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Thread: Bearing Quiz!

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    Bearing Quiz!

    Here's a question an engineer friend of mine posed a few years back:

    Steel bearings require the use of grease. Ceramic bearings do not. Why?

    Answer to follow!

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    Senior Member biker7's Avatar
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    Heat resistance.
    George
    Here's the skinny:

    LIGHTER. The ceramic ball is lighter then the steel ball. This means that
    the ball exerts less force outward against the outer race grove as the bearing
    spins. This reduction in outward force reduces the friction and rolling resistance.
    The lighter ball allows the bearing to rev up faster, and uses less energy
    to maintain it speed.
    HARDER. The ceramic ball is harder then the steel ball. This gives you a
    bearing that will last up to 10 times longer then the steel ball bearing
    because it holds it's finish better.
    SMOOTHER. The ceramic ball has smoother surface properties then the
    steel ball. This means
    less friction between the ball and bearing races giving you a faster spinning
    bearing.
    THERMAL. The ceramic ball has better thermal properties then the steel
    ball. The ceramic ball will not heat up like a steel ball. This helps avoid heat
    build up in the bearing, a primary culprit in skate and skateboard bearing
    performance degradation.
    LUBRICATION. The ceramic balls are impervious to oxidation, chemicals,
    and require essentially no lubricant. The bearings are lubricated with a
    very, very small amount of Aral Aralub oil, not a gel or grease.
    Last edited by biker7; 05-04-05 at 04:18 PM.

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    Senior Member alcahueteria's Avatar
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    hmm, my guess is they are harder and have less friction.

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    Steel, even well made and of high quality has micrscopic imperfections. Could the ceramic surface be engineered to be relatively free of these imperfections? Alhough, there would have to be an interface between ceramic and metal (presumed steel). Is it that ceramic can be made much harder than steel, resisting breakdown?
    The answer doesn't have anthing to do with pixie dust or magic does it?

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    Metal-to-metal contact usually requires "hydrodynamic" lubrication to prevent or minimize microscopic spot welding that tears pits in the surfaces when they separate again. Oil or grease serves this purpose as the hydrodynamic interface. Ceramics do not spot weld so easily due to higher hardness and melting temperature. Just my hunch.

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    Evil Genius capsicum's Avatar
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    Depends on the particular ceramic. And yes to the micro spot welding as well as hardness.
    "Data is not the plural form of annecdote."
    "yuo ned to be deadurcated"

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    And the answer is.....

    Even though steel is hard, it is malleable to a small degree. The bearings touch and roll on a point. However, surrounding that contact point is an area of incidental contact where the two opposing surfaces "scrub" one another. Imagine a fuzzy donut image surrounding the contact point. It's this metal-metal rubbing around the actual rolling point that requires lubrication. Ceramic bearings are significantly harder and less malleable, and thus do not have this surrounding scrubbing contact area. How's that?

    I thought it was in intriguing question because, after all, bearings are ROLLING. You'd think that a high-quality precision bearing wouldn't need lubrication. But it doesn't matter how high the tolerances are if you'll always have the "scrubbing" effect.
    Well, now remember I'M not the engineer, so don't jump all over me if I'm not completely, technically correct. All the posts seemed to come pretty close, especially the issue of micro spot-welding and the need for hydrodynamic lubrication. Also the need for corrosion resistance would necessitate using grease (or other hydrophobic lubricant) in steel bearing applications.
    Thanks for reading and happy trails & roads to you all!

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    cyclist/gearhead/cycli... moxfyre's Avatar
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    Are ceramic bearings used in bikes at all? I'd love to have durable, greaseless hubs, bottom brackets, and headset.
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    Senior Member sydney's Avatar
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    Quote Originally Posted by moxfyre
    Are ceramic bearings used in bikes at all?
    Yeah, an option on Zip hubs for one.

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    cyclist/gearhead/cycli... moxfyre's Avatar
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    Quote Originally Posted by sydney
    Yeah, an option on Zip hubs for one.
    Cool! Know anything about the downsides to them? Cost, durability?
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    Here's something that has irked me over the years....
    Today's average automobile wheel bearings keep going and going and going and.... well, you get the picture. They need service, when, every 50k, if ever? And average bike hub bearings last, at the most, 5k before they're in dire need of overhaul? (Depending obviously on conditions.) But you don't hear people in Seattle overhauling their car's wheel bearings all the time because of all the rain. Also, a car's wheel bearing has a hell of a lot more abuse - much more load, much higher speeds for higher duration, slamming potholes at 70mph, and all the side loads while turning. My Honda Accord went for 200k until I sold it. I never had the wheel bearings serviced, and when I asked my mechanic, he said don't worry about it.
    A bicycle hub has nowhere near the abuse. Less weight, speed, duration and almost no side loading. A car wheel bearing is much beefier and heavier, but how can there be such a big disparity?
    And even the high-end bicycle hubs need much more service than any auto wheel bearing. (My wife's White Ind. rear hub got rough after just a few years of use, and she's 105 lbs.!)
    Something doesn't jive, IMHO.
    Thoughts?

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    cyclist/gearhead/cycli... moxfyre's Avatar
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    Quote Originally Posted by LV2TNDM
    Something doesn't jive, IMHO.
    Thoughts?
    I think it's all about the weight and efficiency. Heavier, more durable bearings and better seals would increase the weight and add drag. Internally geared hubs require less maintenance, but are heavier and less efficient than derailleur gears. Chaincases for singlespeed bikes keep grit out of the chain for 1000s of miles, but are unpopular because they add weight.

    People don't mind if their cars' gas mileage is 20% worse if it'll allow them to do less maintenance. But cyclists do.

    BTW, the XT hubs have really good seals on them, but are heavier than the 105s.
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    Senior Member biker7's Avatar
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    Thanks for the quiz LV2 and well said moxfyre...precisely the tradeoff...seal drag and extra weight for great longevity...cyclists aren't willing to accept that trade off. To keep the quiz going I am surprised LV2 didn't ask the obvious...that is aside from a 4 to 1 cost penalty of ceramic versus steel...what is the mechanical downside of ceramic bearings compared to steel?
    George

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    Quote Originally Posted by LV2TNDM
    Here's something that has irked me over the years....
    Today's average automobile wheel bearings keep going and going and going and.... well, you get the picture. They need service, when, every 50k, if ever? And average bike hub bearings last, at the most, 5k, IMHO.
    Thoughts?
    Interesting comparison, but in all honesty I've never had a bicycle bearing fail. Some of my wheel hubs and bottom brackets have more than 20,000 with no service, no added lubrication. And bicycle components must be very light weight to be competitive in the market place.

    Al

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    cyclist/gearhead/cycli... moxfyre's Avatar
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    Quote Originally Posted by Al1943
    Interesting comparison, but in all honesty I've never had a bicycle bearing fail. Some of my wheel hubs and bottom brackets have more than 20,000 with no service, no added lubrication. And bicycle components must be very light weight to be competitive in the market place.

    Al
    I've had a Shimano cartridge bottom bracket spindle snap after four years of use, had some really crappy freewheel die on me, and I've seen a couple headsets get so worn into their "straight-ahead" position that they couldn't be properly tightened up (more on that peculiar problem head). I've had some badly gummed up derailer pivots too.

    But most bike parts are surprisingly resilient. I've seen some bottom brackets and hubs so dry and dirty I was astonished they still worked...
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    B-b-b-b-b-b-bicicle Rider orange leader's Avatar
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    Quote Originally Posted by moxfyre
    Cool! Know anything about the downsides to them? Cost, durability?
    Please someone correct me if I'm wrong because i'm not a metallurgist...but my thoughts would be that the ceramic bearings are harder, but like metals, since they are harder, they may also be more brittle. they may last longer and roll smoother as long as you don't hit them too hard with a sudden sharp impact.

    I also believe there are different grades of "ceramic". The specialized M2 and M4 frames are aluminum with ceramic imbedded into the alloy of the metal. I believe the ceramics are similar, like alloys. different ceramics for different purposes. I knew a guy in college who was experimenting with ceramics for engine blocks. Funny, he hadn't even heard about the ceramic in the bike frames.

    I once raced R/C cars and trucks (NERD!). I blew up quite a few sets of ceramic bearings just by racing around an oval clay track (no jumps). so I know the bearings can crack and split, just like steel bearings do.

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    Senior Member biker7's Avatar
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    you nailed it orange leader. Ceramic bearings have poorer impact resistance...their benefit which is their hardness resulting in reduced friction is also their achilles heal...suseptibility to fracture when abruptly loaded...why their application has to be chosen carefully. They are perfect for turbocharger bearings for example because there is negligible spike loading, ceramic is one of the best heat insulators...very low coeficient of thermal conductivity...and hence tremendous turbocharger heat from exhaust gases that spin the impeller will not degrade lubrication that is more important with steel bearings with high thermal conductivity and thermal expansion.
    George

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    [QUOTE=LV2TNDM]Here's something that has irked me over the years....
    Today's average automobile wheel bearings keep going and going and going and.... well, you get the picture. They need service, when, every 50k, if ever? QUOTE]

    Excellent point. The reason is that bicycles are not very well engineered, just well marketed. The automotive market is competative enough so that wheel bearings that last 5 000km would be unacceptable (and dangerous). Consumers would not accept such poor performance. Most cyclists don't ride enough to have bearing problems, so I suppose it could be argued that there is no incentive to design more robust equipment. However, I think cyclists are willing to accept poor reliability in exchange for some flashy marketing hook.

    The technology exists, and has existed for a long time, to design and build cheap, reliable bearings for nearly any application. It would be fairly easy to design a hub with bearings that would outlast the owner-bearing design is *very* well understood. The key is adequately sized bearings, and better seals. Grease ports would help as well.

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    yes... You're on my left jstream's Avatar
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    Quote Originally Posted by LV2TNDM
    Here's something that has irked me over the years....
    Today's average automobile wheel bearings keep going and going and going and.... well, you get the picture. They need service, when, every 50k, if ever? And average bike hub bearings last, at the most, 5k before they're in dire need of overhaul? (Depending obviously on conditions.) But you don't hear people in Seattle overhauling their car's wheel bearings all the time because of all the rain. Also, a car's wheel bearing has a hell of a lot more abuse - much more load, much higher speeds for higher duration, slamming potholes at 70mph, and all the side loads while turning. My Honda Accord went for 200k until I sold it. I never had the wheel bearings serviced, and when I asked my mechanic, he said don't worry about it.
    A bicycle hub has nowhere near the abuse. Less weight, speed, duration and almost no side loading. A car wheel bearing is much beefier and heavier, but how can there be such a big disparity?
    And even the high-end bicycle hubs need much more service than any auto wheel bearing. (My wife's White Ind. rear hub got rough after just a few years of use, and she's 105 lbs.!)
    Something doesn't jive, IMHO.
    Thoughts?
    Here's the big difference. Auto wheel bearings are of a different design altogether - they are not ball bearings. They probably have around 1000 times more surface area than bike bearings and can use a grease so heavy that you couldn't turn a bike wheel. Plus there are more bearings in the auto wheel than the bike wheel. Your bike wheel bearings have a total contact patch of about 2mm. That being said, servicing a bike wheel is sooooo easy what's to stop us from doing it several times a year? It takes what?... about 20 minutes max to take apart, clean, inspect, lube, and re-assemble?

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    Quote Originally Posted by jstream
    Here's the big difference. Auto wheel bearings are of a different design altogether - they are not ball bearings. They probably have around 1000 times more surface area than bike bearings and can use a grease so heavy that you couldn't turn a bike wheel. Plus there are more bearings in the auto wheel than the bike wheel. Your bike wheel bearings have a total contact patch of about 2mm. That being said, servicing a bike wheel is sooooo easy what's to stop us from doing it several times a year? It takes what?... about 20 minutes max to take apart, clean, inspect, lube, and re-assemble?
    Knew racers that used light oil instead of grease and cleaned their bearings before each race and added just drops of light lube.

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    cyclist/gearhead/cycli... moxfyre's Avatar
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    Quote Originally Posted by genec
    Knew racers that used light oil instead of grease and cleaned their bearings before each race and added just drops of light lube.
    I've heard of this too, but according to the book Bicycling Science basically it is pointless: they say it reduces the overall power requirement by about 0.01% at racing speeds
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    Senior Member biker7's Avatar
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    typically the higher the viscosity, the longer it will last. Same is true with rollerblade bearings where lubricity and low friction is important. On a bicycle, I tend to believe it is relatively insignificant to run oil in terms of increase in speed and of course oil won't last nearly as long.
    George

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    Thanks for all the insights. The auto industry has a big advantage of economies of scale that make quality bearing manufacturing stay within reason as far as cost is concerned.
    But you'd think that paying $200 plus for a rear hub would equate to something ultra-light, durable and maintenance-free.
    But overall, I can't complain. I love working on (and typing about) bikes. So much that it might be a good idea to acually go out and RIDE once in a while!

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    Senior Member biker7's Avatar
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    yes there is an economy of scale with all automotive parts due to volume but the overriding factor is weight and bearing drag...more than economics. A more robust and better sealed bearing could easily be designed for bicycles to replace hub and BB bearings but not without a weight penalty and a higher friction bearing due to increased bearing surface area to reduce stress and extend bearing life. Larger bearing surface creates more drag as do tighter seals to prevent ingress of road contamination.
    Since weight and drag are such a priority to bicycle performance due to power to weight, the balance between performance and bearing life is really just right for bicycle bearings as it has evolved to its current design level for the past 50 years. If you have ever held or repacked a front automobile wheel bearing or spun a front car hub when having a wheel off for doing a brake job it will become quite apparent. It is all about tradeoffs.
    George
    Last edited by biker7; 05-06-05 at 06:46 PM.

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    My guess is that the "glass" would be sintered aluminous oxide or zirconia which is even stronger. Sure, those materials are strong but, not as strong as steel, e.g., no flexural strength. Even so, if it is strong enough for a bicycle and doesn't need lubricant, how about a glass chain?
    Last edited by wagathon; 05-07-05 at 11:06 AM.

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