I've been thinking about the critique of my constant-torque device since Jeffrey Schnick and John Denker poked some holes in it.
I think the simplified, idealized situation I first observed in Hibbeler's engineering physics book is correct, but my real device suffers the usual failure of the simplified physics analysis to describe a real device 100% accurately.
We're familiar with all those situations for which we ignore air friction, for example when we introduce projectile motion. But in my case the failure is more along the lines of some of the points raised by others when discussing free-body-diagrams within our discussion on vectors. The free-body-diagram does not truly represent the physical reality. The forces acting on an object do not all attach at a common point (often also considered to be the center-of-mass).
I hope the failure of the physical devise I described does not prevent readers from understanding the simplified, idealized situation for which any point of attachment along a line of action results in the same torque on the object. To this end, I have created a second PDF document that shows the simplified analysis in more detail than provided in Hibbeler's text. This second document also points out the things in the real device that prevent it from acting ideally.
I would also point out that in my first set of drawings I made the slot for the sliding pin longer than necessary. I think this led John and Jeffrey to analyze the system over a greater angular displacement than appropriate. The angular displacement can be very limited by how near the line of action approaches the edge of the disk. I believe the real device follows the simplified physics much better as the line of action approaches being tangent to the edge of the disk, but this also results in very small range of motion. This also seems typical... the analysis is often reasonably valid only over some limited range.
Here is the link to the second drawings...
www.bluffton.edu/~edmistonm/ConstantTorque2.pdf
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu