Link Belt LS4300 CII Pilot Pump replacement and failure

As we've been resurrecting our semi-abandoned 1989 beast we've corrected and replaced many items before finally getting to the point of testing our hydraulics. As a number of my previous posts on this machine show, we finally got down to a badly worn pilot pump and subsequent low pilot pressures (50-150 psi). We went ahead and pulled the main pump to replace the original pilot pump, cleaned and put things back together with fresh hydraulic oil (after ensuring that the tank and suction line was clean with no debris or dirt), then fired the beast up with good pilot pressure (over 1200 psi before adjusting the bypass valve down to around 600 psi) and high pressures off the two main pumps of around 5000 psi.

All all functions except travel working great... for about 30 minutes when we lost everything. Some pressure tests showed that our pilot pressure had dropped to around 25 psi. Inspection of the pilot filter showed metal particles that told us the new pump has gone south.

When I first got the new pump and before installing I tried turning the pump shaft by hand and couldn't get it to turn. I then tried some pliers and with a little effort it finally gave, though as the gear teeth meshed I could feel a resistance as if there was insufficient clearance. Not being familiar with this particular style and assuming this was normal for a new pump I put it in and assumed it needed to be run in order to "wear in". Wrong.

I got the pump (around $780) from Hydraulex out of Puyallup WA with O rings, new bolts, and the adapter gear. I was assured that it is a compatible replacement for the original Uchida GZ-C5Z. Has anyone else had something like this happen? I've had many years of experience in repairing and rebuilding automatic transmissions, shuttles, and hydraulics in general without significant problems such as this.

I will know in a couple of days what the new pump looks like when I pull the main pump for inspection and photos of the damage. For comparison I've included photos of the old pump showing what 5000+ hours of operation looks like.

It certainly happens with new pumps... being "locked" from new. Just not enough clearance. They run out fine

Don't think that is the problem you're having though. The old pump looks the shaft bearings were worn out... and gears ran well into the housing.... is that from bad oil cleanliness or viscosity or....

Most all pilot pumps are inline so can't tell you what is typical life expectancy with the offset (A8?) style

When you get the old pump off Id measure the end play in both directions of your pilot pump.... might be something simple like the gear went out...... did you measure gear backlash?

mikebramel said:

It certainly happens with new pumps... being "locked" from new. Just not enough clearance. They run out fine

Don't think that is the problem you're having though. The old pump looks the shaft bearings were worn out... and gears ran well into the housing.... is that from bad oil cleanliness or viscosity or....

Most all pilot pumps are inline so can't tell you what is typical life expectancy with the offset (A8?) style

When you get the old pump off Id measure the end play in both directions of your pilot pump.... might be something simple like the gear went out...... did you measure gear backlash?

Click to expand...

Thanks for your insight and thoughts Mike. This machine was purchased used by the previous owner around 2008 and ran for about three years before many problems became overwhelming for him. It was purchased from a contractor who probably bought it new in 1990 and likely put over 4500 to 5000 hours on it as there are notes and invoices scattered throughout the operator and parts manuals.

The pilot pump is apparently driven by a spur gear off the main input shaft. I can check for play on the spline drive gear but don't want to tear into the housing since that doesn't seem a likely cause for failure of a new pump. When I open the new pump up tomorrow for inspection I'll be able to see any wear on the bronze bushing for the shaft gear and gear faces. Thirty minutes of idle speed operation and less than two minutes run time at full throttle shouldn't have caused the kind of failure we experienced.

But only inspection after teardown will tell more of the story. Yes, we were very careful to ensure that oil, tank, and suction hose were clean and that there were no metallic particles or other contamination before we buttoned things back up prior to running. A picture of the housing and pump location is below.

Thanks again, -Tom

I'm happy to report that the supplier of the new pilot pump has amicably agreed to send a replacement, though disagreed with our assessment of why it failed so soon. But as they said that they valued me as a customer, and as a gesture of good will they're sending a replacement. I plan to run the new one in submerged in a bucket of oil for an hour or so and run it at low speed until it frees up during run-in. Needless to say, I want to take every caution I can so that I don't have to repeat this exercise again. Not too much praise can be offered to a good and reputable parts supplier, so my thanks to the folks at Hydraulex, Puyallup WA for their integrity and honesty!


View attachment 211314

Looks like debris ran through. Before the input shaft bearing failure (cause) or after it? Or is the input shaft overloaded, is the bushing worn to one side

mikebramel said:

Looks like debris ran through. Before the input shaft bearing failure (cause) or after it? Or is the input shaft overloaded, is the bushing worn to one side

Click to expand...

There is a large chunk of the bronze input gear missing that seems to account for softer material contamination running through the chamber which we attribute to inadequate clearance for oil between the shaft and bushing wall. The scoring of the gear shaft is further evidence that there was catastrophic failure within the first minute or so of operation.

We took care to ensure that the tank and intake was clean and clear with no evidence of any contamination, fresh tank filter, fresh fluid, and a later confirmation that all 28 pump plate retainer screw heads were still snuggled where they should be. Much of the failed pilot pump original bluing on the internal parts is still intact. I'm pretty certain that we got a bad pump. Running the new replacement pilot pump in at low speed here at the shop before taking the pump assembly back up the mountain for reinstalling should move us past this present roadblock in getting the beast back into the land of the living. Now that the weather is warming we have work to do!

Thanks for your input Mike. All comments are appreciated and valued.