MARINE ENGINES.

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loosen the parts if not properly secured. For direct-acting singlepiston-rod arrangements a T-shaped piece is formed on the end fortaking the crosshead, or brasses, which are secured to the rod bytwo bolts passing through the T piece, brasses, and cap; the nutsbeing secured at the ends of the bolts with split pins. The nutcan be fitted with a washer with pin let into the cap, and a smallset screw placed at the top of the one and the bottom side of theother, passing through the washer and bearing in a hollow turnedin the reduced part of the nut. With the T form there must notbe any raised part on the piston rod at the piston end, so asto allow the glands andbushes to pass freelyalong; these must ofcourse be placed on therod before it is secured tothe piston, for it is notadvisable, under any cir-cumstances, to cut theglands in two, as someprefer doing in oscillat-ing arrangements.

The gtddes and cross-head for double piston-rod engines, on the returnconnecting-rod principle,are best arranged withtwo guides or motionbars, one on each side ofthe connecting rod. Bythis arrangement thecrosshead is well sup-ported at the ends, andthe strain on the guide bars is in a direct line with the centre line ofthe cylinder. The crosshead is a circular forging, with arms forgedon for taking the piston rods. In some cases it is turned quiteparallel between the arms, these being forged on at right angles tothe main part of the crosshead; in others a collar is formed on eachside between the connecting-rod brasses and the guide blocks.When the arms are forged on at an angle with the crosshead, it isadvisable to make the connecting-rod bearing of a larger diameter,

Fig. 290.—Crosshead Gudgeon and Motion Bars.

, Gudgeon. b b, Slide blocks. cc, Motion bars.