Стандартизация и сертификация в машиностроении

(The Standardization and Certification in Machine-Building)

STANDARDIZATION (7934 characters)

 

Standardization in industry is the development and application of standards that permit large production runs of component parts that can be readily fitted to other parts without adjustment. Standardization allows for clear communication between industry and its suppliers, relatively low cost, and manufacture on the basis of interchangeable parts.

A standard is that which has been selected as a model to which objects or actions may be compared. Standards for industry may be devices and instruments used to regulate colour, size, weight, and other product attributes, or they may be physical models. Standards may also be written mathematical or symbolical descriptions, drawings, or formulas setting forth the important features of objects to be produced or actions to be performed. Standards that are applied in an industrial setting include engineering standards, such as properties of materials, fits and tolerances, terminology, and drafting practices; and product standards intended to describe attributes and ingredients of manufactured items and embodied in drawings, formulas, materials lists, descriptions, or models.

Certain fundamental standards among firms are required to prevent conflict and duplication of effort. The standards activities of governmental departments, trade associations, and technical associations serve in part to meet national standards needs, but one specialized standardizing organization is needed to co-ordinate the diverse standardization activities of many different types of organizations and promote general acceptance of basic standards.

In the United States the American National Standards Institute (ANSI) performs this function. It does not initiate or write standards but provides the means by which national engineering, safety, and industrial standards can be co-ordinated. All interested groups may participate in the decision-making process, and compliance with the national standard is voluntary. The international body that serves this function is the International Organization for Standardization (ISO). Developing an international standard presents the greater challenge because of the breadth of representation and the diversity of needs and viewpoints that must be reconciled.

The International Organization for Standardization

 

The International Organization for Standardization is a specialized international organization founded in Geneva in 1947. It is concerned with standardization in all technical and nontechnical fields except electrical and electronic engineering (the responsibility of the International Electrotechnical Commission). Its membership extends to more than 100 countries, and each member is the national body "most representative of standardization in its country" – in Western industrial countries usually a private organization, such as the American National Standards Institute (ANSI) and the British Standards Institution (BSI), but in most other countries a governmental organization.

Standardization affects units of measurement; alphabetization and transliteration; specifications for parts, materials, surfaces, processes, tools, methods of testing, and machines; and even the form in which specifications are presented. Upon request, the ISO establishes international "technical committees" to investigate and resolve specific issues of standardization and publishes the results as "International Standards" (IS). Because of technological evolution, ISO standards are optimally reviewed (and, if necessary, revised) every five years.

 

(Copyright © 1994-2000 Encyclopædia Britannica, Inc.)

 

MEASUREMENTS

Metric System

Metric System is a decimal system of physical units, named after its unit of length, the metre, the metric sys­tem is adopted as the common system of weights and measures by the majority of countries, and by all coun­tries as the system used in scientific work.

Weights and Measures

Length, capacity, and weight can be measured using standard units. The principal early standards of length were the palm or hand breadth, the foot, and the cubit, which is the length from the elbow to the tip of the mid­dle finger. Such standards were not accurate and defi­nite. Unchanging standards of measurement have been adopted only in modern time.

In the English-speaking world, the everyday units of linear measurement were traditionally the inch, foot, yard and mile. In Great Britain, until recently, these units of length were defined in terms of the imperial standard yard, which was the distance between two lines on a bronze bar made in 1845.

In Britain units of weight (ounces, pounds, and tons) are now also derived from the metric standard — kilogram. This is a solid cylinder of platinum-iridium alloy main­tained at constant temperature at Sevres, near Paris. Cop­ies, as exact as possible, of this standard are maintained by national standards laboratories in many countries.

International System of Units (SI)

International System of Units is a system of meas­urement units based on the MKS (metre-kilogram-second) system. This international system is commonly re­ferred to as SI.

At the Eleventh General Conference on Weights and Measures, held in Paris in 1960 standards were defined for six base units and two supplementary units:

Length

The metre had its origin in the metric system. By in­ternational agreement, the standard metre had been de­fined as the distance between two fine lines on a bar of platinum-iridium alloy. The 1960 conference redefined the metre as 1,650,763.73 wavelengths of the reddish-orange light emitted by the isotope krypton-86. The metre was again redefined in 1983 as the length of the path travelled by light in a vacuum during a time inter­val of 1/299,792,458 of a second.

Mass

When the metric system was created, the kilogram was defined as the mass of 1 cubic decimetre of pure water at the temperature of its maximum density or at 4.0 ° C.

Time

For centuries, time has been universally measured in terms of the rotation of the earth. The second, the basic unit of time, was defined as 1/86,400 of a mean solar day or one complete rotation of the earth on its axis in relation to the sun. Scientists discovered, however, that the rotation of the earth was not constant enough to serve as the basis of the time standard. As a result, the second was redefined in 1967 in terms of the resonant frequency of the caesium atom, that is, the frequency at which this atom absorbs en­ergy: 9,192,631,770 Hz (hertz, or cycles per second).

Temperature

The temperature scale is based on a fixed temperature, that of the triple point of water at which it's solid, liquid and gaseous. The freezing point of water was designated as 273.15 K, equalling exactly 0° on the Celsius tempera­ture scale. The Celsius scale, which is identical to the centigrade scale, is named after the 18th-century Swed­ish astronomer Anders Celsius, who first proposed the use of a scale in which the interval between the freezing and boiling points of water is divided into 100 degrees. By international agreement, the term Celsius has offi­cially replaced centigrade.

One feature of SI is that some units are too large for ordinary use and others too small. To compensate, the prefixes developed for the metric system have been bor­rowed and expanded. These prefixes are used with all three types of units: base, supplementary, and derived. Examples are millimetre (mm), kilometre/hour (km/h), megawatt (MW), and picofarad (pF). Because double pre­fixes are not used, and because the base unit name kilo­gram already contains a prefix, prefixes are used not with kilogram but with gram. The prefixes hecto, deka, deci, and centi are used only rarely, and then usually with metre to express areas and volumes. In accordance with established usage, the centimetre is retained for body measurements and clothing.

In some cases certain other units are allowed for a limited time, subject to future review. These include the nau­tical mile, knot, angstrom, standard atmosphere, hec­tare, and bar.