a 
WHEEL TRAINS 389 
velocity ratio is positive or negative, according as the first -and:last wheels 
; rotate in the same or in opposite directions. 
When the axes of two shafts are parallel 
and near to one another, but not overlapping, 
motion may be transmitted from the one 
shaft to the other by spur wheels, as shown 
in Fig. 606. The broad intermediate wheel Fic. 606. 
M is here called a Marlborough wheel. 
331. Change Speed Gears.—There are many cases in practice where 
it is necessary to drive one shaft from another in a positive manner at 
different speeds at different times. In a screw-cutting lathe, for example, 
the leading screw is driven from the lathe spindle by a train of wheels, 
and the value of the train to be used is the ratio of the pitch of the 
serew to be cut to the pitch of the leading screw. Hence to cut screws 
of different pitch different trains of wheels must be used. In the older 
lathes a set of separate change wheels are used, and different combinations 
of them have to be mounted to suit the work to be done. Modern lathes 
and other machine tools are, however, generally fitted with change gears 
which can be operated by the movement of one or more levers, all the 
M 
wheels being permanently mounted. Not only is this done for the screw 
cutting and feed motions, but the main driving of the machine is now 
largely done by what are called “all-gear drives,” that is, the use of 
stepped pulleys is dispensed with, there being only one driving pulley, 
and all the changes of speed are ‘obtained by putting different toothed 
wheels into gear by the simple movements of one or more levers. One 
important advantage following this substitution of one belt pulley for a 
stepped pulley is that the belt can be run at the speed most suitable to 
‘it, and the power of the machine is not diminished at slower speeds 
through having to reduce the speed of the belt by shifting it to the 
larger steps of the stepped pulley. 
The application of a sliding cotter key to a change speed gear is 
shown in Fig. 607. The driving shaft A carries three wheels C, D, and 
E, which gear with the three wheels F, G, and H respectively, which are 
firmly keyed to the driven shaft 
B. The shaft A is hollow for 
part of its length, and contains 
a rod R, into which is fitted 
the cotter key K, which passes fe A YAIL 
through the slots L in the hollow 
part of A. Each of the wheels 
C, D, and E has six keyways, 
and each is counter bored as ial -- - 
shown. In the position shown 
the cotter key is in two of the 
keyways in D, and the shaft B 
is being driven through the 
wheels D and G. If the rod R Fic. 607. 
be moved to the right a distance ' 
equal to the width of the cotter key the latter will be in the space formed 
by the counter bore in E, and the counter bore in the right-hand side of D 
and all the wheels will be at rest. If the rod R be moved a step further to 
