PISTON AND ORANK EFFORT DIAGRAMS 319 
effort diagrams for the separate cranks must be to the same effort scale 
_ before they can be combined into one effort diagram in the manner 
AP =AB+AC +AD 
Hele 
hp pe = 
2 12 22 | N2}. 
Ie | 18 | 20 | 22 | ¢ 4 14 | N22. 
oe Sa a ee N23. 
Fia. 497. 
shown in Fig. 497. The mean total effort is of course equal to the 
sum of the mean efforts for the separate cranks. 
278. Fluctuation of Energy.—When the direct-acting engine mecha- 
nism is used to transmit the work done on a piston to a shaft, the turning 
effort on the shaft is very variable when only one crank is used, and when 
two or more cranks, inclined to one another, and connected to different 
pistons are used, the turning effort on the shaft, although much more 
nearly uniform, is still variable. This want of uniformity in the turning 
effort on the crank shaft is a characteristic of all heat engines having 
reciprocating pistons, and the result of this is that, except in the very 
improbable case in which the moment of the resistance to the turning of 
the shaft varies so that at every instant it is equal to the turning moment, 
the supply of energy to the shaft over certain intervals must be greater, 
while over other intervals the supply must be less than that required by 
the resistance. 
In most cases in practice the resistance to the rotation of the crank 
shaft of an engine may be considered to be uniform during a complete 
iod or cycle, and the resistance reduced to the crank pin may therefore 
considered as equal to the mean effort on the crank pin during a 
period or cycle. 
Fig. 498 shows a rectangular diagram of crank effort on a base OX, 
L a, a3 M a 5 e N 
Fig. 498. 
representing the path of the crank pin, and the ordinates of the line LMN 
