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46 • ROCK products • August 2016 www.rockproducts.com Background – Conveyor Power Conveyor power usage has been a topic of interest ever since Thomas Robins started developing them in the 19 th century. Conventional wisdom, supported by continuing measure- ment and research since then, understands that the power supplied by the conveyor drives is the sum of several almost independent categories. Individually or as a total, they can be compared by dividing by the weight of the belt and bulk load. The total friction can vary from .040 to .010 for the best conveyor designs. Much of the world estimates the main resistances as "effective" overall friction, without the drag from pulleys, skirtboard etc. based on history. However, each category is affected by many material, design and construction details. The trade group CEMA has long recognized the individual impact of various parts and configurations and included a set of equations addressing each in its "Belt Conveyors for Bulk Materials," including a significant refinement in the 6 th edition in 2005 and later in the 7 th edition. This approach has proven to provide much more information for design accuracy and optimization, especially since computers have allowed the many calculations and checks needed for thorough design over a wide range of cooperating conditions. The main friction losses include: 1. Belt rubber indentation rolling resistance from the hys- teresis friction affected by temperature and compound- ing of the durable, filled rubbers used on conveyors. 2. Idler alignment from imperfect angular alignment during manufacturing and installation. 3. Idler rotating resistance from the design and manufac- ture including seal and temperature sensitive grease drag. 4. Bulk material friction due to internal movement from belt sag between each pair of idlers, especially with belts with fabric carcasses and lower tension. Inclines or declines dominate the power and belt tension when the slope is much greater than the arctangent of .04=2 degrees. For long conveyors, these have a major impact on the capital costs due to the motor size, but more importantly due to the belt strength needed to overcome them and/or the number of idlers needed to control sag. The importance of each varies widely and is only known from detailed calculations and mea- surements. However, from an operating cost point of view, each impacts the energy cost. Other friction from pulleys, skirtboards and other accessories is important for short con- veyors but also vary significantly. A review of the CEMA-defined contributions for the friction losses or the "main" resistances, as they are called, for a range of conveyors indicate that each have a wide range of possi- ble contributions to the total conveyor power, depending on many interacting design details. • Belt Cover Indentation - 25 to 80 percent. • Idler Rolling Resistance - 10 to 50 percent. • Idler Misalignment - 10 to 60 percent. • Material Trampling Movement from Belt Sag - 1 to 55 percent. • Miscellaneous Component Friction - 2 to 10 percent. More on Idler Alignment – Angular Installation Tolerance As shown above in Figure 1, idler misalignment can have a significant impact on the motor power required to run a conveyor and the belt strength required to carry the tension. Idlers are misaligned in several ways, but this article concentrates on the angular installation varia- tion. In an ideal conveyor system, all of the idler axes would all be perfectly perpendicular to the direction of belt travel, but a small error inevitably exists. The angular installation tolerance is a measure of how per- pendicular the idler set is in relation to the belt travel direction or centerline of the belt. Angular installation variation is the angle between the ideal centerline of the idler roll to the installed idler centerline (A ei in the diagram), but is generally measured at the idler mount- ing point, farthest away from the centerline of the belt. The Angular Installation Tolerance was introduced as an indepen- dent variable to be specified for new designs in the significant revision of the CEMA power calculation methodology in its 2005 6 th edition. Any angular misalignment increases the friction and power requirements on the conveyor system by introducing trans- verse slip between the idlers and the belt. This has been measured by the author to be a linear effect. The greater the angle from the ideal idler axis to the actual idler axis, the greater the transverse slip introduced. When the idler is not rolling in the exact same direction as the belt, an addi- tional friction force is created and seen as an energy loss that the motor has to overcome. As more and more idler sets are misaligned along the length of the conveyor, more friction forces and losses are created and more power is required for running the conveyor. Other misalignment types, specifically vertical and horizontal misalignment between idler sets, can negatively affect the conveyor in terms of idler loads and belt tracking, but they do not impact the required power like the angular misalign- ment is capable of. Forward tilting of the idler set, which are sometimes done intentionally to aid belt tracking, or the use of garland idlers can also have a significant negative impact on the required power for a conveyor. Importance of Alignment