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Screw actuators solve a bag-positioning riddle

Flaws in bag consistency make it difficult for this bulk bag filling system to position bags accurately. Screw actuators meet the challenge.
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A bulk-bag filling system from Thiele Technologies (www.thieletech.com) is used to package anything from pet food to salt to dirt. Bags—either multiwall paper or woven polyethylene or polypropylene—move from a hopper holding several hundred bags to a pair of staging trays, one bag per tray. From these two staging trays, a vacuum picking device picks the two bags and presents them to the transfer belt that brings them to the net-weigh filling station.

Because the bags move through the system at a pretty good clip, about 35/min, the bags must be precisely positioned in their trays at the point when the vacuum pick-up cups contact them. Four electric screw actuators from Tolomatic ensure this placement precision.

At the root of this positioning challenge is a lack of consistency in the bags being purchased by Thiele’s customers. The Tolomatic screw actuators compensate for variations in bag dimensions. “The actuators automatically adjust each bag in its tray to compensate for variations in bag length and width,” says Thiele’s Jon Gifford. “The design tolerances for positioning the tops of the bags is only plus-or-minus 0.031 inches, and now we can maintain that precision even when bag dimensions vary.”

Thiele has devised what it calls a “bag-top reference mechanism.” It relies on four Tolomatic B3S10 rodless electric screw actuators. Two of the actuators, each operated by its own servo motor, adjust the vertical position of the bags by lowering the two bag tops to a precise reference point determined by two Cognex (www.cognex.com) vision systems. The vision systems send information to the bagging machine’s Rockwell (www.rockwellautomation.com) PLC, and the PLC then instructs the Tolomatic actuators how far they need to move in order to get the two bag tops to the correct position for vacuum cup pickup.

At the same time the two bags are being adjusted vertically, they’re being moved to the dead center of their staging trays by two horizontal B3S10 screw actuators that have been modified to meet Thiele’s application. Each horizontal actuator has two carriers riding on a screw with right-hand threads on one half and left-hand threads on the other half. As the screw turns, each pair of carriers moves toward each other to center the bag in the tray. The two actuators are joined by a coupler and are operated by one servo motor connected by a compact 180-degree Tolomatic belt drive. Once the bags are vertically aligned and centered, they are picked up by the pneumatically powered vacuum pickup device and presented to the transfer belt that carries them into the filling station.

While many of the other actuators in the bulk bagging system are pneumatic, Gifford says he wanted to use electric screw actuators for positioning of the bags because they are faster and more accurate. He appreciates the way that Tolomatic was able to modify its standard offerings to meet Thiele’s requirements. He also adds that the ganged horizontal actuators operated by a single servo motor was a great space-saving feature.

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