19th Century technology sometimes involves large-scale bearings. I am curious about the "chariot"-style bearings used in lighthouse optics, and the similar bearings used to support rotating telescope domes.

See the illustration of the interior of an observatory dome here: https://www.dunsink.dias.ie/grubbtelescope/ The rotating dome sits on a circle of wheels.

The chariot-style lighthouse lens bearing is shown in this video: https://www.youtube.com/watch?v=6yCZlbND32o

The rotating structure that supports the wheels, in both cases, is a bit like the cage in a ball bearing. But I'm not sure whether there's any mechanical advantage to having it rotate freely. Why not fix the wheels to the object that is being supported? Is this something to do with rolling resistance, or is there some other advantage to mounting the wheels on a separate structure that rotates independently?


1 Answer 1


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The big advantage is that there is very little load on the axles (other than the carrier ring) and therefore there is almost no wheel-axle friction.

Why not fix the wheels to the object that is being supported?

If you do then you need bearings between the wheels and the axles. As it is, the wheels are acting as roller bearings with a much larger diameter than axle bearings would have so the surface stresses are much lower, wear is lower and a very heavy load can be reliably rotated with a moderately sized gravity motor.

  • $\begingroup$ Just to elaborate because it may not be obvious, the friction force is reduced because lever arm is now the diameter of the wheel rather than the radius of the wheel (look at the mechanics of a wheel where contact point on the ground of a wheel is fixed and the wheel rotates around that. The wear should also be reduced since the wear that would otherwise go to the walls of the hole is now spread out over the larger outer circumference. $\endgroup$
    – DKNguyen
    Commented Sep 13, 2022 at 21:15
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    $\begingroup$ @DaveTweed, what was the reason for deleting the image in my answer? There is no image in the question and readers would have to watch a video to understand the question. I included a screengrab to solve that issue. Can you roll back, please? $\endgroup$
    – Transistor
    Commented Sep 14, 2022 at 6:21
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    $\begingroup$ There was no image. There was just a reference to an image, but the corresponding link didn't exist. In the edit you did of the original post, you inadvertently deleted the link when you added the text at the end. I have now restored both the reference and the link. $\endgroup$
    – Dave Tweed
    Commented Sep 14, 2022 at 11:21
  • $\begingroup$ @DaveTweed: Thanks for fixing that. I missed it in the revision history when I checked. $\endgroup$
    – Transistor
    Commented Sep 14, 2022 at 13:42

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