| 
| 
\ 
———————— a a 
ee 
hie 
BALANCING 421 
ing force at right angles to the first. The second disturbing force 
evidently goes through the same variations in magnitude as the first. 
- The first varies from Ww*r/g at the beginning of the stroke to zero at the 
_ middle of the stroke, and from zero at the middle it varies to — Wo*r/g 
at the end of the stroke. The second varies from zero at the beginning 
_ of the stroke to Ww*r/g at the middle of the stroke, and diminishes again 
to zero at the end of the stroke. 
If the mass of DD at crank radius, and opposite to the crank pin, as 
in Fig. 689, be less than the mass of the reciprocating parts, then the 
disturbing force in the line of stroke will be only partly balanced, and the 
disturbing forces will now be, first, a force in the line of stroke equal to 
(W — w)w*r cos 0 , : 
, and second, a force at right angles to the line of stroke 
equal to wor sin 0 
The obvious construction for finding R, the result- 
ant disturbing force for any position of the crank, FE 
is shown in Fig. 690, where 7, =(W — w)w?r/g, and at 
, = wwr'/9. / \ 
In some cases the kind of balancing just de- 
scribed, where the disturbing force in the line 
of stroke is entirely or partially balanced, may ¥ 
be ‘advantageous, as in locomotives, where a ~ 
horizontal disturbing force is generally more 
injurious than a vertical one, but in many other Fic. 690. 
cases there would be no advantage whatever. 
353. Distribution of Weight of Connecting-rod.—The part of the 
connecting-rod in the neighbourhood of the crank pin has almost» pure 
rotary motion with the crank pin, and the part in the neighbour- 
hood of the cross-head has almost pure reciprocating motion with the 
piston. If A is the centre of the cross-head end of the connecting-rod, 
B the centre of the crank pin end, and C the centre of gravity of the 
, where w is the combined weight of the masses DD. 
_~i— 
OF} PO O 
———_.. 
Fig. 691. 
whole rod, and if W is the total weight of the rod, then the usual prac- 
tice is to reckon -* . W as the part of the weight of the rod which is 
to be credited as a revolving weight at the crank pin centre, and the 
remaining part ae . W is to be credited as reciprocating with the piston. 
The position of the centre of gravity of an actual connecting-rod may 
be determined by balancing it in a horizontal position on a knife edge. 
The values of the weights to be reckoned as revolving and reciprocating 
respectively may, however, be obtained by direct weighing, as shown in 
