- SPECIAL REPORTS
Forklifts are like German shorthaired pointers: You can't live with them, and you can't live without them.
Certainly, forklifts are necessary for moving heavy loads. But, like pointers, forklifts come with high operating and maintenance costs. Like pointers, forklifts need room to move, which isn't good for plant managers trying to minimize floor space. And, like pointers, forklifts can be destructive. By some estimates, forklifts cause more than 100 fatalities and 35,000 serious injuries in the United States each year. A typical assembly plant spends thousands of dollars on guardrails, posts and other measures to protect expensive equipment from collisions with the vehicles. Unlike pointers, however, forklifts aren't always available when you need them. If operators have to wait for parts to be delivered-or if forklift drivers are forced to wait for operators-productivity diminishes.
So, when Tower Automotive began building a new assembly plant in Madison, MS, in 2001, engineers laid out the line with a goal of minimizing the number of forklifts needed to service the facility. Operating in two shifts, the plant assembles the frames for Nissan's Titan pickup and Pathfinder sport utility vehicle, both of which are assembled in a sprawling complex in nearby Canton, MS. The frames are built in six variations and supplied to the Nissan plant in the sequence in which they are needed.
The plant consists of three parallel assembly lines. One of the outer lines builds the rear half, or "ladder assembly," of the SUV frame; the other assembles the ladder assembly for the pickup. The front half of the frame for both vehicles is assembled on the central line. When the front half, or "clip," is complete, it meets up with the appropriate ladder assembly, which arrives at the central line via an overhead power-and-free conveyor. A radio frequency identification system ensures that each front clip is joined with the correct ladder assembly.
Originally, the plant was to have five forklift drivers per shift: one for shipping and receiving; one to transfer finished frames from the assembly line to the electrocoating line; and three to service the assembly line. But, Tower engineers were convinced they could do better-and get lean at the same time-through more efficient material handling.
Working with Tower to achieve that goal were Doug Dienberg, a former Tower engineer who now runs his own consulting business, Manufacturing Muscle LLC (Oak Creek, WI); Paul Lendved, a design engineer with Topper Industrial Inc. (Sturtevant, WI), a supplier of material handling equipment; and John Pope, president of Program Management for Industry (Pewaukee, WI). "Our challenge was to simplify material handling so that we could reduce the number of forklifts and drivers in the plant," says Dienberg.
Today, only one forklift driver is needed to service the assembly line. Moreover, most of that driver's deliveries are done between shifts, greatly reducing the risk of accidents with line personnel. (Forklifts are still needed at the end of the line, and in shipping and receiving.) And, because the assembly and parts supply processes are so flexible, the plant is able to produce any frame model at any time.
"The material handling system has been the key to our success," says David Valkanoff, business unit leader at the plant. "Since we don't have to worry about parts distribution, we can just concentrate on improving cycle time."
If the plant was to reduce its dependence on forklifts, part deliveries to each assembly station had to be predictable and they had to be accomplished without disrupting production.
During an 8-hour shift, containers of parts are emptied and exchanged 410 times. Of those, 132 container exchanges do not require a forklift. Parts are delivered in totes on line-side flow racks. Another 64 container exchanges are "forklift-constrained." That is, a forklift is required to change the container at the point of consumption. For these operations, an assembler calls for a forklift when the amount of parts in the container reaches a minimum level, and the driver must arrive at the assembly station before the last part is pulled from the container.
The remaining 214 container exchanges are "forklift-dependent." A forklift is necessary to deliver the parts, but not to move them into position at the point of consumption. Instead, containers of parts are placed onto flexible, mobile carts that operators can quickly move in and out of position as needed. At the end of the shift, a forklift delivers full containers to a staging area near the assembly station, and takes away the empties.
The carts were supplied by Topper Industrial. Called Ergo-Carts, they are custom-made to carry four-sided containers in a variety of shapes and sizes. The carts can be moved manually, or they can be towed. Each cart has six casters and can turn 360 degrees in less than 6 feet of space. The casters have sealed bearings that do not require lubrication. The casters roll so smoothly that an operator can push a 3,000-pound load with less than 35 pounds of effort.
"An operator can back out the empty cart and wheel in a full one with minimal space," says Dienberg. "A container of parts can be changed out in 69 seconds."
The carts can tilt the container to minimize reaching and bending movements by operators. A large cart can tilt a container 15 degrees. Small carts can tilt their loads 45 degrees. For safety, a hydraulic damper controls the speed at which the container tilts. An adjustable bolt stops the tilt at the desired angle. Once the container has been positioned, a pin locks the rack in place to keep it from tilting backwards.
If the containers need to be raised or lowered, the carts can be equipped with lifts, or they can be built to work with external lifts. Another option allows operators to rotate the container to access parts on the opposite side.
"In the old days, carts were something you bought out of a catalog, and you didn't care if the wheels were rickety," says Lendved. "Now, carts are much more custom. You design the cart with the people and the process in mind."
The containers can be made from corrugated steel, wire mesh or heavy-duty plastic. The containers can be equipped with removable gates on one or two sides to give operators easy access to the parts. Containers can also be made with a middle wall for carrying two different parts. When the container is empty, all four sides collapse for shipping back to the supplier.
Part suppliers cooperated with Tower and Topper to set the specifications of the containers. In some cases, Tower even specified how parts should be oriented in the containers, so operators would not have to manipulate them excessively. "Suppliers have been willing to work with us," says Dienberg.
To make part deliveries more predictable and to decrease the amount of parts kept at the plant, Tower established a vendor-managed inventory (VMI) center 2 miles away. This was critical, since most of Tower's suppliers are 2 to 3 days away. In a VMI program, a supplier generates orders for a customer based on demand information sent by the customer. The supplier and the customer agree on objectives for inventory levels, fill rates and transaction costs. The supplier takes on greater responsibility for having parts available when they're needed, but peaks and valleys in demand from the customer even out.
In Tower's case, parts from most suppliers are pulled from the center according to a 4-hour cycle. "That's unprecedented," says Dienberg. "Normally, an automotive plant carries 3 to 4 days of inventory."
Instead of moving parts to a storage location at the plant and then to the assembly line, parts are transferred directly from the VMI center to the assembly stations where they are needed. As a result, Tower has reduced material handling by 40 percent, Dienberg says.
The Ergo-Carts have been instrumental in turning the line into a pull system by simplifying container exchanges. The carts also act as kanbans. Carts with full containers are parked in spots on the shop floor marked by a specific color. Carts with empty containers are parked in spots outlined in a different color. "Anyone walking through the plant can see instantly if we have enough parts," says Dienberg. "If there's no cart on a square, that's a red flag."
Throughout all three lines, operators manually install parts, and, in some cases, apply a few quick welds to tack them in place. Six-axis robots then complete the welds. "Ninety-five percent of the welds on the frame are done automatically," says Valkanoff.
Many of the parts are large and heavy, so ergonomics was a key concern when designing the line. Assembly stations are designed to minimize lifting and turning motions by operators. The Ergo-Carts and containers are large enough to hold several hours' worth of parts, but small enough that several can be positioned within a few feet of the line. Operators never have to take more than two or three steps to get a part. Parts bins tilt to minimize reaching and bending motions, and lifts adjust the height of bins to the height of individual assemblers.
For example, one part for the front clip is more than 4 feet long and weighs 30 pounds. Originally, the container was positioned so that the operator picked up the part, rotated it 90 degrees, and placed the part in a fixture. But, because the bin was on a mobile cart, the operator was able to position the bin so he would only have to rotate the part 15 degrees. "That adjustment didn't cost us anything," quips Dienberg.
Indeed, Dienberg says ergo-nomic improvements don't have to come with a large price tag. Simple devices, such as a support roller to help assemblers move long, heavy parts, are often more practical than gantry-mounted electric or pneumatic assist devices. "Many ergonomic assist devices are expensive and take more time to move a part than simply doing it by hand," he says.
The material handling system at the Madison plant has been so successful that Tower plans to duplicate it at a new plant in Smyrna, TN. That plant will supply frames for a new midsize pickup and SUV from Nissan, which will also be assembled in Smyrna.
For more information on lean manufacturing, call Manufacturing Muscle at 414-803-2549 or send an e-mail to email@example.com. For more information on material handling equipment, call Topper Industrial at 262-886-6931 or visit www.topperindustrial.com.