Another parallel motion

Only the input is driven; every other part of this model moves by physics (MuJoCo), and its ratios are measured. Run it live, change its parameters, or switch the friction off to see the output stop.

Modelled as

The beam D B C rests at B on the top of a pillar, 35 cm tall, that rocks on its footstep F. The pillar is rocked through 0.08 radian each way. The radius bar E A, 24.6 cm long, runs from a fixed centre E on the wall to the pin A on the beam, between B and C. The piston-rod is pinned at the beam's end C, 25.4 cm from B. Every joint is a pin in a ring with a few hundredths of a centimetre play. The cross-head is free to move up, down and across, and only the pin C places it. The pillar is the only driven part.

Measured

The piston-rod rises and falls 9.8 cm, about 0.58 m per radian of the pillar, in step with it. The beam tips faster one way than the other, so C's motion is not quite in proportion to the pillar's. The radius bar swings about 1.5 times as far as the pillar, the other way. C strays across by no more than 0.4 mm. The deepest contact is under 0.01 mm.

Notes

Brown gives no sizes; proportions are from the figure, scaled by 0.7. As drawn, the radius bar is short, and C would stray about 4 mm over its stroke on a line 6 degrees off upright. Here E and A are moved slightly and the radius bar lengthened from about 15 to 24.6 cm, so that on paper C keeps within 0.3 mm of its line. The whole linkage is turned 2.2 degrees about F so that the line stands upright. The pump-rod at D is left out. No steam is simulated. The drive is stiff enough to hold the pillar and beam.

Film (MP4) · Still (PNG) · Spec (YAML) · Model (MJCF)