Motion System Design

A disconnect at the paper mill

As one of Alabama's largest paper recyclers, Weyerhauser Co. recognizes the need to minimize energy costs at its Pine Hill paper mill. The company burns bark from its operations to power its paper and forest product operations. When the chain drives on screw conveyors feeding bark from hoppers to the boilers failed too often, Weyerhauser was forced to switch to higher-priced fuel.

To eliminate chain failures, improve reliability, and maintain continuous operation, Weyerhauser converted the drive system from motors that each drove three augers through a reducer and chain drive to individual Planetgear reducers and ac variable speed motors from Rexnord Industries Inc.

Three bark bins feed and meter via multiple screw conveyors, the largest of which has 18 screws. Shredded bark arrives on a belt conveyor from the mill's drum debarkers or outside sources and is emptied into a large hopper, with the screws feeding to the boiler from the bottom of the hopper.

Previously, a single 30-hp motor, reducer, and chain drive powered sets of three screws each. It was difficult to maintain proper alignment of drive components, which caused repeated premature failures and shutdowns. The original setup required constant maintenance because of stripped sprockets, broken chains, failed couplings, and broken bearing housings. When one screw was damaged, it was necessary to take three screws out of service because one motor and reducer drove all. In addition, because the center sprocket had to be slightly smaller for chain clearance, speeds weren't exact (a 1 rpm difference) for all three screws in a set, which also contributed to wear and chain breakage.

Chain drives on the largest bark bin were replaced with individual Rex Planetgear Polaris-series reducers on every shaft. Each reducer is C-face mounted to a 5-hp ac variable speed motor, which eliminates an input coupling, as well as the need to align components and maintain chain tension. The new system is more energy-efficient, using three 5-hp motors instead of one 30-hp motor.

Driving each screw individually lets users rotate them separately and in opposite directions, providing better control of feed and releasing jams. The ac variable speed motors are computer-controlled, making it easier to adjust screw speeds and change output to meet power changes.

With the new system, Weyerhauser saved more than $500,000 annually in fuel, maintenance, and repair costs.

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