Belts that are designed to meet a light service factor, ranging from 1.1 to 1.4, are sufficient for revolving and vibrating screens such as this one in an ore sorting plant.

You can choose belts from a catalog simply by matching their listed ratings with the drive requirements. But these ratings are based on laboratory tests with standard belt lengths and pulley diameters, plus constant speed and load conditions. If application conditions are not constant, you probably need a belt with more capacity.

To account for more severe conditions, belt manufacturers have developed service factors for common industrial applications of V-belts and synchronous belts. These factors reflect the relative severity of many different types of machines and operating conditions based on field experience.

Service factors let you convert nominal belt ratings from the manufacturer's catalog into more accurate belt horsepower requirements that take into account real-world variables such as start-stop loads, cyclic loads, and shock loads. For example, motor start-stop loads can be two to three times the motor rating. Cyclic loads create more stress reversals in belts, whereas shock loads cause higher stresses than the constant loads represented by the manufacturer's drive design ratings.

Applying a service factor in belt design is easy: just multiply it by the nominal expected load on the drive to obtain the design belt horsepower. The nominal load is usually indicated by the horsepower rating of the electric motor.

Shock loads

Applications with high shock loads can shorten the fatigue life of belts as well as shafts, bearings and other drive components. Using larger service factors in belt selection helps accommodate such loads by requiring belts that are stronger (and larger) than would be needed for the nominal design loads. As an option to larger belts, you can add more belts of the same size in a multiple-belt drive.

Machines such as this jaw-type ore crusher require belts that meet a service factor in the range of 1.3 to 1.8, depending on the type of drive motor and the number of hours the equipment operates per day.

High loads, or loads that vary significantly, can also cause V-belts to slip on their pulleys. For such applications, high service factors result in stronger belts to handle the high loads, plus more belt tension to prevent slipping.

Some machines, particularly pulverizers, paper mill beaters, hammer mills, textile and saw mill machinery, may experience very high shock loads -- higher than those normally encountered in such equipment. Such applications call for higher service factors than those listed in manufacturer's tables. They can range from 1.5 to 2.0 for V-belts and 2.5 to 3.0 for synchronous belts. If you expect unusually high shock loads, its a good idea to ask the manufacturer for a recommendation.

Other severe applications where service factors are probably needed to obtain reasonable belt life include planer feed rolls, paper shredders, cutoff saws, log conveyor debarkers, gang saws, rock crushers, car crushers, oil field pumps, mining, and agricultural equipment.

In cases where equipment clearances, sizes and locations are fixed in the original design, it may not be possible to go back later and substitute a larger belt to extend service life. So be sure to account for abnormal shock loads in the equipment design stage.

Other parameters

Besides accounting for severe load conditions, service factors can be used to compensate for other variables in an application such as pulley diameter, belt length, and speed.

Smaller diameter pulleys cause a belt to flex more, creating higher fatigue stress in the belt and reducing life. Likewise, shorter belts experience more fatigue cycles than long ones because they flex around pulleys more often. At lower speed and higher torque, transmitting the same horsepower requires higher belt tension to prevent slipping (V-belts) or ratcheting (synchronous belts). This higher tension also increases belt stresses.

V-belts are often used to operate paddles in flocculator drives for sewer treatment plants. Here, part of the belt drive is submerged in water. Because water is a lubricant, it reduces the belt's coefficient of friction and lets it slip on the pulleys. Using a higher service factor results in a higher-capacity belt drive with more tension, thereby preventing the slippage.

In addition, a service factor higher than 1.0 can compensate for bending fatigue of a belt caused by idlers. Backside idlers have a much greater effect on fatigue than inside idlers.

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