Date:    Mon, 23 Dec 2002 15:04:39 PST

Bearing Seals

> What is a labyrinth seal?  For that matter, even though I think I
> can picture it, what is a contact seal?

Moving seals are a more complicated than they first appear and are
only slightly related to fixed seals such as beer caps, mason jars,
and gas or radiator caps.  This is best emphasized by the old saying
that "the seal that doesn't leak, leaks" that being the essence of the
problem.  If the seal doesn't leak a little, its flexible sealing lip
will burn for lack of lubrication from the fluid that it is intended
to contain.  Therefore, there must be fluid under the seal lip.

If a seal is intended to contain oil and seal it from water, the
principal problem is one of mixing disparate fluids under the seal
lip.  Because circulation occurs under the seal lip, an emulsion will
develop and even if the volume of oil on the inside is too large to be
contaminated significantly, the shaft will rust when standing,
destroying the seal lip.  Automotive bearings are sealed to retain
grease and oil but are protected from water exposure by splash
shields.

Separating two fluids requires two seal lips separated by a drained
dry space.  This is done on automatic transmission and differential
gears with incompatible oils, to prevent contamination by circulation
under each seal lip.  This is not possible with oil and water on
bicycles because there is no water most of the time, leaving the water
seal lip dry and unlubricated, which renders it useless when exposed
to water.

Most so called sealed bearings are not water tight, mainly because
they have run dry, burning the seal lip which becomes a capillary to
suck water when wet.  Phil Wood used bearings designed for used in
electric motors that use a rubber lip seal to prevent air (dust) flow
that always occurs in rotating machinery that sucks at the axle and
blows at the periphery.  Such bearings were never meant to prevent
water intrusion, something they can do only for a short time when new.
This is the main reason why such "sealed" hubs were not available at
the time he introduced them.  To make this work, one would have to
protect the seal lip from contacting anything but oil by a shield,
otherwise known as a labyrinth seal.

The most common labyrinth seals on bicycles are found on Campagnolo
Pedals, threaded head bearings, and above all on Sturmey Archer
3-Speed hubs that are rust free and working more than 50 years after
manufacture.  Bendix and New Departure coaster brakes are also
examples of excellent water rejection unless submerged.

The nature of a labyrinth seal is that it uses gravity to purge water
from its entrance.  Typically this requires nothing more than two
nested channel cross section washers of two diameters, one rotating in
the other that is anchored in the housing.  To visualize this make a
"C" shape with both hands, interleaving the thumb and forefingers so
they move freely in a rotary motion from the elbows.  You can see
that, vertically, water has no ability to enter, and tilting the pair
either way only enhances the barrier.

The last such device I am aware of was the New Winner Pro Sun Tour
freewheel, whose labyrinth was visible as a tiny brass ring on both
faces.  It's problem was that such a seal must take into account the
wetting angle of water and must have a large enough air gap to prevent
capillary attraction.  The Sun Tour execution lay at the lower limit
with its small spacing but they worked under most conditions.

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