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Drives Help Pick Sticks
Controls give operators all they can handle while cutting waste
Michael Babb -- Design News, August 18, 2003
Maine-based Solon Manufacturing (www.solonme.com) produces sterile cotton-tipped swabs for the medical and electronics industries.
These aren't the double-ended swabs you find in the drugstores to clean babies' ears, insists Solon Process Engineer Peter Martell. "We have to make them to the exact requirements of the medical industry," he says, mostly because laboratory test systems are calibrated for an exact size and mass of the cotton tip.
But while production to exact tolerances is highly automated, packaging isn't. So until now, operators were playing pick-up sticks, but the machines were not laying them straight.
The swabs were dropping directly from production onto a conveyor belt where operators scooped them up and stuffed them into plastic vials. But with the machine producing 500 swabs per minute, even the most experienced operators had trouble identifying bunches of 100, so they would compensate by over packing. When you're producing millions per year, "a few extra for good measure," can add up to a lot of excess product.
To help the operators, Solon introduced a simple system for counting the swabs as they dropped out of the machine. In the gap above the conveyor belt, a photoelectric sensor counted the sticks as they dropped while a pneumatically powered collection arm held them until the desired count was achieved. This solved the counting problem but introduced another difficulty: the little mechanical collection arm's jerky movements often scattered the swabs, and anything that dropped had to be discarded.
"We realized the mechanical sorting process was resulting in cases of wasted product each day," explains Martell. "To be flexible enough to meet our customers' needs and produce the optimum product, we also realized that we needed variable speed control on the machine for some of the specialty fiber swabs."
While the speed of the machines and the conveyor belts could be adjusted, it was a time-consuming process—taking 20 to 40 minutes to change the gears and sprockets. That's a considerable amount of downtime for machines that produce at least 500 swabs per minute.
Solon engineers decided to try ac variable speed drives, and selected Allen-Bradley PowerFlex 4 drives for the machines. They kept the photoelectric sensor and counter but got rid of the mechanical arm. Instead, with the ac drive now powering the conveyor belt, the belt speed was synchronized with the counting operation. While the machine is dropping the swabs onto the conveyor belt, it travels at about 3.75 ft/min. After 100 swabs accumulate, the belt changes speed to 17.5 ft/min for about 2 seconds.
The ability to vary the conveyor speed also allows new machine operators to gradually speed up the process as they learn, says Martell. "Before installing the PowerFlex 4 drives, operators had to shut down the line if the product was being released faster than they could handle. Speed control also saved the swab machines from the extensive wear and tear they'd endure as a result of several shutdowns per shift."
After eight months of usage, Solon was able to report significant progress toward the goals established in 2001. Martell says, "We actually exceeded our goal of 2% reduction of over-pack, which translates into hundreds of cases of finished product. In addition, on the first machine we installed the PowerFlex 4 drive, we achieved a 50% reduction in waste, and an average of 10% increase in overall production."
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