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Flexible Assembly: Saturn's road to success: General Motors Corp's new subcompact automobile.

Flexible assembly systems are helping manufacturers cut production costs, improve quality, increase factory output, and build different versions of a product on the same assembly line. That's why General Motors Corp. (Detroit) decided to make flexible assembly an integral part of the production process for its new Saturn line of subcompact automobiles.

In July 1990, Saturn's assembly plant in Spring Hill, Tenn., began full-scale production, with a goal of making cars that could compete with Japanese models in price, quality, and in meeting ever-varying customer demands. The lineup of cars includes four distinct product types. The base-model SL and SL1 sedans are powered by 85-horse-power single-overhead cam four-cylinder engines that incorporate two valves per cylinder and single-point electronic fuel injection. The engine can be combined with one of two transmission types--either a computer-controlled electronic four-speed automatic or a five-speed manual unit. The high-performance SL2 sedan and Coupe SC are equipped with 124-horsepower dual-overhead cam four-cylinder engines that have four valves per cylinder, a multiport fuel-injection system, and a choice of manual or automatic transmission.

Flexible assembly plays its most important role in assembly of the engines and transmissions. The engine-assembly area employs an automated production system supplied by Ingersoll Rand Co. (Farmington Hills, Mich.) along with robotic equipment produced by GM Fanuc Robotics Corp. (Auburn Hills, Mich.). In the transmission-assembly area, a conveyor and electrified monorail system supplied by Lucas Assembly and Test Systems (Westland, Mich.) is used in a parallel work-cell arrangement that combines both manual and automated assembly processes.

Modular subassembly of components is paramount to maintaining flexibility in the Saturn assembly process. Both transmission and engine parts were specifically designed to be adaptable to modular assembly. For example, the oil pan was manufactured to include small mounts so that robots could easily grab it and position it on the engine.

"One of the prime considerations throughout the entire project," said Bob Harlos, Saturn's manager of powertrain assembly systems, "was the ability to assemble the various engine and transmission types through common systems. We put a lot of effort into trying to balance the assembly lines and keep the work content fairly common between powertrain component types." In engine assembly, for example, both the single- and dual-overhead cam versions use identical lightweight aluminum engine blocks, the same cast-iron cylinder liners, and the same cast-iron crankshaft. For the transmission, both the automatic and manual versions use the same parallel shaft designs. This enables the shaft subassemblies for two transmissions to be fed to the same point on the main assembly line.

All told, the powertrain-assembly system is flexible enough to put together up to 12 different transmission and engine combinations, along with accessories such as air conditioning and power steering.

Engine Assembly

Four major engine components are manufactured at the Spring Hill plant--the block, cylinder heads, crankshaft, and front cover. They are made using a lost-foam casting process designed to produce lightweight high-quality parts.

During engine assembly, the engine parts are stored in a buffering area. They are transported on pallets by a roller conveyor to the different assembly stations on the main production line.

Fitted to the assembly pallets are "memory tags" that are a key element of the entire powertrain-assembly network. These tags are used in both engine and transmission assembly to convey information about the components being carried on individual pallets. When a pallet goes into an assembly station the tag is examined by an electronic read/write unit to identify the components. This information is downloaded to the assembly station's programmable logic controller so that the appropriate production tasks can be carried out. If a part appears at an incorrect assembly station or is damaged, the read/write unit can insert information into the memory tag so that the component is rerouted to the appropriate subassembly or repair station.

During the engine-block-assembly stage, which is one of the first phases of the engine-assembly process, a cleaned aluminum block reaches the assembly line conveyor and approaches the crankshaft installation station. Here, a high degree of automation flexibility is required because so many different functions are performed. "Before it puts the crankshaft in place, the system checks for the presence of main bearing inserts," Harlos said. "It also oils the bearing inserts and crankshaft and positions the engine thrust bearings so that nothing is damaged upon entering the aluminum block."

Main bearing insertion is a critical part of the engine-assembly process. Any contamination or nicks to mating parts must be avoided because tolerances of [+ or -]0.001 inch are involved in the fit between the crankshaft and engine block. After insertion, an in-process verification system equipped with ultrasonic sensors checks that the bearings have been properly seated.

Next, the engine block is moved to the crankshaft-assembly cell. There, automated equipment removes the main bearing caps, inserts the crankshaft, and tightens the caps. The engine block is then conveyed to an automatic torquing station where an electric motor turns the crankshaft and monitors the torque required to ensure that the shaft rotates freely. Simultaneously, the crankshaft is automatically checked for end play.

The engine block then passes through a vision station, where a camera and computer vision system manufactured by Ball Corp. (Mogadore, Ohio) checks the orientation of the crankshaft to ensure correct engine timing. If there is a problem, the shaft can be reoriented before the block is moved to the next assembly station.

Piston Insertion

In the next phase of the assembly process, pistons are placed in the engine block. Automated handling equipment picks up the engine block and rotates it 180 degrees from the "oil pan up" position to the "oil pan down" position, where the oil pan mounting holes are facing the ground and the cylinder bores are facing up. Tight tolerances between the cylinder bores and the pistons make this positioning operation critical.

To ready the pistons for insertion into the engine block, automated equipment and tooling are used to attach a forged-steel connecting rod and rod bearings to the piston. A connecting rod is inserted into the piston pin bore, a pin is inserted to attach the connecting rod to the piston, the piston rings are added, and the entire subassembly is oiled.

After the piston and connecting rod subassembly has been completed, a GM Fanuc A-510 robot picks up the piston and compresses the piston rings so that the piston assembly can fit into the cylinder bore. A second arm steadies the connecting rod while the robot inserts the entire subassembly into the cylinder bore. At the same time, tooling extends from the bottom of the bore alongside the crankshaft to guide the connecting rod into place, preventing damage to the cylinder bore or crankshaft.

Next, the engine is again rotated through 180 degrees. The cap half of the connecting rod is inserted and the cap is automatically tightened to a predetermined torque by automated dc electric nut runners before the engine is rotated back to the "oil pan up" postion.

The partly completed engine then proceeds to a cylinder-head-assembly station. A robot lifts the single- or dual-overhead cam cylinder head over the engine and brings it to rest on the cylinder head gasket. The electric nut runners simultaneously tighten all the cylinder head bolts to the required torque, making use of torque-turn control algorithms to achieve optimum clamping.

In the next step, the engine pallet proceeds along a tilted section of the conveyor. The engine is oriented so that its front section is elevated 15 degrees toward a technician, to allow easy automatic installation of parts such as the crak sprocket, timing gear chain, chain tensioners, and camshaft sprocket. The engine front cover is installed in a semiautomatic operation. Another machine applies sealant to the cover, a technician places the front cover on the engine and inserts the screws, and an automatic nut runner completes the assembly.

In the engine oil-pan-installation station, a walking-beam conveyor belt lines up oil pans that have been designed to include mounting attachments for ease of assembly. The engine is rotated to allow overhead placement of the oil pan. As a nut runner picks up the pan, a robot applies room-temperature vulcanized rubber sealant to the oil pan. A modified nut runner then places the oil pan on the engine and tightens the fasteners to the required torque.

The engine is returned to the pallet in the "oil pan down" position, and final engine dress operations take place, including installation of the flywheel, air intake and exhaust manifolds, alternator, and water pump. All in-process data are updated on the memory tag.

Transmission Assembly

Meanwhile, in a separate assembly area located alongside the engine line, the transmissions are assembled. Four different versions are built: manual or automatic, each with either close or wide gear ratios. The transmission housings are mounted on a fixture attached to a monorail carrier that transport them down the production line.

The transmission is first routed through several manual assembly stations where technicians install gears and other subassemblies. These stations are supplied with the necessary components on trolleys that are identified and guided to the correct location by use of the memory tag.

As in the engine-assembly process, trolleys are directed to the required stations depending on product type and station availability. At each assembly station, product and process status information is written to the memory tag and made available to a host computer for process control. At any time during assembly, the technician can allow a transmission to be rerouted through the system or to a repair station via a process-by-pass path.

After passing through the four manual assembly stations, the transmission proceeds to a series of automated cells for bolt torquing, snap ring insertion, and seal pressing. The monorail carrier then takes the partly assembled transmission to a second manual assembly area. There, technicians mount shift lever actuators, the clutch assembly, and the rear cover on the automatic transmission. The manual transmission is fitted with shift level actuators, synchronizing sleeves, remaining gears, and the rear cover.

The transmission then passes through an automated assembly station where the cover bolts are tightened. Last comes the inspection station where the transmissions are put through a series of functional tests. The inspection station employs compressed air to activate the shift levers and clutch mechanisms on the transmissions.

Operating Conditions

"At this stage of the process, we also have the capability to check out the transmissions in test stands that simulate real vehicle operating conditions," Harlos said. About 10 percent of the transmissions manufactured at Spring Hill are randomly selected for this testing.

One important consideration in keeping the transmission-assembly process running smoothly is the fact that the component content of the manual transmission is less than that of the automatic version. This can cause scheduling problems, particularly when it is time to mate the transmission needs to the engine. "We start the assembly process in sequence, but because the process is nonsynchronous the deck may get shuffled," Harlos explained. "To offset that, we have a buffering area at the end of the transmission-assembly line that collects the four different types of transmissions. We then put them back into sequence and draw off the lot as needed."

Powertrain Connection

The sequence in which engines and transmissions are released to the final assembly area is controlled by a vehicle build manifest, which carries information on customer orders. The build manifest is also used to generate a list of accessories that need to be mounted on the engine. A vehicle-identification number is automatically stamped on both the engine and the transmission, and final powertrain dressing operations are completed before the powertrain is transported to the general auto-assembly area.

Throughout the powertrain-assembly process, status information is available to operators on message-display monitors. Real-time in-process monitoring of quality trends and inventory requirements is built into the web of operations.

The Saturn car-assembly plant is designed to operate at a rate of 60 cars per hour, and all assembly line supplying operations coincide with that rate. This is a lower rate than is typical in powertrain-assembly areas at other automakers, according to Harlos, because many other manufacturers supply powertrains to several assembly plants but the Saturn plant is self-sufficient.

Feasibility Test

Since Saturn began production, the Spring Hill plant has come under intense scrutiny because it is often viewed as a testing ground for flexible assembly systems in U.S. automotive manufacturing. Problems earlier this year involving defective front seats and coolant resulted in limited product recalls. However, Harlos said that the Saturn plant is now firmly back on tract. "At this stage we are focusing on quality, not numbers. The effort is to maintain and improve the quality of the products we are currently producing," he said.

Nevertheless, a wait-and-see attitude on flexible assembly systems may be one that many American manufacturers are adopting, according to Eric Mittelstadt, president and CEO of GM Fanuc. "There has been more and more interest in flexible assembly systems over time, but as with other forms of flexible automation--and indeed automation in general--these systems are being used more often in Europe and Japan than in the United States."
COPYRIGHT 1991 American Society of Mechanical Engineers
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:includes related article about Digital Equipment Corp.
Author:O'Connor, Leo
Publication:Mechanical Engineering-CIME
Date:Nov 1, 1991
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