A parallel motion used only in particular cases

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

Scott Russell's arrangement, as drawn. The beam E C B is 45 cm long. Its middle C rides on the radius arm G C, which is the same length as each half of the beam and is rocked on G through 8 degrees each way. Its end B is held near G's level by a rocker from H, below, which stands in for a slide. E sits directly over G, and carries the cross-head and the piston-rod, which hangs through a gland. The lower bar C K and the bar K H make a parallelogram with the beam's right half and the rocker, as in the figure. 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 beam's end places it. The arm is the only driven part.

Measured

The cross-head rises and falls about 10.5 cm, 0.362 m per radian of the arm, in step with it. The beam turns 1.84 times as far as the arm, the other way. E strays across by no more than 0.4 mm. The deepest contact is 0.003 mm.

Notes

Brown gives no sizes. The proportions follow the figure, with each half of the beam equal to the arm, which is what makes E's path straight. A rocker in place of a true slide under B tilts E's line by about 3 degrees. The whole linkage is turned that much about G so the line stands upright; on paper E then keeps within 0.3 mm of it. The piston-rod hangs in front of the arm's centre. The drive is stiff enough to hold the beam's weight, which otherwise drags the arm slowly down. No steam is simulated.

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