Press fit on plastic
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  1. #1
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    Default Press fit on plastic

    Hi,

    There are charts for metal fittings but if one were to fit say steel bearing to plastic housing those interference dimensions are not enough for adequate hold. So my question is, is there any rule of thumb eg. Use the chart and that interference dimension say times X is enough? Of course it will never be equal to metal - metal fit but at least better.

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    No chart that I know of, but with very few exceptions, I do know that if a metal bearing or bush is pressed in to a plastic housing ''has'' to be mechanically retained - circlips etc etc, ……...otherwise it will ''walk out''

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    be aware that many plastics will stress fracture and/or creep . Too much interference and your part may fail days, weeks or months later.

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    Cold flow will relax any press fit. Also if this is a molded part the bore will probably have a taper. Adhesive and or a retainer ring. Maybe a knurl on the bearing? If the bearing can generate any heat, be aware that the heat will concentrate at the juncture between the bearing and the plastic and that can cause spin out. I had that happen when I used some nice half inch thick PVC I had to make a drill fixture. All the drill bushings spun out due to the interface melting.

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    What kind of plastic?

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    If you look at ink jet printers, they have a bunch of plain bearings, not sure what they do. But it would be well thought out and tested

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    i had a really hard time matching some plastic parts (pa 6.6) to steel recently. the parts would change dimension for three days after machning. i was running back and forth between lathe and freezer.

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    "Plastic" covers allot of ground.
    Thermosetting, Thermoplast ?

    Reinforced with ?

    Also "Molded" ? Or "machined from block" ?

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    Many plastics have a grain that you cannot see. Sheets of HDPE have a layered grain sim to plywood.

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    Quote Originally Posted by Mtndew View Post
    What kind of plastic?
    More of a general question, but mostly POM C is what I use..

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    Quote Originally Posted by Corn View Post
    More of a general question, but mostly POM C is what I use..
    That's a generic kind of Delrin, which is rated for press fits by DuPont. How much of a press fit it will take I cannot say, and a question to the mfgr is in order.

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    Delrin will stand quite a bit of interferance and is pretty stable,how much is really depended on the size and geometry.

    Nylon is probably the worst,the crap changes as it ages.I made some 8" nyloil with moly sulfide wheels with an approximatly 2.5" bearing with a .010" interferance fit and within a week the bearings fell out.I ended up using tolerance rings and that has seemed to work along with not machining to size at one setting;roughing and letting it sit a week then finishing.

    I'm using some Rulon for some bushings now and so far it seems pretty stable,machines like Delrin but a little softer.


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