422 APPLIED MECHANICS 
Fig. 691. The connecting-rod is supported on two knife edges place 1 
at right angles to its axis and passing through the centres of the fe 
of the end brasses. One of the knife edges is placed on the platform ¢ 
a weighing machine, and the other rests on a support on the groun ia 
When the crank or large end of the rod is on the platform, the weig 
indicated is the weight to be credited as revolving, and when the ere 
head or small end is on the platform, the weight indicated is the weig 
to be credited as reciprocating. 
In many cases in practice the weight of the part of the connecti 
rod to be credited as revolving is about two-thirds of the total ieee 
the rod. 
difficulties in the way of esr valent the reciprocating rar 
are so great, that in practice only an approximate solution is attempted. — 
If the horizontal disturbing forces due to the acceleration of the reci- 
procating parts be completely balanced in the manner explained in 
Art. 352, the vertical disturbing forces introduced may be so great as to. 
cause serious damage to the permanent way, to the bridges, and to the 
wheel tyres. As the result of the extensive experience of locomotive — 
engineers, the practice now generally adopted is to balance all the re- — 
volving parts completely and two-thirds of the reciprocating parts. i 
is to say, as regards the reciprocating parts, the horizontal disturbi 
forces due to the acceleration of two-thirds of the reciprocating parts are 
balanced by revolving masses, but it must be remembered that these 
revolving masses introduced cause vertical disturbing forces equal 
those which they balance horizontally. | re 
The balance weights are, in practically all cases, placed between ee 
spokes of the wheels near the rims, 
In what follows, the pistons are assumed to have harmonic motion. 
355. Inside Cylinder Uncoupled Locomotives. — These ae 
have two cylinders placed between the frames, and the driving axle is. 
cranked, the two cranks being at right angles to one another. The . 
important revolving weights which have to be balanced are, the crank — 
arms, the crank pins, and the parts of the connecting-rods, ivaleal 7 
as explained in Art. 353. All these should be reduced to equi 
weights at the crank pin centres. a 
To the equivalent revolving weight at each crank pin centre has to 
be added two-thirds of the weight of the reciprocating parts for one 
cylinder. The problem is then to find the weights to be placed in the 
wheels, at a given radius depending on the diameter of the wheels, oe 
order to balance the weights assumed to be at the crank pin cen 
This problem is a simple case of the one considered in Art. 349, 
on account of its importance the solution for this case will be give 
here. 
Since all the revolving masses have the same angular velocity, the 
centrifugal forces may be represented by the products of the weights of 
these masses and the radii of the respective circles described by heir 
centres of gravity. ; 
Referring to Fig. 692, 1 and 2 are the circles described by the centres” 
of gravity of the required balance weights. A is the left-hand and E Bo 
