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There's a video of assembling a physician's scale here.

And here's a photo of the bottom:

bottom of Detecto scale

I estimate that the poise on the scale's weigh beam have a mass or less than $1/1000th$ of the scale's capacity, and there's no minimum weight for the scale, so the scale must have an mechanical advantage of at least $1000$ between the end of the weigh beam and the platform.

This could be achieved with three levers in series, each with a mechanical advantage of $10,$ or two levers each with a mechanical advantage a little over $30,$ or some other combination.

I see that the weigh beam is one lever, and that it's in series with the long lever under the scale. The load from the platform seems to come in at these places (green arrows):

scale bottom with load locations marked

It looks like the long lever has a mechanical advantage of about 18; I measured 47 pixels and 828 pixels for these two distances (green):

scale bottom with distances marked

If the mechanical advantage is just the long lever in series with the weigh beam, that would leave the weigh beam to have a mechanical advantage of more than $50.$ Is that right? Perhaps there is some mechanism inside the horizontal crossbar beneath the weigh beam giving part of that mechanical advantage?

My guess was that the short lever is not adding mechanical advantage, but instead positioned so that it doesn't matter whether the patient stands near the front of the scale or near the back. Is that right, or is the short lever needed for additional mechanical advantage?

Photo source

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  • $\begingroup$ Are you studying from photos? Or in person? It might be very illustrative to see one in person. $\endgroup$
    – DKNguyen
    Commented Oct 28, 2022 at 17:24
  • $\begingroup$ I don't have one. Perhaps someone here does, though. $\endgroup$ Commented Oct 28, 2022 at 19:42

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Here's an animated GIF I made from a YouTube video by thang010146. I don't know enough to answer your questions, but the illustration may help.

enter image description here

My guess was that the short lever is not adding mechanical advantage, but instead positioned so that it doesn't matter whether the patient stands near the front of the scale or near the back. Is that right, or is the short lever needed for additional mechanical advantage?

I think it's performing several functions:

  • It handles 1/4 of the total weight. (There are two blue levers and two grey levers).
  • It provides stability for the platform.
  • As you point out, it provides an accurate weighing no matter where on the platform the load is placed.
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  • $\begingroup$ Cute junction!! $\endgroup$ Commented Oct 28, 2022 at 17:13
  • $\begingroup$ Thank you! That mostly answers my question. The over all mechanical advantage still seems very high to me, but this seems to confirm what the short lever is doing. $\endgroup$ Commented Oct 28, 2022 at 19:36
  • $\begingroup$ The math! $\endgroup$ Commented Oct 30, 2022 at 19:47

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