Nominal size - the main size, determined from the functional purpose of the part. In accordance with GOST 25346-89 "NVG. ESDP. General Provisions, Series of Tolerances and Major Deviations ”nominal size is the size relative to which the limit dimensions are determined and which serves as the starting point for deviations. The nominal size is obtained from strength calculations or other methods, and then rounded to the standard size and affixed to the drawing.

To reduce the number of sizes of materials, tools and devices in Russia, GOST 6636-96 “ONV. Normal linear dimensions ”, developed in accordance with ISO recommendations. Rows of normal linear dimensions are based on the series of preferred numbers, defined, according to GOST 8032-84 "Preferred numbers and series of preferred numbers", but with some restrictions imposed on it by GOST 6636-96. In accordance with it, the following ranges of normal linear dimensions are provided: Ra 5; Ra 10; Ra 20; Ra 40, and it is recommended to give preference to sizes from rows with a larger gradation (Ra 5 is better than Ra 10, etc.). Each subsequent row includes the previous one.

As an example, we give a fragment of GOST 6636-96 (Table 1.1).

Table 1.1

The required size cannot be sustained in production absolutely exactly. Therefore, the concept of real size is introduced.

Actual size(in accordance with the same G OST25346- 89) is the size set by the measurement.

But the actual size itself may be in some limits, for which limiting sizes are assigned.

Size limits- two maximum allowable sizes between which the actual size should be or which can be equal.

The larger of the two size limits is called the largest size limit - D (d) max,   smaller - the smallest limiting size - D (d) min.

Comparing the actual size with the limiting gives the opportunity to judge the suitability of the details.

Part condition: D (d) mxx > D (d)\u003e D (d) mm.

Limit dimensions are most convenient to set as a deviation value from the nominal size.

A graphic representation of the concepts below is shown in Fig. 1.1.

Fig. 1.1.

Upper deviationcalled the algebraic difference between the maximum limiting and nominal dimensions.

The lower deviation is the algebraic difference between the smallest and nominal dimensions.

The difference between the largest and smallest limiting dimensions is called the tolerance.

In other words, the tolerance is the officially resolved error of the part. In this case, the deviation can be both positive and negative, while the tolerance is always positive. Therefore, before admission, the sign is not put, while it is always put before deviations.

For example:   030 - the nominal size required for ordering

tool.

According to GOST 25346-89, the tolerance is denoted IT   (from English International Tolerance)   or T.

Respectively:

Upper limit deviation -

Lower limit deviation -

where ES   (from fr. Ecart superierir) -   top deviation designation

for the hole ( es -   shaft); EI   (from fr. Ecarl inferierir) -   designation of the lower deviation for the hole (ei -   shaft).

Tolerance field - space bounded by upper and lower deviations. The tolerance field is determined by the tolerance value and its position relative to the nominal size. For a graphic image, the tolerance field is enclosed between two lines corresponding to the upper and lower deviations relative to the zero line.

The zero line is the line corresponding to the nominal size, from which the deviations of dimensions are postponed during the graphic image of tolerances. If the zero line is horizontal, positive deviations are deposited up from it, and negative deviations - down.

Dimension tolerances can also be represented schematically, in the form of tolerance fields, without giving the details themselves (Fig. 1.2).

The deviation will be positive if the size determined by the deviation is larger than the nominal, and negative - if the size is smaller than the nominal.

Fig. 1.2.

In the drawings   maximum deviations are placed in millimeters in smaller type, with the upper deviation above, and the lower below below the designated or nominal size:

In case of equality of absolute values ​​of deviations, their value is indicated once - next to the nominal size in the same font with the sign “±” (50 ± 0.1).

Deviation equal to zero in the drawings do not put. In this case, put only one deviation, each in its place. For example:

In the manufacture of parts that will have mates with each other, the designer takes into account the fact that these parts will have errors and ideally do not fit together. The designer determines in advance in what range the permissible errors. Set in 2 sizes for each mating part, the minimum and maximum values. Within this range and must be the size of the part. The difference between the largest and smallest limiting dimensions is called tolerance

Especially critically important tolerances   manifest themselves in the design of the dimensions of the shafts and the dimensions of the shafts themselves.

Maximum part size or top deviation ES, es   - the difference between the largest and nominal size.

Minimum size or lower deviation EI, ei   - the difference between the smallest and nominal size.

Landings are divided into 3 groups depending on the selected tolerance fields for the shaft and the hole:

• With a gap.   Example:

• With tension. Example:

• Transient. Example:

Landing tolerance fields

For each of the groups described above, there are a number of tolerance fields in accordance with which a shaft-hole interface group is manufactured. Each separately taken tolerance field solves its specific task in a certain industry, therefore there are so many of them. Below is a picture of the types of tolerance fields:

The main deviations of the holes are indicated by capital letters, and shafts - lowercase.

For the formation of landing shaft - hole there is a rule. The meaning of this rule is as follows - the main deviations of the holes are equal in magnitude and opposite in sign to the main deviations of the shafts, indicated by the same letter.

The totality of the tolerances or quality

Qualitet   - a set of tolerances considered as corresponding to the same level of accuracy for all nominal sizes.

Quality implies that the workpieces fall into the same accuracy class, regardless of their size, provided that the manufacture of different parts is carried out on the same machine, and under the same technological conditions, the same cutting tools.

20 qualifications are established (01, 0 - 18).

The most accurate qualifications used for the manufacture of samples of measures and calibers - 01, 0, 1, 2, 3, 4.

The qualifications used for the manufacture of the mating surfaces must be sufficiently accurate, but under normal conditions, special accuracy is not required, therefore, for these purposes apply from 5 to 11 qualifications.

From 11 to 18, the qualifications are not particularly accurate and their use is limited in the manufacture of non-conjugate parts.

Below is a table of accuracy in qualifications.

Difference of tolerances from qualifications

The differences are still there. Tolerances   - these are theoretical deviations, margin of error   within which you need to make a shaft - a hole, depending on the purpose, the size of the shaft and the hole. Qualitet   same is the degree precision manufacturing   the mating surfaces of the shaft - the hole, this is the actual deviations, depending on the machine or method of bringing the surface of the mating parts to the final stage.

For example. You need to make a shaft and a seat for it - a hole with a tolerance field H8 and h8, respectively, taking into account all factors such as shaft diameter and hole, working conditions, material of products. The diameter of the shaft and holes take 21mm. With the tolerance H8, the tolerance field is 0 + 33µm and h8 + -33µm. in order to get into this tolerance zone, you need to select the quality or accuracy class of manufacturing. We take into account that in the manufacture of a machine tool, the non-uniformity of manufacturing a part can deviate both in a positive and in a negative direction, therefore, taking into account the tolerance field H8 and h8, it was 33/2 = 16.5 μm. All values ​​of 6, inclusive, correspond to this value. Consequently, we select the machine and the processing method that allows to achieve the accuracy class corresponding to the 6th grade.

The basic concepts and terms are regulated by GOST 25346–89.

The size   - the numerical value of the linear magnitude (diameter, length, etc.). Valid   call the size set by the measurement with the permissible error.

Two maximum allowable sizes between which the actual size should be or can be equal are called size limits. The biggest of them is called largest limit size, smaller - smallest marginal size.

Nominal size   - size, which serves as a reference point for deviations and relative to which limit sizes are determined. For parts making up a joint, the nominal size is common.

Not any size obtained as a result of the calculation may be taken as nominal. In order to increase the level of interchangeability, reduce the range of products and sizes of blanks, standard or normalized cutting and measuring tools, tooling and gauges, create conditions for specialization and cooperation of enterprises, reduce the cost of production, size values ​​calculated by calculation should be rounded in accordance with the values ​​specified in GOST 6636–69. In this case, the initial value of the size obtained by calculation or otherwise, if it differs from the standard, should be rounded to the nearest larger standard size. The standard for normal linear dimensions is based on the series of preferred numbers GOST 8032-84.

The most widely used series of preferred numbers, built on a geometric progression. Geometric progression provides a rational gradation of the numerical values ​​of parameters and sizes, when you need to set not one value, but a uniform series of values ​​in a certain range. In this case, the number of members of the series turns out to be smaller compared with the arithmetic progression.

Accepted notation:

D(d) – nominal size of the hole (shaft);

D   max, ( d   m ah) D   min, ( d   min) , D   e ( d   e)   D m(d m) - the size of the hole (shaft), the largest (maximum), smallest (minimum), real, medium.

ES(es) - the upper limit deviation of the hole (shaft);

El(ei) - lower limit deviation of the hole (shaft);

S, S   max S   min S   m - gaps, the largest (maximum), the smallest (minimum), average, respectively;

N, N   max, N   min, N   m – tension, the greatest (maximum), the smallest (minimum), the average, respectively;

TD, Td, TS, TN, TSN- tolerances of the hole, shaft, clearance, tension, clearance - tension (in transitional landing), respectively;

IT1, IT2, IT3…Itn……IT18 - quality tolerances are indicated by a combination of letters ITwith a serial number qualitet.

Deviation   - the algebraic difference between the size (real, limit, etc.) and the corresponding nominal size:

For hole ES = D   max - D; EI = D   min - D;

For shaft   es = d   max - d; ei = d   min - d.

Actual deviation   - algebraic difference between real and nominal dimensions. Deviation is positive if the actual size is greater than the nominal and negative if it is less than the nominal. If the actual size is equal to the nominal, then its deviation is zero.

Maximum deviation   called the algebraic difference between the limiting and nominal dimensions. There are upper and lower deviations. Upper deviation   - algebraic difference between the maximum limiting and nominal dimensions. Lower deviation   - the algebraic difference between the smallest limiting and nominal dimensions.

For simplification and convenience of work on the drawings and tables of standards for tolerances and fitments, it is customary to put down the maximum deviation values ​​instead of the limit sizes: upper and lower. Deviations are always indicated with a “+” or “-” sign. The upper limit deviation is set slightly higher than the nominal size, and the lower one is slightly lower. Deviations equal to zero are not stamped on the drawing. If the upper and lower limit deviations are equal in absolute value but opposite in sign, then the numerical value of the deviation is indicated with a “±” sign; the deviation is indicated after the nominal size. For example:

thirty ; 55; 3 + 0.06; 45 ± 0,031.

Main deviation   - one of two deviations (upper or lower), used to determine the tolerance field relative to the zero line. Usually such a deviation is the deviation closest to the zero line.

Zero line   - the line corresponding to the nominal size, from which the deviations of dimensions are postponed in the graphic image of tolerances and landings. If the zero line is horizontal, then positive deviations are deposited up from it, and negative deviations - down.

Size tolerance   - the difference between the largest and smallest limiting dimensions or the absolute value of the algebraic difference between the upper and lower deviations:

For hole   Td= D   max - D   mi n = ES – EI;

For shaft   Td = d   max - d   min   = es - ei.

Tolerance is a measure of size accuracy. The smaller the tolerance, the higher the required accuracy of the part, the less allowed variation in the actual dimensions of the part.

When processing, each part acquires its actual size and can be assessed as suitable if it is in the range of maximum dimensions, or rejected if the actual size has exceeded these limits.

The condition of the validity of parts can be expressed by the following inequality:

D   max ( d   max) ≥ D   e ( d   e)   ≥ D   min ( d   min)

Tolerance is a measure of size accuracy. The smaller the tolerance, the smaller the allowable variation of the actual dimensions, the higher the accuracy of the part and, as a consequence, the complexity of processing and its cost increase

Tolerance   - field bounded by upper and lower deviations. The tolerance field is determined by the numerical tolerance value and its position relative to the nominal size. For a graphic image, the tolerance field is enclosed between two lines corresponding to the upper and lower deviations relative to the zero line (Figure 1.1).

Figure 1.1 - Layout of fields of tolerances:

but   - holes ( ES   and EI   - positive); b   - shaft ( es   and ei - negative)

In the connection of parts included in one another, there are covering and covered surface. Shaft   - the term used to refer to the external (covered) elements of parts. Hole   - The term is conventionally used to refer to the internal (covering) elements of parts. The terms bore and shaft refer not only to cylindrical parts of circular cross section, but also to elements of parts of other shapes, for example, limited to two parallel planes.

Main shaft   - shaft, the upper deviation of which is zero ( es= 0).

Main hole   - hole, the lower deviation of which is zero ( EI= 0).

Gap   - the difference between the sizes of the hole and the shaft, if the size of the hole is larger than the size of the shaft. The gap allows relative movement of the assembled parts.

Preload   - the difference between the sizes of the shaft and the hole before assembly, if the size of the shaft is larger than the size of the hole. Tension provides mutual immobility of parts after assembly.

The largest and smallest gaps (tension)   - two limiting values ​​between which there should be a gap (tension).

Average clearance   there is an arithmetic average between the largest and smallest gap (tension).

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

Landing with clearance   - landing, which always provides clearance in the connection.

In landings with a clearance, the opening tolerance zone is located above the shaft tolerance area. Landings with a clearance also include landings in which the lower limit of the tolerance field of the orifice coincides with the upper limit of the tolerance field of the shaft.

Impact Landing   - landing, which always provides tension in the connection. In squeeze landings, the opening tolerance zone is located below the shaft tolerance area

Transition landing   is called landing, at which it is possible to obtain both a gap and a tightness in the joint. In such a fit, the tolerance fields of the bore and shaft completely or partially overlap each other.

Landing tolerance   - the sum of the tolerances of the bore and shaft making up the connection.

Landing characteristics:

For landings with clearance:

S   min = D   min - d   max = EI – es;

S   max = D   max - d   min = ES – ei;

S   m = 0.5 ( S   max + S   min);

Ts = S   max - S   min = Td + Td;

For landings with interference:

N   min = d   min - D   max = ei – ES;

N   max = d   max - D   min = es – EI;

N   m = 0.5 ( N   max + N   min);

TN = N   max - N   min = Td + Td;

For transitional landings:

S   max = D   max - d   min = ES – ei;

N   max = d   max - D   min = es – EI;

N   m ( S   m) = 0.5 ( N   max - S   max);

a result with a minus sign will mean that the average value for the fit corresponds to S   m.

Ts(N) = TN(S) = S   max + N   max = Td + Td.

In mechanical engineering and instrument making, plantings of all three groups are widely used: with clearance, tension, and transitional. Landing of any group can be obtained either by changing the dimensions of both mating parts, or one mating part.

The combination of landings in which the maximum deviations of the holes of the same nominal size and the same accuracy are the same, while different landings are achieved by changing the maximum deviations of the shafts, are called hole system. For all landings in the hole system, the lower hole deviation EI= 0, i.e. the lower limit of the tolerance field of the main hole coincides with the zero line.

The combination of landings in which the maximum deviations of the shaft of the same nominal size and the same accuracy are the same, while different landings are achieved by changing the maximum deviations of the holes, are called shaft system. For all landings in the shaft system, the upper deviation of the main shaft es= 0, i.e., the upper limit of the shaft tolerance field always coincides with the zero line.

Both systems are equal and have approximately the same nature of the landings of the same name, i.e., marginal clearances and tightness. In each case, the choice of a system is influenced by design, technological and economic considerations. However, you should pay attention to the fact that the exact shafts of different diameters can be machined with one tool when changing only the setup of the machine. The exact holes are machined with a dimensional cutting tool (countersinks, reamers, broaches, etc.), and each hole size requires its own set of tools. In the system, the holes of various sizes of holes are many times smaller than in the shaft system, and, consequently, the range of expensive tools is reduced. Therefore, the predominant distribution system has holes. However, in some cases, you have to use the shaft system. Here are some examples of the preferred use of the shaft system:

In order to avoid stress concentration in the place of transition from one diameter to another for strength reasons, it is undesirable to make a stepped shaft, and then it will perform a constant diameter;

When repairing, when there is a finished shaft and a hole is made under it;

For technological reasons, when the cost of manufacturing the shaft, for example, on centerless grinding machines is small, it is advantageous to use the shaft system;

When using standard assemblies and parts. For example, the outer diameter of rolling bearings is manufactured on a shaft system. If you make the outer diameter of the bearing in the bore system, then it would be necessary to significantly expand their range, and it is not expedient to process the bearing according to the outer diameter;

When on the shaft of the same diameter it is necessary to install several holes with a different type of fit.

Related information.

Size tolerance - called the difference between the largest and smallest limiting dimensions or the algebraic difference between the upper and lower deviations / 2 /.

Admission denoted by the letter "T" (from the Latin. tolerance   - tolerance):

TD = D max - Dmin = ES - EI - hole size tolerance;

Td = dmax - dmin = es - ei - shaft size tolerance.

For examples 1-6 previously discussed (section 1.1), dimensional tolerances are defined as follows:

1) Td = 24.015 - 24.002 = 0.015 - 0.002 = 0.013 mm;

2) Td = 39.975 - 39.950 = (-0.025) - (-0.050) = 0.025 mm;

3) TD = 32.007 - 31.982 = 0.007 - (-0.018) = 0.025 mm;

4) TD = 12.027 - 12 = 0.027 - 0 = 0.027 mm;

5) Td = 78 - 77.954 = 0 - (- 0.046) = 0.046 mm;

6) Td = 100.5 - 99.5 = 0.5 - (- 0.5) = 1 mm.

Tolerance - the value is always positive . The tolerance describes the accuracy of the part. The smaller the tolerance, the harder it is to work on the part, as the demands on the accuracy of the machine, tool, fixtures, and worker’s qualifications increase. Unnecessarily large tolerances reduce the reliability and quality of the product.

In some connections, gaps or tightness may occur with various combinations of the size limits of the bore and shaft. The nature of the connection details, determined by the size of the gaps or tightness resulting in it, called landing . Landing characterizes more or less freedom of relative movement of the parts to be joined or the degree of resistance to their mutual displacement / 1 /.

Distinguish   three groups of landings:

1) with a guaranteed clearance;

2) transitional;

3) with a guaranteed fit.

If the dimensions of the hole are larger than the shaft, then a gap arises in the connection.

Gap – this is the positive difference between the dimensions of the hole and the shaft / 1 /:

S = D - d 0 - clearance;

Smax = Dmax - dmin - the largest gap,

Smin = Dmin - dmax - the smallest gap.

If, prior to assembly, the shaft dimensions are larger than the dimensions of the hole, then a tension occurs in the joint. Preload – this is the positive difference between the dimensions of the shaft and the hole /1/:

N = d - D 0 - interference,

Nmax = dmax - Dmin - the greatest tension;

Nmin = dmin - Dmax - the smallest tension.

Landings in which there is a likelihood of a gap or tension, called transient.

Landing tolerance   - this is the clearance tolerance for plantings with a guaranteed clearance (defined as the difference between the largest and smallest gaps) or the tolerance of interference for plantings with a guaranteed fit (this is determined as the difference between the largest and smallest tension). In transitional landings, the landing tolerance is the clearance or tightness tolerance / 1 /.

Landing tolerance designation:

TS = Smax - Smin - landing tolerance for landings with a guaranteed clearance.

TN = Nmax - Nmin - landing tolerance for landings with guaranteed tightness.

T (S, N) = Smax + Nmax - landing tolerance for transitional landings.

For any group of landings, the landing tolerance can be determined by the formula

When assembling two parts, one inside the other, distinguish the outer-covering and inner-covered surface. One of the dimensions of the contacting surfaces is called enveloping size, and the other is encompassing. For round bodies, the circumferential surface has the general name-bore, and the circumscribed shaft, and the corresponding dimensions are called the bore diameter and shaft diameter.

Movable or fixed connection of parts can be made due to deviations of the coupled dimensions of the shaft or holes in one direction or another from their nominal dimensions.

The calculated size stamped on the drawing, called the nominal size (Fig. 439). Nominal sizes are put in millimeters.

Actual size   called the actual size, obtained by direct measurement after machining the part.

Limitingcalled dimensions, between which the actual size of the same element of the part of the manufactured batch can fluctuate. The larger of these is called the largest size limit, and the smaller one is the smallest size limit.

If the nominal size in the drawing is only one limit size, for example 25 +0.4 or 25 -0.1, it means that the other limit size coincides with the nominal size. The plus sign indicates that the size limit is greater than the nominal, and the minus sign, which is the size limit less than the nominal.

Valid   deviation is the difference between the actual and nominal dimensions.

Top   deviation is the difference between the largest limit size and nominal.

Lower   deviation is the difference between the smallest limiting and nominal dimensions.

By admission   called the difference between the largest and smallest limiting dimensions.

Clearances, tightness and landing. The clearance is the positive difference between the size of the hole and the size of the shaft. The size of the gap determines a greater or lesser degree of freedom of mutual movement of the mating parts.

Tension is the negative difference between the dimensions of the hole and the shaft, creating (after assembly) a fixed connection.

Landing   called the nature or type of connection of two parts inserted one into another.

All landings are divided into two groups: mobile landings and fixed landings.

Rolling landing   called the connection of two parts, providing the freedom of their relative movement.

Fixed landing   called the connection of two parts, providing the appropriate degree of strength of their connection.

There are the following types of landings, differing from each other by a greater or lesser clearance or more or less interference.

Moving landings   Fixed landing

Sliding With Hot Gr

Movement D Pressing Pr

Running X Lightweight Pl

Light running l Deaf

Wide T Shay T

Stress H Dense P

Tolerance system.There are two tolerance systems: the hole system and the shaft system.

The hole system is characterized by the fact that in it for all the fits of the same degree of accuracy (of the same class), referred to the same nominal diameter, the limiting dimensions of the hole remain constant. Implementation of various landings in the hole system is achieved by a corresponding change in the limiting dimensions of the shaft. In a bore system, the smallest bore size of a bore is its nominal size.

The shaft system is characterized by the fact that in it for all the fits of the same system and the degree of accuracy (of one class), assigned to the same nominal diameter, the limiting dimensions of the shaft remain constant. Implementation of various landings in the shaft system is achieved by a corresponding change in the limiting dimensions of the hole. In the shaft system, the maximum size of the shaft is its nominal size.

The tolerance of the hole in the hole system is always directed in the direction of increasing the hole (into the body), and the shaft tolerance in the shaft system in the direction of decreasing the shaft (into the body). The base of the systems is indicated by: the hole is the letter A, the shaft is the letter B. The hole in the shaft system and the shaft in the hole system are indicated by the letters and numbers of their respective fit and accuracy class.

In engineering, the hole system was adopted predominantly.