Hydraulic question

[newtone79]

I used my Komatsu PC28 to dig up my first stump this weekend and I noticed after a while the boom functions got really slow. I read a couple threads about this type of thing and they mentioned pressure relief valves bypassing when they shouldn't for various reasons. I have a diagram of the circuit and I was hoping someone would explain which one I should concentrate on checking. There are two main reliefs it seems like, then a few others. It's not a terribly complicated system but it's still over my head.

 

[Cliffy]

All functions, a couple or just boom up and down?

 

[newtone79]

Boom up was the slowest being almost unusable near the end, but I think everything was slower to some extent. I think the biggest cylinders were the slow ones based on what the print says.

 

[Cliffy]

So looking at the schematic, you have 3 fixed displacement pumps. 2 power the boom functions and the 3rd the swing and blade functions.
There are 2 relief valves in the main valve assembly both set to 2844psi. Third in the 2 spool control valve set to 2489psi.
If all functions slow, i would look at the common denominator, suction.

 

[newtone79]

I found a big 1” low pressure line that was pretty rough looking but not leaking. I replaced that and tested by digging out a pretty good sized hickory stump. By the end my hydraulic oil tank was pretty hot but not too hot to touch. One thing I realized was that if I engage the track motor a little bit, I get some boom lift speed back. I’ll eventually put some gauges on the test points but does that behavior raise any flags?

 

[Tones]

I found a big 1” low pressure line that was pretty rough looking but not leaking. I replaced that and tested by digging out a pretty good sized hickory stump. By the end my hydraulic oil tank was pretty hot but not too hot to touch. One thing I realized was that if I engage the track motor a little bit, I get some boom lift speed back. I’ll eventually put some gauges on the test points but does that behavior raise any flags?

Getting speed back like that suggests to me that the problem is in the pump or what controls it. Flow and pressure tests need to be done to stop the guessing game. Guessing get expensive when you fix what ain't broke.

 

[newtone79]

It has been a while and of course I have ignored this problem until now because I haven't needed to dig any more stumps. This past weekend I dug up a pretty big poplar stump and this issue showed up again. I dug in to the manual some more and found decent information (on previous gen, fingers crossed they didn't change too much). I'm hoping a pressure relief adjustment will help but I just wanted to ask here if I can get some clarification on whether this makes sense to someone more knowledgeable in hydraulics.

Why do they specify that adjustment order? Interesting that RH travel and boom are in the same group after what I see. I do hear some hissing when operating, so is it possible that engaging the travel motor lowers the pressure enough to allow the boom circuit to operate under what the relief valve is set to? Do pressure relief valves wear out? This machine does have a ton of hours on it, so anything is possible. Just trying to understand better before I dig in and possibly make things worse. Thanks for the input!

 

[HarleyHappy]

You’re getting out of line. First thing would be pressure gauges as indicated above, it sounds like one of your pumps are not supplying enough pressure, when hot.
If it has a ton of hours, what’s the hydraulic fluid look like.
A lot of times, you can take an old worn out anything hydraulic and put in thicker hydraulic oil and get some more time out of it, before fixing or replacing pumps and or valves. Think butter on a dish in the sun.

 

[Hffhvg]

Relief valves wouldn’t be likely to cause this. In that case even with cold oil there wouldn’t be force in the cylinder even before ever deadheading the function and in that case looking at the first schematic with the boom also the bucket, right travel and boom offset cylinder would also lack force. These four functions share the same relief valve according to schematic and are all supplied by the first pump from the engine. Like said perform pressure test on these three pumps, when starting to work and also after the problem on the boom arises to compress the results (i would expect at least one quarter of less pressure when the boom gets slow). Like also said before it kinda seems this first pump is the problem…
When engaging right travel the boom gets additional supply of flow from second pump that why you also get a spike of more force with the boom.
But like you said that the machine is old with many hours it can also come down to the basics as always. What is the state of your oil, have you ever changed it? The pump might only get slightly worn, but low viscosity as oil gets hot is great contributor to having less flow. Boom cylinder is probably the biggest and demands more flow to get full force than others, same with drive motors. When oil gets hot and boom weak try traveling straight, just left and right travel no other function engaged. Would you say that right travel is slacking, going slower? That would be easiest to see when traveling uphill…
But like said, make pressure tests first and check state of oil.

 

[newtone79]

Fluid looks good and I changed the filter when I first got it since it looked like it hadn't been changed in a while. I will go through the pressure tests for sure, just trying to get an understanding before I do that. Interesting note on performance that maybe I haven't been too clear on. The only two functions that aren't affected are the blade and swing. They seem to be on their own pump and spool, so I guess that makes sense. The reason I was thinking relief valve is that there's a brief moment where things move faster regardless of the oil temp, then I hear the hissing sound and everything is slower. I'm never in a hurry to do things, so it doesn't bother me. Pressure testing should be interesting regardless. I'll post here when I do it to keep the story going.

 

[John C.]

The relief valve cartridges will get weak over time because the cone will get a groove worn in it or the spring holding the cone down will get weak. You will need to put a gauge in the circuit to see what is going on. You should be able to adjust the setting up be turning screw. When the cone goes bad you will be able to adjust the pressure up some but it will drift down when the oil gets warm. You can remove the cartridge and take it apart to inspect the cone. There will be a ring on it where it seats in the orifice. If it out of round or you can feel it with your finger nail, it will need to be replaced. It can be a lot of other things as well as mentioned above. These machine were built inexpensive but they don't stay that way forever.