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I'm working on a design where I need a ball bearing with an inner diameter of 8mm. The ball bearings will support small 3D printed wheels, looking kind of like a skateboard wheels. In order to fasten these wheels, I'm planning to use 8mm screws. The screws that I have found are threaded all the way, and I'm wondering if it is possible/recommended to use threaded screws to support ball bearings? Ideally I would use screws that are not threaded all the way, so the ball bearing can sit on the part that is not threaded. However, I couldn't find such screws. So what if I would use threaded screws? Would it work at all? Is there some workaround I can do using washers/extra bolts?

Edit:

Here is the design, plus a new design I came up with...

https://www.dropbox.com/s/c86cxw1wyc9zi7v/IMG_3628.JPG

What I have learned so far... There is such a thing as a shaft collar, and that's what I should be using in this case. Here is one that is 3D printed:

https://www.thingiverse.com/thing:400921

And on there there is also an example of how it is used together with ball bearings. That's what I should be inspired by...

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    $\begingroup$ Look for shoulder screws. $\endgroup$ – Eric S Oct 16 '17 at 11:39
  • $\begingroup$ PLease provide a diagram. It's not clear whether you expect the ball bearings, or even the wheels, to move. A ball bearing that's captured by the end of a screw will see a lot of friction if it tries to rotate in any axis. Or do you intend for the ball to orbit the non-threaded part of the bolt's shaft? $\endgroup$ – Carl Witthoft Oct 16 '17 at 15:01
  • $\begingroup$ maybe hes talking about some sealed bearing unit. $\endgroup$ – agentp Oct 17 '17 at 1:44
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So yes, you can - but

I recently made a device using skateboard bearings (608zz) and mounted them onto a M8 bolt. It had noticeable slop despite using a bolt with a smooth shank where the bearings were located. This is because a M8 bolt is designed to fit through an 8mm hole, so the shaft will be smaller than 8mm.

According to my calipers, my M8 bolt's shank was was 7.84mm near the head and 7.65mm near the start of the thread. The bearing inner diameter was 8.03mm. In my device this meant one bearing had a slop of ~0.2mm and the other closer to 0.4mm.

With an offset of 0.2mm:

  • Two bearings 60mm apart mean the shaft can have an angular deviation of 0.5 degrees.
  • A 1kg load will create up to 45 grams of torque.
  • A 1kg load spinning at 1000 RPM offset by 0.4mm will induce 200g of linear force that turns into vibration.

So for low speed, low precision devices - sure, go ahead. But if you are trying to build a precise device or one with high shaft speeds, you'll need to look for other solutions.


In my case it turns out that angle grinder disks are not precision flywheels and electrical tape is a poor flexible shaft coupling, so bearing slop is not a problem yet....

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If the bearing has a machined raceway then it would be fine from a bearing operation standpoint. Caution is needed long term because the tips of the threads might wear out inducing slop in the connection. The bearings might walk out or their inner raceway might start spinning causing wear.

Additionally if the bearing is designed for specific radial clearance after it has been installed via a press fit, then when placed loosely over threads the clearance will be higher and the ball loads will be more concentrated. The life of the bearing will decrease as the clearance increases. This might not be as important in this application. Try to find the loading capacity of the bearing and you can calculate the life of the bearing from the ratio

$$ (\mbox{life in cycles}) =10^6 \left( \frac{\rm capacity}{\rm load} \right)^3 $$

This assumes fully loaded balls (with zero radial clearance).

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    $\begingroup$ If it's to be used in connection with 3D-printed wheels using any common 3D printer and any common 3D-printing material, the wheels will die ages before the bolt or the bearing show any sign of wear. $\endgroup$ – SF. Jan 19 '18 at 11:23

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