It used to be acceptable to design a belt drive simply by determining the required speed ratio and horsepower, selecting belts and pulleys from a catalog, and finding the lowest price. But that process won’t produce the best results. Today, smart designers fine-tune belt drives to optimize their efficiency and keep down the overall cost (initial plus operation).
Consider a 50-hp electric motor with a belt drive operating continuously at a 1:1 speed ratio on a 30-in. center-to-center distance. No matter what belts and pulleys you select, the belt drive package costs less than $400. But it will probably cost about $20,000 annually to operate. If finetuning the design reduces energyconsumption just 2%, it will save $400 each year. Or, instead of energy savings, the drive capacity could be increased with little energy cost increase. Clearly, paying more for efficient drive components can be very beneficial.
This approach isn’t limited to new designs. Taking a fresh look at old installations can lead to upgraded belt drives that far outperform their predecessors.
Drive efficiency depends on several factors, including load capacity, belt flexing resistance, speed, pulley size and belt tension. Here are some general rules of thumb for taking advantage of these factors.
Load capacity. Rule No. 1 says that maximum efficiency is obtained by operating at or very near the belt’s rated load capacity. Belt load ratings are calculated and published in tables by the Rubber Manufacturers Association and belt suppliers. RMA performance ratings tend to be more conservative than those published by manufacturers. Deviating from these guidelines by underbelting (not enough load capacity) or overbelting (too much capacity) can be costly because both conditions cause efficiency to drop.
With underbelting, the belts not only have inadequate capacity for the job, they may also experience excessive stretch and increased slip. Early and possibly catastrophic failure can occur, causing downtime.
Overbelting usually involves a thicker or wider belt, which has more resistance to flexing as it moves around a pulley. This condition is a major cause of energy loss in belt drives. Also, higher flexing stresses may preclude the extra load capacity from providing more life.
Selecting a more flexible belt (one with thinner cross-section) or one with a cogged (grooved) inside periphery are effective ways to reduce flexural losses. In some cases it’s better to use a larger number of small-cross-section belts rather than heavier belts that at first glance appear more robust.
Overbelting can slowly sneak into a system. Many older drives, though properly designed in their original form, now operate at less than optimum efficiency because they haven’t kept pace with advances in belt technology and the resultant rise in belt capacities (See box, “A fresh look at old drives”).
Speed. Rule No. 2 says that the higher the belt speed, the more power it will transmit. If space permits, you can increase belt speed and efficiency by using larger diameter pulleys. Belts operate satisfactorily at high surface speeds ranging from 2,500 to 6,500 fpm, and in some cases even up to 20,000 fpm.
At a modest increase in cost, larger pulleys also increase belt life because they reduce flexural stress and require less tensioning force.
Maintenance. Rule No. 3 says that maximum efficiency is achieved by eliminating or closely controlling maintenance factors that contribute to energy loss. The most obvious maintenance factor is belt tension. Insufficient tension allows belt slip, which reduces speed and performance (horsepower). Chronic under-tension causes wear and abrasion that dramatically shortens the life of belts, and to a lesser extent pulleys. But trying to extend the maintenance interval by overtensioning the drive usually has the opposite effect. It hastens belt stretch and deterioration due to stresses. The increased loads caused by excessive tension may also damage other drive components. For the optimum amount of tension, consult the manufacturer.
In some situations, maintenance may be minimal or unreliable. If so, design the belt drive to be as maintenance-free as possible. For example, a synchronous belt drive, though initially more expensive, delivers high efficiency — about 98% — with minimal attention. Its tooth engagement eliminates belt slip and associated problems. The highstrength tensile member holds tooth pitch geometry and allows little or no loss in belt tension (stretch).
Belt misalignment is a major factor from both the design and maintenance standpoints. Misalignment reduces service life and accelerates wear of all drive components. Also it generates more noise and contributes energy loss. Recommended limits for misalignment depend on the type of belt (V or synchronous) and the application.
An often overlooked maintenance factor is the use of belt guards to improve efficiency. In addition to enhancing safety, guards shield belt drives from the effects of windage (air disturbance) caused by outside sources (air circulation, drafts, or traffic). This reduces small energy losses in belt drives attributed to windage, and therefore adds a slight improvement in efficiency.
Increasing the efficiency of a drive does not always reduce energy cost. In fact, the opposite may occur. Consider a fan drive that operates at only 85% efficiency due to excessive belt slippage. You upgrade the drive by installing a new design belt that eliminates slippage and achieves 97% efficiency. Now the motor performs more work instead of literally just squeaking by. As a result, the energy cost goes up, rather than down as you may have expected. If the increased airflow isn’t needed, you can avoid paying for it by reducing speed, downsizing the fan or selecting a smaller, more efficient motor.
In the future
If a belt drive is expected to last for several years, chances are that newer designs will make it obsolete before then. Examples include higher technology components such as narrow cross-section V-belts that replace classical V-belts, banded sets in place of multiple individual belts, plus highly-flexible V-ribbed belts and noslip synchronous belts.
Other factors may affect belt drive design as well. Energy costs are likely to escalate faster than component costs. Smaller, more efficient electrical motors will lead to more compact drives. And government mandates will play a more prominent role in areas such as noise generation and energy efficiency.
Ron Francis is project engineer, applications for The Goodyear Tire and Rubber Co., Lincoln, Nebr