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

Details of Construction. — Proceeding to the constructive details of the navvy, the main frame has its side girders formed much deeper in the centre that at the ends, and they are duplicated on each side at the front end under the tower. There are three internal transverse girders, and longitudinals again under the engine and gearing ; all are well tied together, and stiffened by angle irons and gussets. Between the inner longitudinals under the tower a large tank is formed, which when filled with water acts as ballast to steady the machine, and forms a convenient supply for the engine. The whole frame is rivetted together at the works ; but to facilitate transport it is divided crossways in the centre, and the two halves are united by bolts and joint plates.

The lower pivot of the jib is carried somewhat in advance of the main frame by a pair of projecting girders rivetted on the top of the transverse members of the frame, and converging in the form of a V to an apex, at which is placed the pivot ; these are further stiffened by plates underneath, and the front end of the frame where they are fixed, is made of double thickness the whole way across. This construction has been found necessary in order to provide for the enormous stress which has to be resisted at this point if the bucket is allowed to jam itself when cutting ; the force is indeed sufficient sometimes to lift the back or engine end of the machine almost off its legs. The plan of carrying the pivot well in advance of the main frame, which is a distinctive feature of the navvy, enables the jib to be swung round further, and renders the ground in front much more accessible when laying down the rails.

At each corner of the frame is a strong jack-screw, and a fifth is placed immediately under the pivot of the jib ; these take the entire weight when at work. The outside wheels on which the machine is mounted, are double-flanged and 10½ ft. gauge ; and for transport from one cutting to another ordinary single-flanged wheels are also furnished inside, to the standard 4 ft. 8½ in. gauge.

The tower may be described as an oblique truncated pyramid, well extended at the base for bolting to the longitudinals of the main frame. It is formed of two plate-sides, stiffened with T irons, and braced together with crossplates and stays ; between them is an opening large in enough for the driver to watch the motion of the bucket, even when the jib is straight ahead. The top of the pyramid is finished with a roof-plate extended forwards in front for taking the top pivot of the jib, and stiffened by a V shaped girder like that for the bottom pivot ; on this table are placed also the guide pulleys for the main chain.