Although classical V-belts can be used individually, classical belts tend to be somewhat overdesigned for drives light enough to demand only one belt. Thus, a special category of belts evolved specifically for single-belt applications, with the terms "single belt," "fractional horsepower" (FHP), or "light duty" denoting this class.
Originally, these belts were developed to serve the needs of OEM consumer applications such as furnace blowers, lawn and garden equipment, and home shop tools. The belts were generally custom designed for each application, but as replacement markets developed, the sizes were standardized. Designations run from 2L (the smallest) to 5L (the largest). The 4L and 5L sections were dimensionally similar to A and B classical belts and can operate interchangeably on A and B sheaves. However, 4L and 5L belts have a lower power rating than classical A and B section belts. The 4L and 5L were originally termed FA (fractional A) and FB (fractional B), and 3L was formerly called FO.
Classical V-belts are the backbone of industrial drives. Most manufacturers offer a standard line rated for fractional horsepower and intended for mass-produced products such as furnace blowers, small tools, and other consumer-oriented applications. Belts are also manufactured to special order for appliances and lawn and garden equipment where the need for maximum performance at lowest cost justifies extensive design and development.
The service life provided by standard single-belt sections often far exceeds that demanded by low duty-cycle consumer equipment. Thus, life can be traded off for the sake of higher power rating. In this way, fractional-horsepower belts can be used to transmit more than one horsepower. For example, up to 10 hp can be transmitted by a single belt for a service life less than 1,000 hr. This practice is common with lawn and garden equipment where belt life is designed for hundreds of hours of service rather than thousands of hours in continuous industrial applications.
When the life sacrifice is too great, the belt can be made more durable through special construction and choice of reinforcing materials. Most manufacturers offer such designs as "premium" stock belts intended primarily for lawn and garden replacement service. But the belts also may be used to extend the life of any single-belt drive.