Excavator hydraulic pump help ?

[racerboy832]

I have a early 90s grey market mini excavator. It has an obsolete load sensing variable displacement axial piston pump. The pump has been rebuilt 3 times and failed 3 times. It was rebuilt by true pump builders and dyno tested. Parts are a nightmare to fine. The bottom line is I am done with the pump.

My assumption is it is a closed center valve block as there is a main relief on both ends of a split valve block. The original pump has two sections and a gear pump.

I want to look into going to a dumb old gear pump. They are cheap, are not as sensitive to contamination and used on a ton mini excavators. I dont care if it is not as fast or responsive. Its my yard machine to play with. Its not a money maker.

My question is? What do I have to do make it an open center system compatible with a gear pump?

The oil tank does keep a slight pressure. Should it be vented with a gear pump.
Any other suggestions?

Thank you.

 

[oldnslo]

Changing from closed center to open center depends on your valve. If it is convertible from closed center to open center easily then you can run a gear pump. If not convertible you will have to replace the valve with an open center valve or install some form of unloading feature to unload the gear pump when no functions are in use.

May also have some concerns with heat or oil capacity in the reservoir since gear pumps are constantly moving oil.

 

[Rich B1]

If the valve was made for an excavator, it is likely built as a closed center valve and isn稚 convertible. Tying to find and size an unloader valve for a gear pump is going to be a frustrating exercise. Plus that will be a crude solution at best, it is going to create heat.

Have you looked for a different piston pump?

And what keeps failing? Piston pumps with clean hydraulics don稚 fail frequently.

 

[wdchyd]

Curious.....when you did the previous hyd pump rebuilds did anyone pull the track final drives/hyd motors off to do a rinse/clean out to rule out damaged drives? You may already know this but the variable displacement piston pumps are mated to a dedicated drive motor that can spew contamination to newly rebuilt pumps.

 

[racerboy832]

I bought the machine needing a pump. Everything had been replaced but it never working I tried myself and couldn稚 get it to work. I sent it out to a big name pump shop and they had no luck. They changed everything and it still would not dyno out. After sourcing Japanese build sheets they found out it was missing two small pins in the regulators. I had to buy one new regulator to have a machinist make a copy of the pin. I put the machine together after going through every valve and spot I could flush. It lasted about 50hrs and I had a Cylinder fail and contaminated the system. Everything but case drains are filtered. I ordered parts from the pump shop. They were used but was told they are good. I built the pump and now everything is filtered. It was running good and one side of the pump exploded. It was a straight up mechanical failure. I am frustrated. It is not a money maker. Just a toy.

 

[IceStationZebra]

Who is the manufacturer of the pumps? Pumps don't normally explode even if they are worn out, sounds like it is dead-heading. You say everything is filtered - I assume you mean before they dump back into the reservoir, did it get emptied and cleaned out as well? Have you checked the output pressure with the system at standby and in use?

As far as switching pumps I echo everyone above. It will be much easier to switch to a different brand of pump than to switch type of pump. But even that could be a pain if they use an odd spline or port location.

ISZ

 

[Rich B1]

There are reasons old excavators become rebar......

Parts availability and trying to keep them alive are 2 of them....

 

[oldnslo]

Racerboy,
What do you mean by one side of the pump exploded? Housing crack, rotary group trash itself, etc.?

Since these are open loop, variable volume load sensing pumps the case drain pressure should be no higher than 5 - 6 PSI greater than inlet pressure. If case pressure gets to high it causes the piston slippers to lift off which will cause catastrophic failure of the rotating group in a few hours run time. Typical evidence of this is the slippers are rounded over, slippers can be pulled off the ends of the pistons in severe cases.

Normally filters are not recommended nor used in case drains on open loop, variable volume pumps for this reason.

Do you have the brand and model code of the pumps on this machine? I would be surprised if another brand of piston pump would not bolt up to your machine. Very likely that the ports will be different size or in slightly different location.

 

[racerboy832]

Here is a picture of the latest failure

 

[racerboy832]

The case drain from the pump is routed into the suction side of the pump. There is no way for it to build case pressure. It’s a Uchida Hydromatik pump. It’s an odd ball even by pump standards. Sure I can put another one on for $3-4K but is it even worth it. If I could slap on a $1000 gear pump and it would work I’d do that. I’ve seen so many pieces of equipment treated like crap, dirty fluid, filters never changed and still going on a gear pump.

 

[Rich B1]

Uchida is Rexroth licensee in Japan. They build good pumps. Find someone who knows Rexroth pumps, you likely can find parts or a suitable replacement.

Got any pics of the lens plate or face of the rotary block? You need to look at your case drain and suction side. Make sure you don稚 have suction screens that may be plugged or holding trash. I really don稚 like suction screens. They likely kill more pumps than they protect, IME.

 

[wdchyd]

My first observation was the pistons deheaded and piston retainer ring (ring with 9 holes), they were broken from excessive vacuum on the suction side of rotating group (depending on what direction of movement). So now I see you say the case drain is routed to the suction side of the pump, I can’t see this going too well if a suction screen or filter is plugged or restricted. The purpose of the charge pump is to keep a positive pressure inside the hose loop between each pump and motor rotating groups, when damage/wear starts blowing excessive amounts of oil out the case drain it’s an indication of too much clearance in the mechanical parts of both motor and pump and is suppose to route the extra oil back to the reservoir. I’ve not seen any case drains routed to the suction side, maybe it is normal for this machine but I have not encountered it.
The broken retainer ring broken and every piston deheaded shows too much vacuum IMHO.
You mention rebuilding the pump several times, but no mention of pulling the final drives. Whenever any catastrophic damage like this is encountered it is mandatory to pull the corresponding component to (at the minimum) do an inspection and complete clean out and repair any damage done by fragments running thru the final drive.
The previous owners failed at finding the root cause so I suspect you just unknowingly carried out the problems to this point. Putting in a positive displacement gear or vane pump is only going to work if you add open center control valves and replumb the existing hyd lines. Just retrofitting a new gear pump is only part of the work.

 

[oldnslo]

Racerboy,
I agree with wdchyd in that this is a system - circuit problem and not a component problem. Case drain connected to inlet is not the best circuit since case drain oil is the hottest and dirtiest oil in the system so running that right back into the inlet of the pump is just adding heat and dirt to the system.

If this pump is as you describe in post #1 a load sense variable volume open loop pump feeding the valve stack that controls all of the functions on your mini ex then you might have an easier solution for going to gear pump.

Use a valve like Sun LRDC or Rexroth Oil Control VLST which are normally closed modulating elements to act as load sense unloading valve for the gear pump. Connect the load sense line from the valve stack to the pilot port of the modulating element. Tee the pressure line to feed this valve and connect 3rd port to tank. As long as the load sense line is vented this valve will unload the gear pump. Start to operate a function and the load sense will cause the modulating valve to regulate the flow to valve and by-pass the excess flow to tank.

A couple words of caution.
1) You will have to verify that there is a relief valve in the system set to regulate maximum pressure. Typically the variable volume pump would have the max pressure limitation in its control and de-stroke maintaining pressure with out forcing the oil over the relief valve.

2) cooler capacity: The gear pump will potentially create more heat since when you are not consuming full pump flow doing work the excess flow is being by-passed at load induced pressure creating heat.

good luck on what ever you decide to do.

 

[racerboy832]

I’ve gone through every inch of the hydraulic system after it fail including the finals. The suction strainer in the tank is new and because the strainer isn’t on the bottom of the reservoir 90 percent of the junk just sat on the bottom. After I had filters on everything returning to the tank the only debris I found was in the area of the pump. The machine was barely above idle when the pump failed.

 

[racerboy832]

If I was a betting man it looks to me that the center cone shattered first. I’m only running 3000psi max. Each side of the pump has a main relief at the valve control body. The machine has a big oil cooler on it. It’s has an extra valve for a hammer and I know those make the oil hot. I have never put a hammer on it though.

 

[racerboy832]

I spend most of my days at work on equipment and wrenching at home on it. It’s always my own crap that gives me a headache. The case drain setup off the pump is factory setup. If it worked for the first 3000 hours I don’t see why it won’t continue to work.

 

[racerboy832]

If I was a betting man it looks to me that the center cone shattered first. I’m only running 3000psi max. Each side of the pump has a main relief at the valve control body. The machine has a big oil cooler on it. It’s has an extra valve for a hammer and I know those make the oil hot. I have never put a hammer on it though.

If that didn’t sound right. The main reliefs are at each end of the valve body that controls the joystick functions.

 

[oldnslo]

If I was a betting man it looks to me that the center cone shattered first. I知 only running 3000psi max.

Racerboy,
I respectfully disagree with your failure analysis. My thoughts in looking at those failed parts is that something is causing the slippers to lift off the swash plate. This can be caused by excessive inlet vacuum, high case pressure, pistons binding in the cylinder bores or combination of the above.

What does the bottom of the cylinder block look like and the mating wear plate?

I have experienced suction hoses failing when the inner tube in the hose collapse and the outer hose looks perfectly fine.

Again, good luck in finding a solution that keeps this machine operating.

 

[wdchyd]

Oldnslo,

Maybe lack of proper charge pump pressure?, or excessive blow by on the valve plates/block contributing to cavitation between the pump and motor. Theoretically to keep the piston shoes tight against the swash plates it would need the positive charge pressure and to prevent excessive vacuum. The pistons can take much pressure but pull apart when trying to suck oil from a starved loop.

I’m just throwing stuff out to see what sticks, I don’t doubt the experiences of the OP, certainly frustrating to have an endless ongoing issue on a machine. Our shop rarely works on any hydrostat pump if we don’t do the complete housekeeping, prime and breakin, Lots of things can go wrong in a short time.

 

[Rich B1]

I would still like to see a pic of the lens. I am guessing it will show significant cavitation.

Another possibility was the pump was not primed completely before engine start. I have seen pumps fail similar to that when they started dry. A lot of equipment has marginal head on the reservoir to pump inlet and don稚 prime the pump without help.

Or the suction line is restricted in some way.