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Power Transmission: Bulk material pulley selection |
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Selecting the right conveyor pulley for bulk material handling applicationsBy Galen Burdeshaw
The author is a Rockwell Automation Dodge Pulley Engineer (dodge-pt.com).
Pulleys used in bulk material handling applications come in a variety of types and styles. With so many options itís sometimes difficult to know where to begin the selection process. Perhaps the best place to start might be a review of the advantages of each pulley and how it might perform in a given application.
The most popular and most basic pulley is the Heavy Duty Drum pulley. This pulley is designed to meet the load limits set by the Conveyor Equipment Manufacturerís Association (CEMA). These pulleys are used throughout the aggregate industry and in some light mining applications. CEMA (cemanet.org) pulleys are welded-steel construction and typically utilize flexible end discs. But how do you know if you need more than a CEMA pulley?
 The end disc design is the most common differentiator between a heavy duty or extra heavy duty construction. The limiting factor that prevents additional load capacity in the pulley is the heat-affected zone at the end disc to hub weld. The heat-affected zone is an area within the base metal that has its mechanical properties or microstructure altered by the heat of the weld itself (below). These zones become the weak link within the pulley and therefore are the subject area for fatigue failure. Cracks in this area are the most common type of failure mode for CEMA pulleys.
Another factor to consider before choosing between a heavy duty or extra heavy duty pulley is the shaft size of the application. The stress increases in the end disc are a function of shaft size, so for a given load, the smaller the shaft, the greater the shaft deflection at the hub. Higher shaft deflection at the hub will result in more distortion within the end disc.
This distortion influences the stress in the end disc welds. As end disc stress increases so does the probability for premature pulley failure, which can be verified through finite element analysis. This type of failure will generally occur when pulley shafting is undersized for the tensions applied to the pulley. Basically the bending stress in the end disc reaches such elevated levels that the material experiences fatigue failure in the area similar to a paper clip bending several times in your hand. Larger shafts with less deflection will minimize the amount of stress in the end disc.
One way to eliminate this end disc failure is to increase the shaft size. The larger shaft will prevent excess deflection. This in turn will minimize the end disc stress and prolong pulley life. Another option is to choose a different style of pulley altogether, for example a pulley that has integral hubs within the end discs. The integral hub eliminates the fatigue point in the heat affected zone and provides much greater service factor to the pulley.
 Engineered Class pulleys are usually stronger than pulleys with an integral hub design. Engineered pulleys are just that; they are custom designed to the tensions, bearing centres and wrap angles of the application. It never hurts to have Engineered Class pulleys quoted when tensions are known and pulley specifications are provided.
T-Section pulleys are larger pulleys and are generally much more expensive. The need for these types of pulleys is generally based upon enhanced reliability and/or elevated tension levels. T-Section pulleys are rare and will generally include Super Pulleys in large mining conveyors.
Wing pulleys are another offering available for belt conveyors. Wing pulleys are essentially combination belt cleaners and belt supporters. Separate contacting surfaces on the pulley beat the conveyor belt through chordal action to remove dirt and debris. As this debris falls off the belt it is directed outward away from the pulley through angled spacer plates. If the debris were not removed the belt and drum pulleys would wear, resulting in decreased product life.
With so many types, styles, and different options available today, choosing the right pulley for your application can be confusing. The bottom line is to choose a pulley that has been designed for your particular application.
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