working, when you put your thumb against the bucket, 40+ Litres/min ( from the pump ) is flowing through your relief valve.
When you put the bucket up against the thumb, and the cylinder comes in, your relief valve has worked correctly, except you haven't got 40 Litres/Min going through it, just 50ml ( as a guess )has relieved back to tank.

Do you need a relief valve on the rod side of the cylinder? Depends
If you are going to use the thumb as intended, then NO.

However, if you are going to use it to lift, or strap it to the bucket and then unintentionally rotate the bucket or put Quick disconnects or flow control valves on the rod side, then I would say YES, due to Pressure Intensification.

If you had quick disconnects or a flow control valve on the rod side and it was not connected properly or the rod side oil was blocked from going back to tank, then pressure intensification will happen.

If you have a 2:1 ratio cylinder and you put 3000psi on the piston side and the rod side oil could not return to tank, then pressure will increase to 6000psi ( 3000 x 2 = 6000). If your hose is rated for 4000 psi and you now have 6000 psi on it, the hose can burst and if someone is hit by high velocity oil, then an oil injection can incur.

All the best
Duff

Excellent explanation. Thank you very much!

 

[Stumblebum]

ROcHE1234 ; Could you post a close up photo of that relief valve ?
Or, perhaps the part number.
I couldn't locate it on the Walvoil site.

 

[ROcHE1234]

ROcHE1234 ; Could you post a close up photo of that relief valve ?
Or, perhaps the part number.
I couldn't locate it on the Walvoil site.

There you go! Big so you can read the numbers. The model line starts with V-VMP

 

[Stumblebum]

Thank You !!

 

[Stumblebum]

Would your "local hydraulics shop" happen to sell that valve on the internet ?
I have searched quite a bit on the internet and cannot find anywhere in North America that sells them.
I need one that has 1/2" or SAE 8 ports.

 

[rScotty]

I agree the valve is on the correct side of the thumb cylinder. On that side it protects the thumb circuit when the bucket curl is pushing the thumb back against the dipper stick.

But I cannot tell from the photo where the reilef pressure is going. With this valve, does the pressurized fluid go back to the sump or is it regeneratively replacing the fluid on the other side of the thumb cylinder? And if it is not, what is?

BTW, I'm a believer in simple relief valves, & surplus center.com has good inexpensive ones. Thumbs and buckets used together get into all sorts of odd pressure and leverage situations. The only downside that I see to adding one to the other side of the thumb circuit is the extra plumbing & mounting.
rScotty

 

[Stumblebum]

I agree the valve is on the correct side of the thumb cylinder. On that side it protects the thumb circuit when the bucket curl is pushing the thumb back against the dipper stick.

But I cannot tell from the photo where the reilef pressure is going. With this valve, does the pressurized fluid go back to the sump or is it regeneratively replacing the fluid on the other side of the thumb cylinder? And if it is not, what is?

BTW, I'm a believer in simple relief valves, & surplus center.com has good inexpensive ones. Thumbs and buckets used together get into all sorts of odd pressure and leverage situations. The only downside that I see to adding one to the other side of the thumb circuit is the extra plumbing & mounting.
rScotty

Perfect timing for your post !
I have the same questions.
Because I read in another forum that the oil needs to be replaced on the negative pressure side of the thumb cylinder to prevent cavitation.
I would appreciate it if someone could locate a vendor for the relief like ROcHE1234 used. Or a different brand that has the same functions.
I have 1/2" NPT hoses on my thumb. I can get adapters to fit whatever the relief valve requires to connect them.
I am attaching the info. I obtained on website a couple of months ago.