. The steam-engine and other heat-motors. lly this motion is com-municated to a cross-head, and thereciprocating motion of the latter isconverted into a rotary motion bymeans of a connecting-rod andcrank. Such an engine is called adouble-acting engine. Where steamis admitted to only one side, theconnecting-rod may be directlyconnected to a pivot-pin in the piston. When a plane can bepassed through the connecting-rod and the crank-arm, the crankis said to be on a center. When the piston is nearest the crankit is on the crank center, when it is farthest away it is on the headcenter. The action o
. The steam-engine and other heat-motors. lly this motion is com-municated to a cross-head, and thereciprocating motion of the latter isconverted into a rotary motion bymeans of a connecting-rod andcrank. Such an engine is called adouble-acting engine. Where steamis admitted to only one side, theconnecting-rod may be directlyconnected to a pivot-pin in the piston. When a plane can bepassed through the connecting-rod and the crank-arm, the crankis said to be on a center. When the piston is nearest the crankit is on the crank center, when it is farthest away it is on the headcenter. The action of steam in a steam-engine is as follows: The pres-sure of the steam from the boiler causes the piston to move througha part of its stroke to a point at which, by some kind of automaticmechanism to be described later, the steam-supply from the boileris cut off. The piston must complete the stroke having only thediminishing steam-pressure and the energy stored up in the mov-ing parts to supply the pressure necessary to overcome Fig. 5. Stratton Separator. 16 THE STEAM-ENGINE AND OTHER HEAT-MOTORS. If the crank-pin has a uniform motion, by Newtons First Lawthe resultant of all the pressures exerted on the crank-pin must ex-actly equal the resultant of all its resistances just as if the crank-pin were at rest. If at any time there is a difference, acceleration(change of velocity or variable speed) positive or negative im-mediately follows. It is evident, then, that variation in rapidityof motion causes the equality that must exist between the driv-ing and resisting forces. When the piston has reached the end of its stroke all the steamthat produced its motion must be allowed to escape to some placeof lower pressure, otherwise the piston on the return-stroke wouldcompress the steam. Compression in excess lessens the amountof external work done, and is neither desirable nor economical,as the object of an engine is the production of external work. The steam may be exhaust
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