Insane Hydraulics

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"Achilles Heel" Pump Design

Just look at this mess!

Rexroth A4VG Pump Catastrophic Failure
Rexroth A4VG Pump Catastrophic Failure
Rexroth A4VG Pump Catastrophic Failure
Rexroth A4VG Pump Catastrophic Failure

This pump is trashed beyond any sensible repair. What a shame! But let me tell you one thing now - not only have I been consistently coming across this same, very peculiar and easily identifiable catastrophic failure pattern ever since I began overhauling hydraulic pumps, but also - in 99 cases out of 100 it's been absolutely avoidable!

I mentioned this before in many other posts, but I keep seeing the same failure over and over (these particular pics are from the last week, for example) - and so I decided to make a separate article on the subject.

All such catastrophic hydraulic pump failures have the same cause - the breakage of one of the races (a.k.a shells) of the roller swashplate bearing! This is why I refer to any pump that employs roller bearings to support the swashplate as the "Achilles Heel Pump Design" - because when such pumps work at their rated pressures - the races are universally the first thing to break!

Don't get me wrong - this is not a bad design. The rollers provide the advantage of low friction and virtually no difference between the starting and the steady state torque - but - the fact that a couple of dozens of pretty narrow needles support a load of several tons, that vibrates at high frequency due to pressure ripple and often stays at the same position for long periods of time means that very often these bearing a subjected to two things:

a) extremely high contact pressure, and

b) zero lubrication under the rollers

something that leads to the appearance of very distinctive marks both on the races and the swashplate sometimes referred to as "brinelling".

The most treacherous thing about this wear is the fact that even when the marks are very deep, the pump operates perfectly fine. However, as soon as such a race cracks - the failure is instant and catastrophic. This is the race from the pump I just showed you:

This race has very deep roller marks

See how deep these roller marks are? This is a big no-no. They should have been replaced, but no one thought of overhauling a pump that worked perfectly fine! Have seen this far too many times. The only way to uncover this wear is to open a pump and inspect the parts.

From my practice, the most common models that show this wear ate the Rexroth A4VGs, the Rexroth A11VOs, and the Danfoss series 90. I get them all the time. But I'm absolutely convinced that any pump that employs a roller bearing system to support the swash plate has the same "Achilles Heel". For example the Danfoss A11VO "clone" - the D1P, or, maybe, the Linde's BPV (if you can still find one). There's more.

So, my plea to all the hydraulic pump owners out there is:

If your hydraulic equipment uses a pump, that has a roller-bearing-supported swashplate (you can easily find this out by looking up the respective parts list), and you don't want your pump to suffer a catastrophic failure, please, please, note well the following two things:

1) An overhaul every 8000 - 10000 hours is mandatory for such units. Even if the pump is operating perfectly fine! It will give you the chance to inspect the bearings and replace them before they break!

2) If you bought used equipment, that uses such a pump, and you don't know the service history of the pump - do schedule an overhaul asap to evaluate the pump's condition, especially the said bearings.

This advice may seem contradictory to some, because, well, we are mechanics, and let's be honest - we make money when clients break things, right? Yes, this is true, but we want our clients to break things "just enough". When a client has a pump or a motor that needs an overhaul - we are happy to provide it, but when an OEM-protected pump is broken beyond repair, he will get a new pump from the respective brand dealer, and not from us! Bad for us, because we waste time opening a unit that we won't be fixing, and bad for the client because he gets hit with an unexpected and painful expense! And we want our clients to be as rich as possible, don't we?