I have a bunch of ball bearings (608 2RS) that I'm planning on using in a project with printed parts. But I'm not sure how to attach and secure them properly.
How is this done usually?
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$\begingroup$ Superglue works fine for most cases, but be careful not to get any glue inside the bearing. $\endgroup$– EzraDec 16, 2018 at 18:53
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$\begingroup$ Please post an image of the part in question. In addition, do the bearings need to move (rotate) , or are they fixed in place? $\endgroup$– Carl WitthoftDec 17, 2018 at 15:57
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2$\begingroup$ @CarlWitthoft 608 2RS are standard Ball Bearings of 22mm diameter. $\endgroup$– TrishDec 18, 2018 at 17:36
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1 Answer
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Ball bearings are usually fitted in one style, no matter what is the material that they are embedded in: press fit.
Press fit
To get a good press fit, the part will have to have a hole that is the diameter of the bearing (22 mm) plus a little margin that depends on your printer and filament. From my own experience, a design with 22.1 mm to 22.4 mm diameter resulted in a near-enough 22mm hole that gave decent fit. Your design will need to be adjusted depending on the material and resolution of the printer.
Note that in the case of machined metal parts, the hole usually is not exactly 22mm but a coupe thau (=1/1000 inch) smaller as the bearings get pressed into the hole with a hydraulic press. They do deliberately deform the bearing and workpiece a tiny bit to sit perfectly. If you make the hole too much too small, the printed parts could break under the stress such treatment puts them under. However, if done just right the ring itself will deform just enough to fit the bearing's casing, as long as it is flexible enough. I have experienced prints of 2 wall thickness with 15-20% infill to allow a little bit of flex, which resulted in them applying quite some tension on a set of three screws: the Slider for 2040 Openbuild V-slot by FabianFriethjoph does use this effect from PLA to force the wheels into the guide rail just enough to prevent wobble.
Since most filaments shrink, you might still need to use a larger-than 22 mm hole in design to get just a couple thau under the 22 mm you want for the perfect press fit.
Adjustable position
In some cases, ball bearings are fitted into larger holes, and then fastened and adjusted with a set of 3 setscrews - their seat can be changed slightly to compensate for warp or shear of the whole item. However, printed plastics are bad at holding a thread, and it would be a very good idea to include a metal insert with the thread. For example, you could include a nut in the middle of the ring holding the bearing, or you might use metal inserts.
Capping
Even if one uses a snug press fit or adjustable position, it can be a good idea to use a cap that makes sure the bearing can't fall out of its area without removing the cap first.
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$\begingroup$ OK, getting a nice tight fit seems to be the way to go then. I might add some thin ribs to the hole to grab it. I suppose I can also roughen the bearings to improve their friction. $\endgroup$ Dec 14, 2018 at 19:46
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$\begingroup$ the ribs might work, but I suggest to actually make the whole thing to be something like 21.95 mm in result. It should not just fit snugly, it should need to be forced in... but without breaking the casing. $\endgroup$– TrishDec 14, 2018 at 19:49
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$\begingroup$ I might actually go for a cap on the outside to hold it in place. Maybe something low-tech like pushing some wooden pegs around it. $\endgroup$ Dec 16, 2018 at 19:17
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$\begingroup$ @BanksySan a cap that secures it can be a good idea $\endgroup$– TrishDec 16, 2018 at 19:28