From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Sealed hubs seal pumping direction
Date: 7 Aug 1997 18:07:18 GMT

Anthony Levand writes:

> I recently replaced the cones and ball bearings in my front wheel
> hub. The hub is a "sealed" type. I noticed that the seals are
> designed to retain the grease, that is that they pump inward.
> Working for a major radial lip seal manufacturer I know that the
> state of the art truck axle grease seals pump outward to exclude
> dust and water, the main contributor to bearing failure.

The direction of the seal lip is more a matter of where you want the
inside volume to be.  How that lip seals is not affected by where the
cup side faces, since sealing is only a function of contact pressure
and lubrication.  The seal must always weep (leak) or the contact edge
will burn and recede.  It is for this reason that such seals fail,
because you cannot successfully separate two fluids with a single
seal.  There is always fluid circulating under the contact edge and
with two fluids, they will mix.

> One can see in which direction the seal pumps by the lip geometry.
> They pump from the steep angle side to the shallower angle side.
> There is a whole science on the tribology of seal pumping that I
> won't get into. I wonder if anyone has had trouble with dirt or
> water migration in there hubs, say on mountain bikes that have been
> through muddy water. I installed my seals backwards.

You'll notice that in most applications elastomer lip seals are not
used in the way they are in bicycles because bicycle seals are
attempting to perform a service for witch they are marginally suited.
In automotive practice, separate seals are used to retain separate
fluids, such as between an automatic transmission and a differential,
or a wheel bearing and dirt where double seals are common.  Once the
oil of a bearing ceases to wet the seal lip, it gets fried and becomes
a capillary gap for any water that comes along.

Jobst Brandt      <jbrandt@hpl.hp.com>