Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 2 Indirect Power Transfer Devices 75
SMOOTHER DRIVE WITHOUT GEARS
The transmission in the motor scooter in Figure 2-3 is torque-sensitive;
motor speed controls the continuously variable drive ratio. The operator
merely works the throttle and brake.
Variable-diameter V-belt pulleys connect the motor and chain drive
sprocket to give a wide range of speed reduction. The front pulley incorporates
a three-ball centrifugal clutch which forces the flanges together
when the engine speeds up. At idle speed the belt rides on a ballbearing
between the retracted flanges of the pulley. During starting and warmup,
a lockout prevents the forward clutch from operating.
Upon initial engagement, the overall drive ratio is approximately 18:1.
As engine speed increases, the belt rides higher up on the forward-pulley
flanges until the overall drive ratio becomes approximately 6:1. The resulting
variations in belt tension are absorbed by the spring-loaded flanges of
the rear pulley. When a clutch is in an idle position, the V-belt is forced to
the outer edge of the rear pulley by a spring force. When the clutch
engages, the floating half of the front pulley moves inward, increasing its
effective diameter and pulling the belt down between the flanges of the
rear pulley.
The transmission is torque-responsive. A sudden engine acceleration
increases the effective diameter of the rear pulley, lowering the drive ratio.
It works this way: An increase in belt tension rotates the floating flange
ahead in relation to the driving flange. The belt now slips slightly on its
driver. At this time nylon rollers on the floating flange engage cams on the
driving flange, pulling the flanges together and increasing the effective
diameter of the pulley.
Figure 2-3
Smoother Drive Without Gears