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www.rockproducts.com ROCK products • August 2016 • 47 While roll misalignment is also introduced when fabricat- ing the idler frames, this should be a significantly smaller error if manufacturers use precise jigs. Different from the manufacturing of the idlers, idlers are installed on exist- ing structure and tape measures are commonly used for alignment. In many cases, during commissioning or with train- ing idlers, the idler sets are slightly, but intentionally misaligned to help the belt track properly. The inten- tional misalignment uses the created additional friction forces from the transverse slip to push the belt along in the right direction. The need to intentionally misalign during commissioning may be a sign of poor idler or structure installation. Calculating Angular Misalignment Losses Accurately estimating the total friction for a conveyor is important so that the motor and belt are sized appro- priately, and calculating the anticipated misalignment losses is an important part of the design. Since each idler set can potentially be misaligned more or less than the previous one, the value used in the alignment loss calculations is the average abso- lute value of the misalignment of each idler set for the entire conveyor. CEMA states that angular installation will usually have an error of 3/8 in. to 1½ in., where the low range of misalignment typically applies to conveyors with permanent rigid structure where precise alignment procedures and tools may be used and the high range of misalignment are possible when moveable or roof-hung structure, or in other difficult installation environments. The prediction of the friction forces created by the angu- lar misalignment also requires the values for the belt width, belt weight, material weight, conveyor length and the sliding friction factor. The belt, material and length information are known during design, and the sliding fric- tion factor has been measured to be 0.5 for steel idlers to rubber belting. Detailed calculations are provided in CEMA's "Belt Conveyors for Bulk Materials" 6 th and 7 th Editions and used in modern conveyor design software such as Overland Conveyors Belt Analyst. Figure 3 shows the power breakdown from Belt Analyst for one example, though the size of the pie pieces varies depending on a range of design and installation details. Conclusion If idler alignment is planned for before construction, and then installed within the specification, conveyors operate at capacity and require less power to run, which allows the use of smaller motors and lighter belts, in addition to decreasing the initial and operating cost of the conveyor. Existing con- veyors can often benefit from energy saving by realigning the idlers. The Idler Alignment Verification Rig (AVR) makes this process accurate and cost effective. Allen V. Reicks. P.E., has spent much of his career at Overland Conveyor Co. and Precision Pulley and Idler (PPI). He is also the primary author of Chapter 6 Belt Tension and Power Engi- neering of the Conveyor Equipment Manufacturers Association (CEMA) "Belt Conveyors for Bulk Materials." He can be con- tacted at avr4idlers.com. Figure 3 – Main resistance breakdown for the con- veyor case study as installed.