MARINE ENGINES.

413

at the top and bottom, with a small rod on each side, on which areleft collars for the bottom part or inside brasses to bear against,while the two outside brasses are bolted hard up against the insideones, with a screwed part on the ends of the rods having nuts andwashers: these rods therefore act as the main connecting rod andtightening-up bolts. This plan, however, has not been muchadopted.

The next detail to consider is the cranked shaft —a term adoptedto distinguish between cranks forged in one piece with the shaft,and plain ones having cranks shrunk on. The crank arms areforged on solid at right anglesto each other, they are then boredacross and slotted out for thepart between the jaws, leaving apart for the crank pin, which isturned out in the turning lathesor cutting-out machines used forthat purpose. There are threemain bearings, one on each sideof the cranks at the outsideand one central bearing betweenthem. When the distance be-tween the cylinders is great, thecranked shaft is separated atthe centre between the cranks,having solid discs forged on, secured with bolts and nuts, and across key for taking the sheering strain off the bolts: in thisarrangement two central bearings are provided, instead of one,as when both cranks are forged on entire. The end parts ofthe crank arms are often finished in the turning lathe, but someexamples have the circular form; the former, however, is the betterplan when counter weights are strapped on for balancing the weightof the crank arms. These straps are quite flat, except where theypass through the cast-iron balancing piece, where they are rounded.The balancing piece is secured with nuts, let into the block at theextreme end, and joggled into the arms at the crank end; the endof the strap should have a round pin rivetted in, with a correspond-ing hole bored on the crank end, which tends to prevent the strapmoving sideways. Some makers leave a joggle on each side ofthe strap by planing out the sides of the crank arm, which makes

A

Fig. 297.—Cranked Shaft.

A a. Cranked shaft. b, Main bearing. c, Crank pin.