REGULATION OF STEAM.
ii 7
THE CONNECTING ROD AND CRANK.
The length of the connecting rod varies considerably, those ofdirect-acting marine engines being much shorter than in otherarrangements. Taking an example, however, where the lengthequals five times the length of the crank from centre to centre; whenthe centre of the cross-head, where the connecting rod is attached,is placed at half stroke, from the centre of the engine shaft to the 'centre of the cross-head is the length of the connecting rod, delin-
a, Centre of engine shaft. b, Half stroke of piston. c, Point on crank path at half stroke.d e, Stroke of piston. f, Point on crank path at ^ths of the stroke.
eated as from A to B. With BAas the radius describe the arc A C,draw a straight line from the centre A to the point C on the crankpath; this is the centre line of the crank at half stroke, the point Cbeing the centre of the crank pin above or below the centre line ofthe engine. It will be seen that there is a great difference of thetravel of the crank pin on its path for the IN and OUT strokes, thearc described for the IN stroke being greater than that for theOUT, as delineated from IN to C above the centre line, and fromOUT to C below the centre line of the engine. There is no remedyfor this Variation; it is inherent in all crank motions, and varies asthe length of the connecting rod. It therefore becomes imperativeto find the point C to suit the length of the rod, as likewise to deter-mine the point on the crank path for the particular part of thestroke of the piston that may be determined on for cutting off thesteam. Thus supposing it is desirable to cut off at five-eighths ofthe stroke of the piston, the arc A C will be greater than for thehalf stroke, and vice versa when the point of cut-off is sooner thanthe half stroke. The particular point is easily found by takingthe radius, and placing the point of the compasses on the first point