qualifications form the basis of the current system of tolerances and landings. quality is a set of tolerances that, for all nominal sizes, correspond to the same degree of accuracy.

Thus, we can say that it is the qualifications that determine how accurately the product as a whole or its individual parts. The name of this technical term comes from the word " qualitas", which in Latin means " quality».

The set of those tolerances that correspond to the same level of accuracy for all nominal sizes is called the qualification system.

The standard established 20 qualifications - 01, 0, 1, 2...18 . As the quality number increases, the tolerance increases, i.e., the accuracy decreases. Qualities from 01 to 5 are intended primarily for calibers. For landings, qualifications are provided from the 5th to the 12th.

The set of tolerances and landings, which was created on the basis of theoretical studies and experimental research, and also built on the basis of practical experience, is called the system of tolerances and landings. Its main purpose is to select such options for tolerances and fits for typical joints of various parts of machines and equipment that are minimally necessary, but completely sufficient.

The basis for the standardization of measuring instruments and cutting tools make up exactly the most optimal gradations of tolerances and landings. In addition, thanks to them, the interchangeability of various parts of machines and equipment is achieved, as well as improving the quality of finished products.

Tables are used to design a unified system of tolerances and landings. They indicate the reasonable values ​​\u200b\u200bof the limit deviations for various nominal sizes.

When designing various machines and mechanisms, developers proceed from the fact that all parts must meet the requirements of repeatability, applicability and interchangeability, as well as be unified and comply with accepted standards. One of the most rational ways to fulfill all these conditions is to use at the design stage the largest possible number of such constituent parts, the production of which has already been mastered by the industry. This allows, among other things, to significantly reduce development time and costs. At the same time, it is necessary to provide high precision interchangeable components, assemblies and parts in terms of their compliance with geometric parameters.

Using such a technical method as modular layout, which is one of the standardization methods, it is possible to effectively ensure the interchangeability of components, parts and assemblies. In addition, it greatly facilitates repairs, which greatly simplifies the work of the relevant personnel (especially in difficult conditions), and allows you to organize the supply of spare parts.

Modern industrial production is focused mainly on the mass production of products. One of its prerequisites is the timely receipt of such components on the assembly line. finished products, which do not require additional fitting for their installation. In addition, interchangeability must be ensured that does not affect the functional and other characteristics of the finished product.

Drawing tolerances and landings on the drawings. The principle of interchangeability.

The tolerance field is the field limited by the upper and lower deviations. The tolerance field is determined by the tolerance value and its position relative to the nominal size. In a graphical representation, it is concluded between the lines corresponding to the upper and lower deviations of the zero line.

When drawing dimensions with upper and lower deviations, certain rules must be observed:

Upper or lower deviations equal to zero are not indicated.

The number of characters in the upper and lower deviations is equalized, if necessary, zeros are added to the right to maintain a single number of characters, for example Æ .

The upper and lower deviations are recorded in two lines, with the upper deviation placed above the lower one; the height of the deviation digits is approximately half the digits of the nominal size;

In the case of a symmetrical location of the tolerance field relative to the zero line, i.e. when the upper deviation is equal in absolute value to the lower deviation, but opposite in sign, their value is indicated after the ± sign by digits equal in height to the digits of the nominal size;

The tolerance field characterizes not only the tolerance value, but also its location relative to the nominal size or zero line. It can be located above, below, symmetrically, unilaterally and asymmetrically relative to the zero line. For clarity, in the drawings of parts above the dimension line, after the nominal size, it is customary to indicate the upper and lower deviations in millimeters with their signs, and for clarity, they build diagrams for the location of the shaft or hole tolerance field relative to the zero line; while the upper and lower deviations are set aside in micrometers, and not in millimeters.

Landing- the nature of the connection of the part, determined by the magnitude of the gaps or interferences resulting in it. There are three tick landings:

with a gap,

with interference

transitional.

Note that the shaft and hole forming the fit have the same nominal size and differ in upper and lower deviations. For this reason, in the drawings above the dimension line, the landing is indicated after the nominal size by a fraction, in the numerators of which the maximum deviations for the hole are recorded, and in the denominator - similar data for the shaft.

The difference between the dimensions of the shaft and the hole before assembly, if the size of the shaft is larger than the size of the hole, is called preload N. Interference landing – this is a fit at which an interference is provided in the connection, and the tolerance field of the hole is located under the shaft tolerance field.

Least N min and the greatest N max preloads are important for an interference fit:

N min takes place in the connection, if in the hole with the largest limit size D max the shaft of the smallest limit size will be pressed d min ;

N max takes place at the smallest limit size of the hole D min and the largest limit size of the shaft d max .

The difference between the dimensions of the hole and the shaft before assembly, if the size of the hole is larger than the hole in the shaft, is called gap S. A fit in which a clearance is provided in the joint and the hole tolerance field is located above the shaft tolerance field is called a clearance fit. It is characterized by the smallest S min and the greatest S max clearances:

S min takes place in the connection of the hole with the shaft is formed if in the hole with the smallest limit size D min, the shaft with the largest size limit will be installed d max;

S max takes place at the largest limit size of the hole D max and the smallest limit size of the shaft d min .

The difference between the largest smallest gaps or the sum of the tolerances of the hole and the shaft that make up the connection is called landing tolerance.

A landing, in which it is possible to obtain both a gap and an interference fit, is called transition fit. In this case, the tolerance fields of the hole and the shaft overlap partially or completely.

Due to the inevitable fluctuations in the dimensions of the shaft and hole from the largest to the smallest values, when assembling parts, there is a fluctuation in gaps and interference. The largest and smallest gaps, as well as tightness, are calculated by the formulas. And the smaller the fluctuation of gaps or tightness, the higher the fit accuracy.

The principle of interchangeability

The property of the design of the component part of the product, providing the possibility of its use instead of another without additional processing, while maintaining the specified quality of the product, of which it is included, is called interchangeability. With full interchangeability, parts of the same type, products, for example, bolts, studs, can be manufactured and installed in “their places” without additional processing or preliminary fitting.

Along with complete interchangeability, it is allowed to assemble products by methods of incomplete and group interchangeability, regulation and fitting.

Incomplete interchangeability refers to the assembly of products based on probabilistic calculations.

With group interchangeability, parts manufactured on common machine equipment with technologically made tolerances are sorted by size into several size groups; then check the assembly of the part of the same group number.

The control method involves the assembly with regulation of the position or size of one or more separate, pre-selected parts of the product, called compensators.

The fitting method is the assembly of products with the fitting of one and the assembled parts. Interchangeability provides high quality products and reduces their cost, while contributing to the development of advanced technology and measuring technology. Without interchangeability, modern production is impossible. Interchangeability is based on standardization- finding a solution for repetitive tasks in the field of science, technology and economics, aimed at achieving the optimal degree of streamlining in a particular area. Standardization is aimed at improving and managing the national economy, improving the technical level and quality of products, etc. The main task of standardization is to create a system of normative and technical documentation that establishes requirements for standardization objects that is mandatory for use in certain areas of activity. The most important normative and technical standardization document is a standard developed on the basis of the achievements of domestic and foreign science, technology, best practices and providing solutions that are optimal for the economic and social development of the country.

Tolerances and landings are normalized by state standards that are part of two systems: ESDP - “Unified system of tolerances and landings” and ONV - “Basic standards of interchangeability”. The ESDP applies to the tolerances and fits of the dimensions of smooth elements of parts and to the fits formed when these parts are joined. ONV regulates the tolerances and fits of keyed, splined, threaded and conical connections, as well as gears and wheels.

Tolerances and landings are indicated on the drawings, sketches of technological maps and in other technological documentation. Based on tolerances and fits, technological processes for manufacturing parts and controlling their dimensions, as well as assembling products, are developed.

On the working drawing, the details are affixed with dimensions called nominal, maximum deviations of dimensions and symbols of tolerance fields. The nominal hole size is denoted by D, and the nominal size of the shaft - d. In cases where the shaft and hole form one connection for the nominal size of the connection, take the total size of the shaft and hole, denoted d(D). The nominal size is chosen from a number of normal linear sizes according to GOST 6636-69. limiting the number of sizes used. For sizes in the range 0.001-0.009mm row installed: 0.001; 0.002; 0.003;..0.009 mm. There are four main rows of normal sizes (Ra5; Ra10; Ra20; Ra40) and one row of additional sizes. Rows with a larger gradation of sizes are preferred, i.e. row Ra5 reduce to prefer a number Ra10 etc.

It is almost impossible to machine a part exactly to the nominal size due to the numerous errors that affect the processing web. Workpiece dimensions deviate from the specified nominal dimension. Therefore, they are limited to two limit sizes, one of which (larger) is called the largest limit size, and the other (smaller) is called the smallest limit size. The largest hole size limit is indicated D max, shaft d max; correspondingly the smallest limit hole size D min, and shaft d min .

Measuring a hole or shaft with an allowable error determines their actual size. A part is eligible if its actual size is greater than the smallest size limit, but does not exceed the largest size limit.

In the drawings, instead of the limiting dimensions, two limit deviations are indicated next to the nominal size, for example .

deviation called the algebraic difference between the dimensions and the corresponding nominal size. Thus, the nominal size also serves as the starting point for deviations and determines the position of the zero line.

Actual deviation- algebraic difference between the actual and nominal size.

Limit deviation- algebraic difference between actual and nominal sizes. One of the two limit deviations is called the upper, and the other is called the lower.

The upper and lower deviations can be positive, i.e. with a plus sign, negative, i.e. with a minus sign and equal to zero.

Zero line- a line corresponding to the nominal size, from which dimensional deviations are plotted in the graphic representation of tolerances and fits (GOST 25346-82). If the zero line is located horizontally, then the positive deviation is laid off from it, and the negative deviation is laid down.

Tolerance and landing system

ESDP standards apply to smooth mating and non-mating elements of parts with nominal dimensions up to 10,000 mm (Table 1)

Tab. 1 ESDP standards

qualifications

The classes (levels, degrees) of accuracy in the ESDP are called qualifications, which distinguishes them from the accuracy classes in the OST system. quality(degree of accuracy) - degree of gradation of system tolerance values.

Tolerances in each quality increase with an increase in nominal sizes, but they correspond to the same level of accuracy, determined by the quality (its serial number).

For a given nominal size, the tolerance for different qualifications is not the same, since each qualification determines the need to use certain methods and means of processing products.

The ESDP has 19 qualifications, designated by a serial number: 01; 0; one; 2; 3; 4; five; 6; 7; 8; nine; 10; eleven; 12; 13; fourteen; 15; 16 and 17. The highest accuracy corresponds to grade 01, and the lowest to grade 17. Accuracy decreases from grade 01 to grade 17.

The qualification tolerance is conventionally denoted in capital Latin letters ІТ with the qualification number, for example, ІТ6 - admission of the 6th qualification. In what follows, the word tolerance refers to the tolerance of the system. Qualities 01, 0 and 1 are provided for assessing the accuracy of plane-parallel end measures of length, and qualifications 2, 3 and 4 - for assessing smooth plug gauges and staple gauges. The dimensions of parts of high-precision critical joints, for example, rolling bearings, crankshaft journals, parts connected to rolling bearings of high accuracy classes, spindles of precision and precision metal-cutting machines, and others are performed according to the 5th and 6th grades. Qualities 7 and 8 are the most common. They are provided for the dimensions of precise critical joints in instrument making and mechanical engineering, for example, parts of internal combustion engines, automobiles, aircraft, machine tools, measuring instruments. The dimensions of the parts of diesel locomotives, steam engines, hoisting and transport mechanisms, printing, textile and agricultural machines are mainly performed according to the 9th grade. Quality 10 is intended for the dimensions of non-critical connections, for example, for the dimensions of parts of agricultural machines, tractors and wagons. The dimensions of parts that form irresponsible connections, in which large gaps and their fluctuations are permissible, for example, the dimensions of covers, flanges, parts obtained by casting or stamping, are assigned according to the 11th and 12th qualifications.

Qualities 13-17 are intended for non-critical dimensions of parts that are not included in connections with other parts, i.e. for free sizes, as well as for interoperational sizes.

Tolerances in grades 5-17 are determined by the general formula:

1Tq = ai, (1)

where q- qualification number; but- a dimensionless coefficient established for each quality and independent of the nominal size (it is called the “number of tolerance units”); і - tolerance unit (µm) - a multiplier depending on the nominal size;

for sizes 1-500 µm

for sizes St. 500 to 10,000 mm

where D from- geometric mean of boundary values

where D min And D max- the smallest and largest boundary value of the interval of nominal sizes, mm.

With a given quality and range of nominal dimensions, the tolerance value is constant for shafts and holes (their tolerance fields are the same). Starting from the 5th grade, the tolerances for the transition to the neighboring less accurate grade increase by 60% (the denominator of a geometric progression is 1.6). Every five qualifications, tolerances increase 10 times. For example, for parts of nominal dimensions St. 1 to 3 mm admission of the 5th qualification IT5 = 4 µm; after five qualifications, it increases 10 times, i.e. ІТ1О = .40 µm etc.

Intervals of nominal sizes in the ranges of St. 3 to 180 and St. 500 to 10000 mm in systems OST and ESDP coincide.

In the OST system up to 3 mm the following size intervals are established: up to 0.01; St. 0.01 to 0.03; St. 0.03 to 0.06; St. 0.06 to 0.1 (exception); from 0.1 to 0.3; St. 0.3 to 0.6; St. 0.6 to 1 (exception) and 1 to 3 mm. Interval St. 180 to 260 mm divided into two intermediate intervals: St. 180 to 220 and St. 220 to 260 mm. Interval St.-260 to 360 mm divided into intervals: St. 260 to 310 and St. 310 to 360 mm. Interval St. 360 to 500 mm divided into intervals: St. 360 to 440 and St. 440 to 500 mm.

When converting accuracy classes according to OST to qualifications according to ESDP, you need to know the following. Since in the OST system tolerances were calculated according to formulas that differed from formulas (2) and (3), there is no exact match of tolerances for accuracy classes and qualifications. Initially, the following accuracy classes were established in the OST system: 1; 2; 2a; 3; 3a; 4; five; 7; 8; and 9. Later, the OST system was supplemented with more accurate classes 10 and 11. In the OST system, shaft tolerances of 1, 2, and 2a accuracy classes are set to be smaller than for holes of the same accuracy classes.

This is due to the difficulty of machining holes compared to shafts.

Main deviations

Basic deviation- one of two deviations (upper or lower) used to determine the position of the tolerance field relative to the zero line. This deviation is the nearest deviation from the zero line. For shaft (hole) tolerance fields located above the zero line, the main deviation is the lower deviation, shaft ei (for hole EI) with a plus sign, and for tolerance fields located below the zero line, the main deviation is the upper shaft deviation е* (for hole ES) with a minus sign. From the border of the main deviation, the tolerance field begins. The position of the second boundary of the tolerance field (i.e., the second limit deviation) is determined as the algebraic sum of the value of the main deviation and the accuracy tolerance.

For shafts, 28 basic deviations are established and the same number of basic deviations for holes (GOST 25346 - 82). The main deviations are indicated by one or two letters of the Latin alphabet: for the shaft - in lowercase letters from a to zc, and for the hole - in capital letters from A to ZC (Fig. 1, d). The values ​​of the main deviations are given in the tables.

The main deviations of the shafts from a to g (upper deviations es with a minus sign) and the main deviation of the shaft h (es equal to zero) are designed to form shaft tolerance fields in landings with a gap; from ј (ј ѕ) to n - in transitional landings from р to zс (lower deviations ei with a plus sign) - in interference fit. Similarly, the main deviations of holes from A to G (lower deviations ЕІ with a plus sign) and the main deviation of the hole H (for it ЕІ = 0) are designed to form tolerance fields for holes in landings with a gap; from Ј (Ј *) to N - in transitional landings and from P to ZC (upper deviations ES with a minus sign) - in interference fit. The letters ј * and Ј * indicate the symmetrical location of the tolerance relative to the zero line. In this case, the numerical values ​​of the upper es (ES) and lower ei (EI) deviations of the shaft (hole) are numerically equal, but opposite in sign (the upper deviation is with a “plus” sign, and the lower one is with a “minus” sign).

The main deviations of the shaft and the hole, indicated by the letter of the same name (for a given size range), are equal in magnitude, but opposite in sign; they increase with increasing size interval value.

Hole system and shaft system

A combination of tolerance fields for shafts and holes can be obtained big number landings. There are landings in the hole system and in the shaft system.

Landings in the hole system- landings in which various gaps and interferences are obtained by connecting shafts of different sizes with one main hole (Fig. 1, a), the tolerance field of which (for a given quality and size interval) is constant for the entire set of landings. The tolerance field of the main hole is located invariably relative to zero

line so that its lower deviation ЕІ = 0 (it is the main deviation H), and the upper deviation ЕЅ with a + “plus” sign is numerically equal to the tolerance of the main hole. Shaft tolerance fields in clearance fits are located below the zero line (under the tolerance field of the main hole), and in interference fit - above the tolerance field of the main hole (Fig. 1, b). In transitional landings, the tolerance fields of the shafts partially or completely overlap the tolerance field of the main hole.

Fits in the shaft system- landings in which various gaps and interferences are obtained by connecting holes of various sizes with one main shaft, the tolerance field of which (for a given quality and size range) is constant for the entire set of landings. The tolerance field of the main shaft is located invariably relative to the zero line so that its upper deviation еѕ = 0, and the lower deviation еі with a minus sign is numerically equal to the tolerance of the main shaft. The tolerance fields of holes in landings with a gap are located above the tolerance field of the main shaft, and in interference fit - below the tolerance field of the main shaft.

The hole system is characterized by a simpler technology for manufacturing products compared to the shaft system, and therefore it has received predominant use. According to the shaft system, rolling bearings are connected to the holes of the bushings or product cases, as well as the piston pin to the piston and connecting rod, etc.

In some cases, to obtain connections with very large gaps, use combined landings- landings formed by the tolerance fields of the holes from the shaft system and the tolerance fields of the shafts from the hole system.

For nominal sizes less than 1 and St. 3150 mm, as well as for the 9th-12th grades with nominal sizes of 1-3150 mm, landings are formed by a combination of tolerance fields for holes and shafts of the same accuracy grade, for example, H6 / p6; H7/e7; E8/h8; H9/e9 and B11/h1. In the 6th and 7th grades with nominal sizes of 1-3150 mm, for technological reasons, it is recommended to choose the hole tolerance field one grade rougher than the shaft tolerance field, for example, H7 / k6; E8/h7.

In addition to the landings indicated in the tables, in technically justified cases, other landings formed from the ESDP tolerance fields are allowed to be used. The fit should refer to the hole system or the shaft system, and if the tolerances of the hole and the shaft are not the same, the hole should have a larger tolerance. The tolerances of the hole and the shaft may differ by no more than two qualifications.

The choice and assignment of tolerances and landings is carried out on the basis of calculations of the necessary clearances or tightness, taking into account the operating experience of such joints.

The property of independently manufactured parts (or units) to take their place in the unit (or machine) without additional processing during assembly and perform their functions in accordance with technical requirements to the operation of this node (or machine)
Incomplete or limited interchangeability is determined by the selection or additional processing parts during assembly

Hole system

A set of fits in which different gaps and interferences are obtained by connecting different shafts to the main hole (hole, the lower deviation of which is zero)

Shaft system

A set of landings in which various gaps and interferences are obtained by connecting various holes with the main shaft (a shaft whose upper deviation is zero)

In order to increase the level of interchangeability of products, reduce the range normal instrument established tolerance fields for shafts and holes of preferred application.
The nature of the connection (fit) is determined by the difference in the dimensions of the hole and the shaft

Terms and definitions according to GOST 25346

Size- numerical value of a linear quantity (diameter, length, etc.) in the selected units of measurement

actual size is the element size set by the measurement

Limit dimensions- two maximum allowable sizes of the element, between which there must be (or which may be equal to) the actual size

The largest (smallest) size limit- the largest (smallest) allowable element size

Nominal size- the size relative to which deviations are determined

Deviation- algebraic difference between the size (actual or limit size) and the corresponding nominal size

Actual deviation- algebraic difference between the actual and the corresponding nominal dimensions

Limit deviation- algebraic difference between the limit and the corresponding nominal size. Distinguish between upper and lower limit deviations

Upper deviation ES, es- algebraic difference between the largest limit and the corresponding nominal size
ES- upper deviation of the hole; es- upper shaft deflection

Lower deviation EI, ei- algebraic difference between the smallest limit and the corresponding nominal size
EI- lower deviation of the hole; ei- lower shaft deflection

Basic deviation- one of two limit deviations (upper or lower), which determines the position of the tolerance field relative to the zero line. In this system of tolerances and landings, the main deviation is the closest to the zero line

Zero line- a line corresponding to the nominal size, from which dimensional deviations are plotted when graphic image tolerance and landing fields. If the zero line is horizontal, then positive deviations are plotted up from it, and negative deviations are plotted down.

Tolerance T- the difference between the largest and smallest limit sizes or the algebraic difference between the upper and lower deviations
Tolerance is an absolute value without a sign

Standard IT approval- any of the tolerances established by this system of tolerances and landings. (Hereinafter, the term "tolerance" means "standard tolerance")

Tolerance field- a field limited by the largest and smallest limit sizes and determined by the tolerance value and its position relative to the nominal size. With a graphic representation, the tolerance field is enclosed between two lines corresponding to the upper and lower deviations relative to the zero line

Quality (degree of accuracy)- a set of tolerances considered as corresponding to the same level of accuracy for all nominal sizes

Tolerance unit i, I- a multiplier in the tolerance formulas, which is a function of the nominal size and serves to determine the numerical value of the tolerance
i- tolerance unit for nominal sizes up to 500 mm, I- tolerance unit for nominal sizes of St. 500 mm

Shaft- a term conventionally used to refer to the external elements of parts, including non-cylindrical elements

Hole- a term conventionally used to refer to the internal elements of parts, including non-cylindrical elements

main shaft- shaft, the upper deviation of which is equal to zero

Main hole- hole, the lower deviation of which is zero

Maximum (minimum) material limit- a term referring to that of the limiting dimensions, which corresponds to the largest (smallest) volume of material, i.e. the largest (smallest) limit size of the shaft or the smallest (largest) limit size of the hole

Landing- the nature of the connection of two parts, determined by the difference in their sizes before assembly

Nominal fit size- nominal size common to the hole and shaft that make up the connection

fit tolerance- the sum of the tolerances of the hole and the shaft that make up the connection

Gap- the difference between the dimensions of the hole and the shaft before assembly, if the size of the hole is larger than the size of the shaft

Preload- the difference between the dimensions of the shaft and the hole before assembly, if the size of the shaft is larger than the size of the hole
Preload can be defined as the negative difference between the dimensions of the hole and the shaft

Landing with clearance- landing, in which a gap is always formed in the connection, i.e. the smallest hole size limit is greater than or equal to the largest shaft size limit. In the graphical representation, the hole tolerance field is located above the shaft tolerance field

Landing with interference - fit, in which there is always an interference in the connection, i.e. the largest hole size limit is less than or equal to the smallest shaft size limit. In the graphical representation, the hole tolerance field is located under the shaft tolerance field

transition fit- landing, in which it is possible to obtain both a gap and an interference fit in the connection, depending on the actual dimensions of the hole and shaft. With a graphical representation of the tolerance field, the hole and the shaft overlap completely or partially

Landings in the hole system

- landings in which the required clearances and interferences are obtained by combining different shaft tolerance fields with the tolerance field of the main hole

Fits in the shaft system

- landings in which the required clearances and interferences are obtained by a combination of different tolerance fields of the holes with the tolerance field of the main shaft

normal temperature- tolerances and limit deviations established in this standard refer to the dimensions of parts at a temperature of 20 degrees C