Komatsu hyraulic thumb

I just got a very old Komatsu PC05 mini excavator. It had a new thumb installed before I got it. They got pressure for the thumb by cutting and welding a cap on the ?? pipe return line, taking pressure from that line, running it to the thumb valve with 3/8? hose, and back to the filter and tank, effectively making the main return line 3/8? hose. I don?t think restricting the return line is good. I want to put a T in the main pressure line (manual says main line max is 2489 pounds,) install a Prince R1850H relief valve in the new line, set it to 1000 pounds, and run that hose to the thumb valve. Then when you activate the thumb, it will only get 1000 pounds pressure. When you curl the bucket, the thumb cylinder will over-pressurize, causing the Prince valve to relieve pressure to 1000 pounds, with the blown-off oil going back to the tank. I will restore the original main return line to ??. Am I off base here? Suggestions?

Neither plan sounds right, is it open or closed center hydraulics that makes a big difference. A t in the return as they did wouldn't provide any pressure other than residual i.e. resistance against filter etc

If it's working the way it's plumbed, evidently it has an open center system.

That means when there is no demand for pressure, the oil is free flowing through the valves and back to the tank.

Tapping into the pressure line would not give pressure to the thumb unless there was another valve closed.

You will need to leave it plumbed as it is, taking power to the thumb off the return line, but do get rid of the restrictive 3/8 hose.

You can use the 1000lb relief on the thumb circuit as a dump back to the return down stream of the thumb valve. The relief valve would be ahead the thumb control valve.

This is all depending on it being an open center system. Your manual should have that info.

For a relief to work on the thumb when you curl the bucket, it needs to be either a port relief in the control valve or a cross port relief. A cross port relief is a separate valve in the lines that go to the thumb cylinder.

As others have said, we need to know if the system is open or closed center to figure out the rest of it.

This is a two-pump open system, one pump going to each side of a 6-valve bank, with a common return in the middle, giving full flow for multiple movements of the bucket-stick-boom-swing with no loss of pressure. All valves have open centers. The relief valve I want to use has three ports: in, out, and relief to tank. I called Prince Manufacturing (excellent customer service!) and spoke to Andy in the hydraulic department. He said what I want to do will work. There is no manual for this machine - it's about 35 years old, and very little information is available anywhere except for later -3, -5, -7 models that appear to have made many changes. The hoses use Komatsu 30 degree fittings, which are different from JIC, SAE, and NPT, so it's a bit of a challenge to modify. All in all, it's quite an education - thank you all for your advice, comments, and help.

The idea you described will not work. Putting a T into an open center pressure line will not provide enough pressure, unless you also simultaneously work another function on the valve bank that that pressure line feeds. Also, the relief valve plumbed as you describe will relieve the pump pressure on that side of the circuit for any function to 1000 psi, and will have no effect on the oil trapped in your thumb cylinder.

The proper way to do it would be to use a power beyond valve to operate the thumb. (I am making an assumption here that the factory system relief valve is in the control valve block.) One of the supply lines going to your valve bank would need to be connected to the IN port on the accessory valve. Then a line would connect the power beyond valve to the inlet port on the original valve block. The relief valve in the accessory valve would need to be set to match your current system pressure. There would also be a return line from the accessory valve to the tank. You could then put a relief valve like you describe in the ?close? line of the thumb to allow it to release when the bucket is curled against it.

A valve like a Gresen V10 sectional would be really easy to configure for what you need and has the bonus of being able to put the work port relief right in the control valve. Not the cheapest but possibly the simplest.

The easiest solution would be to keep the setup like it is, just upgrade lines and possibly valve to handle the return flow from both pumps. Then add a relief valve in the hose going to your thumb.

Thank you for your clear explanation. ?Oil trapped? is the part I was missing. I agree increasing the hose size on the current system would be the best solution, and I am getting the type of fitting the valve currently has. It is from Japan; it could be any of several size/thread/types. If you want a mind-trip down hydraulic hose lane, go to this site: https://nnalert.com/tqxblz5
I never dreamed there were that many different types of hoses.

I get to work with hydraulics built all over the world. Some days are more fun than others.