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

To simplify the setting of dimensions in the drawings, instead of the maximum dimensions, maximum deviations are indicated: upper deviation- algebraic difference between the largest limit and nominal sizes; lower deviation - algebraic difference between the smallest limit and nominal sizes.

The upper deviation is indicated ES(Ecart Superieur) for holes and es- for shafts; the lower deviation is indicated El(Ecart Interieur) for holes and ei- for shafts.

According to definitions: for holes ES=D max -D; EI= D min -D; for shafts es=d max –d; ei= d mln -d

The peculiarity of deviations is that they always have a sign (+) or (-). In a particular case, one of the deviations may be equal to zero, i.e. one of the maximum dimensions may coincide with the nominal value.

Admission size is the difference between the largest and smallest limit sizes or the algebraic difference between the upper and lower deviations.

The tolerance is indicated by IT (International Tolerance) or T D - hole tolerance and T d - shaft tolerance.

According to the definition: hole tolerance T D =D max -D min ; shaft tolerance Td=d max -d min . The size tolerance is always positive.

The size tolerance expresses the spread of actual dimensions ranging from the largest to the smallest limiting dimensions; it physically determines the magnitude of the officially permitted error in the actual size of a part element during its manufacturing process.

Tolerance field- this is a field limited by upper and lower deviations. The tolerance field is determined by the size of the tolerance and its position relative to the nominal size. With the same tolerance for the same nominal size, there may be different tolerance fields.

For a graphical representation of tolerance fields, allowing one to understand the relationship between nominal and maximum dimensions, maximum deviations and tolerance, the concept of a zero line has been introduced.

Zero line is called a line corresponding to the nominal size, from which the maximum deviations of dimensions are plotted when graphically depicting tolerance fields. Positive deviations are laid upward, and negative deviations are laid down from it (Fig. 1.4 and 1.5)

The surfaces along which parts are connected during assembly are called mating , the rest - unmatched, or free . Of two mating surfaces, the enclosing surface is called hole , and the covered one is shaft (Fig. 7.1).

In this case, in the designation of hole parameters, capital letters of the Latin alphabet are used ( D, E, S), and shafts – lowercase ( d, e,s).

The mating surfaces are characterized by a common size called nominal connection size (D, d).

Valid part size is the size obtained during manufacturing and measurement with an acceptable error.

Limit dimensions are the maximum ( D max And d max) and minimum ( D min And d min ) permissible dimensions, between which the actual size of a suitable part must lie. The difference between the largest and smallest limit sizes is called admission hole size T.D. and shaft Td .

TD (Td) = D max (d max ) – D min (d min ).

The size tolerance determines the specified boundaries (maximum deviations) of the actual size of a suitable part.

Tolerances are depicted as fields limited by the upper and lower size deviations. In this case, the nominal size corresponds to zero line . The deviation closest to the zero line is called main . The main deviation of the holes is indicated in capital letters of the Latin alphabet A, B, C, Z, shafts – lowercase a, b, c, … , z.

Hole size tolerances T.D. and shaft Td can be defined as the algebraic difference between the upper and lower limit deviations:

TD(Td) = ES(es) – EI(ei).

The tolerance depends on the size and required level of manufacturing accuracy of the part, which is determined quality (degree of accuracy).

Quality is a set of tolerances corresponding to the same degree of accuracy.

The standard establishes 20 qualifications in decreasing order of accuracy: 01; 0; 1; 2…18. Qualities are designated by a combination of capital letters IT with the serial number of qualification: IT 01, IT 0, IT 1, …, IT 18. As the quality number increases, the tolerance for the manufacture of the part increases.

The cost of manufacturing parts and the quality of the connection depend on the correct assignment of quality. Below are the recommended areas of application of qualifications:

– from 01 to 5 – for standards, gauge blocks and gauges;

– from 6 to 8 – to form fits for critical parts, widely used in mechanical engineering;

– from 9 to 11 – to create landings of non-critical units operating at low speeds and loads;

– from 12 to 14 – for tolerances on free dimensions;

– from 15 to 18 – for tolerances on workpieces.

On working drawings of parts, tolerances are indicated next to the nominal size. In this case, the letter specifies the main deviation, and the number specifies the quality of accuracy. For example:

 25 k6;  25 N7;  30 h8 ;  30 F8 .

7.2. The concept of plantings and planting systems

Landing is the nature of the connection of two parts, determined by the freedom of their relative movement. Depending on the relative position of the tolerance fields, the holes and the landing shaft can be of three types.

1. With guaranteed clearance S given that: D min ≥ d max :

– maximum clearance S max = D max – d min ;

– minimum clearance S min = D min – d max .

Landings with clearance are designed to form movable and fixed detachable connections. Provide ease of assembly and disassembly of units. Fixed connections require additional fastening with screws, dowels, etc.

2. With guaranteed tension N given that: D max d min :

– maximum tension N max = d max – D min ;

– minimum interference N min = d min – D max .

Interference fits ensure the formation of permanent connections more often without the use of additional fastening.

3. Transitional landings , at which it is possible to obtain both a gap and an interference in the connection:

– maximum clearance S max = D max – d min ;

– maximum tension N max = d max – D min .

Transitional fits are intended for fixed detachable connections. Provides high centering accuracy. They require additional fastening with screws, dowels, etc.

The ESDP provides for fits in the hole system and in the shaft system.

Landings in the hole system main hole N with different shaft tolerances: a, b, c, d, e, f, g, h(landing with clearance); j S , k, m, n(transitional landings); p, r, s, t, u, v, x, y, z(pressure fit).

Fittings in the shaft system are formed by a combination of tolerance fields main shaft h with different hole tolerances: A, B, C, D, E, F, G, H(landing with clearance); J s , K, M, N(transitional landings); P, R, S, T, U, V, X, Y, Z(pressure fit).

The fits are indicated on the assembly drawings next to the nominal mating size in the form of a fraction: the hole tolerance is in the numerator, the shaft tolerance is in the denominator. For example:

30or30

.

It should be noted that in the designation of the fit in the hole system the letter must be present in the numerator N, and in the shaft system the denominator is the letter h. If the designation contains both letters N And h, for example  20 N6/h5 , then in this case preference is given to the hole system.

Metrological practice has established that it is impossible to produce absolutely accurate dimensions of a part, and there is no need to always have a very accurate value of the size of the processed part.

It must be remembered that the more precisely the size must be processed, the more expensive the production. Apparently, there is no need to particularly explain that in different mechanisms and machines there are parts that must be processed especially carefully, and there are parts that do not require careful manufacturing. Therefore, there is a need to talk about dimensional accuracy.

As in every business, when it comes to dimensional accuracy, there are a number of concepts and definitions that are necessary to speak the same language and express your thoughts more briefly.

Let's consider a number of practically used definitions and concepts of sizes and their deviations.

Size is a numerical value of a physical quantity obtained as a result of measuring a characteristic or parameter of an object (process) in selected units of measurement. In most cases, it represents the difference in the states of an object or process according to a selected parameter, characteristic, indicator in time compared to a measure, standard, true or actual value of a physical quantity.

Actual size is the size established by measurement with permissible error. A size is only called valid when it is measured with an error that can be allowed by any regulatory document. This term refers to the case where a measurement is made to determine the suitability of the dimensions of an object or process to certain requirements. When such requirements are not established and measurements are not made for the purpose of product acceptance, the term measured size is sometimes used, i.e. the size obtained from measurements, instead of the term "actual size". In this case, the measurement accuracy is selected depending on the goal set before the measurement.

True size is a size obtained as a result of processing, manufacturing, the value of which is unknown to us, although it exists, since it is impossible to measure completely without error. Therefore, the concept of “true size” is replaced by the concept of “actual size”, which is close to the true one under the conditions of the goal.

Limit sizes are the maximum permissible sizes between which the actual size must be or can be equal to. From this definition it is clear that when it is necessary to manufacture a part, its size must be specified in two values, i.e. acceptable values. And these two values ​​are called the largest maximum size - the larger of the two maximum sizes and the smallest maximum size - the smaller of the two maximum sizes. A suitable part must have a size between these limiting sizes. However, specifying the requirements for manufacturing accuracy in two dimensional values ​​is very inconvenient when preparing drawings, although in the USA the size is specified this way. Therefore, in most countries of the world the concepts of “nominal size”, “deviations” and “tolerance” are used.

Nominal size is the size relative to which the maximum dimensions are determined and which serves as the starting point for deviations. The size indicated in the drawing is nominal. The nominal size is determined by the designer as a result of calculations of overall dimensions either for strength or rigidity, or taking into account design and technological considerations.

However, you cannot take as nominal any size that was obtained during the calculation.

It must be remembered that the economic efficiency of metrological support is achieved when it is possible to get by with a small range of sizes without compromising quality. So, if we imagine that the designer will put any nominal size on the drawing, for example the size of the holes, then it will be almost impossible to produce drills centrally in tool factories, since there will be an infinite number of drill sizes.

In this regard, the industry uses the concepts of preferred numbers and series of preferred numbers, i.e. values ​​to which calculated values ​​should be rounded. Typically round up to the nearest higher number. This approach makes it possible to reduce the number of standard sizes of parts and assemblies, the number of cutting tools and other technological and control equipment.

The series of preferred numbers throughout the world are accepted to be the same and represent geometric progressions with denominators Ш; “VWVW 4 VlO, which is approximately equal to 1.6; 1.25; 1.12; 1.06 (a geometric progression is a series of numbers in which each subsequent number is obtained by multiplying the previous one by the same number - the denominator of the progression). These rows are conventionally called R5; RIO; R20; R40.

Preferred numbers are widely used in standardization when it is necessary to establish a number of values ​​for standardized parameters or properties within certain ranges. Nominal values ​​of linear dimensions in existing standards are also taken from the specified series of preferred numbers with certain rounding. For example, for R5 (denominator 1.6), values ​​of 10 are taken; 16; 25; 40; 63; 100; 160; 250; 400; 630, etc.

Deviation is the algebraic difference between the limit and the real, i.e. measured, sizes. Therefore, deviation should be understood as how much the size differs from the permissible value when standardizing requirements or according to measurement results.

Since when normalizing by permissible deviations there are two limiting sizes - the largest and the smallest, the terms upper and lower deviations are accepted when normalizing permissible deviations, i.e. indications of requirements within the size tolerance. The upper deviation is the algebraic difference between the largest limit and nominal sizes. The lower deviation is the algebraic difference between the actual and the smallest maximum dimensions when normalized by the tolerance value.

The peculiarity of deviations is that they always have a plus or minus sign. The indication in the definition of an algebraic difference shows that both deviations, i.e. both upper and lower can have positive values, i.e. the largest and smallest limit sizes will be greater than the nominal, or minus values ​​(both less than the nominal), or the upper deviation may have a positive deviation, and the lower one - a negative deviation.

At the same time, there may be cases when the upper deviation is greater than the nominal, then the deviation will take a plus sign, and the lower deviation is less than the nominal, then it will have a minus sign.

The upper deviation is denoted by ES at the holes and es at the shafts, and sometimes - VO.

The lower deviation is denoted by EI at the holes, ei at the shafts, or - BUT.

Tolerance (usually denoted T) is the difference between the largest and smallest limit sizes, or the absolute value of the algebraic difference between the upper and lower deviations. A special feature of the tolerance is that it does not have a sign. This is like a zone of size values ​​between which the actual size should be, i.e. suitable part size.

Synonyms of this term can be the following: “permissible value”, “dimensions”, “characteristics”, “parameters”.

If we are talking about a tolerance of 10 microns, this means that a suitable batch may contain parts whose dimensions, in the extreme case, differ from each other by no more than 10 microns.

The concept of tolerance is very important and is used as a criterion for the accuracy of parts manufacturing. The tighter the tolerance, the more accurately the part will be manufactured. The larger the tolerance, the rougher the part. But at the same time, the smaller the tolerance, the more difficult, complex, and hence more expensive the production of parts; The greater the tolerances, the easier and cheaper it is to manufacture the part. So there is, to a certain extent, a contradiction between developers and manufacturers. Designers want tight tolerances (more accurate product) and manufacturers want tight tolerances (easier to manufacture).

Therefore, the choice of tolerance must be justified. Larger tolerances should be used whenever possible, as this is economically beneficial for production, provided that the quality of the product is not compromised.

Very often, along with the term “tolerance” and instead (not entirely correctly), the term “tolerance field” is used, since, as mentioned above, tolerance is a zone (field) within which the dimensions of a suitable part are located.

The tolerance field, or the acceptable value field, is a field limited by upper and lower deviations. The tolerance field is determined by the size of the tolerance and its position relative to the nominal size.

Basic concepts about tolerances and fits

The mechanisms of machines and devices consist of parts that perform certain relative movements during operation or are connected motionlessly. Parts that, to one degree or another, interact with each other in a mechanism are called conjugated.
Absolutely accurate production of any part is impossible, just as it is impossible to measure its absolute size, since the accuracy of any measurement is limited by the capabilities of measuring instruments at a given stage of scientific and technological progress, and there is no limit to this accuracy. However, making parts of mechanisms with the greatest precision is often impractical, first of all, from an economic point of view, since high-precision products are much more expensive to manufacture, and for normal functioning in a mechanism it is quite enough to make a part with less precision, i.e. cheaper.

Production experience has shown that the problem of choosing optimal accuracy can be solved by establishing for each part size (especially for its matching sizes) the limits within which its actual size may vary; At the same time, it is assumed that the assembly into which the part is included must correspond to its purpose and not lose its functionality under the required operating conditions with the required resource.

Recommendations for the selection of maximum deviations in the dimensions of parts were developed on the basis of many years of experience in the manufacture and operation of various mechanisms and instruments and scientific research, and are set out in the unified system of tolerances and fits (USDP CMEA). Tolerances and landings established ESDP CMEA
Let's consider the basic concepts from this system.

The nominal size is the main size, obtained from the calculation of strength, rigidity, or chosen structurally and marked on the drawing. Simply put, the nominal size of the part was obtained by designers and developers by calculation (based on the requirements of strength, rigidity, etc.) and is indicated on the part drawing as the main dimension.
The nominal size of a connection is common to the hole and shaft that make up the connection. Based on nominal dimensions, drawings of parts, assembly units and devices are made on one scale or another.

For unification and standardization, series of nominal sizes have been established (GOST 8032-84 "Preferred numbers and series of preferred numbers"). The calculated or selected size should be rounded to the nearest value from the standard range. This especially applies to the dimensions of parts obtained with standard or normalized tools, or connecting to other standard parts or assemblies.
To reduce the range of cutting and measuring tools used in production, it is first recommended to use dimensions ending in 0 And 5 , and then - to 0; 2; 5 And 8 .

The size obtained as a result of measuring a part with the greatest possible accuracy is called actual.
Do not confuse the actual size of a part with its absolute size.
Absolute size – real (actual) size of the part; it cannot be measured by any ultra-precise measuring instruments, since there will always be an error due, first of all, to the level of development of science, technology and technology. In addition, any material body at a temperature above absolute zero “breathes” - microparticles, molecules and atoms constantly move on its surface, breaking away from the body and returning back. Therefore, even with ultra-precise measuring instruments at our disposal, it is impossible to determine the absolute size of the part; we can only talk about the real size in an infinitely small period (moment) of time.
The conclusion is obvious - the absolute size of a part (like any body) is an abstract concept.

The dimensions between which the actual size of the manufactured part may lie are called limiting, and a distinction is made between the largest and smallest limiting dimensions.
A part made within the range of the maximum dimensions is considered suitable. If its size exceeds the maximum limits, it is considered a defect.
The maximum dimensions determine the type of connection of parts and the permissible inaccuracy of their manufacture.
For convenience, the drawings indicate the nominal size of the part, and each of the two maximum sizes is determined by its deviation from this size. The magnitude and sign of the deviation are obtained by subtracting the nominal size from the corresponding maximum size.

The difference between the largest limit and nominal sizes is called the upper deviation (denoted es or ES), the difference between the smallest limit and nominal - lower deviation (denoted ei or EI).
The upper deviation corresponds to the largest limit size, and the lower one corresponds to the smallest.

All mating (interacting) In the mechanism, parts are divided into two groups - shafts and holes.
The shaft denotes the outer (male) element of the part. In this case, the shaft does not have to have a round shape: the concept of “shaft” includes, for example, a key, and the keyway in this case is called a “hole”. The main shaft is the one whose upper deviation is zero.
Shaft dimensions in diagrams and in calculations are indicated in lowercase (small) letters: d, dmax, dmin, es, ei, etc.

A hole denotes the internal (female) element of a part. As with the shaft, the hole does not have to be round - it can be any shape. The main hole is the hole whose lower deviation is zero.
Hole sizes in diagrams and in calculations are indicated in capital letters: D, Dmax, Dmin, ES, EI, etc.

Tolerance (T) is the difference between the largest and smallest limiting dimensions of a part. That is, a tolerance is the interval between the maximum dimensions, within which the part is not considered defective.
The tolerance on the shaft size is denoted Td, holes - TD. Obviously, the larger the dimensional tolerance, the easier it is to manufacture the part.
The tolerance on the size of a part can be defined as the difference between the maximum dimensions or as the sum of the maximum deviations:

TD(d) = D(d)max – D(d)min = ES(es) + EI(ei) ,

in this case, the signs of maximum deviations should be taken into account, since the tolerance on the size of the part is always positive (cannot be less than zero).

Landings

The nature of the connection, determined by the difference between the male and female dimensions, is called fit.
The positive difference between the diameters of the hole and the shaft is called clearance (denoted by the letter S), and negative – by interference (denoted by the letter N).
In other words, if the diameter of the shaft is less than the diameter of the hole, there is a gap, but if the diameter of the shaft exceeds the diameter of the hole, there is interference in the mating.
The gap determines the nature of the mutual mobility of the mating parts, and the tension determines the nature of their fixed connection.

Depending on the ratio of the actual dimensions of the shaft and the hole, there are movable fits - with a gap, fixed fits - with interference, and transitional fits, i.e. fits in which both clearance and interference may be present (depending on what deviations the actual dimensions of the mating parts have from the nominal dimensions).
Fittings in which there is necessarily a gap are called landings with guaranteed clearance, and landings in which interference is required are called with guaranteed interference.
In the first case, the maximum dimensions of the hole and shaft are chosen so that there is a guaranteed gap in the interface.
The difference between the largest maximum hole size (Dmax) and the smallest maximum shaft size (dmin) determines the maximum clearance (Smax):

Smax = Dmax – dmin.

The difference between the smallest maximum hole size (Dmin) and the largest maximum shaft size (dmax) is the smallest gap (Smin):

Smin = Dmin – dmax.

The actual clearance will be between the specified limits, i.e. between the maximum and minimum clearance. The clearance is necessary to ensure mobility of the connection and placement of lubricant. The higher the speed and the higher the viscosity of the lubricant, the larger the gap should be.

In interference fits, the maximum dimensions of the shaft and hole are chosen so that the mating has a guaranteed interference, limited by the minimum and maximum values ​​- Nmax and Nmin:

Nmax = dmax – Dmin, Nmin = dmin – Dmax.

Transitional fits can give a small gap or interference. Before the parts are manufactured, it is impossible to say what will be paired. This only becomes clear during assembly. The gap should not exceed the maximum gap value, and the interference should not exceed the maximum interference value. Transitional fits are used if it is necessary to ensure precise centering of the hole and shaft.
Total in ESDP CMEA provided 28 types of main deviations for shafts and the same for holes. Each of them is designated by a lowercase Latin letter (GOST 2.304 - 81) if the deviation relates to the shaft, or capital if the deviation relates to the hole.
The letter designations of the main deviations are taken in alphabetical order, starting from the deviations that provide the largest gaps in the connection. By combining different shaft and hole deviations, different types of fits can be obtained. (clearance, interference or transition).

Fit in hole system and shaft system

Plantings installed ESDP CMEA, can be carried out using hole or shaft systems.

The hole system is characterized by the fact that for all fits the maximum hole dimensions remain constant, and fits are carried out by corresponding changes in the maximum shaft dimensions (i.e. the shaft is adjusted to the hole). The hole size is called the main one, and the shaft size is called the landing size.

The shaft system is characterized by the fact that for all fits the maximum dimensions of the shaft remain constant, and fits are carried out by changing the hole (i.e. the hole is adjusted to the size of the shaft). The shaft size is called the main one, and the holes are called the landing size.

In industrial enterprises, the hole system is mainly used, since it requires fewer cutting and measuring tools, i.e., it is more economical. In addition, it is technologically more convenient to adjust the shaft to the hole, and not vice versa, since it is more convenient to process and control measurements of the outer surface rather than the inner one.
The shaft system is usually used for the outer rings of ball bearings and in cases where several parts with different fits are mounted on a smooth shaft.

In mechanical engineering, the most common fits are arranged in descending order of tension and increasing clearance: press (Pr), light press (Pl), blind (G), tight (T), tense (N), tight (P), sliding (S), movement (D), chassis (X), light travel (L), wide travel (W).
Press fits provide guaranteed tightness. Blind, tight, tense and tight fits are transitional, while the rest have guaranteed clearance.
For a sliding fit, the guaranteed clearance is zero.

To assess the accuracy of connections (fits), we use the concept of fit tolerance, which is the difference between the largest and smallest gaps (in landings with clearance) or the largest and smallest interference (in interference fits). In transitional fits, the fit tolerance is equal to the difference between the largest and smallest interference or the sum of the largest interference and the largest gap.
The fit tolerance is also equal to the sum of the hole and shaft tolerances.

Qualities

The set of tolerances corresponding to the same degree of accuracy for all nominal sizes is called quality (I). In other words, quality is the degree of accuracy with which a part is made, taking into account the size of this part.
Obviously, if you make a very large and a very small part with the same tolerance, then the relative accuracy of manufacturing the large part will be higher. Therefore, the quality system takes into account the fact that (with the same tolerances) the ratio of the tolerance value to the nominal size of a large part will be less than the ratio of the tolerance to the nominal size of a small part (Fig. 2), i.e. a conventionally large part is made more accurately relative to of their sizes. If, for example, for a shaft with a nominal diameter of 3 meters, a millimeter deviation from the size can be considered insignificant, then for a shaft with a diameter of 10 mm such a deviation will be very noticeable.
The introduction of a system of qualifications allows us to avoid such confusion, since the accuracy of manufacturing parts is tied to their dimensions.

By ESDP CMEA qualifications are standardized in the form 19 rows. Each qualification is designated by a serial number 01; 0; 1; 2; 3;...; 17 , increasing with increasing tolerance.
The two most accurate qualifications - 01 And 0 .
Link to qualification qualifications ESDP CMEA can be abbreviated as IT “International Admission” with the qualification number.
For example, IT7 means tolerance 7 -th quality.

In the CMEA system, the following symbols are used to designate tolerances indicating qualifications:

• Letters of the Latin alphabet are used, with holes identified in uppercase letters and shafts in lowercase letters.
• Hole in hole system (main hole) denoted by the letter N and in numbers - the number of the qualification. For example, H6, H11 etc.
• The shaft in the hole system is indicated by a fit symbol and numbers - the quality number. For example, g6, d11 etc.
• The connection between the hole and the shaft in the hole system is indicated fractionally: in the numerator - the tolerance of the hole, in the denominator - the tolerance of the shaft.

Graphic representation of tolerances and fits

For clarity, a graphical representation of tolerances and fits is often used using so-called tolerance fields (see Fig. 3).

The construction is carried out as follows.
From the horizontal line, conventionally depicting the surface of the part at its nominal size, the maximum deviations are plotted on an arbitrarily chosen scale. Typically, on diagrams, deviation values ​​are indicated in microns, but tolerance fields can also be constructed in millimeters if the deviations are large enough.

The line that, when constructing tolerance zone diagrams, corresponds to the nominal size and serves as the starting point for measuring dimensional deviations is called zero (0-0) .
Tolerance field is a field limited by upper and lower deviations, i.e., when displayed graphically, tolerance fields show zones that are limited by two lines drawn at distances corresponding to the upper and lower deviations on a selected scale.
Obviously, the tolerance field is determined by the size of the tolerance and its position relative to the nominal size.
In the diagrams, the tolerance fields have the form of rectangles, the upper and lower sides of which are parallel to the zero line and display maximum deviations, and the side sides on a selected scale correspond to the size tolerance.

The diagrams indicate the nominal D and maximum (Dmax, Dmin, dmax, dmin) dimensions, maximum deviations (ES, EI, es, ei), tolerance fields and other parameters.

The maximum deviation, which is closer to the zero line, is called the main (top or bottom). It determines the position of the tolerance field relative to the zero line. For tolerance fields located below the zero line, the main deviation is the upper deviation.
For tolerance fields located above the zero line, the main deviation is the lower deviation.

The principle of formation of tolerance fields adopted in ESDP, allows a combination of any basic deviations with any qualifications. For example, you can create tolerance fields a11, u14, c15 and others not specified in the standard. The exception is the main deviations J and j, which are replaced by the main deviations Js, and js.

Using all the main deviations and qualifications allows you to get 490 tolerance fields for shafts and 489 for holes. Such wide possibilities for creating tolerance fields make it possible to use ESDP in various special cases. This is its significant advantage. However, in practice, the use of all tolerance fields is uneconomical, as it will cause an excessive variety of fits and special technological equipment.

When developing national systems of admissions and landings based on systems ISO From the entire variety of tolerance fields, only those fields are selected that meet the needs of the country’s industry and its foreign economic relations.

• h and H - upper and lower deviations of the shaft and holes, equal to zero (tolerances with basic deviations h and H are accepted for the main shafts and holes).
• a - h (A - H) - deviations that form tolerance fields for landings with gaps.
• js - n (Js - N) - deviations forming tolerance fields for transitional fits.
• p – zc (P - ZC) - deviations that form tolerance fields for interference fits.

The main deviations are shown schematically in Fig. 4 .

The tolerance field in the CMEA ESDP is formed by a combination of one of the main deviations with a tolerance for one of the qualifications. In accordance with this, the tolerance field is indicated by the letter of the main deviation and the quality number, for example 65f6; 65e11- for the shaft; 65Р6; 65H7- for the hole.
The main deviations depend on the nominal dimensions of the parts and remain constant for all grades. The exception is the main deviations of the holes J, K, M, N and shafts j And k, which, with the same nominal sizes, have different meanings in different grades. Therefore, in the diagrams of tolerance fields with deviations J, K, M, N, j, k, are usually divided into parts and shown in steps.

Type tolerance fields are specific js6, Js8, Js9 etc. They actually do not have a main deviation, since they are located symmetrically relative to the zero line. By definition, the main deviation is the deviation closest to the zero line. This means that both deviations of such specific tolerance fields can be considered basic, which is unacceptable.

The main deviations are of particular importance H And h, which are equal to zero (figure). Tolerance fields with such basic deviations are located from the nominal value “into the body” of the part; they are called the tolerance fields of the main hole and the main shaft.
Landing designations are constructed as fractions, and the numerator always contains the designation of the tolerance field of the female surface (hole), and the denominator always contains the tolerance field of the male surface (shaft).

When choosing the quality of the connection and the type of fit, the designer should take into account the nature of the interface, operating conditions, the presence of vibration, service life, temperature fluctuations and manufacturing costs.
It is recommended to select the quality and type of fit by analogy with those parts and assemblies whose operation is well known, or be guided by the recommendations of reference literature and regulatory documents (OST).
In accordance with the quality of the fit, the surface cleanliness of the mating parts is selected.

Tolerances and fits are established for four ranges of nominal sizes:

• small - up to 1 mm;
• average - from 1 before 500 mm;
• big - from 500 before 3150 mm;
• very large - from 3150 before 10 000 mm.

The mid range is the most important because it is used much more often.

Designation of tolerances on drawings

Indications and designations on drawings of maximum deviations of the shape and location of surfaces are regulated by GOST 2.308-79, which provides special signs and symbols for these purposes.
The main provisions of this standard, the signs and symbols used to indicate maximum deviations, can be found in this document ( WORD format, 400 kB).

It is more convenient to consider the basic concepts of interchangeability in geometric parameters using the example of shafts and holes and their connections.

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

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

The geometric parameters of parts are quantitatively assessed through dimensions.

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

Dimensions are divided into nominal, actual and limiting.

Definitions are given in accordance with GOST 25346-89 "Unified system of tolerances and landings. General provisions, series of tolerances and main deviations."

The nominal size is the size relative to which deviations are determined.

The nominal size is obtained as a result of calculations (strength, dynamic, kinematic, etc.) or selected from any other considerations (aesthetic, structural, technological, etc.). The size thus obtained should be rounded to the nearest value from the range of normal sizes (see section "Standardization"). The main share of numerical characteristics used in technology are linear dimensions. Due to the large proportion of linear dimensions and their role in ensuring interchangeability, series of normal linear dimensions were established. The series of normal linear dimensions are regulated throughout the entire range, which is widely used.

The basis for normal linear dimensions is the preferred numbers, and in some cases their rounded values.

Actual size is the size of the element as determined by the measurement. This term refers to the case where a measurement is made to determine the suitability of the dimensions of a part to specified requirements. Measurement is the process of finding the values ​​of a physical quantity experimentally using special technical means, and measurement error is the deviation of the measurement result from the true value of the measured quantity. True size is the size obtained as a result of processing the part. The true size is unknown because it is impossible to measure without error. In this regard, the concept of “true size” is replaced by the concept of “actual size”.

Limit dimensions - two maximum permissible dimensions of an element, between which the actual size must be (or can be equal to). For the limit size that corresponds to the largest volume of material, i.e. the largest limit size of the shaft or the smallest limit size of the hole, the term maximum material limit is provided; for the limit size to which the smallest volume of material corresponds, i.e. the smallest limit size of the shaft or the largest limit size of the hole, the minimum material limit.

The largest limit size is the largest allowable size of an element (Fig. 5.1)

The smallest size limit is the smallest allowable element size.

From these definitions it follows that when it is necessary to manufacture a part, its size must be specified by two permissible values ​​- the largest and the smallest. A valid part must have a size between these limit values.

Deviation is the algebraic difference between the size (actual or maximum size) and the nominal size.

The actual deviation is the algebraic difference between the actual and the corresponding nominal dimensions.

The maximum deviation is the algebraic difference between the maximum and nominal sizes.

Deviations are divided into upper and lower. The upper deviation E8, ea (Fig. 5.2) is the algebraic difference between the largest limit and nominal sizes. (EA is the upper deviation of the hole, EG is the upper deviation of the shaft).

The lower deviation E1, e (Fig. 5.2) is the algebraic difference between the smallest limit and nominal sizes. (E1 is the lower deviation of the hole, e is the lower deviation of the shaft).

Tolerance T is the difference between the largest and smallest limit sizes or the algebraic difference between the upper and lower deviations (Fig. 5.2).

Standard tolerance P - any of the tolerances established by this system of tolerances and landings.

Tolerance characterizes the accuracy of the size.

Tolerance field - a field limited by the largest and smallest maximum sizes and determined by the value of the tolerance and its position relative to the nominal size. In a graphical representation, the tolerance field is enclosed between two lines corresponding to the upper and lower deviations relative to the zero line (Fig. 5.2).

It is almost impossible to depict deviations and tolerances on the same scale as the dimensions of the part.

To indicate the nominal size, the so-called zero line is used.

Zero line - a line corresponding to the nominal size, from which dimensional deviations are plotted when graphically depicting tolerance and fit fields. If the zero line is located horizontally, then positive deviations are laid up from it, and negative deviations are laid down (Fig. 5.2).

Using the above definitions, the following characteristics of shafts and holes can be calculated.

Schematic designation of tolerance fields

For clarity, it is convenient to present all the concepts considered graphically (Fig. 5.3).

On the drawings, instead of maximum dimensions, maximum deviations from the nominal size are indicated. Considering that deviations can

can be positive (+), negative (-) and one of them can be equal to zero, then there are five possible cases of the position of the tolerance field in a graphical representation:

• 1) the upper and lower deviations are positive;
• 2) the upper deviation is positive, and the lower one is zero;
• 3) the upper deviation is positive, and the lower deviation is zero;
• 4) the upper deviation is zero, and the lower deviation is negative;
• 5) the upper and lower deviations are negative.

In Fig. 5.4, ​​a shows the listed cases for a hole, and in Fig. 5.4, ​​b - for the shaft.

For convenience of standardization, one deviation is identified, which characterizes the position of the tolerance field relative to the nominal size. This deviation is called the main one.

The main deviation is one of two maximum 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 one is the deviation closest to the zero line.

From formulas (5.1) - (5.8) it follows that the requirements for dimensional accuracy can be normalized in several ways. You can set two limit sizes, between which the distances must be

a - holes; b-shaft

measures of suitable parts; you can set the nominal size and two maximum deviations from it (upper and lower); you can set the nominal size, one of the maximum deviations (upper or lower) and size tolerance.

In mechanical engineering, all parts are conventionally divided into two groups:

1. "shafts" – external (male) elements of the part, the nominal size of the shaft is usually denoted d;

2. "holes" – internal (enclosing) elements of the part, the nominal size of the hole is indicated D.

The terms “shaft” and “hole” refer not only to cylindrical parts with a circular cross-section, but also to elements of parts of any other shape.

The geometric parameters of parts are quantitatively assessed through dimensions. Size – this is the numerical value of a linear quantity (diameter, length, height, etc.) in selected units. In mechanical engineering, dimensions are indicated in millimeters. The following sizes are available:

Nominal size ( D, d, l) – the size that serves as the starting point for deviations and relative to which the maximum dimensions are determined. For the parts making up the connection, the nominal size is common. Nominal dimensions are determined by calculating their strength and rigidity, as well as based on the perfection of geometric shapes and ensuring the manufacturability of product designs.

To reduce the number of standard sizes of workpieces and parts, cutting and measuring tools, dies, fixtures, as well as to facilitate the typification of technological processes, the size values ​​obtained by calculation should be rounded (usually up) in accordance with the values ​​of a number of normal linear dimensions.

Actual size - size established by measurement with permissible error. This term was introduced because it is impossible to manufacture a part with absolutely accurate required dimensions and measure them without introducing an error. The actual size of a part in a working machine due to wear, elastic, residual, thermal deformation and other reasons differs from the size determined in a static state or during assembly. This circumstance must be taken into account when accurately analyzing the mechanism as a whole.

Limit dimensions of the part - two maximum permissible sizes, between which the actual size of a suitable part must be or can be equal to. The larger one is called the largest limit size, smaller – smallest size limit. Accepted designations for them D max and D min for hole, d max and d min – for the shaft. Comparing the actual size with the maximum makes it possible to judge the suitability of the part.

Reject size– the size at which the part is removed from work. The rejection size is usually specified in standards through the wear limit or wear limit.

Deviation called the algebraic difference between the size (real, limit, etc.) and the corresponding nominal size. Deviations are vectors that show how much the maximum size differs from the nominal size. Deviations are always specified with a “+” or “–” sign.

Actual deviation - algebraic difference between real and nominal sizes.

Maximum deviation - algebraic difference between the maximum and nominal sizes. One of the two maximum deviations is called top, and the other - lower Designations of deviations, their definitions and formulas are given in table. 8.1.

The upper and lower deviations can be positive (located above the nominal size or zero line), negative (located below the zero line), and equal to zero (coincide with the nominal size - zero line).