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Rec.Bicycles Frequently Asked Questions Posting Part 4/5
Section - 8f.6 "Sealed" Bearings

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> Has anyone had any major problems with the Shimono XT "sealed" Bottom
> bracket besides me?

This subject comes up often and has been beat around a bit.  There is a 
basic misconception about seals.  The seals commonly sold in the bicycle
business are not capable of sealing out water because they were never
designed for that purpose.  These seals are designed to prevent air from
being drawn through the bearing when used in, typically, electric motors
where the motor rotation pumps air that would centrifugally be drawn 
through the bearing.  If this were permitted, the lubricant would act as
fly paper and capture all the dust that passes, rendering the lubricant
uselessly contaminated.

Seal practice requires a seal to leak if it is to work.  The seepage
lubricates the interface between shaft and seal and without this small
amount of weeping, the seal lip would burn and develop a gap.  In the
presence of water on the outside, the weeping oil emulsifies and
circulates back under the lip to introduce moisture into the bearing.
This is usually not fatal because it is only a small amount, but the
displaced grease on the lip dries out and leaves the lip unlubricated.

The next time water contacts the interface, it wicks into the gap by
capillary action and begins to fill the bearing.  This is an expected
result for seal manufacturers who live by the rule that no two fluids
can be effectively separated by a single seal lip.  Two oils, for
instance, must have separate seals with a ventilated air gap between
them.  If a seal is to work with only one lip the contained fluid must
be at a higher pressure so that the flow is biased to prevent 
circulation.

None of the effective methods are used in the so called 'sealed'
bearings that Phil Wood introduced into bicycling years ago.  His 
components failed at least as often as non sealed units and probably
more often because they make field repair difficult.  These are not
liquid seals but merely air dams.

jobst.brandt@stanfordalumni.org

 [More from Ben Escoto <bescoto@stanford.edu>]

 Date:    Sat, 07 Nov 1998 21:31:31 -0800
 Subject: Additional entry on bearings for FAQ


	Although the entry on "Sealed" Bearings (8.44 as of the
10/7/98 FAQ) provides useful technical information on seals, many
readers may not be able to directly apply it to bicycling on a
practical level.  I asked about this on rec.bicycles.tech and received
helpful responses from Jobst Brandt, Matt O'Toole, and Hans-Joachim
Zierke, among others.  I hope the following summary will be an
interesting and useful supplement to the entry mentioned above.

	Firstly, it is important to distinguish between bearings that
are protected by a seal and bearings that cannot be individually
removed because they are locked in a larger structure.  The first I
will call "sealed bearings"; the second are more properly called
"cartridge bearings."  Bearings in hubs, bottom brackets, etc (whether
cartridge or cup-and-cone) on modern quality bicycles are usually
sealed.  For a better description of the difference between
cup-and-cone and cartridge bearings, see the entries under "Cartridge
Bearings" and "Cup-and-Cone Bearing" in Sheldon Brown's excellent
bicycle glossary (http://www.sheldonbrown.com/glossary.html).


	So, for the reasons Mr. Brandt explained in the other entry,
bearings on bicycles are not truly sealed, in the sense that water and
dirt cannot enter under any circumstances.  The best designs include
two seals: a contact seal closer to the bearing, and then either a
labyrinth or a second contact seal further out.  The outer seal in
hubs with double contact sealing should be oiled when the hub is
serviced, because this seal is not lubricated by the bearing grease
like the inner seal.

	But even well-sealed bearings (of any type) can be
contaminated if exposed to pressurized water, as can happen in heavy
rain, if the bearings are submerged, or if you spray your hubs with
water as you clean your bike.

	Given this, both cup-and-cone bearings and cartridge bearings
will occasionally need to be serviced.  Here are some pros and cons of
cartridge and cup-and-cone bearings regarding their maintenance.


	Cup-and-Cone:  Cup and cone bearings are usually easily
disassembled and serviced by cleaning the races, replacing the
bearings, relubing, and reassembling.  Also, individual bearings are
quite cheap to replace.

Although the cup and cone races are usually resist pitting better than
their cartridge bearing counterparts and rarely need to be replaced, a
ruined cup in a cup-and-cone hub, for example, may require that the
whole hub be scrapped.  Campagnolo is one manufacturer who makes hubs
with replaceable cups and keeps spare parts available enough that
repairing hubs in this way is often feasible.


	Cartridge: Cartridge bearings are usually harder to service.
The cartridge seal is easier to break during disassembly and often the
cartridge is not removable so the bearings are much harder to clean.
Additionally, the races inside the cartridge are often more poorly
made than the races in cup-and-cone bearings and more prone to damage
and rust.  Components with irreplacable cartridge bearings are much
less maintainable than those with cup-and-cone bearings.

However, the cartridges in some components (for instance the hubs made
by Phil Wood, Syncros, and others) can be replaced without a bearing
press.  These cartridges are much easier to repack and can be replaced
easily if damaged.


	So, what practical significance does this have?  Cup-and-cone
bearings are superior (in terms of maintainance) to irreplacable
cartridge bearings.  There doesn't seem to be a consensus on
cup-and-cone bearings vs the cartridge bearings found in, e.g., Phil
Wood's hubs.  As of this writing (Nov 98) both Campagnolo and Shimano
have stuck with cup-and-cone bearings for their hubs, while most third
parties are manufacturing cartridge bearings, probably because
cartridges are much easier to manufacture than cup or cone races.

	Right now Shimano makes the best inexpensive hubs:  they are
sealed correctly (double contact or contact/labyrinth), are fairly
durable, and are quite serviceable.  Hubs such as Phil Wood's are much
more expensive, but may be better in some respects (see above).


--
Ben Escoto
PGP/MIME mail welcome - finger bescoto@leland.stanford.edu for key

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Top Document: Rec.Bicycles Frequently Asked Questions Posting Part 4/5
Previous Document: 8f.5 Cassette or Freewheel Hubs take 2
Next Document: 8f.7 Ball Bearing Grades

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