screw airend manufacturers have used many types of shaft seals over the years.
In most early airend designs, there were two distinct categories of shaft seals:
seals made from various types of PTFE commonly
known as Teflon, came into the market in the early 80s as manufacturers
discovered that the much stiffer PTFE shaft seals could withstand high pressure
as well as synthetic compressor oils. They work equally well in oil free or
lubricated compressors, and are easy to mass produce in those custom OEM sizes.
a shaft seal leak problem requires that you know what type of seal is used in
your airend. To help in this discussion, we will divide them up according to
types mentioned above.
mechanical seals are designed as a lubricated part. Usually there is an oil feed
to and an oil scavenge from the shaft seal. The shaft seal may weep oil
continuously while running if this scavenge line should become plugged. This
seal scavenge line usually attaches to a fitting on the bottom of the shaft seal
housing. The fitting and line should be checked before replacing the seal. In
some airends, a check valve may be installed in the seal scavenge line (or in
the inlet valve area where it connects) which will cause a seal to leak if it
a leaky mechanical seal is accompanied by a jingling sound from the drive end.
This might point to a broken seal spring. These coil springs are responsible for
pushing the rotating seal up against the stationary seat, compensating for wear
throughout it’s lifetime. However, they occasionally break, allowing the
components to flop around, thus the jingling sound. Time for a new seal here,
and hurry before the seal components get into the drive end bearings, creating
another set of problems!
seals can handle more shaft runout than lip seals. However, before replacing
that leaking seal, isolate the
compressor electrically, disconnect the coupling or slacken the drive belts and
check the shaft side play with a dial indicator. If you have more than
.005” side play in this input shaft, the leaky seal may be pointing to
an impending drive end bearing problem.
the problem may not be the seal itself but instead the leak may result from a
failed O-Ring. In many styles of mechanical seals, O-Rings are often used to
seal the stationary seat to the seal housing, as well as the housing to the
airend. A “weeping seal” problem may turn out to be a failed O-Ring or seal
housing gasket. Make sure when replacing this O-Ring that the replacement is
made of the proper material to be
compatible with your compressor oil and use gasket adhesive on the seal housing
the seal manufacturers instructions when replacing a mechanical seal. Be sure to
lube up the inside of the rubber “boot” before assembly. Some seal
manufacturers recommend using petroleum jelly, others suggest using the
compressor oil or light mineral oil. The
lubricant allows the rubber sealing boot to slide on the shaft during assembly
and bond to the shaft shortly afterward.
When replacing the seal housing, be certain the housing and gasket line
up properly with the scavenge line porting on the airend.
type shaft seals are widely used in rotary screw airends and come in a variety
of designs and materials. This seal functions to retain the oil inside the
airend, but also to keep air, dust and dirt from sucking in along the rotating
shaft when the machine is off load. (When the inlet valve closes, if the airend
continues to rotate, a vacuum may be pulled on the inlet end.)
general, lip seals are much more sensitive than mechanical seals to shaft runout
or surface irregularities. A lip seal will often leak oil if the shaft has a
total indicated runout of more than
only a few thousandths of an inch. Therefore it is even more critical before
replacing a leaking lip seal to check the radial or side play in the shaft. You
cannot assume because the shaft runs true when spun by hand that it will do so
at speed. An out of balance coupling or drive belt pulley will cause shaft
runout in an airend with a worn bearing. Tight belts will also pull the shaft
away from the original centerline as bearing wear increases, putting all excess
clearance on one side of the seal. Again, you should
isolate the machine, and loosen the belts or coupling. Physically push
the shaft back and forth while measuring shaft deflection with a dial indicator
to check for bearing wear Remember, excessive movement in this shaft, especially
in belt driven airends is warning that more serious damage may be occurring
inside the airend. Better to find it now.
type shaft seals cannot withstand high pressures. A seal of this type which
leaks profusely upon compressor shutdown may be an indicator of a bad airend
check valve or oil stop valve, allowing full discharge pressure to reach the
replacing a rubber lip type shaft seal, after checking to be certain that the
shaft is round and running true we use a Loctite sealant between the seal and
the seal housing. Press the seal into the housing and lube it up with compressor
oil. We oil the shaft and, after installing new seal housing gaskets or O-Rings,
slide the seal and housing onto the shaft. On shafts with no reduction to the
keyed drive area, a dummy keyway plug may be required to avoid shaft damage.
Take care to center
the seal housing while sliding it on, ease the seal over the shaft, and
carefully tighten down the housing bolts in a crossing pattern.
shaft seals are becoming very common in newer airend designs. The use of a PTFE
single or double lip shaft seal allows the compressor manufacturer to eliminate
the airend check valve as well as an oil stop valve from the package. In a rare
case of less is more, this not only reduces OEM cost, but eliminates without
penalty, two components which have sent many airends to a premature grave.
the rubber lip seals mentioned above, PTFE seals cannot tolerate shaft runout.
In addition, PTFE seals have a tendency to cut a groove in the shaft due to the
abrasive nature of the tough seal material and the contaminants in the oil.
Therefore, most better airend manufacturers design removable shaft seal sleeves
to take this wear. We highly recommend the replacement of this seal sleeve every
time a PTFE seal is replaced. The seal sleeve may also have a small O-Ring seal
behind it which must be replaced. If not we recommend that a Loctite sealant be
used to seal the sleeve to the shaft, as hot, pressurized oil can weep under the
sleeve even if the PTFE seal has seated perfectly.