Thin wall linear ball bushing press fits
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  1. #1
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    Default Thin wall linear ball bushing press fits

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    We have a parts catcher on one Austrian lathe that slides from back behind the tailstock up to the chuck end and then swings under the chuck. It does this rolling on two induction hardened chrome plated 25 MM shafts, The linear ball bushings were pressed into an aluminum carrier, two of the bearings in one bore and one in the other for stabilization.
    There was no accommodation for lubrication of any kind, so I assume they were considered to be "Lifetime sealed".
    The bearing balls eventually were surrounded by coolant that got past the seals, naturally. Then they seized, of course.
    We finally got the old thin wall linear bushings out, (what a nightmare!) and the bores have some slight damage, some of which is corrosion and some caused by the corroded balls traveling back down the outside during returns. Since the return pathways are exposed to the balls that meant hardened corroded balls trying to slide across the aluminum bore walls, so there are slight tracks wore along the bores in 6 lines for two of the bores.
    My thought was to polish the corrosion away from the bores and install new bushings clocked so that the balls have new surfaces to contact during the return, the 25MM shafts are new and close toleranced but my worry is the amount of press the bushings had when new.
    The old ones had to be welded in lines to shrink enough to press out, so I think the corrosion made the bores smaller.
    These thin shells are extremely hard and brittle so we'll need to make pressing dies to make sure the stresses are spread all the way around the rims as we press them in.


    In addition to all this we plan to add a couple of zerk fittings in the hope that this won't need redoing soon. Since original replacement seals need another red label $100 in order to be here tomorrow we want to reinstall the old seals and hope the daily addition of fresh grease will keep coolant and corrosion out.
    The shells are nominally 35 MM OD so I've been looking for a website that has info on installation requirements. So far I haven't found them.
    Any help on press fits for these greatly appreciated!
    parts

  2. #2
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    If I had no other info, light press fit. Metal to metal dimensions if possible. I don't have time to research now.

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    Thanks, that was about what I was thinking.

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    Using this search string "Lubrication of thin wall linear ball bushings" I found this;

    https://www.thomsonlinear.com/downlo...nance_taen.pdf

    Which explains a lot about this failure, these were "Lubricated for life" type bushings and the life expectancy has limits based on variables, in my case the influx of coolant past the seals.
    For the future I'll be looking for the first signs of any increased friction when pulled by hand, or gritty feel.
    This parts catcher is not used often in our operations so it's not a break down situation. Perhaps my experiences will prompt others to catch failures before they happen.

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    BTW, this is the second "Sealed for life" or "Lubed for life" situation we've had with this machine, in the future before purchasing a new machine I'll be asking focused questions about lubrication.
    I've noticed that rotary bearings often seem to outlive the original owners by considerable times, but it seems the technology of linear ball bushings is suspect, for me at least.

  6. #6
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    Quote Originally Posted by partsproduction View Post
    BTW, this is the second "Sealed for life" or "Lubed for life" situation we've had with this machine, in the future before purchasing
    a new machine I'll be asking focused questions about lubrication.
    I've noticed that rotary bearings often seem to outlive the original owners by considerable times, but it seems the technology of linear ball bushings is suspect, for me at least.
    What kind/brand of coolant ?

    what materials do you machine on it ?

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    I'm guessing that there's no room to bore the parts out to fit 40mm OD bearings?
    I'm using solid ceramic linear bushings on thompson rod in a hydraulic pull broach and they been working fine for 10-15+ years. They get constantly showered with cutting oil and steel chips, have no seals, and have never needed attention so far. These replaced destroyed ball bushings. Maybe you could find some the size you need?

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