Wooden model of ladder connecting rod

Wooden model of ladder connecting rod by James Watt, 1781

This model shows another of the mechanisms schemed by James Watt about 1781 as an equivalent for the crank. The end of the connecting-rod is made in the form of a rack or ladder, the rungs of which are capable of acting on either side of a pinion fixed on the fly- wheel shaft of the engine. The connecting-rod is guided by a roller at the lower end, which works in a large opening of special form in a fixed guide plate which keeps the rack in gear with the pinion on the fly-wheel shaft. The arrangement somewhat resembles the mangle-wheel motion reversed.