suspended from a 
CHAPTER XX 
GOVERNORS 
281. Function of a Governor.—The function of a governor is to 
_ regulate the mean speed of a machine or prime mover, or to keep the 
mean speed within certain limits, the limits of variation depending on the 
nature of the work which the machine or prime mover has to do. The 
limits of variation of mean speed will also depend on the sensitiveness of 
_ the governor used. 
e function of the governor differs from that of the fly-wheel. The 
fly-wheel limits the variation of speed, during a cycle, which may be per- 
formed during a fraction of a revolution or during several revolutions, but 
the function of the fly-wheel is not to regulate the speed when a permanent 
__ change takes place in the load, or when the change in the load lasts for 
_ more than a cycle of operations of the machine or prime mover ; this is the 
function of the governor, which should regulate the supply of power to 
the demand. For example, in a steam-engine the fly-wheel controls the 
variation of speed due to the difference between the effort on the crank 
and the resistance at the crank pin due to the load when the work 
done by the effort, during a cycle, is equal to the work done on the 
resistance. 
A change in the average resistance should be accompanied or followed 
as soon as possible by a corresponding change in the average effort which 
is effected by the governor altering the point of cut off, or altering the 
initial pressure by operating a throttle valve. The governor of a recipro- 
cating steam-engine can only act during the period of admission of steam 
to the cylinder, and if a permanent change in the load occurs between 
the periods of admission, the fly-wheel exerts a controlling influence on the 
speed until the governor can act. 
282. Revolving Pendulum.—lIn its simplest form the revolving 
pendulum consists of 
a small body A re- 
volving about a ver- 
tical axis OY, and 
point B by a thread 
or slender rod. In 
Fig. 511 the point B 
is on the axis OY, 
mae in Figs. 512 
and 513 B is at some . 611. 512. Fra. 513. 
om gerne tira FiG 511 Fia. 512 
OY. When B is outside OY it rotates about OY with the same 
angular velocity as A by being on an arm fixed to a rotating spindle, 
329 
