Volume Eleven - Pre-Victorian to the Present Day - some more aspects - The Steam Navvy and the Dock Extension of 1883 . . .

The jib is stiffened laterally by rivetting to its lower member on each side a broad plate placed on edge so as to form an section in plan, the two sides of the angle being united with knees and gussets. This enables it securely to withstand the heavy twisting strains when cutting in any such material as tough clay mixed with boulders. Round the platform at the base of the post are led the chains connected with the swinging gear.

The engine is of the ordinary vertical type, with a cross-tube boiler carrying usually 80 lb. pressure, and a pair of cylinders of 10 horse-power nominal ; it runs up to 160 or 170 revolutions per minute under the control of a governor. On the crankshaft is keyed a pinion, gearing into a spur-wheel four times its size on the main drum shaft, from which all the other motions are transmitted.

The main drum is tapered, so as to give the engine the most power when the chain is pulling at least advantage, and vice versa ; it is loose on the shaft, and is driven by a clutch, and controlled by a powerful foot brake. By a pair of equal spur-wheels, motion is given to a second shaft, on which is mounted a small drum having its own clutch and brake, and winding a light chain for pulling the bucket back ; this however, is very seldom required, except when the machine is used for dredging.

On the other end of the second shaft is a double-cone friction clutch with a pair of reverse bevel wheels, for driving in either direction a short longitudinal shaft, which at its front end drives a third cross-shaft giving motion to the swinging and propelling gears through a double-ended clutch. A drum on the shaft winds the swinging chain that is led round the circular platform on the foot of the jib post ; and this drum also has a brake under command of the driver.

A pitch-chain from a pinion upon the shaft drives a wheel keyed on the axle of the front travelling wheels. By means therefore of the bevel wheels and friction cones the driver can move the navvy backwards or forwards, or swing the jib in either direction ; and all the different movements can be effected without reversing or stopping the engine, so that no time is lost. A strong cast-iron framework, firmly bolted to the main frame, carries the various shafts.