Machine Kinematics
solved, then drawn
Concept

Constant velocity — where it appears

A joint between two shafts is constant-velocity when its output turns at exactly its input's rate at every instant, not only on average over a turn. Between parallel shafts that means hubs that only translate against each other; between shafts that meet it is a symmetry swapping the hubs that fixes every configuration.

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

A universal joint at 40° input, shafts 25° apart. Two shafts meeting at 25°, joined by a cross whose two pins are at right angles to each other and each at right angles to the shaft it carries. That is the whole geometry, and everything else follows from it. This is a four-joint spatial loop with all four axes through one point: Kutzbach counts −2 and the screw system has rank 3, so it has one degree of freedom and turns. At this instant the output shaft is at 42.79° while the input is at 40°, and the output is turning 1.0124 times as fast — which is not 1, and never is except at the four points of each turn where the curves cross.

The joint that is not constant velocity

A universal joint is the spatial mechanism everybody has met and almost nobody has been told the truth about. Its output shaft runs fast, then slow, twice per revolution, and the amount depends only on the angle between the shafts — which is why cars have two of them and why the second one has to be fitted the right way round.

spatial · Spatial A double Cardan joint in the W arrangement. Two universal joints on an intermediate shaft, the input arriving at 15° to it and the output leaving at 15°, bent the same way so that the input and output lines meet, with the intermediate shaft's two yokes in one plane. Each pin is found as the line square to its own shaft and to the pin it crosses, so the output's angle is solved rather than taken from a formula. The dashed marks are one on each hub. The shaded square is the plane that swaps the input's approach with the output's departure. At an input angle of 40° the output is at 40.00°, exactly in step.

Constant velocity is a mirror

Between parallel shafts a coupling keeps its output in step exactly when its hubs only translate relative to each other, which is membership of a group. Between shafts that meet, no group will do: a single universal joint at 30° and a correctly phased double Cardan joint both close to the spherical group, and one wanders by 8.2° a turn while the other keeps step to 10⁻¹⁵. What separates them is a mirror that swaps the two hubs. It either fixes every configuration of the joint or fixes none, so one symmetric position certifies a whole turn — which is what the phasing mark on a propshaft checks.

pairs · Pair

Named alongside it

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

Spherical linkageUniversal jointVelocity ratioDisplacement subgroupGrashof's conditionthe Hooke jointLie bracketRatioReflectionScrewSymmetry

All concepts