Machinedesign 1451 Light Service Factor998 0 0

Factoring in belt loads

Sept. 1, 2000
When designing V-belt or synchronous belt drives for demanding applications, be sure to use service factors. They can make a big difference in belt life.

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.

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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Another problem can occur when a designer uses too large a service factor, either inadvertently or in an attempt to be conservative. In such situations, the larger service factor requires a higher capacity belt with more tension. The higher tension due to overdesign causes higher stresses on drive components, especially the shaft and bearings, even when the drive is not operating at rated loads. These high stresses shorten the life of the drive components.

Longer life in lumber and paper mills

Sure, you say, using service factors will increase belt life in severe applications. But how much can I expect? This is hard to say because it depends so much on the application. There's no rule of thumb, although increases of 50 to 100% are not uncommon.

Applications with the highest shock loads generally benefit the most. This is particularly evident in the lumber industry where companies have been converting chain and V-belt drives to synchronous belt drives on equipment such as chip and saw heads, planer feed rolls, and cutoff saws. Here are some examples.

At one midwestern paper mill, rough operating conditions caused excessive wear on a chain drive used to rotate the feed rolls on a log conveyor. The chains were being replaced every six weeks and the sprockets every 12 weeks. So the company decided to switch to a synchronous belt drive. Recognizing that shock loads were unusually high in this application, they selected a belt based on a higher-than-normal service factor. After more than a year of operation, the mill reported few signs of belt wear.

At a large southern plywood mill, a chain drive on a conveyor generated considerable noise around the roller drive cases (housings) and required weekly lubrication, which created a messy work area. The company switched to synchronous belts that were designed using a high service factor. The belts require no lubrication or retensioning. In addition, they reduced the noise level appreciably and they last up to five times longer than the chains they replaced.

In a southeastern lumber mill, the roller chain driving a chip conveyor required constant retensioning and lubrication, and rarely lasted more than two months. A replacement synchronous belt drive designed with a high service factor has eliminated the labor involved in retensioning, lubricating, and replacing the chains. The belts also last longer.

What the tables show

Service factor tables can be obtained from most belt manufacturers. The tables for V-belt drives list service factors that generally range from 1.0 to 1.8 depending on the type of driver (ac motor, dc motor, or internal combustion engine), the type of driven machine, and the duty cycle. Three duty cycles are usually included -- intermittent service (3 to 5 hr/day), normal service (8 to 10 hr/day), and continuous service (16 to 24 hr/day). Service factors for synchronous belts run a bit higher, ranging from 1.0 to 2.4.

Another synchronous belt consideration

Service factors for synchronous belts are higher than those for V-belts. This ensures that the belts have enough tension to keep the belt teeth fully engaged in the sprockets under variable shock load conditions. With insufficient tension, high loads can cause the teeth to ratchet or jump out of their sprocket grooves. Even if the belt doesn't ratchet, friction on the belt teeth when they don't fit into the sprocket grooves wears the teeth down.

Gary Porter is an application engineer for the Power Transmission Div. of The Gates Rubber Co., Denver, Colo.

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