"Handling reliability is a non-negotiable demand here." (Roller and belt conveyor systems key to just-in-time delivery system at Lear Seating)
"I've found that a lot of manufacturing operations that are described as being JIT use that term pretty loosely. In this plant, what we mean by JIT (dust-in-time) is that within three hours of receiving an order, we have manufactured the product, packaged it, loaded it onto trailers, and delivered it to our customer. Now that's our idea of JIT," says Mike Edwards, plant manager at Lear Seating.
Located in Janesville, Wis., Lear Seating supplies a nearby General Motors plant with seats for Suburban and Blazer trucks at the rate of 880 sets of seats per day. The Lear plant combines a high rate of throughout with a relatively labor-intensive manufacturing process. One key to making the whole operation a success is the company's choice of materials handling systems.
Rellability was the driver
Although company managers considered a number of different types of handling systems - ranging from fully automated to largely manual - their final choice was largely driven by the need for system reliability. Tom Lattomus, manager of engineering services and planning, explains, "Our contractural agreement with our customer does not allow for time-out for repairs to a materials handling system. This is a no-excuses type of operation.
"When we examined the different types of handling systems available, we took a hard look at reliability. You see, if we were to lose an hour of production because one of our handling systems was down, there's no way to make up that time. Our commitment to supplying seat sets within no more than three hours is absolute, and that's why we can't tolerate downtime."
Given that need, the company chose to use a series of roller and belt conveyor systems (Rapistan Demag Corp.) for virtually all of its materials handling operations. During the design phase, one of the primary drivers was this requirement for guaranteed up-time. As Mike Edwards phrases it, "Conveyor reliability is a non-negotiable demand here."
Conveyors at work
The plant's manufacturing operation is based on a modular work-cell, team-build approach, rather than the more traditional line-build process.
Production begins as soon as an order is received from the GM plant. Lear Seating manufactures a total of 78 different part numbers. The production volume of each part varies from day to day, as do styles, colors, upholstery materials, and other details.
In the first step, metal frames are assembled into seat backs and seat cushions in eight work cells. These work cells, which are located offline, are positioned on either side of a series of 36-in. belt conveyors. The use of belt conveyors at this stage permits the products to be transported without needing a carrier or pallet to prevent soiling.
As the seat back/cushion sub-assemblies are completed, they are placed on a conveyor and carried to the next work cell. At a second set of eight cells, the seat backs and cushions are married with hardware to become finished products. The sets are then kitted by hand onto plastic pallets, and held in a buffer area.
When the team in one of the final assembly work cells needs a kit, the conveyor control system releases the correct one from the buffer onto a 36-in. live roller conveyor. At the pallet's destination, a transfer mechanism automatically shuttles the pallet off of the roller conveyor to the correct work-cell spur.
Next, the palletized kit is slid off of the conveyor spur onto a wheeled cart for transfer to an assembly station. The kit is then off-loaded.
Once the final assembly work is finished, the completed seat is returned to the same plastic pallet and slid back onto the conveyor spur. The conveyor control system automatically indexes the pallet back onto the main roller conveyor.
Completed seats are conveyed in sequence through inspection, then into an infrared tunnel oven for wrinkle removal, and finally, a bagging station. The oven conveyor is a two-strand chain design that offers little mass for heat build-up.
The next stop for the seats is accumulation. The accumulation conveyor features separate lanes for front, center, and rear seat pallets, and uses dual-width lanes to permit pallets to be arranged back-to-back, rather than end-to-end. This accumulation/buffer section can hold seats representing approximately thirty minutes of production.
As seats are released from this buffer, they are transported onto a lift and placed into a four-tier flow-rack installation. The flow-rack includes 82 gravity conveyor lanes, with each lane dedicated to one specific seat type, style, color, and upholstery material. Each lane has a capacity of 7 pallets stored end-to-end.
On to the customer
"We don't know the precise sequence in which the seat part numbers will be loaded and delivered until our computer, which is directly linked to the GM plant, prints shipping tickets," explains Tom Lattomus. "But we do receive two earlier transmissions that tell us which seat types and colors will be required."
The flow rack therefore contains the proper assemblies, but not in the correct sequence.
The seats are removed from the front end of the flow rack, inserted into steel shipping frames, and loaded aboard trailers for shipping to the customer's assembly plant.
According to Edwards, the entire process, from the time that Lear first receives an order until the arrival of a completed seat assembly at GM, takes three hours or less. "Our customer's production depends on our ability to supply the right products in the right sequence at the moment they're needed. That's JIT manufacturing in the true sense of the term."
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|Publication:||Modern Materials Handling|
|Date:||Sep 1, 1993|
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