Classical Multiple V-Belts

Nov. 15, 2002
In addition to the flat leather and fabric belts used in belt drives of the early 1900s, industry used hemp or wire rope running in grooved pulleys.

In addition to the flat leather and fabric belts used in belt drives of the early 1900s, industry used hemp or wire rope running in grooved pulleys. These rope systems inspired development of the rubber V-belt, which first appeared in the 1920s. V-belts make use of a wedging action between belt and sheave, thereby developing high driving forces with considerably less belt tension than required by flat belts. This feature reduces loads on bearings. Also, V-belts inherently track better than flat belts and, thus, do not have to be aligned as carefully.

When V-belts first appeared, most applications were for large industrial drives typically requiring 10 to 15 belts between a single pair of shafts. Thus, belts for these industrial drives became known as "multiple" belts, and today they are referred to as "classical multiple" or "heavy-duty conventional" belts. V-belts and sheaves have been standardized, with letter designations running from A through E. These standard sizes are recognized worldwide.

Classical multiple V-belts offer the broadest range of power ratings and generally are the first type of belt drive considered. They are readily available from local distributors and engineered and rated for long, low-maintenance service. Their only drawback when compared with more modern designs is relatively high weight and space requirements. The belt, being of heavy construction, generates high centrifugal forces that place relatively low limits on top speed. Also, the thickness of the belt limits bend radius, thus requiring the need for relatively large sheaves. In practice, these constraints rarely are serious limitations.

Most manufacturers offer two lines of multiple classical belts. One is the "standard" classification based on a fabric-wrapped construction. The wrap provides a protective envelope that tends to prevent damage and prolong life. The other classification is the "premium" or nonwrapped construction. The advantage of this design is that for a given standard size, none of the cross section is allocated to a wrap; thus, the total section consists of working tensile material. This allows rating belts of nonwrapped construction at higher power, section for section.

The lack of wrap makes a belt more vulnerable to damage; therefore, wrapped construction can be thought of as a more conservative selection. In OEM applications, however, service conditions tend to be well defined, and drive systems normally undergo thorough analysis and rigorous testing. Thus, these applications tend to favor nonwrapped construction, where the higher power rating can be used safely by virtue of the thorough engineering applied to possible environmental effects on the belt.

Belts for in-plant use, in contrast, are normally selected according to simple catalog ratings or to fit existing sheaves. Little, if any, attention is given to life testing or environmental effects, so wrapped construction is recommended.

Belts of nonwrapped construction, sometimes called bandless or raw-edge belts, are made with cogs or molded notches on the underside. Cogged belts permit more severe bends, thus allowing operation over smaller pulleys. Belts of standard wrapped construction are not available cogged because the protective envelope is both difficult to manufacture in cog form and also prevents severe bends. Notched belts are more expensive than those having a flat base. However, because they have a higher load-carrying capacity and run on less-expensive, smaller-diameter pulleys, the drive may require fewer notched belts.

Classical V-belts are frequently used individually, particularly in A and B sizes, smallest of the five cross sections. The larger sections, C, D, and E, generally are not used in single-belt drives because of certain cost penalties and inefficiencies encountered when scaling drives upward. The economics are such that for equivalent ratings, multiple belt drives based on A or B sections usually cost less than single-belt drives with C, D, or E sections.

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