Axial pump loses HUGE volume before pressure loss?

[fastline]

Sorry to start another thread here but I think we are homing in on our slow stick problem on an excavator. As a bench mark, new performance of the stick should be 4 sec from full retract to full extend, ours is at 15 sec. That is a HUGE volume loss.

For reference, estimating on the rod side that this is about 3-4 gal of oil, that is 45-60gpm from the pump. At 15sec, we would be about 10-15gpm. That is a HUGE volume difference.

When the stick is put in a bind, it will pull the engine right down indicating it is pressuring up. I would think if a pump was bypassing this bad, it would be loud, hot, and have no pressure at all. Can anyone confirm any of this or is this typical of piston pumps to lose a drastic amount of volume when worn out?

I should note that this is a variable pump but shares a signal pressure from another pump that is operating decent.

 

[willie59]

fastline, is this about the 225 excavator you've been working on? If so, it's not really a problem that you started a new thread. But in a way, you're cutting yourself off at the knees to get info you need. You now have a number of threads addressing various aspects of your machine and the problem you're having, which means there are responses hidden in numerous threads which can make it difficult to connect the dots.

To use an overly simple analogy, some mechanical problems, like a flat tire, are pretty easy to diagnose. But with many problems, it can be a list of various "clues" that a technician considers to get an idea of what the possible causes are. Therein lies your problem with numerous threads, all the little tidbits of info, for you to look at and us for to consider as well in helping you, are scattered across various threads making it problematic for both you and forum members who might be able to help you.

For your own benefit, I would try to keep things concerning a problem neat and tidy within one thread. But if you have a different problem, like engine suddenly started using oil, by all means, start a new thread about that problem.

 

[fastline]

yeah, I kind of debated starting different threads but there is only about 185 problems with this machine so things might get windy. I think I am trying to group problems together by association but as I learn more by the hour on it, I am determining what is related and what is not.

I will throw in a general update in this post about associate issues and I guess try to keep the hydraulic issues in this thread.

UPDATE: SN - 51U, Model - CAT 225, HRS - estimated 10-12K, neglected.

I verified that rear pump runs stick and left tract. I confirmed that both the left track and stick run slower than normal. Boom, bucket, and RH track run off the front pump and seem to operate decent for the age. Both pumps make normal noise which is not loud, both run about the same temp which is relatively cool. Both pumps receive parallel signal pressure for swash plate and since front pump seems ok, I am leaning towards signal pressure being ok. I have not put gauges on the machine yet but I do know if I put the stick in a bind, it WILL pull the engine down. In general, it seems like I have a huge lack of volume but seemingly normal pressure under load.



I also have been dealing with an inability to lower the stick slowly. It will jump down about 6" at a time. This is very likely unrelated to the pump volume issues but something to fix. Need to test pilot pressures but leaning towards air in pilot circuit or pilot value issue. No creep at all in suspect cylinder and no other odd losses of control.

 

[motrack]

Something else that will cause slow cycle times is problem with regenerative circuits. I dont Have hyd schematics for you unit so cant help with trouble shooting.
Only bringing up something I did not see mentioned as a possible cause of slow cycle times.

Rob

 

[fastline]

I will admit that I do not have experience building hydro pumps but we design and build race engine parts as well as engineer a wide range of parts. After studying the diagrams for the pump, I cannot for the life of me get myself to believe that the pump is so weak that it can only move 1/4 of the target oil or less yet still have pressure to get work done. I would expect decent volume until load is introduced and pressure would fall off drastically as volume also decreased. I would think this level of bypassing would cause noise and heat that would be obvious. Just trying to grasp some new concepts here.

One thing that is bugging me is that swash plate. Even if the pressure to the pump decreases which should let it run at full stroke per my understanding, I have to wonder if that swash plate is truly rotating where it should. I wish there was a visible rotator on the side to see it turn. The issues seems just like a pump that works just fine but does not pump enough oil.

 

[willie59]

fastline, you're muddying up the water in your own mind because your ignorance (lack of knowledge) of the way pumps work. A faulty pump, not a completely trashed pump, whether positive displacement or variable displacement, can still develop pressure, just takes longer to do it compared to a tight pump. A worn pumps deficiency is flow output. As for the generation of heat, it is true you will generate heat where oil is being forced by a surface of high restriction, like leakage of a pump, but with a variable displacement pump (hydrostat), they have a case drain to drain off internal leakage. This case drain would drain off excessive leakage of a worn rotating group and replace it with "cool oil" from system long before you would notice heat generation.

The dilemma for you is to determine if you have a faulty pump group. The easiest way to do this on an excavator circuit, though not always "easy", is to switch the output lines of the pumps and see if problems move to other functions.

 

[fastline]

F'N NAILED IT!!!!!

Servo valve to actuate swash plate. Swash plate was being pinned at min stroke. Removed signal line to servo and now have full function of stick and track. I would have to guess that the springs are either broke or weak and my guess is weak since it pushed the swash plate to max with no signal pressure.