308 APPLIED MECHANICS 
new arrangement, but as the engine is single acting, the forces to be dealt 
with during the return stroke are much smaller. It is easy to show that 
the stroke of the piston is now greater than twice the radius of the crank. 
. The “dead centres” still occur when the piston is at the ends of its 
stroke, but the two dead centres are no longer on the same diameter of the 
crank pin circle, and, as will be seen by reference to Fig. 478, the time 
taken for the return stroke is less than that taken for the forward stroke 
if the crank is turning uniformly. 
The construction for determining the piston velocity and piston 
acceleration are unaltered, except that CL, the acceleration, is now shown 
on a line through C parallel to the line of stroke, instead of on the line 
of stroke. . . 
270. Inversions of the Direct-Acting Engine Mechanism.—W hat, 
in preceding Articles, has been called the direct-acting engine mechanism 
is also known as one form of the s/¢der-crank chain, namely, the turning 
slider-crank chain. The slider-crank chain consists of four parts, three 
links and a block or slider. In the direct-acting engine mechanism 
(Fig. 479) the links are; the crank a, the connecting-rod ¢, and the 
‘a Cc 
~ dat bt += 
b 
Fig. 479. Fig. 480. 
frame b of the engine. ‘The cross-head d is the block or slider, but the 
slider may include the piston-rod and piston, and, as in most internal 
combustion engines, the slider may consist of the piston only. 
Various mechanisms may be obtained from the slider-crank chain by 
the process of inversion, which pial Re 
involves the exchange of one at . 
fixed part or link for another. 
The oscillating engine mech- 
anism (Fig. 480) is obtained from 7 
the direct-acting engine mech- Zine of. “Stroke of Ram 
anism by making ¢ the fixed link iE 
instead of b. The crank is still 
a, but the crank shaft is now at 
B instead of at C. The block d ¥ 
is now the cylinder which oscil- \ 
lates on trunnions at A. The aoa 
link 4 oscillates with the cylinder, Me ; 
\ 
but the relative motion between 
dand 6 is still that of sliding ; in 
fact, the motion of any one link 
relative to that of any other link 
of the chain is unaltered by the process of inversion. The oscillating engine 
mechanism is known as the swinging-block slider-crank chain. 
The mechanism shown in Fig. 481 is known as the Whitworth quick 
Fia. 481. 
