- SPECIAL REPORTS
This year marks the centennial of the moving assembly line. The manually operated magneto workstation, which was pioneered by engineers at Ford Motor Co., relied on metal rollers.
Today, there are three options for moving a pallet from one station to another: belts, plastic chain or rollers. Each conveying medium has pros and cons that engineers must consider.
Many factors need to be considered when evaluating different assembly conveyor options. The application, coupled with each engineer’s preferences, ultimately define the specific requirements.
However, other factors, such as part accumulation, placement and orientation, play a role. Other key application considerations include product size and payload dimensions, such as the weight of the pallet or fixture. Available floor space is also important, as is the operating environment. For example, products assembled in clean rooms demand unique setups.
“Two of the most important things to consider are number of parts per pallet and throughput,” notes Noel Dehne, vice president of automotive sales at Daifuku Webb Holding Co. “Engineers also need to specify how accurately a product needs to be positioned for the assembly task at each workstation.”
Flat and toothed belts can be used to move pallets from one station to another on an assembly line. Typically, this conveyance method is used for applications that involve small parts or light payloads. Products assembled this way include auto parts, such as wiper motors and spark plugs, and small consumer goods, such as razors.
“The advantages of belts are cost, accuracy and speed,” says Mike Hosch, director of new product development at Dorner Manufacturing Corp. “Belts are lower in cost than plastic chains and more accurate, without additional components required, and offer much higher speeds.”
“Flat belts offer higher surface friction, resulting in better pallet acceleration out of a station,” adds Mark Dinges, group product manager for conveyor systems at Bosch Rexroth Corp. “In addition, they are lightweight, which allows engineers to create long section lengths up to 50 meters; good for use in clean rooms; and economical, requiring a low initial investment.”
However, the most cost-effective type of conveyor, both in terms of the initial capital investment and total cost of ownership over the life the conveyor is a flat belt.
“While the initial cost to install a belt is higher than flattop or roller chain, the cost per meter simply makes a belt the most economical of all the conveyor types,” claims Dinges.
Another conveyor option that engineers should consider is plastic flattop chain. Products that rely heavily on fasteners, such car seats, typically use this conveyance method.
“One advantage is its side-flexing design, which is compatible with curve sections and provides leading-edge pallet orientation,” claims Dinges. “This type of conveyor is also fastener-friendly, because overlapping caps reduce the chance for fasteners to fall between the chain links.”
In addition, plastic flattop chain conveyors allow higher point loading and higher payload carrying capacity than flat belts. And, they’re easier to install or replace. Rolls of flattop chain can be quickly connected with a master link.
“Plastic chains offer a more robust mounting system and flexibility for locating pallets and are more durable in certain applications,” says Hosch. “However, plastic chains are less accurate without additional components. In addition, speeds are limited.”
Steel or plastic roller chain can also be used to move pallets from one station to another on an assembly line. Products typically assembled with this type of robust conveyor include automotive powertrain subassemblies.
Metal chain can be used for transferring parts through an oven, such as a heat-treatment process, or part washing system. Steel is also used for assembly lines that are exposed to radiation or other harsh environments that could break down plastic.
Roller chain conveyors feature power-and-free designs where minimal surface friction allows for higher point loading and higher payload carrying capacity. They are available with or without filler blocks located between the transport rollers to reduce the infiltration of small fasteners.
Powered rollers can also be used on assembly lines. Dishwashers, refrigerators and other large household appliances are typically assembled on these conveyors.
“This type of conveyor offers the highest point loading and highest payload carrying capacity,” Dinges points out. “It’s also fastener-free. Unlike chains, there’s no place for a fastener to become lodged. Powered rollers are also bidirectional, which allows for reversing applications [when necessary]. And, low-cost layouts can be achieved through the use of curves, merges and diverts.”
Conveyor maintenance is often overlooked by manufacturing engineers. Dinges claims that the easiest type of conveyor to maintain is roller chain. “Roller chain is very easy to handle and install,” he explains. “Generally, it’s the most durable and long-lasting of the conveyor media.
“Roller chain is also easy to lubricate, especially when performed with an automatic lubrication module,” notes Dinges. “The low coefficient of friction of the rollers results in low wear to the chain and workpiece pallets.”
When evaluating different types of conveyance options, manufacturing engineers should always try to keep an open mind. “Too many people are used to doing things the same old way,” laments Dehne. “Lack of knowledge and a willingness to learn something new can be a hindrance.”
“Personal preference and past history are the biggest mistake that engineers make when choosing between belts, plastic chain or rollers,” adds Dinges.
“Engineers sometimes have a predetermined media in mind, which in many cases is based on a past experience. It is the responsibility of the applications engineer to objectively look at the entire conveyor system and to make the best selection or recommendation based on the application, the payload, the environment and other factors.”