Synchronous belts transmit power through positive engagement between the profiled teeth of the belt and pulley. Although this engagement of the teeth (along with proper belt tension) prevents the belt from ratcheting, the belt is free to track or move from side to side on the pulley.
To prevent the belt from slipping off the pulley and to resist lateral forces caused by sideways movement of the belt, synchronous belt drive systems typically require one or more flanged pulleys. The orientation of the cord twist can cause the belt to move to one side of the pulleys. To reduce this tendency, synchronous belts often use traction cords with “S” and “Z” twists.
Causes of belt tracking
The drive wires in a synchronous belt are twisted in an “S” (right-hand) or “Z” (left-hand) twist pattern. The direction of the twist determines which side of the pulley the belt will tend to follow. To reduce this tendency, synchronous belts often use traction cables with alternating twists.
However, even when drag due to drive cord orientation is minimized, synchronous belts may still “favor” one side of the pulley, typically the side that provides a shorter wheelbase and therefore lower tension.
Ribbon tracking can also be caused by varying loads, mainly due to distortion of tensioned wires. But varying loads can also cause angular misalignment between pulley axes (which is also sometimes the product of mounting inaccuracies) and deflection in the drive system structure, both of which contribute to the belt's tendency to track one way.
It is important to prevent the belt from causing significant forces against the pulley flanges, which could result in belt edge wear or flange failure. The tracking force is typically greater for shorter belts than for longer belts because the helix angle of the traction cord decreases as the belt length increases. Likewise, wide straps tend to track harder than narrow straps.
The relationship between belt width and pulley diameter also affects drag forces: smaller diameter pulleys (relative to belt width) tend to cause belts to pull with greater forces than larger diameter pulleys. Manufacturers do not recommend using pulleys with diameters smaller than the belt width because this can result in excessive drag forces.
When to use flanged pulleys
The general guidance provided by manufacturers is that in all synchronous belt drive systems, at least one pulley must have flanges. Alternatively, for short distance drives with two pulleys, each of the pulleys may have a flange on one side. When the spacing (center distance between shafts) is eight times or more the diameter of the smallest pulley, both pulleys must have flanges.
For serpentine configurations, which use more than two pulleys, correct alignment becomes even more critical, as there are more cases of belt engagement with the pulley. In these arrangements, flanges must be included on all other pulleys. Alternatively, each pulley should have a flange on alternate sides.
When belts are used on pulleys with vertical shafts (i.e. the belt runs on one side), gravity tends to pull the belt downward, so vertical shaft systems must have at least one pulley with flanges on both sides and the remaining pulleys must be flanged at least at the bottom.
In conveying applications, it may not be possible to use flanged pulleys due to the orientation of the product on the belt. In these cases, a flanged pulley can be used as a rear idle, positioned close to the guide pulley (for unidirectional travel) or halfway between the two pulleys (for bidirectional travel).
And when flanged pulleys may not be necessary
In some cases, synchronous belt drives can function correctly without flanged pulleys. Considering the factors discussed above, large diameter pulleys without flanges can sometimes be used, provided the flange face is sufficiently wider than the belt. And idler pulleys generally do not need to be flanged, but may include flanges if lateral belt control is required.
