Precision low speed rotary plain bearing
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    Default Precision low speed rotary plain bearing

    Let's suppose I want to allow a shaft to rotate and slide with minimal clearance. The shaft won't rotate quickly, maybe 1 RPM maximum, but I can't tolerate much, if any, radial clearance. Let's also say, for fun, that I want to stick with plain bearings. I can think of two solutions -- one in which I ream and lap the inside of a bronze bushing until I get the right clearance and just leave it. We say it'll wear slowly and the clearance is lapped in and there's no issue.

    I prefer to make things adjustable, however. Is there any chance I could get away with using a split bushing? I usually only see split housings used for clamping and not as bearings, but can this be done and is it a good idea? What's the closest thing to "rotary gibs"

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    What about having tapered journals and bushings with an axial adjustment? What type of loads will you have and how are you lubricating this shaft? I just finished refurbishing a 70 year old Delta Toolmaker surface grinder that uses a tapered bushing for the main spindle support behind the grinding wheel. The steel shaft journal still showed the original grinding pattern where it ran in the bronze bearing. This system has recirculating lubrication by having a ring that rides and rotates on the horizontal journal and carries up spindle oil from a self contained sump. For low speed loads you could probably get away with a greased system.

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    Automotive crankshaft bearings are split plain bearings, but not adjustable.

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    1. Tapered cones with a split and something to drive them together is adjustable.
    2. You haven't specified loads, etc. If it survives just make it a very mild press fit out of something conformable and low friction. IE, slightly undersized hole in plastic with a screw to tighten across a split. Your friction goes up, but that wasn't a listed requirement and a bit more push overcomes that.
    3. Following on overland's idea, look at how bushing based lathe headstocks are done. Large bushing, shim to adjust the gap.

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    Pre WW2 automotive crankshaft bearings were adjustable with shim packs .....when oil pressure fell,off with the sump ,and take out a couple of 002 shims from each bearing,then tighten up the caps again.

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    If you wanted to spend a bit materials-wise, I'd look at a hardened, polished shaft and a slight (couple tenths) interference fit augmented PEEK bearing.

    PEEK (POLYETHERETHERKETONE) – BEARING GRADE | Sterling Plastics Inc.- Blaine MN plastics distributor

    As long as it's not a really dirty environment, properly done it should be a lifetime set-and-forget.

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    I've used Thorplas Blue material. Same idea as the PEEK Milland mentioned above. The Thordon stuff machines beautifully and costs and arm and a leg. Made in Canada, I think. The question I have is with the 'slide' requirement. Never tried that with this material but it should work. this doesn't address the 'adjustable' issue, but with this stuff you may never see enough wear to need adjusting.

    Mining Bearings

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    I looked up how the myford spindle goes together and now it makes sense. You have two halves and shim them open.

    Someone mentioned split conical plain bearings. How would that work? Why split them if you have axial displacement?

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    Quote Originally Posted by sp1nm0nkey View Post
    I looked up how the myford spindle goes together and now it makes sense. You have two halves and shim them open.

    Someone mentioned split conical plain bearings. How would that work? Why split them if you have axial displacement?

    They're designed to be able to control radial clearance, just as your situation requires. Picture a sleeve bearing that's tapered on the OD. It fits in the housing (headstock of whatever) which has an identical ID taper. It's retained by a screw-on cap and as the cap is screwed on tighter it compresses the bearing reducing the bearing ID and setting the clearance with the shaft. The bearing itself will be moved axially so thrust is addressed separately if required. It sounds like not required in your case with both rotational and axial motion.

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    Bronze or oil lite bushings can often fill the bill for a simple but high precising bearing.

    Amazon.com: The Hillman Group 58106 Flange Bearing, Bronze 3/4 X 1 X 1-1/4 X 1-Inch, 4-Pack : Industrial & Scientific

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    Quote Originally Posted by michiganbuck View Post
    Bronze or oil lite bushings can often fill the bill for a simple but high precising bearing.
    The OP wants minimal (no) radial clearance, and a bronze bushing usually has to have some due to need for an oil film. Too tight and there's risk of abrasive wear between bushing and shaft, or having it seize up.

    A plastic bushing/bearing usually is more forgiving in this sort of use, and can actually function with a slight interference fit due to the lower modulus of elasticity and non-seizing nature of a plastic/hard steel shaft mate.

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    As far as I understand, polymer bearings are used in metrology equipment as long as they’re very thin. That way you get metal/granite stiffness but polymer lubricity. Is that right?

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    If you use a spring preload on the bearing bigger as the actual load you end up with zero clearance And as the speed is this low it could be done probbably
    You can do it by making a plain bearing with 3 high spots And then one of these spots is preloaded Depending on the actual situation even gravity can function as a load Imagen a V block with a shaft Weught of the shaft is the preload that creates a bearing with no clearance as long as the load is less as weight of the shaft

    Peter

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    Quote Originally Posted by sp1nm0nkey View Post
    As far as I understand, polymer bearings are used in metrology equipment as long as they’re very thin. That way you get metal/granite stiffness but polymer lubricity. Is that right?
    Not only for joint stiffness. Temp and load affects the polymer and cause it to flow out of the load zone. Saw this with Garlock dx bushings. The acetal thickness was about 0.006” above the porous bronze substrate. Loads and impacts were high and some of the acetal moved out of the load zone to the no load zone. Amazingly the diameter stayed the same.

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    Quote Originally Posted by sp1nm0nkey View Post
    ... I can't tolerate much, if any, radial clearance. ....
    You are unlikely to be able to get that kind of sub-micron performance from a home-made mechanism. Give up now.

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    Old-time "circular graduating engines" (aka circular dividing engines") used conical bearings. The highest-grade graduating engines were capable of scribing graduation lines within one arcsecond of the nominal position.

    The US National Bureau of Standards (now NIST) published a paper, "The Graduation of Precise Circles", that included a cutaway diagram of the spindle and directions fir the adjustment of the tapered spindle bearing of their SIP graduating engine shortly after the end of WWII. Unfortunately, I've misplaced my copy and thus can't provide a more-specific reference.

    Beyond that, I can say that David St John, the Cheif Engineer of Berger Instruments (Boston, MA) told me in a mid-1980s telephone conversation that the dividing-engine and highest-grade surveying instrument conical bearings were lapped together using ". . . the finest grade garnet powder mixed into lamb-tallow." for their final fitting.

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    I see no problem with a split bearing and shims.....if the OP sizes the shaft to suit,off the peg stuff could be used ,along with a few bits from a old engine(main caps and bolts).....The sliding shaft may be the main problem as it will have to be cylindrical within say a few microns over the entire bearing. ..the obvious way of fitting is scraping and blueing ,and removal of shims.

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    My Berco line boring machine uses boring bars that are hard chrome plated on teh OD...
    Bearings that support the boring bar allow rotation and linear movement...
    Bearings are adjustable....There are round carriers that fit into the supports at each end of the work....(position can be moved to accommodate larger or smaller work)
    The carriers are split along one side and have an adjusting screw to spread the split.
    Bearings are high tin Babbitt precision bored that the boring bar runs directly on..
    The bearing carriers are held in a cylindrical housing , closed with an adjustable cap that squeezes the carriers against the adjusting screws....

    Produces round holes and has good dampening qualities....(Babbitt with large surface area against the boring bar.)....
    There are different carriers for different sized boring bars.....Outer housing all being standard...
    Cheers Ross

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    Split bearings feasible

    You can make a tight fit between shaft and plain bearing without having to fear wear, if you give either the journal or the bearing an oil groove. This would have to be a helical groove that oils the entire bearing surface within one revolution of the journal.

    An other possibility is the use of dry plain bearings. See Bronze Wrapped Plain Bearings | Metric Wrapped Bronze | Bowman

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    Sounds like he needs to get the radial play below 1/64 inch. Should be dead easy.


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