Machine Kinematics
solved, then drawn
Concept

Radius of curvature — where it appears

The radius of the circle that best fits a curve at a point, the inverse of how sharply it bends there. For a cam it decides contact stress and whether a follower can stay on the surface, and it passing through nought is the moment a cam stops being convex.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A 34-unit roller on a 20-unit base: the cam cannot be made. The outer curve is the pitch curve — where the roller's centre has to go, which is just the base radius plus the follower's displacement, plotted in polar coordinates. The inner curve is what has to be cut: the pitch curve offset inward by the roller's radius along its own normal. Where the pitch curve bends more tightly than the roller is wide, that offset crosses itself, and the metal the profile needs has already been removed. The tightest radius of curvature here is 19.43 at 85°, against a roller of 34 — so the cam is undercut by 14.57 and no amount of care in manufacture recovers it. The pressure angle, meanwhile, peaks at a perfectly comfortable 21.2°: this cam fails a test it never looked like failing.

The cam that cannot be cut

A cam has to be big enough for two reasons, and they are not the same reason. One is that the follower will jam in its guide if the pressure angle is steep. The other is that the roller will gouge the profile if the curvature is tight — and there is a combination where the pressure angle is comfortable and the cam still cannot be manufactured at all.

cams · Cam The same 45° rise on an arm, with the quick return and with a return chosen for the arm. A cycloidal rise of 20 over 45°, cut for a flat face on an arm pivoted 90 from the cam's centre and lifting against its turning, each at the least base that keeps the cam convex. Left: the quick programme's return, 170° cycloidal, base 23.26, with 1.0% of room before the return goes wrong. Right: a 170° simple harmonic return, base 18.08, with 4.9% of room before the rise limits it.

A gentle return cannot widen a steep rise

An arm lifting against a cam on a 45° rise had its least base pinned by the return: raise it a hundredth and the return went concave. Give it a return chosen for the arm, slower or gentler, and the least base falls from 23.3 to 18.1, a fifth less. But the window above it only grows from 1% to 4.9%, because once the return is out of the way the rise closes the window itself, and the arm's own reach closes the next one. With a tenth's room the best return saves 8% of base, and no return of nine gives a quarter's room. Constant acceleration, the law with the lowest peak deceleration, is the worst return of the three.

cams · Cam

Named alongside it

The objects these essays reach for when they reach for this one.

FollowerAccelerationBase circleCamCam profileConstraintConvexityCycloidal motionDwellOffset curvePitchPitch curve

All concepts