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Design Features
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A variable speed pulley is a V-grooved sheave in which the groove width can be mechanically increased or decreased to allow a V-belt to ride in the groove at various diameters. Its function is to vary the speed of and transmit power to a driven shaft through a V-belt and a Companion Sheave. The speed is varied by changing the diameter at which the belt rides in the variable speed pulley and therefore the ratio between it and the fixed diameter Companion Sheave.

power rating

The power rating of a variable speed pulley depends on the force it can transmit through a v-belt and companion sheave to a driven shaft. This force is identified as BELT TENSION (Te) in V-belt drive calculations.

belt tension The most common means of providing belt tension is by an axial force applied by a compression spring against the moveable flange of the variable speed pulley. The larger the load requirement of a drive, the greater the required tension and the greater the required axial force (normally 3 times the effective belt tension).
short duration loads Variable speed pulleys are commonly subjected to short duration loads, such as high starting loads and cyclical load which are considerably larger than normal running loads. To transmit these loads, without slippage or loss of performance, belt tension well in excess of required running load tension must be developed. However, if a belt is run continuously under the tension required for peak loads, its fatigue life, the fatigue life of the drive components and the efficiency of the drive is substantially reduced.
  Hi-Lo pulleys have solved this problem
mechanism operation

Incorporated in their design is a cam mechanism which provides belt tension as demanded. The load requirement itself determines how much axial force and resulting belt tension is needed. Consequently a Hi-Lo Pulley subjects the components in a variable speed drive to a minimal tension.
When a Hi-Lo Pulley is mounted on a motor shaft the pulley shaft with the cam housing attached becomes integral with the motor shaft. The sliding pulley faces, however are free to move axially and rotate until the cam followers, due to the effective belt tension, contact the cams. When the belt tension requirement increases, the cam followers rotate up the helical cam ramp causing additional axial pressure between the faces of the belt. The belt is consequently gripped tighter and the increased load can be transmitted without slippage of loss of pitch diameter setting. When the belt tension requirement decreases there is an opposite reaction and the axial force is reduced.
Hi-Lo pulley
drive types
There are two basic types of Hi-Lo Variable Speed Pulley Drives; ADJUSTABLE CENTER DISTANCE DRIVES and FIXED CENTER DISTANCE DRIVES
distance drive
One shaft is fixed and the other is moveable. the variable speed pulley is normally mounted on the moveable shaft and a fixed diameter sheave on the stationary shaft. Speed variation of the driven shaft is accomplished by mechanically changing the center distance between the shafts. An increase in center distance will pull the belt to a smaller diameter in the variable speed pulley. This increases the drive ratio and reduces the driven shaft RPM
fixed center distance drive
The driver and the driven shafts are both stationary. The drive consists of two adjustable pitch diameter pulleys. One pulley is manually adjustable by means of an adjustable screw and the other adjustable by means of an adjusting screw in the mechanical pulley, usually mounted on the motor shaft, causing the pulley faces to move in or out and therefore changing the diameter at which the belt diameter in the automatic pulley. The RPM of the driven shaft changes with the change of ration in the belt diameters of the two pulleys.
Produced by the Internet Power Company, Seattle, WA