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356 PULLEYS.
When the back gears are in action the speed will be
:
zj! = 33^ revolutions per minute.
8
1 65
Jr_ = 20 ^ revolutions per minute.
8
= 13% revolutions per minute.
_L = §yi revolutions per minute.
8
= 5% revolutions per minute.
8
These speeds are all within the practical requirements of
the problem, and now the next operation is to modify the diam-
eters slightly in order to get proper tension on the belt. (See
page 352.)
FLY=WHEELS.
Fly-wheels are used to regulate the motion in machinery by
storing up energy during increasing velocity, and giving out
energy during decreasing velocity. Fly-wheels cannot perform
either of these functions without a corresponding change in
velocity. The rim of the wheel may be very heavy and moving
at a high velocity, the change in speed may be small and hardly
perceptible if the energy absorbed and given out is small, but
there must always be a change in velocity to enable a fly-wheel
to act. The common expression of gaining power by a heavy
fly-wheel is very misleading, to say the least. There is no
power gained by a fly-wheel but, on the contrary, considerable
power is absorbed by friction in the bearings when a shaft is
loaded with a heavy fly-wheel, (see example in calculating fric-
tion, page 305). Nevertheless, a fly-wheel performs a very use-
ful function in machinery by storing up energy when the supply
exceeds the demand and giving it out at the time it is needed to
do the work. ( For momentum of fly-wheels see example, page
300. For kinetic energy, see example, page 301 ).
Weight of Rim of a Fly=Wheel.
The weight of a rim of a cast-iron fly-wheel will be
:
W—tr1
X 0.7854 X D X 3.1416 X 0.26 ; this reduces to,
W=D X d2
X 0.64
D = Middle diameter of rim in inches.
d= Diameter of section of rim in inches.
W= Weight in pounds.
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