Komatsu PC50uu-1 hydraulic problem

[apm]

OK, I tried this post in the main excavator forum and got no response. Maybe it should be here, instead?

I've got a 1993 Komatsu PC50UU-1 gray market machine. I've got the pdf manual for it. It's got a hydraulic problem that I've been unable to figure out.

This machine is a direct hydraulic machine, no electronics, solenoids, etc. in the hydraulic circuit. I believe it uses a gear pump. The pump is 3 stage and is supposedly brand new (installed by the previous owner). There are two control valves that operate all of the functions. One is a 6 spool valve and one is a 3 spool valve. The first two stages of the pump feed the 6 spool valve, and all functions seem to work as they should. The third stage of the pump feeds the 3 spool valve. this valve controls the blade, the swing motor and the rapid travel function. The blade is weak as well as the swing (slow). The rapid travel functions. I hooked a hydraulic gauge into the line from the pump to the 3 spool valve. There is a test plug there for that purpose. When operating the blade or the swing against the stop pin, the pressure never gets over 1500 psi. Relief is supposed to be about 3100 psi.

I assume I'm bypassing oil either in the pump or the 3 spool valve. The pump is supposed to be brand new. I can hold a function open or against the stop until pressure comes up to 1500 and I'd expect heat to build if I was bypassing oil somewhere. The pump does not seem to get overly hot, nor the valve.

I've swapped out the relief valve from the 6 spool valve (same part number) into the 3 spool valve, and it made no difference. I've had the 3 spool valve out of the machine and totally disassembled and cleaned it. I found nothing wrong, nothing broken, no excessive clearances, etc. There is also a suction and safety valve on the 3 spool that I disassembled and cleaned. It appeared to be fine but I've got no way of testing or swapping that one.

What would you try next?

Thanks,
Greg

 

[jackie606]

i am not the technician,we are the parts supplier for Komatsu excavator.I am not able to answer your questions,hope other ones can help you.

 

[GaryHoff]

Check your cycle times for the swing and blade. Also check your other cycle times (travel, boom, ect.)? Does the blade or swing seem to drift? What is your pilot pressure at? Komatsu seems to like to put little screens in all their pilot fittings, there should be a screen on the outlet of your pilot pump or at its first valve in, as well as screens in all your pilot control pressure in lines. Theres also usually screens in the pilot lines into the control valve. Does your swing motor have reliefs on it? If the swing motor has reliefs that could be your low swing pressure (the swing pressure is suppose to be lower than main pressure though)

 

[apm]

Thanks, Gary for the reply. There is no pilot system on this machine. It's just straight hydraulics with spool valve control. There are two spool valves, a 3 spool and a 6 spool. All of the functions resulting from the 6 spool valve seem to be fine. All of the functions from the 3 spool valve are weak, ie; the blade and swing. I'm trying to determine if I have a pump problem or a valve problem. One stage of the pump (3 stage) feeds the 3 spool valve. The pressure to that valve won't go over 1500 psi. By the book, should be about 3100 psi.

Greg

 

[apm]

The cycle times for the swing and blade are within the book spec. They just don't have any power. The blade won't lift the machine and the swing will barely move with a load. The swing appears to get worse as the oil heats up. May be dealing with two problems, hydraulic system and swing motor.

Greg

 

[GaryHoff]

Good cycle times suggest a good pump. A bad gear pump will generally still build good pressure, but slow cycle times. Sounds like your safety valve is weak. I dont have a pc50uu-1 book (my books start from -2). Does the swing drift at all? Can you push of the stick and turn the house? Does the blade drift? When you are reading the 1500psi, is it when the swing is stalled or the blade, or either, both dirrections?

 

[capnkel]

Check your pressure on the 6 spool bank,if thats where it should be,try swapping the pressure lines on the 2 valve banks if possible.If the problem switches valve banks,then the pump is bad,if it stays the same,the relief is weak.

 

[apm]

The swing doesn't seem to drift at all. The swing feels normal with no load, but slows down as the oil heats up and is weak. The blade does not drift. If I lift the front with the boom, then put the blade down, it will hold the machine up after I've lifted the boom. The 1500 psi reading is when either the blade or the swing is stalled.

capnkel, the 3rd stage of the pump that feeds the 3 spool valve is smaller and lower flow than the other two stages. I can't physically swap the hoses without having new hoses made. Is it worth a try even though it's lower flow?

thanks guys for the input, I'm not having much luck, yet.

Greg

 

[capnkel]

I think its worth a shot swapping lines around,a few adaptors should be easy to adapt the different hose sizes.It would give you a much better idea where the trouble lies.Check the pressures on all 3 pumps before swapping hoses around.The machine does not need to be throttled up just to see how the pressures compare,so the higher flow pump going into the lower flow valve wont hurt a thing.

 

[Tony Lovegrove]

Hey there I know this problem with the pc50uu-1 was posted 6 months ago I am having the same problem. I've re built the hydraulic motor so I know it's not that, I've worked through the whole machine and now I'm up to the swivel motor I'm wondering if I've got a broken seal which is by passing the hydraulic oil? Can someone please help me cheers

 

[Tony Lovegrove]

Greg its Tony here I'm having the same problem with my pc50uu-1 how did you solve the issue I'm at my wits end cheers mate.

 

[apm]

Hi Tony,

I'm still working on it. I just got a set of test hoses made to be able to swap the pump stages between valves. Hope to get a chance to try it this weekend. I'll post if I come up with an answer.

Greg

 

[apm]

OK, so I finally got a chance to swap the pump stages to the control valves. The stage that normally operates the blade and the swing, I moved to the arm and the right hand drive motor. The problem moved. Previously the blade and swing were very weak, now they're strong as they can be. The blade will pop the machine right up off the ground. The swing is faster and snappier than I've ever seen it. But the arm is weak and the right hand drive motor barely turns the track, so I guess that means one stage of the pump is bad? I can't think of any other cause, there are no electrics or electronics on the pumps or valves, just straight hydraulic.

Anybody have any input?

Thanks,
Greg

 

[mikebramel]

Sounds like the pump is bad. Maybe the gears are spinning on the shafts

 

[capnkel]

Sounds like that section of the pump is worn out.You should be able to replace just that section if you can find the parts.You would likely need a new set of gears,gear housing,wear plates and a seal kit.If you happen to be lucky enought to have a pump that the gear section can be flipped in,that will get you back most of you power.Its not a hard job to do at all.A couple pics of the pump might help us identify it.If its a common brand,you may not need to buy from a dealer at their inflated pricing.

 

[apm]

It's a Kayaba pump. I got it apart this weekend and it really doesn't look bad at all to me. I took it to my Komatsu dealer this afternoon and he looked at the parts and said "you better look somewhere else for your problem, there's nothing wrong with this pump". The only other thing I can think to check is some kind of restriction on the suction side. Maybe collapsed hose or something. Anybody else got any ideas of things to check?

Thanks,
Greg

 

[capnkel]

Make sure there is nothing between the pump output and the weak valve section.maybe you missed a valve somewhere?Are there any valves built into that pump?Some have what looks like just a plug,but contain flow or relief valves built into the pump housing.If the problem swiched sides by switching pumps,that pretty much eliminates any issues with the control valve.One thing you could try is putting a spare valve inline hooked to the weak pump section,and see if your pressure is ok right at the pump.Pictures sure would be a big help.

 

[apm]

It's pretty much just a "plain Jane" pump, no valves or reliefs. It's fixed displacement gear pump. I'll see if I can figure out the picture thing. The gear is made as all one piece, can't be slipping on the shaft.
Thanks,
Greg

 

[apm]

This should be a link to a copy of the pump parts drawing, showing the third stage that I'm having trouble with.
http://komatsupartsbook.com/#t=p&p=/root/25146/6960854/6960969/6960972

Thanks,
Greg

 

[apm]

to add to the mystery, the Komatsu dealer put the pump on their test stand and all three stages checked out good. I think that's good news, but I sure don't know where to go from here.

Thanks,
Greg