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axle, rod and connecting

CRANK, in mechanics, is a fundamental form of the lever. The word means primarily a bend, the simple form of crank being a bend of an axle for turning it. On a windlass the crank is the cross-pieCe that joins the axle with the handle. In a steam-engine the crank may take the form shown in the diagram at 1, in which the broad end of the crank is keyed fast to a rotating shaft and the smaller end is loosely bolted to a connecting rod, for pro ducing reciprocating motion. Sometimes the crank takes the form of a wheel, when it is termed a wheel-crank, as at 2. Here the other end of the connecting rod shows connection with a piston. The double crank shown as 3 is employed where the crank interrupts the axle lengthwise. B and B are the cranks and A the connecting rod. A three-throw crank is constituted by three of these double cranks on one axle. When there are two or more cranks on one axle they are placed oppositely so as to balance. The one theoretical difficulty with a single crank is that it has two dead centres in every revolution, the dead centre being a point at which if stopped the parts arc so in line that neither a push nor a pull on the connecting-rod will start rotation. With a three-throw crank

the dead centres are so timed that one crank is always in working.position. The crank is the simplest device known for altering reciprocating (or back and forth) motion into rotary mo tion, or vice-versa, as it has few parts and nothing to get out of order, while the recipro cating part slows up and starts again with the most desirable speed ratio, obviating all shocks in the machine. A crank-pin is the pin or bolt that joins the crank proper to the connecting rod. A bell-crank is a bent lever somewhat re sembling an engine crank, for changing the di rection of motion of a wire that pulls a mechan ically operated bell.