Hi everyone, I have an old school excavator and I'm having some issues with the hydraulics. It is a 1984 Kubota KH-28L. I don't know much about hydraulics, but here's what I've observed. Keep in mind that this is old-school, non pilot controls, no pressure reliefs, no fancy nothin! When I try to extend or crowd the dipper, it will travel (somewhat slowly from what i'm used to: KX121-3) until it reaches vertical position, and then pause before completing the swing. My guess is that there's air in it somewhere and it will move with the aid of gravity, and then pauses to compress the air before it builds enough pressure to complete the swing. Sometimes if I leave the dipper in the vertical position and drive the machine around a little, the dipper will swing back and forth a bit like a pendulum. Occasionally the bucket function acts similarly. The main boom seems slow to me, but it doesn't act the same as the others. Another big problem is that the functions don't seem to share much. It may be caused by air in the lines, or it may be not related at all. The worst is that the swing function and dipper cannot be used at the same time. If I'm swinging and try to extend or crowd the dipper, it stops dead mid-swing and won't keep going until I stop using the dipper. In one swing direction, it doesn't matter how much I feather the controls - it does it every time. The other direction if I feather it carefully, I can sometimes make them cooperate, though not satisfactorily. Also, if I am traveling either direction with the tracks and try to swing, one track slows to almost a stop and the other keeps going full speed. Sorry it's a long winded description, but I wanted to explain it carefully. Any thoughts or advice are welcome! Thanks!
Your old machine has slow hyd and the stick and bucket are getting ahead of the oil, it should have Make Up valves to help out with that but they rely on a head of pressure in the tank. Check that out when oil is warm. Are the hyd pumps variable displacement piston pumps? Sounds like one is at minimum output thus stick/swing travel slow down. That's all I can say you need a Koboda man now. Later Bob
My parts book calls it a gear pump. And there is a vent on the hydraulic tank, so I don't think it could be pressurized. I'm not good with hydraulics except for changing hoses I'm hoping there are some people on here that are familiar with the old iron!
From what I could tell from the Kubota parts pages (before the website quit working:beatsme) is that machine uses tandem gear pumps for hydraulics. Didn't see any type of sequence or pressure control valves in the supply piping, so it's likely the main control valves are open center porting. Two main control valves, one gear pump per control valve. I don't know this machine, but a typical system like this, two pumps, two control valves, split the work load between the two pumps. For example, pump A feeds control valve A, which work left drive. Pump B feeds control valve B, which works right drive. As for your arm problem, not knowing your machine, I can't say what's causing your problem. Maybe a failing gear pump? Hydraulic tank may have an inlet strainer that's clogged restricting flow of oil to feed pumps? Faulty load check valves in control valves? (although I didn't see any on the parts pages, don't mean they ain't there though). If a gear pump is worn, they typically get hot really quick when under load. And testing a worn gear pump can be tricky unless it's totally worn slap out as a worn gear pump can still develop pressure, it just takes longer than a gear pump that's tight.
Wille, I have found most sound Fixed Displacement systems will hold 90% of their relief valve setting at full throttle as they are throttled down to low idle if the engine doesn't stall from the load. Good for hyd or trans systems in my opinion. Later Bob
Thanks for the input and I hope I can get this worked out. If I were to try to bleed air out of a circuit, how should I do it?
Your machine has a main relief valve alright, no hyd system can work safely without one of some sort. It limits the system pressure so lines don't blow off and pumps break open. Your main valve body also has port reliefs too to protect every circuit from external forces when the spool is in hold. What I mean't was if you have a gear pump feeding your system(fixed output) and you have a gauge in the pump outlet line. Stall a circuit and run at full throttle, you will see main relief setting on the gauge. Then throttle down as low as you can without the engine stalling. If your system is good the two readings should be within 10% of each other. If the reading when you throttle down falls to less than half of the full throttle reading something is worn out. Later Bob
Ok, but I don't think that'll work because the engine will stall at full throttle under load... Any arm function will do that and the tracks will stall it too if in high speed mode.
Well if that is happening you don't have enough power in the engine or the main relief valves are set TOO HIGH. If your throttle control is a cable make sure it is moving the lever on the governor all the way to the stop. The cable might need adjustment and that's were to start. Get the engine right first. Later Bob
Wow, been busy at work and haven't had time to play with my own toys! Yes, the throttle does move the lever to the stop and the motor seems to run pretty well for its age. It seems like the hydraulics are strong enough for the size of the machine, based on the other Kubotas I've run. So should I try to adjust my relief valves? I'm not even sure where they are or how to adjust them. I'm still fairly sure that my main problem is air because I was running the machine yesterday and at certain points the dipper would swing in and out like a pendulum. Then after holding the lever for a second, it would continue to move normally. Thanks for the help.
Get a parts pic from your dealer and it should point out the relief valves to you. Then while you are there ask service what the pressure settings should be. Later Bob