Excavator hydraulic pump help ?

[oldnslo]

Wdchyd
From the pump description there is no charge pump. This is not a closed loop hydrostatic drive but and open loop variable volume pump that uses closed center valves.

Gets confusing since there are also open center systems that have fixed displacement pumps and require power beyond.

 

[wdchyd]

.................The original pump has two sections and a gear pump.

Oldnslo, I was assuming it was a tandem piston hydrostat with aux gear pump.

 

[oldnslo]

Oldnslo, I was assuming it was a tandem piston hydrostat with aux gear pump.

wdchy,
Racerboy also stated: I have a early 90s grey market mini excavator. It has an obsolete load sensing variable displacement axial piston pump. So based on this I presumed it was an open loop load sense pump. If tandem hydrostatic then he definitely has charge pump issues.

Sure be nice if the hydraulic world didn't have the potential for confusion on system styles.

My definitions of different systems styles:

1) Open center: This is a circuit where valves are plumbed in series and is constantly flowing through the valves in the spring loaded condition from the pump to the reservoir and typically has a fixed displacement pump.

2) Closed center: This is a circuit where the valves are plumbed in parallel and block all flow in the spring loaded condition. These typically have a variable volume pressure compensated pump.

3) Open loop: Can be either open center or closed center but the oil flows from the pump back to the reservoir.

4) Closed loop: Oil flows from the pump directly to the motor and from the motor directly to the pump. These require a charge pump to make up for oil losses - leakage in the system and typically called hydrostatic drives.

I believe Racerboy has systems 2 & 3. Possible that he has system 4

 

[rScotty]

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.

This has been a fascinating discussion on closed center systems for us hydraulic enthusiasts. But I'm wondering what you've decided to do?

Getting back to the original question....it seems easy enough to change to a closed system. Nice simple pump (how to drive it?), new control valves (probably, but check that), and vent the reservoir. Limit the pressure in the system with a relief valve or two. Sounds like you have cooling and reservoir capacity but check.
Then it's good to go.

There's reasons why simple, inexpensive, & rugged hydraulic systems on most tractors are open center.

 

[wdchyd]

“Re: Help with swapping hydraulic hoses to check weak. Mine has 3 sections. Gear for pilot controls, 2 piston pumps for main functions. Yours may just have one pump for all functions. If that is the case it is probably a worn wheel motor. I cannot see how your pump gets fluid. Its possible one is the feed, one pressure and other is the case drain.“

This quote by racerboy from another recent thread

 

[racerboy832]

Wcchyd has it correct. Mine is a two section piston pump with a little gear pump that controls the pilot controls. The valve plate side of the rotating group looks like new. There is a couple tiny little nicks in it but I think that was from after that it let go. I do not believe that I have a charge pump. Positive pressure in the reservoir keeps the fluid in the inlet side of the pump. Even the other side of the main pump that shares a case drain looks great.

 

[racerboy832]

I could be completely wrong but I would think my current setup is a closed loop system and I would want to make it an open loop to run a dumb old gear pump.

 

[racerboy832]

Is there anyway to verify what type of system I have?

 

[racerboy832]

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.

The valve side of the rotating group looks good. Only a couple nicks in my opinion looks like it happened when the pieces were bouncing around. The other rotating group looks like I could reuse it if I wanted to. I am open to all theories. The intake side of the pump is all steel tube except for a 6 inch rubber coupler. It looks good. Thank you got the help.

 

[racerboy832]

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.

I alway open the highest points at the pump and bleed the air out. The tank is higher. I won’t start anything till all the bubbles stop and it’s solid fluid. The inlet side is steel pipe. Only one rubber coupler.