Subject: "Sealed" Bearings
From: Jobst Brandt
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.
[More from Ben Escoto]
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://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
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