However, they were heavy and had a tendency to
exhaust oil onto the pressure side of the pump. This oil
vapor in the exhaust is messy, and if the pressure side is
to be used for de-icing boots, it must be separated out
before the air reaches the boots, since oil can cause the
rubber boots to deteriorate.
Eventually, dry pumps were created as an alternative
solution. Dry pumps are a lighter and simpler form of
vacuum power than wet pumps. Dry vacuum pumps consist of an elliptical chamber, within which a circular rotor
spins and has floating vanes to seal against the sides of
the chamber. The vanes ride up and down in their slots,
following the contour of the pump chamber, pulling air
out of the intake ports and pushing it out of the exhaust
ports. In most pumps, both the rotor and vanes are made
from a self-lubricating carbon material.
Dry pumps have become the standard source of
vacuum power in modern piston aircraft. However, they
have also proven to be a weak link in a very critical aircraft navigation system.
Since vane wear is the single, predictable failure mode,
some pump manufacturers have made improvements in
the ability to inspect the length of the vanes. This way,
pumps can be removed from service before the vanes
become critically short. One example is the Tempest
Tornado line of vacuum pumps, which include a wear-indication port (WIP) on the back of the pump housing
that allows a mechanic to check the wear of the rotor
vanes. It’s a simple check that does not require removing
the pump from the engine. You simply unscrew the WIP
and conduct a visual check of the remaining length of
the vanes, making a simple go/no-go decision.
Contamination—Aside from pump wear, failures
can often be traced back to some form of pump contamination. Anything that gets inside the pump can cause
the vanes to jam and fracture. This includes tiny pieces
of rubber hose, Teflon tape, oil, water, or even carbon
shards from a previous pump failure.
When a pump fails, it can send carbon pieces back
upstream into the vacuum lines. It is critical to clean or
replace all vacuum hoses in order to eliminate carbon
pieces that could quickly destroy the new pump. All
hoses should also be inspected to ensure that they have
no internal deterioration that could cause rubber pieces
to be ingested into the pump. If you do find significant
carbon in the system, you might want to have the gyros
overhauled at the same time. The same contamination
that can cause a pump to fail can cause early gyro failure
Why Pumps Fail
As I mentioned earlier, the challenge with a dry pump
is that when it fails, it fails catastrophically and without
warning. Vacuum pumps are designed to extremely tight
tolerances, and the carbon rotor and vanes that make
up the power section of a dry pump are very brittle.
Anything that puts abnormal stress on the rotor of vanes
can cause them to fracture and fail.
That said, dry vacuum pumps not only fail
spontaneously, but also due to one of four
things: excessive wear, contamination, mishandling, or overheating.
Excessive Wear—Wear is the one predictable, and preventable, factor in vacuum pump
failures. The carbon vanes slide in and out of
very tight tolerance slots in the carbon rotor as
they follow the curvature of the pump chamber. They are designed to wear at a predictable
rate, providing slick carbon dust that serves
as lubrication for the system. Over hundreds
of hours, the vanes become shorter until they
reach a point where they are no longer properly supported by the rotor. Once this happens,
the vanes can chatter and become cocked at
an angle in the slot, jamming in place and
shattering as the pump continues to turn. The
vanes can also wear in the slot, allowing play,
and contribute to the problem. This is known
as flank wear, and it progresses as the vanes
get shorter. Excessive pressure from either poor
pressure regulation or from high-demand applications can also contribute to flank wear.
When a rotor vane wears down too much, it can become cocked in the slot and
then jam and fracture.