Synchronous belts transmit power via positive engagement between profiled teeth on the belt and pulley. Although this tooth engagement (along with proper belt tension) prevents the belt from ratcheting, the belt is free to track, or move side-to-side, on the pulley.
To prevent the belt from riding off the pulley, and to resist the lateral forces caused by the belt’s side-to-side motion, synchronous belt drive systems typically require one or more flanged pulleys.
Causes of belt tracking
The tensile cords in a synchronous belt are twisted in either an “S” (right-hand) or a “Z” (left-hand) twist pattern. The direction of the twist determines to which side of the pulley the belt will tend to track. To reduce this tendency, synchronous belts often use tensile cords with alternating twists.
However, even when tracking due to tensile cord orientation is minimized, synchronous belts may still “favor” one side of a pulley — typically the side that provides a shorter center distance, and therefore, a lower tension.
Belt tracking can also be caused by varying loads, primarily due to distortion of the tensile cords. But varying loads can also cause angular misalignment between pulley shafts (which is also sometimes a product of mounting inaccuracies) and deflection in the drive system structure, both of which contribute to a belt’s tendency to track to one side.
It’s important to prevent the belt from causing significant forces against the pulley flanges, which can result in belt edge wear or flange failure. The tracking force is typically higher for shorter belts than for longer belts because the helix angle of the tensile cord decreases as the belt length increases. Similarly, wide belts tend to track with more force than do narrow belts.
The relationship between belt width and pulley diameter also affects tracking forces: smaller diameter pulleys (relative to belt width) tend to cause belts to track with higher forces than do larger diameter pulleys. Manufacturers advise against using pulleys with diameters less than the belt width because this can result in excessive tracking forces.
When to use flanged pulleys…
The general guideline provided by manufacturers is that in all synchronous belt drive systems, at least one pulley should have flanges. Alternatively, for short-span drives with two-pulleys, each of the pulleys can have a flange on one side. When the span (center distance between shafts) is eight times or more the diameter of the smaller pulley, both pulleys should have flanges.
For serpentine configurations, which use more than two pulleys, proper tracking becomes even more critical, since there are more instances of belt-pulley engagement. In these arrangements, flanges should be included on every other pulley. Alternatively, each pulley should have a flange on alternating sides.
When belts are used on pulleys with vertical shafts (i.e., the belt is riding on its side), gravity tends to pull the belt downward, so vertical shaft systems should have at least one pulley with flanges on both sides, and the remaining pulleys should be flanged at least on the bottom side.
In conveying applications, it may not be possible to use flanged pulleys due to the product’s orientation on the belt. In these cases, a flanged pulley can be used as a back-side idler, placed either near the lead pulley (for unidirectional travel) or midway between the two pulleys (for bi-directional travel).
…And when flanged pulleys may not be necessary
In some cases, it is possible for synchronous belt drives to run properly without flanged pulleys. Considering the factors discussed above, large diameter pulleys can sometimes be used without flanges, as long as the flange face is sufficiently wider than the belt. And idler pulleys generally don’t need to be flanged, but they can include flanges if lateral control of the belt is required.
Feature image credit: Altra Industrial Motion Corp.