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Estimating torque on microscope stage Answered


I am involved in a project, based on the technologie used in microscope stages.


These have an X and Y axis which can move independently and are driven by rack and pinion gear, controlled by a translation unit with two knobs. The consistency of the grease used in the shafts that connect the knobs with the gear generates great damping and resistence when the stage is moved, so that the movement is nicely smooth and slow. I am not a technician/engineer but a designer and know nothing about torque. I need to know the torque that is generated by this sort of system, especially when the stage is moved BY HAND, so not using the knobs. I would be interested in the minimum and maximum torque that is generated when pushing the stage by hand.

I am thinking of using a slip clutch/torque limiter to replace the translation unit, because I don't want to depend on grease. That is why I need to know the torque. Anyway, maybe somebody on this forum has a better idea. I'm all open.




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Jack A Lopez
Jack A Lopez

2 years ago

One very old, and reliable, method for producing small amounts of force, in a laboratory setting, is that of a mass m tied to a string, pulling the string downwards with force m*g, where "little g" is the local acceleration of gravity in your laboratory.

If you would prefer a force acting in a direction other than downward, then run the string over a pulley, and that can make it so you have a string pulling sideways, or even upward, or some other direction, if you want.

If you would prefer a torque rather than a force, just have the string pull on the edge of a wheel, or shaft, with radius r.

Then you get torque, tau = m*g*r


Regarding this question of ways to produce resistance torque, have you tried a permanent magnet DC motor with its inputs shorted, or connected to a small resistor? Turning the shaft pushes current through the resistor, which wastes power as heat, but at the shaft it "feels like" ordinary mechanical resistance wasting power as heat; i.e. a torque pushing back against whoever, or whatever, is trying to turn the shaft.

Final thing I was going to mention is this concept of, "mechanical advantage"


For simple machines, like your racks moved by gears, moved by knobs, you can measure mechanical advantage, just by measuring the ratio of one movement to another; e.g. the radius of a knob, multiplied by 2*pi, multiplied by some number of knob turns, is the distance of the input motion, xin. The actual, linear distance the slide moves, is the distance of the output motion, xout.

Anyway, the magic of mechanical advantage is essentially an expression of conservation of energy, or power, like this:

Work = Fin*xin = Fout*xout,

<=> Fout/Fin = xin/xout

where {Fin, xin} are force and distance on the input side, and {Fout, xout} are force and distance on the output side.

Also, for rotating things, work is torque*angle.

You know, the traditional symbols for those are the Greek letters, "tau" and "theta", for torque and angle respectively. I have textified them here, because, I'm using an English language keyboard, without subscripts, so:

Work = tauin*thetain = tauout*thetaout

<=> tauout/tauin = thetain/thetaout

It looks a little messy with the letters all running together, but you probablyknowwhatimsaying.