September 30, 2008
Manufacturing companies must walk a fine line between choosing a conveyor system suited for the application, while maintaining flexibility so the system can handle future jobs.
For stampers—and anyone in manufacturing moving parts from point A to point B, for that matter—choosing a conveyor involves working with a dichotomy: customization versus flexibility. On the one hand, no one conveyor is suitable for all applications; part size, lubricant, temperature, automation, and material handling, to name a few, must all be considered. On the other hand, you need flexibility. You might not know what you'll be producing in six months let alone in six years. This creates a conundrum: How do you choose a conveyor that's suited for the current application yet flexible enough to account for unknown jobs over the horizon?
The truth is it involves balancing myriad factors. If your shop makes an unbalanced decision, the resulting material handling systems may quickly become obsolete, and often make their way to a conveyor "graveyard" out back. True, sometimes scrapping a conveyor can't be avoided. Not every business plan can predict all jobs and the different part geometries they require.
Nevertheless, a balanced decision at least can help you minimize that graveyard risk and perhaps result in a conveyor system that stays in service for years.
Consider an application with large, high-production stamping presses. With all the noise and vibration, not to mention the safety hazards, management wants to ensure workers do not get too close to the press, so they need to transport large parts—40 in. wide in some cases—far enough away to ensure safety as well as maintain or improve productivity.
What material handling technology would suit best? As with any capital investment, material handling configuration depends on the application, and with conveyors, there are no shortages of alternatives. Just a few examples follow.
The downside of separate drives is that the belts can gradually go out of synch, with one belt moving slightly slower than another and, hence, causing parts to twist or turn. This occurs from drive timing problems and, more commonly, heat. Tension drives most conventional conveyor belts, and as those belts heat from use, they expand slightly. Once they do, their speed can change.
How detrimental is this? As always, it depends on the application. If you convey parts to robots with vision sensors, it may not be that serious, because the robots' sensors can account for slight changes in part position. However, if parts are conveyed to a less flexible, mechanized pick-and-place system, part positioning requirements might call for another conveyor setup.
This option usually is the least expensive alternative for large-part handling, so if slight changes in part position don't affect quality or productivity, why not go with the cheaper alternative?
The principal disadvantage is cost. Typically, these setups are pricier than comparable systems thanks to, among other things, the special nonelastic material these timed belts require. Friction heats conventional belt material, causing it to expand and slip. So without the nonelastic material, the timed belts would still eventually slip out of synch.
Accurate part positioning—achievable through the timed-belt setup as well as other conveyor configurations—means conveyors potentially can carry parts through certain processes, such as shearing of light-gauge parts. Here, the belt itself doubles as a fixture, with magnets or vacuums holding parts in place during processing. In these cases, conveyors also can incorporate servo drives, allowing for precise motion control.
Parts can stick to the belt—particularly small ones coated in lubricant—and they can get caught under the belt; however, proper belt and accessory selection can overcome these problems. For instance, small ripples in the belt texture create a surface that, unlike a completely smooth belt, prevents those small, lubricated parts from sticking.
Small parts can also have sharp edges that cut into the belt. In this case, plastic chain-style belts—those with thin links of woven plastic—may be a good choice. Even if those small parts do cut into the plastic, only a few links would need to be replaced rather than an entire belt. On the other hand, extremely small parts could actually become lodged within the plastic-chain links, so in these cases, a pleated belt might suit.
Accessories such as wipers and scrapers help remove parts from the conveyor and ensure they don't become lodged under the belt. Side-wall belts, another option in which corrugated side walls are bonded to the belt on either end, move right along with the belt and, thus, prevent parts from lodging underneath.
Spindle diameters on the end of conveyors should also be considered. Generally speaking, the smaller the part, the smaller the spindle diameter should be, and vice versa. Those smaller spindles help break the suction between the small part and the belt as the part falls off the end. Larger spindle diameters, on the other hand, allow larger parts to fall smoothly off the end.
Lubricant type should enter the picture. Certain chemicals, particularly those in oil-based lubricants, can literally eat away at some belt materials. Sulfurs in drawing compounds can present complications, as can chlorinated chemicals used in parts cleaning. Conveyor belts are made of multiple materials laminated together, and some chemicals actually eat through and delaminate the belt, elongate it, and, ultimately, start peeling off layers of the belt.
Finally, consider the heat the application generates. Belts are rated for a wide range of temperatures; conventional materials may work in environments up to 100 degrees F, while specialty nylon and Teflon® belts work up to 450 degrees F. Beyond this are urethane and plastic-chain belts that work at even higher temperatures, while some wire and chain belts are rated to work in temperatures 1,500 degrees F or more.
Modular conveyors give flexibility as well as customization (see Figure 2). You can change conveyor length, add or subtract curves, or add a specialized Z frame to elevate the conveyor in certain places, such as up to an elevated press bed. The curves that run to the right in a 90-degree turn can be modified to make a 90-degree turn to the left. You can shorten the conveyor by removing the plastic-chain belt, spindle, and support bracket, then sawing the frame to the desired length. This means you can turn a 10-ft. conveyor into a 5-ft. conveyor, a straight one into a curved one, or a flat one into an inclined one.
As always, the application dictates selection. But when looking for material handling alternatives, be sure to consider all variables. This not only includes technical factors—heat, part position, throughput, part geometry, and so on—but business considerations as well. What parts will your shop specialize in not only today but five years from now? Business plans change, but as long as your shop has an idea of where it will be years from now, you will make better decisions about capital outlays for the shop floor, material handling equipment included.
Mike Hosch is director of engineering, Matt Jones is director of channel sales, and Gary Wemmert is director of business development for Dorner Mfg. Corp., 975 Cottonwood Ave., Hartland, WI 53029, 262-367-7600, www.dorner.com.
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