Kubota U17 Thumb Install

[Jonness]

I'm trying to figure out how to hookup a Showtime hydraulic thumb to my Kubota U17 mini excavator. There are two factory hydraulic lines running from the boom to a 2-way foot pedal on the floorboard. From there they run back to the control block. Unfortunately, there's no relief valve on the control block. However, there is a lever that switches the auxiliary circuit from one-way (for breaker) to two-way (for thumb).

Nearest I can tell, what's left to figure out is how to install the relief valve (or valves) and the return line back to the tank.

Do I need one relief valve for each auxiliary line, and if so, what type? Should I mount the valve(s) under the floor near the control block and then run a separate return line to the tank? Or should I put the valves on the boom and run the return line back to the tank from there?

Any help will be greatly appreciated!

 

[lantraxco]

First, are you sure there is no port relief at the foot pedal valve? Sounds like you have pressure and return from the foot pedal valve back to the main valve stack. Second, you really only need a relief on the butt end line to the thumb cylinder, it's not likely anything will pull out on the thumb hard enough to create excess pressure. Could it happen? Sure. Will it happen? Not bloody likely.

I would tee the relief in under the floor, tee it into the line on the output side of the foot pedal valve that goes out to the butt end of the thumb cylinder, run the outlet to a tank port or teed into any return line going to the return filter.

Either of these should work well, they have through ports so you don't need a separate tee:
http://www.surpluscenter.com/Hydrau...-16-GPM-1000-2500-PSI-RELIEF-VALVE-9-1556.axd
http://www.surpluscenter.com/Hydrau...-16-GPM-1000-2500-PSI-RELIEF-VALVE-9-1556.axd

 

[Jonness]

Nearest I can tell, there's no mention in the WSM of a separate adjustable relief valve for the service port circuit. Therefore, I'll rig the valve as you've indicated.

Thanks for your help!

 

[f311fr1]

Use a cross port relief valve. No tank line needed 1/2 NPT 30 GPM 1500-3000 PSI HYD CUSHION VALVE
ITEM NUMBER: 9-4019-50-H Surpluss center also

 

[lumberjack]

A cross port relief won't work on a single cylinder, there isn't anywhere for the fluid to go.

 

[lantraxco]

Use a cross port relief valve. No tank line needed 1/2 NPT 30 GPM 1500-3000 PSI HYD CUSHION VALVE
ITEM NUMBER: 9-4019-50-H Surpluss center also

NO! As Lumberjack stated, the rod end is smaller then the piston end, you can't put a quart of oil in a pint jar, something will give. The difference in effective areas also will create a situation called pressure intensification, when you push back on the thumb you can create pressures high enough to blow a hose or shatter the cylinder. I have seen it done. You must have a relief that goes to tank. They do make twin relief blocks with a common tank line though, those will work.

 

[f311fr1]

Look at the schematics. Any excess flow when the valve is deadheaded in one direction or the other goes to the work port connected to tank.

 

[lantraxco]

View attachment 160751View attachment 160752 Look at the schematics. Any excess flow when the valve is deadheaded in one direction or the other goes to the work port connected to tank.

Is this the schematic for the aux valve on this machine?

 

[f311fr1]

No generic cross port relief valves

 

[f311fr1]

First one is generic cross port relief. The second one just shows two single relief valves plumbed to each work port and the tank ports plumbed before the directional valve. With the first valve plumbed on the work ports only you could get intensification if you used the thumb like a pry bar. In the secon pic if you used the thumb as a pry bar and the relief poped due to excess pressure the flow would go directly to tanl and the cylinder would retract with no damage.

 

[lantraxco]

Okay, I think I'm following you now. Yes, the drawing on the right is the correct setup, and technically you also need makeup checks to backfill the cylinder when the reliefs are opened up. Just on the rod end really, but typically both work ports.

 

[lumberjack]

When the machine's valve is in neutral, both work ports are blocked, neither is to tank. That means with a crossover relief, your relief oil is sent to the rod end, which means intensification happens.

 

[Jonness]

When the machine's valve is in neutral, both work ports are blocked, neither is to tank. That means with a crossover relief, your relief oil is sent to the rod end, which means intensification happens.

If I'm reading you properly, you're saying, when the thumb-control valve is in neutral, and the bucket comes up against the thumb with enough pressure to exceed the crossover relief pressure, the oil has nowhere to flow, so it intensifies pressure within the line instead of allowing oil to flow back to the tank; thus, retracting the thumb? And this would could bend the thumb cylinder rod or burst the line?

 

[lumberjack]

In general, yes. It wouldn't retract the cylinder appreciably, but the pressure intensification would happen. I don't know if the bucket cylinder would have enough force to cause a mechanical failure or not, but that's more academic than anything.

 

[lantraxco]

If I'm reading you properly, you're saying, when the thumb-control valve is in neutral, and the bucket comes up against the thumb with enough pressure to exceed the crossover relief pressure, the oil has nowhere to flow, so it intensifies pressure within the line instead of allowing oil to flow back to the tank; thus, retracting the thumb? And this would could bend the thumb cylinder rod or burst the line?

Not having any relief can cause high induced pressures and possible rod, etc. failure, using crossport reliefs with a double acting cylinder can cause extreme high pressures.

It's the difference between the piston area and the rod end piston minus the rod area, once the force acting on the back side of the piston produces enough pressure to open the relief up to the rod end with nowhere else for oil to go the pressure has to balance, and since the effective area of the piston on the rod end is reduced by the area of the rod, pressure keeps rising. Depending on the rod size, the bigger the rod the higher the intensification, you may see two or three times higher pressure than what would normally occur at the butt end if the relief simply weren't there at all.

I went to work for an outfit years ago, had a crossport on a thumb cylinder on a Deere 690B. I politely informed them it was wrong and dangerous. They responded that though they had experienced lots of trouble with hoses, since they had installed CAT high pressure lines, all was well and to leave it alone. Several months later, they had the thumb cylinder grenade from the rod end port all the way to the butt end weld, luckily it was winter because it covered the cab in hot oil, the operator had the window closed up, so he only had to change underwear. Once again I politely informed them what the problem was, but again overruled, they merely had the cylinder retubed with a heavier wall tube. They finally split the crossport relief block flat in two pieces. I replaced it without asking with the proper valve, lol.

 

[joispoi]

I'd be very surprised if a <2 ton mini requires running a return line back to the tank for a relief valve. I've never heard of anyone having to do that for a machine that size.

I would investigate how the factory pressure relief functions on that circuit before spending any money on possibly redundant parts.

 

[Jonness]

I'd be very surprised if a <2 ton mini requires running a return line back to the tank for a relief valve. I've never heard of anyone having to do that for a machine that size.

I would investigate how the factory pressure relief functions on that circuit before spending any money on possibly redundant parts.

According to the WSM (which I could be mistakenly interpreting), the service port lines are relieved by piggybacking on other system relief valves of about 3400 lbs.

The U17 bucket curls at 3400 lbs, which is greater than the max rating of the thumb cylinder. I've confirmed when the SP foot control is in the middle position, there is no return to the tank. So it seems to me, in this position, the bucket force could bend the thumb cylinder.

This brings me to the following question. If there is no return to the tank when the SP foot pedal is in the middle position, and I only install the relief valve on the butt end of the thumb, are both lines protected when the bucket curls against them?

 

[Jonness]

Here's a link to a youtube video of a machine smaller than mine where the dealer installed a thumb and needed to use a crossover relief valve:

https://www.youtube.com/watch?v=w7mmUejH1tk

I called this dealer and inquired about installing a thumb for me, and he suggested a werk brau thumb and said a crossover relief valve was necessary on the U17 due to not having a dedicated relief valve on the service ports. However, as stated in this thread, a crossover relief valve is not suitable for this application, which is one of the reasons why I want to install a thumb myself.

 

[lantraxco]

The rod end of the cylinder will only build pressure if force is applied in some manner to pull the thumb cylinder out which should be near impossible? The butt end is pressurized when you curl the bucket back against it or you push out with the stick and catch the thumb on something.

There are crossport relief blocks with both reliefs vented to a tank port, but they're rare so we have to say the typical crossover or cushion valve doesn't work with a cylinder. The thumb on that machine in the video seems to be working the way it's supposed to.

Here's a link to a SUN manifold that has two reliefs vented to a tank port, with makeup checks to backfill the cylinder so it doesn't cause a vacuum and try to suck air someplace: http://www.sunhydraulics.com/model/YRDK

 

[Jonness]

That makes sense. Thanks!