The
hypocycloidal annulus must have exactly twice as many teeth as the orbiting
planet gear. No Meccano gear ring has an even number of teeth,
so a built-up ring was used - a 60-link loop of sprocket chain round the
inner rim of a 3 ½" circular girder, secured by twists of thin fuse wire.
The 60T planet gear engages in it two-teeth-to-a-link, becoming effectively
a 30-tooth gear. The action is fascinating to watch.
A
larger-scale version ► using a 95T planetary gear and 95-link chain inside a
5 ½" circular girder.
The crankshaft carries a single eccentric to operate the valve gear, and a counterbalance weight (behind).
Builder's comment: The model still runs faultlessly 11 years on.