REGULATION OF STEAM.

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Fig. 49.—Eccentric Sheave.a, Centre of engine-shaft. b, Centre of eccentric,e f, Thickness of metal round shaft.c d equals a b, multiplied by 2 .

the engine-shaft, it becomes imperative to use an eccentric sheave,over which is placed a loose strap, to which is attached the eccentricrod; thus the eccentric sheave revolves inside of the strap, and theformer being firmly attached to themain engine-shaft, communicatesreciprocating motion to the valveand its adjuncts; the jointed endof the valve having suitable guides,so that the valve-rod moves in astraight line. To set out the ec-centric, we will suppose the dia-meter of the engine-shaft is given,and consequently its centre, whichwe will term A; draw a straightline through the centre of circledelineating the engine-shaft, andon this line set off ab; this we willname the crank of the eccentric, thepoint B denoting the centre from which the eccentric sheave isdescribed; the distance from A to B equals the outside cover, or lap,plus the full opening of the port by valve; then set off a properthickness of metal for the eccentric sheave, around the shaft, andfrom the point B describe a circle touching the periphery of thecircumscribed thickness around the shaft, and the circle describedis the full size of the eccentric sheave; thus the basis is given forconstructing the eccentric motion.

The eccentric is certainly not the best method of imparting motionto the slide-valve, as cams give a better cut off; but considering thegreat number of revolutions per minute many engine-shafts revolveat, it is the only motion that gives satisfaction, being regulär andeasy; whereas with high speeds the cams impart a succession ofblows that would soon shatter the machine, and the noise would be-come intolerable. However, some steam-engines of the fire-enginedass have neither eccentric nor cam motion, but simply an armkeyed on the piston-rod, having an oblong eye, working on a twistedflat bar of iron, thus imparting motion to the slide-valve; an arm isfixed on the end of the twisted bar for talcing the valve-spindle,a short link being interposed between, with the necessary pins,guides, &c.

Having now considered some of the leading features demanding