When to use a shielded bearing
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  1. #1
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    Default When to use a shielded bearing

    At present I have a milling machine gearbox apart and plan to replace two 6206 size bearings. The bearings in question would probably last me the rest of my life but for less than 8 bucks each it's cheap insurance. The bearings are on either end of a spline shaft. The bearings are "splash-lubricated" by 90W in the gearbox. Since the bearings are different brands, I assume one or both have been replaced. One bearing is open and the other is double-shielded. The rest of the bearings in the gearbox are open.

    My question is: In this application is a shielded bearing appropriate or could it possibly be detrimental for them to be shielded or maybe it doesn't matter?

    Thanks,
    Tom

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    Hello Flathead4
    Take an awl or small screw driver & just pop out the seal & you have an open bearing. I would image a bearing running in oil constantly lubricated would last longer than a sealed bearing with just a dab of grease inside.

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    So the shielded or sealed bearings would not typically be used in this type of gearbox, right?

    Tom

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    Default shielded bearing

    I'm certainly not an expert on bearing application but I have absorbed some general understandings.
    my understanding:
    sealed bearings have a positive sealing(more or less) ability but the seals contribute friction and have definate life that is not as long as you might think in running hours-also not reliable as the only seal provided in serious service.
    shielded bearing not a positive seal but acts as a labyrinth to exclude unwanted things from the bearing but since no contact, no wear so lasts as long as the bearing but a labyrinth doesn't clear itself when at rest it can leak at this time.Also allows lube or purge for the rolling elements if desired.Will allow contaminants be driven in from outside sources like air blast or water splash/wash.
    shielded is used almost exclusively in motors instead of sealed so excessive grease will be worked out if over greased.
    in your gearbox I would expect to find open bearings if the lube comes from oil in the box- but if the bearing happens to be submerged in oil ,not a good design because of the tendency to heat the bearing/oil,froth the oil, and waste power, the shield can possibly help lessen these effects.The desired level of oil in a bearing is to have about half the lowest ball submerged-no more.
    I don't see any bad side to the shielded bearing but consider where it was placed in relation to oil level.I wouldn't have value for sealed unless needed for oil control.

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    I prefer shielded bearings almost as a rule. Ball bearings are sensitive to dirt and contamination - especially in multi-speed geared transmissions. People shifting transmissions don\'t always use the gentlest technique and a flake from a gear tooth bull nose can cause real trouble in a ball bearing.

    I can't think of many places I'd use an open bearing. Maybe a spindle bearing protected with laby or lipped seals. I prefer them lubricated for life 'cause they generally last longest in that config regardless of setting.

    I think its perfectly OK to installed fully shielded bearings in a geared transmsission. Splash will eventually wash out the grease and the narrow clearance between shield and inner race ensures against the entry of not much that isn't liquid.

    Shielded bearings in a dusty environment tend to live a short life because the blowing dust wicks the oil from the soap starving the bearing for grease. Motors, shafts, and spindles in woodworking machinery suffer this fate more often than I like to recall. I figure them for a half-life of ten years in a full time environment. I generally replaced them with bearings furnished with rubber lip seals.

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    Just an additive here but the -2RS rubber sealed bearings seem to have a design rpm of 3600...any over speed via belt pulleys etc and one risks melting down the seal material simply from "surface feet per minute"

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    Hello Forest Addy
    In my main line, agriculture, I have yet to see a sealed or a shielded bearing that was running in oil. I have taken apart many a gear box, outboard planetary final drive & transmission & all were open ball ,needle or tapered needle roller bearings. Many a times I would be at a bearing supplier picking up bearings & they didn't have open bearings. No problem, they all told me, just pop out the seal & now you've got your open bearing.
    Hello Enginenut2
    Many of these open bearings on the lower shafts in transmissions are totally submerged in oil.

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    Mr Ciszewski, I gotta confess to a prejudice here, thanks for making me aware of it. Yeah, Sorry guys. My prejudices regarding bearing closures don't square with Mr C's experience and, come to think back on it, it doesn't agree with mine either. My contentions may have some small basis in fact but I blathered to the point whatever sense I could have made was lost in eggregious over-statement.

    I know where my obssession came from: a thoroughly abused auromotive transmission whose syncros were shot yo begin with and the trans was crunched from gear to gear thereafter by its idiot owner until it failed completely. The bearings were total wrecks I took apart for examination. They were so beat up and battered the balls and races looked like moonscapes - except for the front bearing which was double shielded and it was pristine.

    Goes to show how an early traumatic event can lead even a handsome devil like me to an unsupportable obsession. I'm leaving my hysterial BS posted as is for an object lesson.

    We in the technical world have enough to deal with making and mending the stuff that keeps civilization running. Obsessive BS lurks where we least expect it and it attacks otherwise sensible people without warning. I propose we leave unsupportable obsessions (except for seeking the best diesel trucks,machine tool hauling trailer, and fixing clapped out CNC control circuit boards of course - anyone's ears heating up?) to the nutcases and the ideologues for that is their most prominant characteristic..

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    Blathetorious Rex

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    Shields are used where the bearing needs to accept oil lube from the system it is in,
    yet the designer wants to provide some rejection for trash metal. Example that
    flathead4 will appreciate is the front nose bearing of a T84 jeep transmission. That
    bearing was spec'd by ford to be shielded.

    Open bearings are used in places where full access for lube is needed. An example of
    that is my motorbike gearboxes. They run in a puddle of 90wt.

    Sealed bearings are used where environmental contamination is a big issue and the
    bearings are basically thow-aways after some number of hours.

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    Personally i like shielded or sealed in splash lubed or submerged gear box applications. Experience has taught me that keeping foreign matter out is far more important that what its lubed with. Gear boxes by there very nature fill up with junk! Equaly exposed stand alone bearings fail in short order unless rubber sealed version in the paper industry. Sheilded is not enough in a environment rich in fine absorbent dust. Im never sure if its dust that gets in that does it or dust that just sucks up all the grease till it runs dry and seizes. You can clearly see the latter build up of dust that's soaked in oil around them though so its definitely the cause some how!

    The disadvantage with sealed is the seal friction. Weather that's enough to be a problem in real life is another story. My customers prefer reliability a lot more than a extra Kilo Watt hour on the electric bill at the end of the year.

    Now high speed - demanding applications and that all changes. But 99% of the bearings i see are operating under 5K rpm and in a pretty normal all be it dusty environment.

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    Jim,

    That was the kind of general info I was looking for. My concern was that although a shielded bearing would keep out crud it could also limit the lubrication. It sounds like the shields are used to keep out the big chunks but still let the lube pass through.

    Tom

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    Hello Flathead4
    On gear boxes, transmission, etc. that don't have a filtration system, I install magnetic drain plugs on my machinery. This helps eliminate ferrous particles from constantly being circulated in the oil.

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    Shields or sealed are both better in a gear box than open.

    Personally, I go sealed every time, for just about every application. I even "stuffed" a to stroke engine crank case with double sealed bearings back in the 60's with no problems at all. The bearings even survived a fragged piston.

    As for "surface speed" limits? Look at the accessory belt or timing belt in typical automotive applications. RPMs way over 3600, sealed two sides. Good for 3000 hours easily. It's a rough life that includes Catastrophic failures from time to time though ;-)

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    Quote Originally Posted by CalG View Post
    Shields or sealed are both better in a gear box than open.
    Welllllll, maybe. There is a well known german sport car manufacturer that has failing intermediate shaft bearings with all fingers pointing to the shield. Its above the level of the oil, but splashing eventually washes the grease out (as I understand it common shields/seals don't protect against liquid-liquid interchange since they dont have dissimilar surface tension.) If the bearing sits unused for long periods of time the (now thin oil) lubrication runs out and doesn't refill fast enough so it ends up running dry most of the time. The aftermarket solutions dont use a shield and rely on more typ splash lubrication.

    So if the gearbox gets use intermittently shields could possibly do more harm than good. Then again, if a hobbyist, even a new one running almost bone dry may still last a lifetime. Most car transmissions have open bearings. I'd be inclined to do the same.


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