oe ae Er * 
PISTON AND CRANK EFFORT DIAGRAMS 315 
inte per square inch per inch. Also let B be an indicator diagram for 
} other side of the same piston, or for one side of another piston, the 
. effective area of that side being a,, and let p, be the pressure scale of this 
_ diagram in Ibs. per square inch per inch. Then in constructing piston 
effort diagrams which shall be comparable, either the ordinates of B 
_ must be multiplied by a or the ordinates of A must be multiplied by 
pb 1 
Py 
273. Correction of Piston Effort Diagrams for Weight of Recip- 
rocating Parts in Vertical Engines.—In a vertical engine the weight of 
the piston, piston-rod, cross-head, and a part of the connecting-rod 
increases the effort during the down stroke, and diminishes it during the 
up stroke by an amount equal to that weight. Let w equal the weight 
of the reciprocating parts in lbs. per square inch of piston, then the 
effort due to the fluid pressure per square inch of piston must be in- 
Fia. 490. Fig. 491. 
; creased by an amount w for the down stroke, and the effort diagram 
is altered, as shown in Fig. 490, by lowering the base line from XX a 
distance equal to w on the force scale, and the effort diagram for the up 
stroke is corrected, as shown in Fig. 491, by raising the base line from 
q XX an equal amount. 
‘ Frequently half the weight of the connecting-rod is reckoned as 
belonging to the reciprocating parts. 
—- 274. Forces due to Inertia of Reciprocating Parts.—The deter- 
mination of the acceleration of the piston was fully discussed in Arts. 
. 261 to 265. Let f-denote the acceleration of the piston in feet per 
) second per second, W the total weight of the reciprocating parts, in lbs., 
and P the force, in lbs., required to produce the acceleration f, then 
wre and pan. It was shown in Art. 262 that, at the ends of 
2 
the stroke, r( +*), where V is the velocity of the crank pin in 
feet per second, and v is the ratio of the length of the connecting-rod to 
v, the radius of the crank, the plus sign applying to the outer, and the 
minus sign to the inner, end of the stroke. Hence, at the ends of the 
wv: 
stroke, P = — (1 + =). If wis the weight of the reciprocating parts, in 
Tbs. per square inch of piston, and p is the accelerating force, in Ibs. per 
