Fig. 39.
Built-up Cranks.—The form of cranked shaft shown in fig. 38 is largely used for marine engines, but for the very powerful engines now fitted in large ships this design of shaft is very unreliable, the built-up crank shown in fig. 39 being preferred, although it is much heavier than the other. It will be seen from the figure that the shaft, crank arms, and crank-pin are made separately. The arms are shrunk on to the pin and the shaft, and secured to the latter by sunk keys. These heavy shafts and cranks are generally made of steel.
Exercise 39.—Keeping to the dimensions marked in fig. 39, draw the views there shown of a built-up crank-shaft for a marine engine. Scale 3⁄4 inch to a foot.
XI. ECCENTRICS.
The eccentric is a particular form of crank, being a crank in which the crank-pin is large enough to embrace the crank-shaft. In the eccentric what corresponds to the crank-pin is called the sheave or pulley. The advantage which an eccentric possesses over a crank is that the shaft does not require to be divided at the point where the eccentric is put on. The crank, however, has this advantage over the eccentric, namely, that it can be used for converting circular into reciprocating motion, or vice versâ, while the eccentric can only be used for converting circular into reciprocating motion. This is owing to the great leverage at which the friction of the eccentric acts.
The chief application of the eccentric is in the steam-engine, where it is used for working the valve gear.
To permit of the sheave being placed on the shaft without going over the end (which could not be done at all in the case of a cranked axle, and would be a troublesome operation in most cases) it is generally made in two pieces, as shown in fig. 40, which represents one of the eccentrics of a locomotive. The two parts of the sheave are connected by two cotter bolts. The part which embraces the sheave is called the eccentric strap, and corresponds to, and is, in fact, a connecting rod end: the rod proceeding from this is called the eccentric rod.
The distance from the centre of the sheave to the centre of the shaft is called the radius or eccentricity of the eccentric. The throw is twice the eccentricity.