Job Sheet 3 – Variable-Speed Belt Drives
© Festo Didactic 54977-3C
41
Figure 33. Variable-speed belt drive with fixed center-to-center distance.
The flanges of the adjustable pulley can be moved mechanically (usually by
hand) to control the speed ratio. The automatic pulley then changes its position
to keep belt tension constant without modifying the center-to-center distance
between the shafts.
When the flanges are moved to set the adjustable pulley to its open position, the
automatic pulley closes to compensate the belt tension. When the pitch diameter
of the adjustable pulley is at its minimum, the pitch diameter of the automatic
pulley is at its maximum and the speed ratio of the drive is reduced. The opposite
change in the pitch of both pulleys keeps the center-to-center distance between
the shafts the same.
Conversely, setting the adjustable pulley to its closed position opens the
automatic pulley to compensate the change in the belt tension. When the pitch
diameter of the adjustable pulley is at its maximum and the pitch diameter of the
automatic pulley is at its minimum, the speed ratio of the drive increases. Again,
the opposite change in the pitch of both pulleys keeps the center-to-center
distance between the shafts the same.
Adjustable variable-
pitch pulley
Automatic variable-
pitch pulley
Summary of Contents for Belt Drives 2
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