2
$\begingroup$

There are many places I see a knob used to set the vertical position of an object (like in a microscope stand), but how does it not just simply slide down due to gravity once the knob is released. It's not like they first have to pull/push out the knob before turning it so as to lock it first. Instead once you turn it, it just remains in that position. I believe it's a rack and pinion mechanism, but I don't understand how it locks itself in place after it's been set.

$\endgroup$
7
  • 1
    $\begingroup$ Friction is the most likely. If you push on the device will it move down again? $\endgroup$ – Transistor Mar 5 at 13:54
  • $\begingroup$ @Transistor no. It won't move down if I push on it. But I haven't tried using too much force (don't wanna break anything). $\endgroup$ – Suryetto Mar 5 at 14:21
  • $\begingroup$ Not all R&P are self-stopping. A typical photog tripod will, when the clamp is released, descend to bottom position rather quickly. $\endgroup$ – Carl Witthoft Mar 5 at 15:24
  • $\begingroup$ Many microscope supports of this type do actually back-drive (i.e. move and cause the knob to turn fast) if you push harder. So it is simply friction in those cases. If you have a right-angle drive of some kind like a worm gear it may be inherently able to avoid this. $\endgroup$ – Pete W Mar 5 at 17:59
  • $\begingroup$ Some microscopes (i.e. inverted) move only the objective and that sample support stage doesn't move. I suspect that this is to limit focus "creep." The mass of the objective can be partially supported with a spring so the force the rack & pinion sees is just a small part its mass. $\endgroup$ – D Duck Mar 5 at 23:26
3
$\begingroup$

There are many ways this can be achieved, and it will be depended on the actual implementation.

One very prominent example is the lead (or helix) angle in lead/power/acme screws.

enter image description here

If you do the analysis you will end up with the following

[ enter image description here

enter image description here enter image description here

where:

  • T = torque
  • F = load on the screw
  • dm = mean diameter
  • $\mu$ = coefficient of friction
  • l = lead
  • $\phi$ angle of friction
  • $\lambda$ lead angle

The screw is self-locking when the coefficient of friction $\tan\phi$ is greater than the tangent of the lead angle $\tan\lambda$. In that case, the torque to lower the load $(T_{lower})$ will be either zero (barely moving) or negative ( meaning that either you need to apply torque to keep the screw from moving downwards).

$\endgroup$
4
  • $\begingroup$ How would I search for more detailed formulas to design a system with this self-locking screw mechanism? Are there like ISO standards with this? BTW, this is the second time you've answered one of my questions. Thank you for the high quality answers each time! $\endgroup$ – Suryetto Mar 5 at 14:19
  • $\begingroup$ @Suryetto, you were looking for rack and pinion. This is a lead screw solution. Are you sure you were looking at rack and pinion? $\endgroup$ – Transistor Mar 5 at 14:49
  • $\begingroup$ I thought it was a rack and pinion solution, but since the mechanism is hidden, I didn't know which one it is. If we imagine 2 perpendicular axes, the table moves up-down (z-axis) and the knob spins on the horizontal x-axis. I think it's a combination of lead screw and rack and pinion, because I've found one animation where there's a worm gear (basically a screw) which turns a pinion which slides up and down a rack. If it were simply a rack-and-pinion, the table would slide down, but attaching a worm gear allows the friction on the thread to stop the sliding. However, I don't know the name yet $\endgroup$ – Suryetto Mar 5 at 14:58
  • $\begingroup$ As i said the actual implementation matters. For all we know it might be a friction held potentiometer with a knob. $\endgroup$ – NMech Mar 5 at 15:15
1
$\begingroup$

table lifting mechanism (thang010146 youtube)

I've found this method on YouTube (Channel: thang010146), but as NMech shows there are a few other methods like the lead screw and the worm drive screw jack. But all look like they work with the same principle of using the friction of the thread to stop sliding. So we just have to match the friction to the max load capacity we want.

worm drive screw jack

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.