Reduce pressure to thumb (bends pivot pin)

[oldnslo]

Sodo,
Any brand name on a label on that valve stack? If possible you can get a work port relief from some source beside the dealer.

Yes you can plumb a relief or reducing valve in external to solve this problem. Either of these valves would simply reduce the pressure to thumb limiting the force it can generate. NOTE: most pressure reducing valves require a drain line to tank which could be a plumbing problem.

 

[BertZegers]

I have a KX 41 as well. I have used the axillary connections for the hydraulic thumb. There also was a "return to tank" on the boom that I used.
I bought a pressure relief valve and have put it in the "extend" line to the thumb cylinder. I have adjusted the pressure relief valve so that I can pick up a big rock.
This way I have never more pressure on the pins/bushings than needed, even if I hold the pedal.
I would like to add a picture, but the excavator is not home. Cost less than $ 100 Can.

 

[Sodo]

Thanks you guys. Bert that's funny we have the same machine (Kubota KX41). I'm lucky you happened upon this thread, thankful for your input. OldnSlo I'll look at the valve stack, see if I can find a brand.

I have never actually done a "hydraulic mod", this would be my first. Next thing I have to know is the fitting name/size/type. Can you guys tell me why this wouldn't work? Doesn't this "dump to return" while cyl. is extending? I don't think I have to worry about the retract direction, the goal is to limit the "extend" force.

Haven't seen inside one of these, but wouldn't a 2-port relief valve "check" in the backflow direction anyway?

 

[muddstopper]

Can you guys tell me why this wouldn't work? Doesn't this "dump to return" while cyl. is extending? I don't think I have to worry about the retract direction, the goal is to limit the "extend" force.

Haven't seen inside one of these, but wouldn't a 2-port relief valve "check" in the backflow direction anyway?

If I am reading this right, you want to add the relief to the base port of your cylinder, and then tie the tank side of the relief into the retract port of the cyl so that relief oil will flow back to tank thru the control valve. . It will do what you say, but it wont work like you are thinking it will. Your problem will be when you try to retract the cyl. A relief valve is normally closed and opens when pressure exceeds the pressure setting. When you try to retract your cyl, you are putting pressure on the spring side of the relief would most likey cause the relief to open and dump pressurized oil into the base end of the cyl. If this happens, then the cyl would act like it was on regen and actually extend instead of retract. You can use the relief valve you pictured as long as you run a line from the tank port on the valve to another return line or port tht empties to tank.
Edit because I am rethinking what I just wrote. If pressure was applied to the tank port of the relief, it should keep the relief closed, instead of opening it. No I am not sure if what you want to do will work or not, Your making my head hurt. I wonder what the pressure rating is for the tank port on a relief valve???

 

[ronbourd]

Sodo ,My opinion... what if you add the adjustable release valve (like the one you showed) on that pressure hose that is pushing pressure fluid to your cylinder for the extension stroke .The return hose to the tank could be very short if that release valve is located at the control valve.Just adding a "t" leaving the control port and installing the release valve with the the overpressure to flow to the tank. Ronald. good luck.

 

[muddstopper]

Sodo ,My opinion... what if you add the adjustable release valve (like the one you showed) on that pressure hose that is pushing pressure fluid to your cylinder for the extension stroke .The return hose to the tank could be very short if that release valve is located at the control valve.Just adding a "t" leaving the control port and installing the release valve with the the overpressure to flow to the tank. Ronald. good luck.

I too think this would be the best method instead of trying to plumb to the retract port back to CV. If you dump relief oil directly back to tank, in the advent that you curled the bucket with the thumb closed, the relief would let fluid go back to tank and maybe prevent the thumb cyl from being damaged. The relief couldnt do this if it was plumbed to the retract side of cyl since oil flow would be blocked if CV was in center position.

 

[Sodo]

Yes mudd I see, the bucket could always back-drive the thumb whereas the way I had it, the bucket can only backdrive the thumb while I'm stepping on the thumb valve.

Sodo ,My opinion... what if you add the adjustable release valve (like the one you showed) on that pressure hose that is pushing pressure fluid to your cylinder for the extension stroke .The return hose to the tank could be very short if that release valve is located at the control valve.Just adding a "t" leaving the control port and installing the release valve with the the overpressure to flow to the tank. Ronald. good luck.

Thats a really good idea. Basically same as the other valves in the stack except $500 less. And no funny business up on the boom where its visible (and can get tangled in trees etc.) Just have to see if there's room for it (tiny machine!) and how to plumb into the rtn.

 

[Sodo]

OK Ron I looked under the floormat with that in mind. It's going to work GREAT, as you'll see in the pics below. Now that I'm back in the house I realized I don't know which hose (A or B) is "extend" but pretty sure I can figure it out when it's time to install the valve.

Will be a short, clean run to Tee into the return. Theres a lot more space for a Tee in it at the junctions shown in the first pic. I know the relief valve is plumbed backwards in this pic.

 

[dfkrug]

Haven't seen inside one of these, but wouldn't a 2-port relief valve "check" in the backflow direction
anyway?

You need a 3-port valve as others have stated, SODO. Ideally on the valve stack. To provide
work-port relief inline, you need something like this, a "crossover" RV. An RV must dump
to a low-pressure line to the tank.

I had these made up for my hydraulic chainsaw projects, which needed to be limited to
2000 psi. Definitely less than $100, but you have to find a place to mount them. This
is about as small as you are going to get with an inline setup.

 

[Sodo]

Can you explain why 3-port is necessary? Or what it does?! Doesn't this 2-port relief valve simply dump excess pressure to return?