408

MODERN STEAM PRACTtCE.

finishing up the arms quite square, and fitting the sliding blocks ofbrass to them. When this part of the crosshead is turned, thesliding blocks are bored out, and are held in position with a sideflange; the blocks are planed at the top and bottom. In someinstances cast-iron blocks have been used, lined on the rubbingsurfaces with white metal. The motion bars are of cast iron, thebottom one is cast along with the condenser, and the top one, of anx section, is bolted down at each end with one large bolt, passingdown through the vertical distance pieces cast along with the bar;or two smaller bolts passing through flanges at the bottom may beconveniently adopted, in which case the bottom motion bar isgenerally raised up from the condenser Casting, having strength-ening ribs cast along with it. One or more oil cups, providedwith covers and wick siphon pipes, should be cast on the top ofthe guide bar, to lubricate the sliding surfaces. The crosshead inother arrangements is let into the pillow-block pieces (fitted withcaps and bolts), which are cast along with a T piece at the bottom,placed centrally, and which is fitted with a separate brass casting,having clips at the end to take the sliding strain. This guide blockhas a large flat surface at the bottom, but the surface at the top isnot of so large an area, owing to the thrust of the connecting rodbeing neutralized by the weight of the crosshead and adjuncts.Some engineers, indeed, have left a small space between the topsurfaces, thus showing clearly that the top thrust is but little feit.The guide bar is of a trough section, and is bolted down to thecondenser casting, having means of adjusting it to suit the wear ofthe brass sliding piece. The lubrication of the parts is effected bymeans of an open oil well at each end, which is kept constantly full;the bottom sliding piece travels a somewhat greater length thanthe surface provided on the trough guide plate, consequently itsbottom skims the oil in the receivers at each end, thus keeping theguide plate thoroughly lubricated. All these arrangements ofcrossheads are suited for plain connecting rods; some builders,however, adopt forked connecting rods, having the crosshead, orrather the pin for the connecting rod, fixed firmly on the rod,working in a single pillow block and guide piece, provided withcaps and brasses, as in the previous example. The crosshead fortaking the piston rods can by this means be forged and finishedquite flat, and it is bolted to the guide block with the two large boltstaking the cap and securing the brasses for the connecting-rod pin.