az@ada-uts.UUCP (07/31/85)
Anybody knows what 'sealed bearings' mean? How exactly are they sealed?
diego@cca.UUCP (Diego Gonzalez) (08/05/85)
> > Anybody knows what 'sealed bearings' mean? How exactly are they sealed? Yes. Sealed bearings are low-friction, load-bearing devices with a provision for sealing in lubricant and sealing out "dirt." There are several types used in the manufacture of bicycle parts. Typically, references to sealed bearings mean one of the following designs: Precision ball-in-race bearing cassettes, Non-precision ball-in-race bearing cassettes, Loose ball-in-race with gasket or labyrinth seal. The first, precision bearings, refers to very high quality machine bearings which were once found primarily in expensive mechanical equipment. Carefully ground and polished balls (frequently stainless steel) are inserted in machined, honed, and polished bearing races and the assembly is forged to final tolerance and machined on the outside to specification (diameter and thickness). A high-quality grease is packed into the voids around the bearings in the races and rubber, plastic, or teflon* seals inserted in channels machined in the race flanges (a picture would be worth all these words, wouldn't it?). These bearings are reliable, smooth, replaceable (usually), require little maintenance, but can be heavier than other designs, limit design options, and are relatively expensive. The extremely low friction of the rolling surfaces more than offsets the friction of the seals, if the design is executed properly. They began to appear on bicycles in the mid-1970s, as the current interest in bicycle technology began to grow. Currently, hubs, bottom bracket sets, pedals, and derailleur "jockey" wheels are commonly available with this type of bearing. The second type, non-precision bearings, are essentially the same type of assembly manufactured to less stringent standards. While they retain some of the low-maintenance characteristics, they are more suitable for youth bicycles than for serious adult machines. The main reason for this is that they are not as smooth rolling as the best conventional (unsealed) units and cost about the same. Although I cannot prove this, I suspect that some low and medium priced components boasting sealed bearings actually use these bearings. However, as technology has advanced, it is also possible that some acceptable items may use less expensive means of manufacture. My rule of thumb for bicycle components is "You get what you pay for." The third type of sealed bearing is really a more conventional bike bearing (loose or caged balls on separate races) with an added sealing feature. These differ from the designs above in several ways. Whereas the mechanisms I described earlier had both races and bearings all in alignment 180 degrees to the load, the races and bearings in this last design are oblique to the load. In fact, some of the loose bearing designs that have appeared use roller or needle bearings instead of balls. What makes them different from earlier bicycle bearing sets, however, is that sealing features have been added to keep out dirt and to help retain the lubricant. Friction seals, similar to those used in precision and non-precision bearing assemblies are used in some systems. In others, labyrinths of metal or plastic are arranged in the outer flanges of the bearing race body. When the two races are fitted together, the labyrinths mesh like a maze. The overlapping ridges do not touch each other, but they inhibit dirt from entering by making it difficult to negotiate the channels between them. Camagnolo has incorporated a variation of this technique in their bottom brackets for some time. By machining spiral grooves in the spindle at the point where it passes through the cups and making the spiral wind in the right direction, rotation of the spindle forced the grease back into the cup area while dirt was forced the opposite direction. These bearings are somewhat effective at lengthening the period between maintenance, use standard parts and are easily disassembled and cleaned, and are relatively inexpensive. They are seeing greater use in the better bicycles and replacement components and allow component designers considerable latitude. When combined with good manufacturing methods, these bearings can be on a par with the precision units, at least from a quality standpoint. Both of my bikes have both precision, sealed loose ball, and conventional ball bearings. They were selected for their performance for a particular application. As long as they are properly maintained, each will work as intended for a good while. I realy don't view any one type as inherently better; it's really a matter of the effectiveness of an overall design and the quality of manufacturing.
barrys@tekecs.UUCP (Barry Steel) (08/07/85)
>Anybody knows what 'sealed bearings' mean? How exactly are they sealed? Sealed bearings are tapered roller bearings (like the kind you have on the wheels of your car). They require (almost) no adjustment and dirt can not get into the bearing, as each one is a sealed unit. They do, however require special tools to service. One the other hand, "sealed hubs" are more common and cheaper. They consist of a "maze" that is supposed to keep out the dirt. They have the standard, ball and cone bearing set up. These should do a better job of keeping out dirt than regular ball and cone bearings, which simply have a plastic shield to keep out the dirt. | | <---- dirt must go through "maze" | | |-----------| | | | |o o| | ========<=========>======== axle | |o o| | | | |-----------| | | | | Sealed Hub I have bikes with each type of bearing. The good quality ball and cone bearings seem to roll as well as the sealed bearings. In rainy Portland, the sealed bearings seem to be impervious to the wet, while the regular bearings require more maintenance in the wet weather (no data yet on the sealed hubs). My preference, given free choice would be the sealed bearings. I believe these will be standard on all *good* bikes in the future (like permanentantly sealed bearings on cars are now). Hope this helps, barry steel