Variable displacement pump catastrophic failure

My backhoe is using to combination pumps. This is my third replacement in less than 2 months.kindly assist me trouble shoot the possible cause on my combination piston pump. Using 1/2" horse combined as case drain.


1st failure of combination pumps

Pump 1

· Low pressure of 900 psi

· Wear on the valve plate

Pump 2

· Retainer plate bend and wear on the pistons holes

· Pistons shoes rounded, crashed and one piston separated from piston shoe

· Swash plate had cut on the inlet half circumference

· Slight wear on valve plate



2nd failure of combination pumps

Pump 2

· Pistons rounded on shoes

· Retainer plate cracked on kidneys and peeling on the drive shaft hole

3rd failure of combination pumps

Pump 1

· Retainer plate crack on kidneys and wear on the pistons holes

· Pistons shoes rounded, crashed and one piston separated from piston shoe

· Swash plate/cradle had cut on the inlet half circumference

· Wear on lower edge corner of the ball guide

Pump 2

· Retainer plate wear on the pistons holes

· Pistons shoes rounded

· Swash plate/ cradle had slight pitting wear on the low pressure half circumference

· Wear on lower edge corner of the ball guide

Did you flush the system?

I flashed for most of the pipes during the first replacement, but not the return to filter/tank main lines

Bearing in mind the lack of machine specific details, the two top causes of piston pump failure are, in my experience;

1. Contamination
2. Lack of flow to the pump inlet caused by, for example, a blocked suction screen or lack of tank head pressure.

This is rexroth A 10 VO 28 combination pumps. I managed the tank level well. The swash plates had cut on the low pressure half as shown on attached photos. Could that give a clue on the position of swash plate/craddle at the time of failure

When you have a pump failure putting that many contaminates in the hydraulic system a complete flush of the entire system is mandatory.
Do have or have you read the installation manual.
Did you follow the Rexroth installation instructions exactly as it is described in the manual ?
Particularly about filling the pump with oil and bleeding the air out of the system before start-up ?

I filled the pump through case drain with oil. Then I connected the case drain connection to tank which is designed to be below the oil tank level. 1/2" horse pipe to tank is connected to case drain to both pumps? Or the case drain pipe too small to serve the two pumps?

Those are damaged parts on opening

Start around page #26 I thinks.
https://md.boschrexroth.com/modules...AED1F692440.s142-vm&sch=M&id=14528,20,1208238

Is there any factory warranty on those pumps ?
Are you using the factory recommended oil ?
Other than new filters is there a screen in the hydraulic circuit that should be cleaned.

Looks to me like you have A cavitation problem as in supply or suction line restriction or high case drain pressure.
Look for
1. Oil too thick or high viscosity
2. Restriction in supply line
3. Lack of tank pressure on pressurized system.
4. Blocked case drain or plugged case drain filter

The case drain should be very little on a good pump and 1/2 inch line should do it . Just make sure there is nothing blocking it. Also make sure it’s not connected to the main return line causing back pressure.
Also is there any motors on the same case drain line that are failing and causing high case pressure. Check the case drain pressure with a gauge while working it if you can .

This pumps are installed on venieri 12.23 backhoe. 1/2 case drain for two pumps is connected to an approximately 1" horse hydrolic pipe from hydrolic rediator and this return horse enters the the tank below fluid level. As for inlet condition, first failure may have been due to cavitation because of the gun like sound. 2rd failure and third doubt because no machine gun like noise.

Suction and case drain lines has no filters. Filter installed only on main line to the radiator which connects back to tank.

Lack of a filter on the pump case drain line and lack of a screen on the suction side screams "Cheap" to me, especially when using a piston pump. Dunno about anyone else. Why the fcuk didn't the manufacturer use a gear pump if they were going to skimp on filtration..?

I think you are in for a complete system cleanout. I may miss something in the list below. If I do someone please shout out:-

1. Remove and clean hydraulic tank.
2. Remove and clean cooler
3. Remove, disassemble, and clean every hydraulic cylinder on the machine
4. Remove, disassemble, and clean every control valve on the machine.
5. Remove and clean every hydraulic hose & fitting.

NOTE: Order a few cases of aerosol brake cleaner - you're going to need them.

Just out of interest, what oil are you using in the hydraulic system..?

Pump 1 is connected to a priority valve which gives priority to steering wheel. Then any excess joins pump2 flow through a non return valve for work hydraulics. Pump 2 flow has a non return valve before joining excess pump 1 flow.

I am using shell tellus S2 m 46

That system has to be cleaner than a hospital operating room table when you get done doing everything Nige says.
I would be looking closely at those non-return check valves.

Eric Rop said:

I am using shell tellus S2 m 46

Click to expand...

Shouldn't be an issue with the oil provided it agrees with what the manufacturer recommends. What does the manual recommend..?

However IMHO you could go to an ISO32 (equivalent to SAE10W) with no ill effects. That would satisfy all lingering doubts as to oil being "too thick".

Otherwise refer to post #14 above. You may not like it, but you are in for a major amount of work.

Also to note, I think I am using close centre valves with compensator and load sensing dfr. Pump 1 which is connected to the priority valve had loop immediately at the output horse looped to its load sense port, I disconnected the loop because to me it was up stroking the pump unnecessarily if even when you required no flow. The steering valve has ls ports also the shovel valve and the backhoe valves.

What part of "completely clean every single piece of the system" is unclear to you..?

The only factor that system complexity is going to affect is how long the cleaning process takes to accomplish to an acceptable level of cleanliness when completed.
My bet is on at least a week, maybe two, even allowing for (super-clean) workshop facilities and tooling to be available. This is not a job that can be done anywhere.

Once you've got it up and running my suggestion would be to sell it PDQ. Or alternatively if you have a large boat attach a chain to it and use it as an anchor.