Full-complement (cageless) roller bearings, used in many hydraulic
pumps as shaft bearings (Rexroth A4VG series for one), are extremely
robust and assure failure free operation for many thousand hours. They
hardly ever get replaced during overhauls, which is probably why some
mechanics and machine owners think that these bearings should never be replaced (alternatively - replacing such bearings is a waste of money), since they are way too tuff and easily outlive the rest of the pump. This is nonsense, and here is why.
Indeed, full-complement bearings are super-heavy-duty due
to the fact that load is distributed on a higher (compared to the caged
design) amount of rollers, but let us not forget that hydraulic pump
shaft bearing failure most of the times is caused by oil contamination,
which, in its place, more often than not has its origin in the warn-out
internal components - the very ones our bearing is supposed to
"outlive"...
It is absolutely true that such a bearing can outlive its "host" pump - but it can only do it once -
and since a pump can get "many lives" through its "career", cageless
bearings must be disassembled and inspected very carefully every time a
pump equipped with one gets overhauled. If any signs (even minor) of
pitting are present the bearing MUST be replaced. Damage to the
internal pump components, caused by a "simple bearing failure" can be
quite painful!
Take a look at these pictures - here you have
some rotary group parts from a Rexroth A4VG closed loop pump. During a
routine filter replacement fine brass fragments were detected in one of
the filters. Despite the machine showing no signs of performance loss,
the owner opted for disassembling the transmission to evaluate the
condition of the pump and the motor. Upon inspecting the pump, some
serious abrasive particle wear of components was discovered, especially
noticeable on brass parts. The ball guide was scored, the retainer plate was scored, the piston shoes were grinded down to half of their normal height, and the cylinder block bores
were severely scratched by particles caught in the clearance gap
between the piston and the brass sleeve.
All the damage suggested one thing - case oil hard
particle contamination. And where do you think these hard particles
originated from? That's right - from the shaft bearing!
The pump had hardly done a thousand hours since the last major failure.
Want to make a wild guess about which part hadn't been replaced?
So, cageless roller bearings are robust, yes, they do last long, yes, but they are nothing but bearings, therefore should be treated as such - on the "must inspect and replace when necessary" basis.
