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Drives
An estimated 50% of all new auger systems sales have traditional clutch/brake drives, but difficult powder characteristics and/or onerous duty requirements dictate the use of one of a number of alternatives. Clutch/brake drives, whether fixed speed (perhaps variable via stepped pulley) or variable speed via invertors or DC motors, offer the best combination of low purchase cost, high accuracy, torque and acceleration/deceleration performance. Where they score less well is with the repair costs and maintenance often associated with clutch/brake assemblies The direct flux vector invertor and stepper motor drives, offer variable speed with zero maintenance but have poor torque and acceleration/deceleration performance. Direct servo drive scores the best in terms of technical performance. It has the disadvantage of high purchase and repair costs but is becoming increasing popular in pharmaceutical applications.
Much is sometimes made of the accuracy improvements offered by servo drive with its positional stopping precision, over the variation in "coast angle" inherent with clutch/brake units. The calculation below, however, shows that in a typical 10 auger revolution dose, the percentage loss in accuracy contributed by the variation in coast angle is insignificant as a proportion of the overall filling accuracy.
Where it does become significant is when dosing at high speed where perhaps only one auger revolution is used and the variation in clutch/brake coast will add almost 1% to the filling accuracy.
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