BALANCING | 425 
$57. Coupled Locomotives.—In order to increase the adhesion or 
_ resistance to slipping, and thus enable a larger tractive force to be 
utilised, two or more pairs of wheels of a locomotive may be coupled 
a: The coupling together of two wheels is effected by a coupling- 
1 connecting the crank pins on two equal cranks, formed one in each 
_ wheel. The cranks may, however, be separate from the wheels and be 
fixed to the axles. It is obvious that coupled wheels must be of the 
same diameter. 
; So far as the coupling-rods affect the balancing of the engine they 
_ behave exactly as revolving weights at their crank pins, the amount of 
weight at any one crank pin being equal to the portion of the weight of 
the rod supported by that pin. Hence the coupling-rods with their 
eranks and crank pins may be completely balanced in the manner ex- 
plained in Art. 349 by weights in the wheels. In the case of driving 
wheels, the balance weights for the coupling-rod cranks, with their pins 
and the weights of the coupling-rods carried by them, may be combined 
with the balance weights determined, as in the two preceding Articles, 
and resultant balance weights found. The resultant balance weights on 
the driving wheels of a coupled engine may, however, be obtained directly 
by simply including in the planes 1 and 2 (Figs. 692 and 693) the forces 
which will balance the centrifugal forces of the coupling-rod cranks, crank 
pins, and the weights of the coupling-rods which they carry. 
Fig. 694 shows the arrangement of the various cranks in an inside 
cylinder engine with two pairs of wheels coupled. It will be observed 
that the cranks connected by the coupling-rod CD are at right angles to 
— Te Se QC aa eee Eo . ™ 
: Fia. 694, Fiq. 695. 
the cranks connected by the coupling-rod EF, also the crank pins C and E 
__ are opposite to the main crank pins A and B respectively. 
: In the case of an outside cylinder coupled engine (Fig. 695), the 
driving cranks serve also as coupling-rod cranks, each crank pin on these 
cranks being long enough to carry a connecting-rod end and a coupling- 
rod end. Here also the cranks on one side of the engine are at right 
angles to those on the other side. 
The coupling-rod cranks on an inside cylinder engine may be, and 
generally are, shorter than the driving cranks, but on outside cylinder 
engines all the cranks are of the same radius. 
358. Balancing Reciprocating Parts in Coupled Locomotives.— 
The reciprocating parts to be balanced in a coupled engine may be 
balanced in the driving wheels, or this balancing may be distributed 
amongst all the coupled wheels. All that is necessary is to consider each 
axle as a driving axle, with imaginary cranks parallel to the respective 
cranks on the real driving axle, the imaginary crank pins on a particular 
