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.