Conveyors incorporating timing mechanisms abound, but in recent years, conveyor drive technology has become increasingly sophisticated, allowing for highly accurate motion. Such functionality is useful in assembly, manufacturing, and packaging industries that call for accurate movement of product at specific distances and intervals. More specifically, such conveying is enabling new levels of machine integration in robotic pick-and-place, indexing, part positioning, timed conveying, vision inspection, and wide-part handling.
Conveyor basics — All about friction
No matter the specific design, all conveyors use the power of friction for their operation: Typically, two motordriven rollers with a belt tensioner leverage friction to drive a belt.
However, slipping is an issue with flat belts; there’s always the possibility that the belt won’t advance in perfect synchrony with the rollers. Furthermore, as product is loaded on a conveyor and started and stopped, the motion design can be accurately described by Newton’s Second Law of Motion — bodies at rest tend to stay at rest, while those in motion tend to stay in motion. Until friction overcomes inertia and accelerates (or decelerates) the parts loaded on the conveyor, they will briefly undergo micro-slippage on the belt.
For 90% of conveyor applications, such effects are negligible. However, as we’ll explore, high-precision applications require the conveyor to eliminate these modes of lost motion.
Precision versus traditional conveying
Modern conveying began in the early 1900s, when conveyors were basic, relatively clunky steel devices that were designed to simply move materials and discrete product from A to B. In fact, the technology underwent very little little change until the 1970s, when the first major innovation of new manufacturing came — low-profile conveyance. This idea was spurred by the aim to use conveyors to move smaller parts through substations of a manufacturing facility.
Today, the latest innovations boost conveyor accuracy and precision. After change until the 1970s, when the first major innovation of new manufacturing came — low-profile conveyance. This idea was spurred by the aim to use conveyors to move smaller parts through substations of a manufacturing facility.
In the 1990s came innovation to make conveyors suitable for more applications. Most designs went from steel to aluminum, because aluminum conveyors with extruded-style frames allow for modularity and standardization, speedier setup, and easy attachment of motion-design components — including photo-eyes, sensors, and E-stops.
Today, the latest innovations boost conveyor accuracy and precision. After all, many modern applications don’t just move product from A to B; they benefit from the ability to track the exact position of individual parts on a line. What’s more, it’s increasingly important in automated
settings that once an individual part is moved to a destination along the line, it’s within a few thousandths of an inch of the final target location. Such positioning allows full integration of conveyor functions and workstations, so that the parts being manufactured can be gripped by a robotic arm, for example, or even undergo assembly, welding, printing, and scanning while on the belt.
Another precision-conveyor application involves speed matching of tandem belts — using two parallel conveyors to move particularly wide pieces suchas long wooden boards or solar panels,for example. If the twin conveyors aren’t running at the same exact speed, or exerting the same amount of friction on the work piece, the part can begin to skew. Precision conveyors are useful in such applications, as they can be slaved together by one controller and maintain synchronicity. In fact, several conveyors can be run in parallel where required.
Finally, in some sorting facilities, discrete items ride on a conveyor until they are pushed off into pockets. Precision conveying is suitable here as well. Following are traditional conveyor limitations compared to key elements of precision conveyors.
Feedback versus inherent traceability
• Some traditional solutions incorporate encoders on the belt to monitor speed. In addition, traditional belt conveyors often have different capacities in different directions: When driven forward, they can effectively move more load than when driven in reverse.
• Precision conveyors integrate positively driven belts. Here, the drive spindle has teeth on it, which engage those on the belt. This eliminates the belt slipping on the spindle — doing away with the need for an encoder. How is this different than traditional timing-belt designs? It’s new to the conveyance world — as materials technology now allows the toothed belts to be wider, while also exhibiting zero stretch. In fact, until just a few years ago, some integrators customcrafted such belts as specialty subcomponents to achieve better conveyor performance.
What’s more, positively driven belts exhibit the same load capacity in both the forward and reverse directions. Printing and labeling applications benefit from this consistent belt speed. Automated installations incorporating Delta or spider robots for pick-and-place may also benefit. For example, there are cases in which pairing a precision conveyor with a Delta robot delivers precision high enough to render machine vision for robots unnecessary — sometimes saving $30,000 or more in installation costs.
Eliminating slippage and friction
• With traditional setups, designers typically accept the small amount of micro-slippage that occurs between the conveyor belt and the parts riding on it, or minimize it with textured or specially formulated belt surfaces. Another common characteristic is slight friction between the underside of the belt and the surface that supports it.
• In contrast, some precision conveyors can integrate molded fixtures
called pucks. Here, lugs are bolted onto the conveyor belt, while the pucks — custom molded to cradle parts to be conveyed — are mounted onto the lugs. Attachment accuracy reaches ±0.005 in. Key to using pucks is a larger-diameter pulley drive, which allows the attached lugs to ride around the ends of the conveyor without issue.
Addressing the belt-to-table friction issue, some precision conveyors integrate ultra-high-molecular-weight (UHMW) polyethylene beds upon which the conveyor belt rides. Paired with nylon belt backings, this reduces friction for more predictable output and lower overall load on the drive.
Total conveyor design accuracy
• Standard variable-frequency drive (VFD) and motor pairings can drive conveyors with accuracy to 1/16 in., satisfying most applications. However, for those requiring higher performance, accuracies listed for some conveyors are difficult to quantitatively predict, as tolerance buildup of subcomponents — servomotor, power transmission devices, and the conveyor belt itself — is not explicitly listed.
• In contrast, some conveyor packages and units designed to facilitate integration of the timing belt, servomotor and drive, controls, and gearmotor have confirmed accuracy to ±0.20 in. Software programs also exist to facilitate servomotor sizing. Here, the designer supplies the distance to be moved, load, startup inertia, and other known parameters, and the software outputs a range of appropriate setups.
Cheers to better drinkware manufacture — at Tervis
Forget about it, beer cozies: Doubled-walled plastic drinkware is the reigning beverage accessory. Founded in 1946, Tervis, North Venice, Fla., manufactures the wildly popular insulated cups that keep drinks colder or hotter longer.
To accommodate growth, Tervis recently expanded production floor space by 60%, increased workforce more than 40%, and further automated production. For design assistance, Tervis turned to RND Automation and Engineering of Sarasota, Fla., an integrator specializing in custom factory automation and robotic workcells for packaging and material handling. The direction from Tervis was to implement new automation for a steady, phased approach to assemble inner tumblers and outer tumblers — automation production steps requiring no human touch, and reducing repetitive-motion labor.
Today, multiple RND-designed lines run at Tervis.
A conveyor delivery system unites inner and outer tumblers at the robotic stations: Dorner’s 3200 Series Precision Move conveyor package (with new timing belt technology) provides repeatable movement accuracies of ±0.02-in. up to 100 indexes per min. — to accurately index the tumblers for smooth, uninterrupted production flow. The servo-driven conveyor has a flexible fixture attachment system, which accurately locates fixtures to within ±0.005 in. In addition, the precise conveyor has side guides that hold side-to-side tolerance ±0.01-in. for loading, unloading, or locating at assembly stations.
Once fully operational, several more of the machines are expected to process a substantial portion of Tervis’ product offering, reducing cycle times and production footprint.