出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/05/09 02:55:28」(JST)
An assembly line is a manufacturing process (most of the time called a progressive assembly) in which parts (usually interchangeable parts) are added to a product in a sequential manner to create a finished product much faster than with handcrafting-type methods. The division of labour was initially discussed by Adam Smith, regarding the manufacture of pins, in his book The Wealth of Nations (published in 1776). The assembly line was first mechanized in the U.S. by Eli Whitney, in 1797, who also patented a type of cotton gin. Whitney began using the assembly line to manufacture muskets that had interchangeable parts. He was then contracted to supply 10,000 muskets for the U.S. government in two years.
Prior to Whitney's mechanization of the assembly line, craftsmen made muskets one at a time. Due to the handmade & custom nature of this process, each musket was unique. If a single part of the musket broke, it could not be easily replaced, but instead required a custom repair. Because parts manufactured by Whitney's assembly line parts were interchangeable, common parts could be used to replace broken ones. This was a vast improvement in both manufacturing and maintenance of produced goods. In the early 1900s, Ford Motor Company adopted the assembly line to mass-produce the Model T.
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Assembly lines are designed for the sequential organization of workers, tools or machines, and parts. The motion of workers is minimized to the extent possible. All parts or assemblies are handled either by conveyors or motorized vehicles such as fork lifts, or gravity, with no manual trucking. Heavy lifting is done by machines such as overhead cranes or fork lifts. Each worker typically performs one simple operation.
According to Henry Ford:
The principles of assembly are these:
- (1) Place the tools and the men in the sequence of the operation so that each component part shall travel the least possible distance while in the process of finishing.
- (2) Use work slides or some other form of carrier so that when a workman completes his operation, he drops the part always in the same place—which place must always be the most convenient place to his hand—and if possible have gravity carry the part to the next workman for his operation.
- (3) Use sliding assembling lines by which the parts to be assembled are delivered at convenient distances.[1]
Consider the assembly of a car: assume that certain steps in the assembly line are to install the engine, install the hood, and install the wheels (in that order, with arbitrary interstitial steps); only one of these steps can be done at a time. In traditional production, only one car would be assembled at a time. If engine installation takes 20 minutes, hood installation takes five minutes, and wheel installation takes 10 minutes, then a car can be produced every 35 minutes.
In an assembly line, car assembly is split between several stations, all working simultaneously. When one station is finished with a car, it passes it on to the next. By having three stations, a total of three different cars can be operated on at the same time, each one at a different stage of its assembly.
After finishing its work on the first car, the engine installation crew can begin working on the second car. While the engine installation crew works on the second car, the first car can be moved to the hood station and fitted with a hood, then to the wheels station and be fitted with wheels. After the engine has been installed on the second car, the second car moves to the hood assembly. At the same time, the third car moves to the engine assembly. When the third car’s engine has been mounted, it then can be moved to the hood station; meanwhile, subsequent cars (if any) can be moved to the engine installation station.
Assuming no loss of time when moving a car from one station to another, the longest stage on the assembly line determines the throughput (20 minutes for the engine installation) so a car can be produced every 20 minutes, once the first car taking 35 minutes has been produced.
Before the 20th century, most manufactured products were made individually by hand. A single craftsman or team of craftsmen would create each part of a product. They would use their skills and tools such as files and knives to create the individual parts. They would then assemble them into the final product, making cut-and-try changes in the parts until they fit and could work together (craft production).
The assembly line concept was independently redeveloped throughout history and not "invented" at one time by one person. Its exponentially larger development at the end of the 19th century and beginning of the 20th occurred among various people over decades, as other aspects of technology allowed. The development of toolpath control via jigs, fixtures, for machine tools (such as the screw-cutting lathe, metal planer and milling machine) during the early 19th century provided the prerequisites for the modern assembly line by making interchangeable parts a practical reality. The transition to other methods began as manufacturers took advantage of the opportunities that the aforementioned machining developments presented. Thus, before the modern assembly line took shape, there were prototypical forms in various industries, as outlined below.
The Terracotta Army commissioned by the first Chinese Emperor Qin Shi Huangdi is a collection of about 8000 life-sized ceramic soldiers and horses buried with the emperor. The figures had their separate body parts manufactured by different workshops that were later assembled to completion. Notably, each workshop inscribed its name on the part they manufactured to add traceability for quality construction.
At the peak of its efficiency in the early 16th century, the Venetian Arsenal employed some 16,000 people who apparently were able to produce nearly one ship each day, and could fit out, arm, and provision a newly-built galley with standardized parts on an assembly-line basis not seen again until the Industrial Revolution.
The Industrial Revolution in Western Europe and North America, but perhaps most especially in Great Britain and in New England, led to a proliferation of manufacturing and invention. Many industries, notably textiles, firearms, clocks and watches,[2] buttons, horse-drawn vehicles, railroad cars and locomotives, sewing machines, and bicycles, saw expeditious improvement in materials handling, machining, and assembly during the 19th century, although modern concepts such as industrial engineering and logistics had not yet been named.
Adam Smith discusses the division of labour in the manufacture of pins at length in his book The Wealth of Nations (published in 1776).
Probably the first linear and continuous assembly process of post-Renaissance times were the Portsmouth Block Mills built between 1801 and 1803. Marc Isambard Brunel (father of Isambard Kingdom Brunel), with the help of Henry Maudslay and others, designed 22 types of machine tools to make the parts for the blocks used by the Royal Navy. This factory was so successful it remained in use until the 1960s, with the workshop still visible at HM Dockyard in Portsmouth, and still containing some of the original machinery.
One of the earliest examples of almost modern factory layout, designed for easy material handling, was the Bridgewater Foundry. The factory grounds were bordered by the Bridgewater Canal and the Liverpool and Manchester Railway. The buildings were arranged in a line with a railway for carrying the work going through the buildings. Cranes were used for lifting the heavy work, which sometimes weighed in the tens of tons. The work passed sequentially through to erection of framework and final assembly.[3]
Eli Whitney is sometimes credited[by whom?] with developing the armory system of manufacturing in 1801, using the ideas of division of labor, engineering tolerance, and interchangeable parts to create assemblies from parts in a repeatable manner. But Whitney's contribution was mostly as a popularizer rather than "the inventor" of repeatability. He was inspired by several others (including Honoré Blanc), or at least by the contemporary zeitgeist that was building around such ideas. Thomas Jefferson had tried to bring a French mechanic (almost certainly Blanc) and his methods to America in 1785, but the project never went anywhere.[4] A few years later Whitney and his American contemporaries succeeded in introducing the relevant concepts (interchangeable parts, tool-path control via machine tools and jigs, transfer of skill to the equipment, allowing use of semi-skilled or unskilled machine operators) to American firearm-manufacture.
The automatic flour mill built by Oliver Evans in 1785 was called the beginning of modern bulk material handling by Roe (1916). Evans's mill used a leather belt bucket elevator, screw conveyors, canvas belt conveyors, and other mechanical devices to completely automate the process of making flour. The innovation spread to other mills and breweries.[5][6]
Steam powered conveyor lifts began being used for loading and unloading ships some time in the last quarter of the 19th century.[7] Hounshell (1984) shows a ca. 1885 sketch of an electric powered conveyor moving cans through a filling line in a canning factory.
The meatpacking industry of Chicago is believed to be one of the first industrial assembly lines (or dis-assembly lines) to be utilized in the United States starting in 1867. Workers would stand at fixed stations and a pulley system would bring the meat to each worker and they would complete one task. Henry Ford and others have written about the influence of this slaughterhouse practice on the later developments at Ford Motor Company (see below at Ford Motor Company (1908-1915)).
According to a book entitled Michigan Yesterday & Today authored by Robert W. Domm, the modern assembly line and its basic concept is credited to Ransom Olds, who used it to build the first mass-produced automobile, the Oldsmobile Curved Dash.[8]
Olds patented the assembly line concept, which he put to work in his Olds Motor Vehicle Company factory in 1901.[9] This development is often overshadowed by Henry Ford, who perfected the assembly line by installing driven conveyor belts that could produce a Model T in ninety-three minutes.[8]
| Watch video: Ford Model T assembly |
The assembly line developed for the Ford Model T began operation on December 1, 1913.[10] It had immense influence on the world. Despite oversimplistic attempts to attribute it to one man or another, it was in fact a composite development based on logic that took 7 years and plenty of intelligent men. The principal leaders are discussed below.
The basic kernel of an assembly line concept was introduced to Ford Motor Company by William "Pa" Klann upon his return from visiting Swift & Company's slaughterhouse in Chicago and viewing what was referred to as the "disassembly line", where animals were butchered as they moved along a conveyor. The efficiency of one person removing the same piece over and over caught his attention. He reported the idea to Peter E. Martin, soon to be head of Ford production, who was doubtful at the time but encouraged him to proceed. Others at Ford have claimed to have put the idea forth to Henry Ford, but Pa Klann's slaughterhouse revelation is well documented in the archives at the Henry Ford Museum[11] and elsewhere, making him an important contributor to the modern automated assembly line concept. The process was an evolution by trial and error of a team consisting primarily of Peter E. Martin, the factory superintendent; Charles E. Sorensen, Martin's assistant; C. Harold Wills, draftsman and toolmaker; Clarence W. Avery; Charles Ebender; and József Galamb. Some of the groundwork for such development had recently been laid by the intelligent layout of machine tool placement that Walter Flanders had been doing at Ford up to 1908.
In 1922 Ford (via his ghostwriter Crowther) said of his 1913 assembly line:
Charles E. Sorensen, in his 1956 memoir My Forty Years with Ford, presented a different version of development that was not so much about individual “inventors” as a gradual, logical development of industrial engineering:
As a result of these developments in method, Ford's cars came off the line in three minute intervals. This was much faster than previous methods, increasing production by eight to one (requiring 12.5 man-hours before, 1 hour 33 minutes after), while using less manpower.[2] It was so successful, paint became a bottleneck. Only japan black would dry fast enough, forcing the company to drop the variety of colors available before 1914, until fast-drying Duco lacquer was developed in 1926.[2] In 1914, an assembly line worker could buy a Model T with four months' pay.[2]
The assembly line technique was an integral part of the diffusion of the automobile into American society. Decreased costs of production allowed the cost of the Model T to drop within the budget of the American middle class. In 1908, the price of a Model T was around $825, and by 1912 it had dropped to around $575. This price reduction is comparable to a drop from $15,000 to $10,000 in dollar terms from the year 2000.
Ford's complex safety procedures—especially assigning each worker to a specific location instead of allowing them to roam about—dramatically reduced the rate of injury. The combination of high wages and high efficiency is called "Fordism", and was copied by most major industries. The efficiency gains from the assembly line also coincided with the take-off of the United States. The assembly line forced workers to work at a certain pace with very repetitive motions which led to more output per worker while other countries were using less productive methods.
Ford at one point considered suing other car companies because they used the assembly line in their production, but decided against, realizing it was essential to creation and expansion of the industry as a whole.[citation needed]
In the automotive industry, its success was dominating, and quickly spread worldwide. Ford France and Ford Britain in 1911, Ford Denmark 1923, Ford Germany 1925; in 1919, Vulcan (Southport, Lancashire) was the first native European manufacturer to adopt it. Soon, companies had to have assembly lines, or risk going broke by not being able to compete; by 1930, 250 companies which did not had disappeared.[2]
Sociological work has explored the social alienation and boredom that many workers feel because of the repetition of doing the same specialized task all day long.[14] Because workers have to stand in the same place for hours and repeat the same motion hundreds of times per day, repetitive stress injuries are a possible pathology of occupational safety. Industrial noise also proved dangerous. When it was not too high, workers were often prohibited from talking. Charles Piaget, a skilled worker at the LIP factory, recalled that beside being prohibited from speaking, the semi-skilled workers had only 25 centimeters in which to move.[15] Industrial ergonomics later tried to minimize physical trauma.
In his autobiography Henry Ford (1922) mentions several benefits of the assembly line including:
The gains in productivity allowed Ford to increase worker pay from $1.50 per day to $5.00 per day once employees reached three years of service on the assembly line. Ford continued on to reduce the hourly work week while continuously lowering the Model T price. These goals appear altruistic; however, it has been argued that they were implemented by Ford in order to reduce high employee turnover: when the assembly line was introduced in 1913, it was discovered that “every time the company wanted to add 100 men to its factory personnel, it was necessary to hire 963” in order to counteract the natural distaste the assembly line seems to have inspired.[16]
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