No Servos Allowed

We Shouldn't Forget That Mechanical Speed Controls Sometimes Can Be a Better Solution Than Electronics

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Dan HebertBy Dan Hebert, PE, Senior Technical Editor

Before servos, before steppers, before variable-frequency drives and even before dc drives, motor speeds were controlled by a variety of mechanical means. Nowadays, in most machine and robot applications, the machine builder will tell you that electronic speed controls have proven to be a superior alternative.
But in a surprisingly large number of instances, including perhaps yours, mechanical speed control is the best option.

Mechanical Solution Superior

Lantech (www.lantech.com) produces a full line of stretch-packaging, shrink-packaging, and case-erecting machines. The company was challenged by a Fortune 100 client to produce a stretch-wrap machine (Figure 1) that could wrap and unitize up to 180 loads per hour of a very soft product.

To achieve this high rate of production and not damage the product, Lantech needed to rethink the way film is applied to the load. "We tried our traditional control systems, including load cells, dc motor control, ac frequency drives and torque control," says Brian Limberg, product innovation director at Lantech. "But none of these electronic systems were able to run at the high speed required without damaging the product."

Lantech's machine required some sort of feedback from the film to determine how fast to pay film out of the film delivery system to keep from damaging the load. The problem was that at a rate of one revolution per second, the electronic feedback systems couldn't keep up and loads were being damaged or not wrapped tightly enough to survive transport.

"So we turned to a mechanical solution," Limberg says. "By using a variable-speed mechanical drive linked to the machine drive system, we were able to precisely control the amount of film being paid out per revolution. This new film delivery system was patented and is available only from Lantech."

The challenge in using a mechanical drive was linking it in a way that ensured a robust design and provided maximum uptime in such a high-speed environment, Limburg says. "We achieved this by using a combination of timing belts and flat-style drive belts, including a unique 90° belt drive arrangement. We used belts everywhere possible to eliminate the need for regular maintenance and lubrication."

A typical feedback system for an electronically controlled film delivery system includes dancer bars, springs and sensors connected to a controller of some sort to control the speed at which the film is delivered to the load being wrapped. With the mechanical solution, Lantech eliminates all the variables and delays included in the electronic version along with all the expensive hardware needed to make the feedback loop work. All that was required was a way to tell the drive to engage or disengage, and Lantech accomplished that with a simple pivoting roller.

"The beauty of the mechanical drive system is that it doesn't have to interface with the machine controls for speed regulation," Limberg says. "All the controller needs to know is whether the film is being delivered or not, and that simple feedback was accomplished using photoelectric sensors, and a flag on the rotating film delivery system that actuates only when material isn't being dispensed."

Besides the technical challenges involved, there was also the need to change the mindset of Lantech and its customers. "Traditional control systems require and provide some sort of feedback, and the industry had become accustomed to this information requirement, but it simply wasn't needed or available with our purely mechanical system," Limberg explains. "So we had to shift our way of thinking, and we found that the ability to dial a machine in can lead to variances in machine performance and make the machine tweaky and less operator-friendly."

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