A parallel motion commonly used for stationary beam engines

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

Watt's parallel motion. The beam turns on O and is rocked through 0.2 radian each way. The main link hangs 9.5 cm from the beam's end pin E, 28.3 cm from O, to the piston-rod's pin P. The back link hangs the same length from pin D, 17 cm from O, to Q. The parallel bar P Q makes E D Q P a parallelogram. The radius rod runs from Q to a fixed centre G beyond the piston-rod, level with Q. Its length is OD squared over ED, 25.6 cm, which is what puts P on the line. 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 main link's pin places it. The beam is the only driven part.

Measured

The piston-rod rises and falls 11.3 cm, 0.2815 m per radian of the beam, in step with it. The radius rod swings 0.66 times the beam's angle, the other way. P strays across by no more than 0.16 mm. The deepest contact is 0.006 mm.

Notes

Brown gives no sizes. The beam and the links follow the figure's proportions. D was moved a little toward O, and the radius rod kept as drawn, so that P falls on the line; on paper it then keeps within 0.04 mm of it over the stroke. The beam is cut off beyond O, as in the figure. No steam is simulated. The drive is stiff enough to hold the beam's weight.

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