Just the other day I was asked an opinion on a hydraulic pump shaft
seal failure. The client was claiming warranty, as the pump was only
three months old, and the shaft seal was already leaking "severely". It
was an all-too-common you-find-em-everywhere PZB bent axis fixed
displacement pump, a very common type of pump for commercial vehicles.
There are many brands of such pumps on the market, many of them
interchangeable, and all have one common feature - they don't require
drain connection as the case is internally connected to the suction port.
Normally this pump would be connected to the vehicle's PTO through a
pneumatic clutch.
This particular PZB (cut view)
has the outer shaft bearing isolated from the case oil. That is why the
bearing is sealed and has grease inside. A quick glance at the pump in
question rulled out any warranty, as the bearing's outer shield had been
pushed out by oil pressure, which meant that the pressure inside the
pump rose at least higher than maximum pressure the shaft seal can
take, which for these seals is around 5 bars.
Further analysis of the pump showed severe scoring
of the valve plate, cylinder bores and piston snap rings. So, there were
two problems present. First - somehow a rather high positive pressure
was created inside the pump's casing (connected internally to the
suction line), and, second - particle contaminated oil.
When the client heard about contaminated oil, he reassured that the oil was clean, as the whole hydraulic installation was NEW!!!
This is one situation I see too often. There are many people, even
people who have worked around hydraulics for years, who still believe
that new hydraulic installations are clean. In hydraulics "NEW" most of
the times means "CONTAMINATED", especially when talking about new pipes,
tanks and hoses. Even the NEW oil that comes from a sealed drum has
cleanliness level that's not clean enough for most hydraulic
components.
Now let us think for a moment about how it is
possible to create a positive pressure inside a pump that has casing
connected internally to the suction line. Obviously, we have to create
the pressure inside the suction line. First thing that comes in mind is
the client confusing the pressure and suction ports, and driving the
pump in the wrong direction. But what if the pump is connected to a diesel
powered PTO, which rotates in one direction only?
Well, first of all, there is a chance
that the PTO might change rotation due to some unusual circumstances,
but it's way remote. Then there's a remot-er chance of a
pressure rise inside the oil tank, probably due to high oil temperature
and malfunctioning breather. But there is one another possibility for
the pump shaft to rotate in the wrong direction - the system back flow,
which is possible in poorly designed or "economically simplified" (dirt
cheap) systems.
Imagine that you disengaged the clutch, and
still have the crane arm lifted. Now imagine that the distributor valve
has no load drop check valve inside (or the valve is stuck-open)
and the lifting cylinder has no load holding valve, with the only thing
holding it in the air being the closed center spool. If you
move the spool in the way, that connects the load holding line to the
pressure gallery, the load induced pressure will connect to the pump's
pressure line and, as the pump is disconnected from the PTO, the pump
will start turning like a motor, creating positive pressure inside the
suction line and the pump casing. If, at the moment, you have the suction
line ball valve closed, you'll have instant seal failure. If the line
isn't closed, the pressure inside the casing can still climb high
enough to bust the seal, depending on oil flow, the suction line
design, and, of course, oil viscosity.
In a properly designed system, there would be no way for such a failure to happen, but
this situation is still an example to consider. In any case, when
a shaft-seal failure happens, the whole circuit must be checked for possible causes.
