Conveyors and Lean
The trend toward lean manufacturing and work cells has, in some instances, been hard on the conveyor industry. An emphasis on flexibility and the ruthless elimination of waste doesn’t always leave room for powered material transport.
For example, traditional fixed conveyors can represent a substantial capital investment and require periodic maintenance-a source of waste according to the lean manufacturing philosophy. They also take up valuable production space and can be difficult or even impossible to reconfigure in response to product or volume changes.
Then there is the danger of that most insidious of wastes, overproduction. Saddled with an expensive conveyor-based system, production managers may be tempted to run a line at a higher than optimal rate to reduce unit costs and create the illusion of efficiency.
Nonetheless, conveyor manufacturers have not been sitting still. Over the years, their products have continued to evolve, so that today’s conveyors offer a host of technologies allowing them to play a roll in a variety of production environments.
One of the common rebuttals conveyor supplier get is "we already work with lean cells, and we don’t want to use conveyors.” However, suppliers emphasize that it’s important to make sure that investment decisions are based on a close evaluation of a particular manufacturing process. Just because you’re doing lean doesn’t mean you have to go with a manual build-and-pass process.
Along these same lines, workstation manufacturer Lista International Corp. (Holliston, MA) recently began offering its Arlink 8000 series with an optional supplied conveyor system feature. Fully modular and marketed specifically with lean manufacturing needs in mind, Lista engineers have configured the workstations so they can be used with a wide range of conveyor types. Clearly, in their opinion, powered conveyors still have a role to play in the world of lean manufacturing.
When thinking about material handling in a workcell or lean production setting, there are a number of factors to keep in mind before making a final decision. These include everything from production cycle times to the presence or absence of feeder lines, throughput, product mix, batch sizes, and product size and weight. Then there are process-specific issues to take into consideration. For example, does a build process include an automated pressing or dispensing station where parts need to be fixtured?
“While manual production may be the optimal solution for some assembly applications, others are better served with a mix of manual and automated systems,” says Mark Dinges, product manager for material flow automation technologies at Bosch Rexroth Corp. (Buchanan, MI). “It’s critical that any manufacturer considering lean production approach it from the standpoint of reducing waste. Finding the most economic assembly solution depends on a variety of factors such as part size, part weight, cycle time, Takt time, product mix, changeover time and volume.”
With regard to size and weight, for example, Lista product manager James Anderson says there comes a point when it simply doesn’t make sense to have human operators doing the grunt work of moving product down the line. According to Anderson, specific weight limits vary widely, depending on cycle times and throughput. But generally, manufacturers will find it falls somewhere between 15 and 40 pounds.
Even in those cases where weight doesn’t pose an undue burden, it still pays to look at just how much time operators are spending moving product down the line-an activity that adds zero value to the process. Even a couple of seconds of transfer time can add up, given high enough throughputs. This kind of waste is especially costly when you consider the fact that time spent pushing a pallet down a bunch of rollers is time that could be better spent doing something else, like organizing parts and tools. When the conveyor is doing the moving, the operator can be getting ready for the next assembly.
Anderson notes that conveyors can be especially effective at carrying product to or from a workcell-say, from a subassembly feeder line, or to an inspection station-chores that might otherwise be performed by human operators pushing around sets of carts. The idea, Anderson says, is to eliminate the “walking wages” that result from people having to spend an inordinate amount of time delivering things.
Along these same lines, manufacturers need to take a close look at how operators are spending their time in mixed-mode manufacturing production lines where assemblies are passing through one or more automated workstations.
Because an automated station generally requires some kind of fixturing--whether that station is dispensing an adhesive, driving threaded fasteners, executing some kind of a test, or performing some kind of pressing or welding operation--an operator in a passive material transport setting will have to devote at least a portion of his time to loading and unloading work-in-process from the station.
In contrast, when using a powered, pallet-based conveying system, engineers can install a positioning and fixturing element, so that the machine can do its job without human intervention. You no longer have a situation in which a machine and a person are doing what the machine could be doing by itself.
Dinges notes that this arrangement provides an added safety benefit, because you no longer have a human being working in such close proximity to a potentially dangerous machine.
Modularity and Flexibility
With regard to flexibility, conveyors have evolved so that they are increasingly able to respond to changes in production volumes, product lines and product mix. Of course, these are areas in which manual workcells truly excel. Being relatively inexpensive and low-tech, workcells are easily reconfigured. Engineers can even add or subtract entire cells, if necessary. Nonetheless, today’s modular conveyors are fully capable of being reconfigured to meet changing market needs.
For example, engineers requiring basic material transport can go with easy-to-use portable conveyor sections, like those offered by Dynamic Conveyor Corp. (Muskegon, MI) or SmartMove Conveyors (Fall River, MA). The former manufactures what it calls its DynaCon conveyor, which can be literally snapped together and then rolled into place with the help of casters. The latter manufactures plug-and-play belt conveyor sections that can be mounted on interchangeable legs or simply placed on a tabletop.
Assemblers can also go with programmable, sectional powered conveyors, like those in Quickdraw Conveyor Systems' lineup. Similar systems include the TB, KTB and GTB belt conveyor systems manufactured by Montech LLC. All of these conveyors--which can be used with or without workpiece pallets--include a wide range of modules that can do everything from locate and fixture pallets for a pressing operation to elevate, rotate and divert pallets along multiple tracks, depending on production needs.
Then there are those conveyors built around a system of standardized extruded aluminum framing. Each of these lines includes an array of standardized belts, chains, drives and other accessories that allow users to mix and match parts as they please. Slots running the length of the extrusions make it possible to install or remove accessories like parts shelves or RFID readers using simple hand tools.
One of the leaders in the category is FlexLink Systems Inc., which manufactures the XT line of twin-rail, or twin track, conveyors, as well as the “X” series of single-track, chain conveyors. Similarly, Bosch Rexroth offers what it calls its TS1, TSplus and TS4plus twin-rail lines, as well as its VarioFlow single-track series. Another example is Automation & Modular Components Inc. (AMC, Davisburg, MI), which offers the Modu-Flex 83 single-track, chain-based conveyor, as well as a number of different twin-track models as part of its Modu-Con line.
For those engineers assembling larger, heavier products, there are also LOGO!MAT S, XL and WalkOn pallet conveyors manufactured by Logomat Automation Systems Inc. (Hebron, KY). Although these friction-roller based systems are capable of handling loads as heavy as 600 kilograms, they are still fully modular in design. Even the WalkOn, which includes a fully enclosed, low-profile track, can be fully reconfigured as needs change.
“In the old days, if you wanted to make changes, you had to start drilling holes,” says Ray Johnson, president of Automated Conveying Systems Inc. (Bloomington, MN), which distributes a number of conveyor types and manufactures the Link series of conveyors. “Now, with T-bolts and slot nuts, you can add equipment as you need it.”
Any Which Way
Finally, powered conveyors offer a ready means by which engineers can get a handle on process information, regulate production cycles and support lean, demand-based production flow principles. Many of today’s conveyors can be fully integrated into a plant’s process monitoring system. This allows the conveyors to ensure that production doesn’t outstrip customer demand. They can also be equipped with “smart” RFID pallets and read-write units that make it possible for engineers to obtain real-time information on shop floor logistics, enhance quality through the use of electronic work instructions, and enable traceability.
A prime example of this kind of technology is FlexLink’s Dynamic Assembly System (DAS) concept, which includes interfaces that allow it to be used with both manual and fully automated workstations. With this product, the software that controls production flow can be easily integrated with a customer’s order systems, enabling variations in product mix and a “pull” approach to production. DAS offers the highest level of flexibility. You can run a batch size of one or a batch size of 1,000.
Another example is Montech’s Montrac system, which turns the basic idea of a conveyor system on its head by employing a battery of semi-autonomous, self-propelled pallets that make their way along a fixed monorail. Fully modular and installed using surprisingly simple tools-mostly Allen wrenches-the Montrac’s motor-driven, smart pallets can be programmed so that they behave differently, depending on the type of assembly they are carrying.
These kinds of technologies are especially powerful when manufacturing a mix of products in a single facility. In a manual workcell environment, engineers create this kind of flexibility by building multiple cells or equipping a single cell to produce a variety of assemblies. With a smart conveyor, on the other hand, engineers can accomplish the same thing by using diverters to guide different product types through various loops or spurs where they can be subjected to specialized processing.
Say, for example, an assembler is producing two product variants in which one version receives a particular component or feature, but the other doesn’t. Engineers can position the workstation performing that operation off on a siding where only those assemblies requiring this special treatment will receive it. Those that don’t require the component can simply continue down the main line, bypassing the spur or siding.
Again, these capabilities can be applied to products of all weights and sizes. For example, at the high end of the weight spectrum, all three Logomat conveyor lines come with a complete battery of lifting, turning, diverting and elevation options to direct pallets wherever they need to go. In addition to facilitating multiple product lines, these features can be used to optimize flow and pacing: After all, one of keys to a lean manufacturing assembly cell is rhythm and balance.