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22 • ROCK products • October 2016 www.rockproducts.com Bearings D owntime is often blamed on poor maintenance or lack of maintenance. Undoubtedly proper maintenance is important in keep- ing your conveyor in uptime and not downtime. However, there are many operators with exceptional mainte- nance practices who struggle with downtime. A contributing and often overlooked cause of downtime is the conveyor's bearing selection. Conveyors vary from a few feet to several miles long, and the design requirements and complexity increase with length. However, regardless of size or scale, all conveyors require bearings that are properly selected for the load and environment. Knowing the basics of bearing design will help operators determine if a conveyor's bearings are suitable for the application. The Conveyor Equipment Manufactur- er's Association (CEMA) has published a great deal of literature that details all aspects of conveyor design, includ- ing bearing selection. Unfortunately many operators will not have the time or resources to study and apply all of this information. However, even a basic understanding of bearing ratings and characteristics can allow operators to properly select bearings and reduce their costs and downtime. When specifying bearings for convey- ors, it's important to choose not only the brand but also the type of bearing to be used. For example, a ½-ton, ¾-ton and 1-ton truck may all be able to pull a 15,000-lb. load. However, shorter life and earlier failure are to be expected in the ½-ton truck as compared to the 1-ton. In the same way, a pulley equipped with ball bearings may be capable of carrying the same load as a pulley with spherical roller bearings, but failures and downtime will generally be much more frequent and severe with the ball bearing as opposed to the spherical. Types of Bearings There are many different types of roller bearings. For conveyors, there are generally three basic bearing types to consider: ball, spherical roller and tapered roller. Understanding the basic characteristics of each will help you identify problem areas in your oper- ation and aid in specifying suitable bearings for future purchases. Ball Bearing Characteristics Think of a ball bearing as the ½-ton truck in our analogy. Ball bearings can carry both radial and axial loads. The load in a ball bearing is carried by a number of metallic balls, riding in a curved, tunnel-shaped raceway. Because the raceways have a slightly larger radius than the balls, the load is concentrated at a single point of contact. In other words, the bearing is "point loaded," and the load is not uniformly distributed over the width of the raceway. Because the load is only supported by such a small area, the load ratings and fatigue life are limited compared to tapered and spherical roller bear- ings. Ball bearings can usually handle misalignment, but typically do not have shaft expansion capabilities. Ball bearings are generally used for lighter applications, such as unit handling and package handling conveyors. Spherical Roller Bearing Characteristics A spherical roller bearing would be akin to the ¾-ton truck. Spherical roller bear- ings have two rows of barrel-shaped rollers which, when assembled, form a spherical profile. The outer raceway is also machined with a spherical shape, so the rollers contour to the outer ring and form a sort of ball joint. Because of this geometry, spheri- cal roller bearings have a high radial capacity but are not well suited to high axial loads. This makes them ideal for heavily loaded, horizontal shafts, such as in mining, aggregate and construc- tion applications. They are fully self-aligning, mean- ing that the rollers can adjust freely with misalignment. This is useful for applications where some degree of misalignment would be common, such Engineer Uptime into Your Conveyor's Bearings Regardless Of Size Or Scale, All Conveyors Require Bearings That Are Properly Selected For The Load And Environment. By Brad Droegmiller and Michael Dunn