Festo Belt Drives 2, Промышленное обслуживание

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Job Sheet 3 – Variable-Speed Belt Drives
40
© Festo Didactic 54977-3C
The variable-pitch pulley is usually a spring-loaded pulley installed on the driving
shaft. The springs inside the variable-pitch pulley automatically adjust the belt
tension when the flanges of the pulley are moved. The number of springs defines
if one or both flanges of the variable-pitch pulley are movable. If the
variable-pitch pulley has one spring, then only one of its flanges is movable. If the
variable-pitch pulley has two springs, then both of its flanges are movable.
When the flanges are moved to set the variable-pitch pulley to its open position,
the center-to-center distance between the shafts is slightly increased using the
motor base to compensate for the belt tension. The pitch diameter of the
variable-pitch pulley is at its minimum, hence reducing the speed ratio of the
drive.
Conversely, setting the variable-pitch pulley to its closed position requires the
center-to-center distance between the shafts to be slightly decreased using the
motor base to compensate for the belt tension. The pitch diameter of the
variable-pitch pulley is at its maximum, which increases the speed ratio of the
drive.
There are different variations for variable-speed belt drives with an adjustable
center-to-center distance. In some cases, both the variable-pitch and the fixed
pulleys can have grooves, while in others the variable-pitch pulley can be flat.
When the variable-pitch pulley is mounted on the driven shaft, the power is kept
constant through most of the speed range. In such a case, the torque on the
variable-pitch pulley is decreased.
Variable-speed belt drives with a fixed center-to-center distance
Variable-speed belt drives with a fixed center-to-center distance, such as the one
shown in Figure 33, consist of two variable-pitch pulleys: one that is adjustable
and the other that is automatic. As the name implies, the center-to-center
distance between the shafts does not change in these drives, which are also
known as compound drives. Variable-speed belt drives with a fixed center-to-
center distance are used when bigger ranges of speed variation are required.