Some unusual machinery configurations are best served by belts able to deliver power from both top and bottom surfaces. Drives using such belts are referred to as dual drives. Flat belts inherently serve in dual drives, provided an adequate wrap around each pulley. V-belt configurations generally must be manufactured in special sections for dual drives.
Classical sections A through D are available in double or hexagonal configurations termed AA through DD, which operate from either side over standard V-belt sheaves. Drives may be single or multiple as with conventional belts.
Because of the increased thickness and mass of dual-drive belts, they have lower power ratings than the same sections in conventional belts. Dual-drive designs also are more complex and require an analysis of tension and wrap angles at each shaft.
Synchronous belts of XL, L, and H sections are available in dual-drive sections, termed DXL, DL, and DH. Conventional synchronous belts can be used in dual drives when the flat side of the belt can be used as a working surface for shafts not requiring positive ratio relationships. Synchronous belts also can be manufactured with V-ribbed backs, with the V-ribs requiring less tension and providing more positive tracking than a flat surface.
Double-sided V-ribbed belts have been built on an experimental basis, but so far they have demonstrated little advantage over conventional V-ribbed belts with the backside drive provided by the standard flat surface. Dual drives based on conventional V-ribbed belts have already been applied successfully in agricultural machinery, home laundry equipment, and automotive accessory drives.