Numerical values ​​of tolerances table. Tolerances and fits.Measuring tools

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

Hole system

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

Shaft system

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

In order to increase the level of interchangeability of products and reduce the range of standard tools, tolerance fields for shafts and holes for preferred applications have been established.
The nature of the connection (fit) is determined by the difference in the sizes of the hole and the shaft

Terms and definitions according to GOST 25346

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

Actual size— element size established by measurement

Limit dimensions- two maximum permissible sizes of an element, between which the actual size must be (or can be equal to)

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 maximum size) and the corresponding nominal size

Actual deviation- algebraic difference between the real and the corresponding nominal sizes

Maximum deviation— algebraic difference between the limit and the corresponding nominal sizes. There are upper and lower limit deviations

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

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

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

Zero line- a line corresponding to the nominal size, from which dimensional deviations are plotted when graphically depicting tolerance fields and fits. If the zero line is horizontal, then positive deviations are laid up from it, and negative deviations are laid 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 sign

IT standard 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 maximum dimensions and determined by the tolerance value 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

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

Tolerance unit i, I- a multiplier in 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 dimensions up to 500 mm, I— tolerance unit for nominal dimensions St. 500 mm

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

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

Main shaft- a shaft whose upper deviation is zero

Main hole- a hole whose lower deviation is zero

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

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 making up the connection

Fit tolerance- the sum of the tolerances of the hole and shaft making up the connection

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

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

Clearance fit- a fit that always creates a gap in the connection, i.e. the smallest limit size of the hole is greater than or equal to the largest limit size of the shaft. When shown graphically, the tolerance field of the hole is located above the tolerance field of the shaft

Pressure landing - a landing in which interference is always formed in the connection, i.e. The largest maximum hole size is less than or equal to the smallest maximum shaft size. When shown graphically, the tolerance field of the hole is located below the tolerance field of the shaft

Transitional landing- a fit 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. When graphically depicting the tolerance fields of the hole and shaft, they overlap completely or partially

Landings in the hole system

— fits in which the required clearances and interferences are obtained by combining different tolerance fields of the shafts with the tolerance field of the main hole

Fittings in the shaft system

— fits in which the required clearances and interferences are obtained by combining different tolerance fields of the holes with the tolerance field of the main shaft

Normal temperature— the tolerances and maximum deviations established in this standard refer to the dimensions of parts at a temperature of 20 degrees C

Basic concepts and terms are regulated by GOST 25346–89.

Size– numerical value of a linear quantity (diameter, length, etc.). Valid called the size established by measurement with permissible error.

Two maximum permissible sizes, between which the actual size must be or can be equal to, are called maximum dimensions. The larger one is called largest size limit, smaller – smallest size limit.

Nominal size– 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.

Not any size obtained as a result of calculation can be accepted as nominal. In order to increase the level of interchangeability, reduce the range of products and standard sizes of workpieces, standard or normalized cutting and measuring tools, equipment and gauges, create conditions for specialization and cooperation of enterprises, reduce the cost of products, the size values ​​​​obtained by calculation should be rounded in accordance with the values ​​​​specified in GOST 6636–69. In this case, the original size value obtained by calculation or other means, if it differs from the standard one, 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 are constructed according to a geometric progression. Geometric progression provides a rational gradation of numerical values ​​of parameters and sizes when it is necessary to set not one value, but a uniform series of values ​​in a certain range. In this case, the number of terms of the series is smaller compared to an arithmetic progression.

Accepted designations:

D(d) – nominal hole (shaft) size;

D max ,( d m ah), D min ,( d min) , D e ( d e), D m(d m) – dimensions of the hole (shaft), largest (maximum), smallest (minimum), real, average.

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

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

S, S max , S min , S m – gaps, largest (maximum), smallest (minimal), average, respectively;

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

TD, Td, TS, TN, TSN– tolerances of hole, shaft, clearance, interference, clearance – interference (in transitional fit), respectively;

IT 1, IT 2, IT 3…ITn……IT 18 – qualification tolerances are indicated by a combination of letters IT with the serial number of the qualification.

Deviation– 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 sizes. The deviation is positive if the actual size is greater than the nominal size and negative if it is less than the nominal size. If the actual size is equal to the nominal size, then its deviation is zero.

Maximum deviation is called the algebraic difference between the maximum and nominal sizes. There are upper and lower deviations. Upper deviation– algebraic difference between the largest limit and nominal sizes. Lower deviation– algebraic difference between the smallest limit and nominal sizes.

To simplify and conveniently work, in drawings and tables of standards for tolerances and fits, instead of maximum dimensions, it is customary to indicate the values ​​of maximum deviations: 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 limit – slightly lower. Deviations equal to zero are not indicated 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 the sign “±”; the deviation is indicated following the nominal size. For example:

30 ; 55; 3 +0.06; 45±0.031.

Main deviation– one of two deviations (upper or lower), used to determine the tolerance range relative to the zero line. Typically this deviation is the deviation closest to the zero line.

Zero line– a line corresponding to the nominal size, from which dimensional deviations are plotted when graphically depicting tolerances and fits. If the zero line is located horizontally, then positive deviations are laid up from it, and negative deviations are laid down.

Size tolerance– the difference between the largest and smallest limit sizes or the absolute value of the algebraic difference between the upper and lower deviations:

For hole T.D.= D max – D mi n = ES – EI;

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

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

During processing, each part acquires its actual size and can be assessed as acceptable if it is within the range of maximum sizes, or rejected if the actual size is outside these limits.

The condition for the suitability 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 dimensional accuracy. The smaller the tolerance, the smaller the permissible fluctuation in actual dimensions, the higher the accuracy of the part and, as a result, the complexity of processing and its cost increase.

Tolerance field– field limited by upper and lower deviations. The tolerance field is determined by the numerical value of the tolerance and its position relative to the nominal size. When depicted graphically, 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 tolerance fields:

A– holes ( ES And EI– positive); b– shaft ( es And ei– negative)

In the connection of parts that fit into one another, there are female and male surfaces. Shaft– a term used to designate the external (male) elements of parts. Hole– a term conventionally used to designate the internal (encompassing) elements of parts. The terms hole and shaft refer not only to cylindrical parts with a circular cross-section, but also to parts of other shapes, for example those limited by two parallel planes.

Main shaft– a shaft whose upper deviation 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 dimensions of the shaft and the hole before assembly, if the size of the shaft is larger than the size of the hole. The tension ensures the mutual immobility of the parts after their assembly.

The largest and smallest clearances (preferences)– two limit values ​​between which there must be a gap (preference).

Average clearance (preference) is the arithmetic mean between the largest and smallest gap (interference).

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

Clearance fit– a fit that always ensures a gap in the connection.

In clearance fits, the tolerance field of the hole is located above the tolerance field of the shaft. Landings with clearance also include fits in which the lower limit of the hole tolerance field coincides with the upper limit of the shaft tolerance field.

Interference fit– a fit that always ensures tension in the connection. In interference fits, the tolerance field of the hole is located below the tolerance field of the shaft

Transitional landing called a fit in which it is possible to obtain both a gap and an interference fit in the connection. In such a fit, the tolerance fields of the hole and shaft completely or partially overlap each other.

Fit tolerance– the sum of the tolerances of the hole and shaft that make 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 = T.D. + Td;

For interference fits:

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 = T.D. + 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 landing corresponds to S m.

TS(N) = TN(S) = S max + N max = T.D. + Td.

In mechanical engineering and instrument making, fits of all three groups are widely used: with clearance, interference and transitional. The fit of any group can be achieved either by changing the dimensions of both mating parts or one mating part.

A set of fits in which the maximum deviations of holes of the same nominal size and the same accuracy are the same, and different fits are achieved by changing the maximum deviations of the shafts, is called hole system. For all fits 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.

A set of fits in which the maximum deviations of a shaft of the same nominal size and the same accuracy are the same, and different fits are achieved by changing the maximum deviations of the holes, is called shaft system. For all fits 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 character of the same landings, i.e., maximum clearances and interferences. In each specific case, the choice of a particular system is influenced by design, technological and economic considerations. At the same time, you should pay attention to the fact that precision shafts of different diameters can be processed on machines with one tool by changing only the machine setup. Precise holes are machined with measuring cutting tools (countersinks, reamers, broaches, etc.), and each hole size requires its own set of tools. In the system, the holes of various maximum sizes are many times smaller than in the shaft system, and, consequently, the range of expensive tools is reduced. Therefore, the hole system has become more widespread. However, in some cases it is necessary to use a shaft system. Here are some examples of preferred shaft system applications:

To avoid stress concentration at the point of transition from one diameter to another, for strength reasons, it is undesirable to make a stepped shaft, and then it is made of a constant diameter;

During repairs, when there is a ready-made shaft and a hole is made for it;

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

When using standard components and parts. For example, the outer diameter of rolling bearings is manufactured according to the shaft system. If we make the outer diameter of the bearing in a hole system, then it would be necessary to significantly expand their range, and it is impractical to process the bearing along the outer diameter;

When it is necessary to install several holes on a shaft of the same diameter different types landing

Related information.

Initially, production was a one-man business. One person made any mechanism from start to finish, without resorting to outside help. Connections were customized individually. It was impossible to find 2 identical parts in one factory. This continued until the mid-18th century, until people realized the effectiveness of the division of labor. This gave greater productivity, but then the question arose about the interchangeability of products. For this purpose, we developed a system for standardizing the levels of precision in manufacturing parts. The ESDP establishes qualifications (otherwise, degrees of accuracy).

Standardization of accuracy levels

The development of production standardization methods—this includes tolerances, fits, and accuracy grades—is carried out by metrological services. Before you begin to study them directly, you need to understand the meaning of the word “interchangeability.” What is hidden under this definition?

Interchangeability is the ability of parts to be assembled into a single unit and perform their functions without mechanical processing. Relatively speaking, one part is manufactured at one plant, another at a second, and at the same time they can be assembled at a third and fit together.

The purpose of this division is to increase productivity, which is formed due to the following reasons:

• Development of cooperation and specialization. The more diverse the production range, the more time is needed to set up equipment for each specific part.
• Reducing tool varieties. Fewer tool types also improve the efficiency of machine manufacturing. This happens due to the reduction in time for replacing it during the production process.

The concept of admission and qualifications

It is difficult to understand the physical meaning of tolerance without introducing the term “size”. Size is physical quantity, characterizing the distance between two points lying on the same surface. In metrology, there are the following types of it:

• The actual size is obtained by direct measurement of the part: with a ruler, caliper and other measuring tools.
• The nominal size is shown directly on the drawing. It is ideal in terms of accuracy, so obtaining it in reality is impossible due to the presence of a certain equipment error.
• Deviation is the difference between the nominal and actual sizes.
• The lower limit deviation shows the difference between the smallest and nominal size.
• The upper limit deviation indicates the difference between the largest and nominal sizes.

For clarity, let's look at these parameters using an example. Let's imagine there is a shaft with a diameter of 14 mm. It has been technically determined that it will not lose its performance if its manufacturing accuracy is from 15 to 13 mm. In the design documentation this is designated 〖∅14〗_(-1)^(+1).

Diameter 14 is the nominal size, "+1" is the upper limit deviation, and "-1" is the lower limit deviation. Then subtracting the lower limit deviation from the upper limit will give us the shaft tolerance value. That is, in our case it will be +1- (-1) = 2.

All tolerance sizes are standardized and grouped into groups - qualifications. In other words, quality shows the accuracy of the manufactured part. There are 19 such groups or classes in total. Their designation scheme is represented by a certain sequence of numbers: 01, 00, 1, 2, 3...17. The more precise the size, the less quality it has.

Accuracy quality table

Numerical tolerance values
Interval nominal sizes mm		Quality
		01	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
St.	To	µm													mm
	3	0.3	0.5	0.8	1.2	2	3	4	6	10	14	25	40	60	0.10	0.14	0.25	0.40	0.60	1.00	1.40
3	6	0.4	0.6	1	1.5	2.5	4	5	8	12	18	30	48	75	0.12	0.18	0.30	0.48	0.75	1.20	1.80
6	10	0.4	0.6	1	1.5	2.5	4	6	9	15	22	36	58	90	0.15	0.22	0.36	0.58	0.90	1.50	2.20
10	18	0.5	0.8	1.2	2	3	5	8	11	18	27	43	70	110	0.18	0.27	0.43	0.70	1.10	1.80	2.70
18	30	0.6	1	1.5	2.5	4	6	9	13	21	33	52	84	130	0.21	0.33	0.52	0.84	1.30	2.10	3.30
30	50	0.6	1	1.5	2.5	4	7	11	16	25	39	62	100	160	0.25	0.39	0.62	1.00	1.60	2.50	3.90
50	80	0.8	1.2	2	3	5	8	13	19	30	46	74	120	190	0.30	0.46	0.74	1.20	1.90	3.00	4.60
80	120	1	1.5	2.5	4	6	10	15	22	35	54	87	140	220	0.35	0.54	0.87	1.40	2.20	3.50	5.40
120	180	1.2	2	3.5	5	8	12	18	25	40	63	100	160	250	0.40	0.63	1.00	1.60	2.50	4.00	6.30
180	250	2	3	4.5	7	10	14	20	29	46	72	115	185	290	0.46	0.72	1.15	1.85	2.90	4.60	7.20
250	315	2.5	4	6	8	12	16	23	32	52	81	130	210	320	0.52	0.81	1.30	2.10	3.20	5.20	8.10
315	400	3	5	7	9	13	18	25	36	57	89	140	230	360	0.57	0.89	1.40	2.30	3.60	5.70	8.90
400	500	4	6	8	10	15	20	27	40	63	97	155	250	400	0.63	0.97	1.55	2.50	4.00	6.30	9.70
500	630	4.5	6	9	11	16	22	30	44	70	110	175	280	440	0.70	1.10	1.75	2.80	4.40	7.00	11.00
630	800	5	7	10	13	18	25	35	50	80	125	200	320	500	0.80	1.25	2.00	3.20	5.00	8.00	12.50
800	1000	5.5	8	11	15	21	29	40	56	90	140	230	360	560	0.90	1.40	2.30	3.60	5.60	9.00	14.00
1000	1250	6.5	9	13	18	24	34	46	66	105	165	260	420	660	1.05	1.65	2.60	4.20	6.60	10.50	16.50
1250	1600	8	11	15	21	29	40	54	78	125	195	310	500	780	1.25	1.95	3.10	5.00	7.80	12.50	19.50
1600	2000	9	13	18	25	35	48	65	92	150	230	370	600	920	1.50	2.30	3.70	6.00	9.20	15.00	23.00
2000	2500	11	15	22	30	41	57	77	110	175	280	440	700	1100	1.75	2.80	4.40	7.00	11.00	17.50	28.00
2500	3150	13	18	26	36	50	69	93	135	210	330	540	860	1350	2.10	3.30	5.40	8.60	13.50	21.00	33.00

Landing concept

Previously, we considered the accuracy of one part, which was determined only by tolerance. What happens to the accuracy when connecting several parts into one assembly? How will they interact with each other? And so, here it is necessary to introduce a new term “fit”, which will characterize the location of the tolerances of the parts relative to each other.

The selection of fits is made in the shaft and hole system

Shaft system is a set of fits in which the amount of clearance and interference is selected by changing the size of the hole, but the shaft tolerance remains unchanged. In the hole system everything is the other way around. The nature of the connection is determined by the selection of shaft dimensions; the hole tolerance is considered constant.

In mechanical engineering, 90% of products are produced in a hole system. The reason for this is more complex process making a hole from a technological point of view, compared to a shaft. The shaft system is used when difficulties arise in processing the outer surface of a part. A prime example of this is the balls of a rolling bearing.

All types of landing connections are regulated by standards and also have accuracy ratings. The purpose of this division of plantings into groups is to increase productivity by increasing the efficiency of interchangeability.

Types of plantings

The type of fit and its accuracy quality are selected based on the operating conditions and the method of assembly of the unit. In mechanical engineering, they are divided into the following types:

• Clearance fits are connections that are guaranteed to create a gap between the surface of the shaft and the hole. They are designated by Latin letters: A, B…H. They are used in assemblies in which parts “move” relative to each other and when centering surfaces.
• Interference fits are connections in which the shaft tolerance exceeds the hole tolerance, resulting in additional compressive stresses. An interference fit refers to non-separable connection types. They are used in highly loaded units, the main parameter of which is strength. This includes attaching metal sealing rings and valve seats of the cylinder head to the shaft, installing large couplings and keys under gears, etc., etc. There are two ways to fit the shaft onto the hole with interference. The simplest of them is pressing. The shaft is centered along the hole and then placed under a press. With greater tension, the properties of metals are used to expand when exposed to elevated temperatures and contract when the temperature decreases. This method is characterized by greater accuracy of surface mating. Immediately before joining, the shaft is pre-cooled and the hole is heated. Next, parts are installed, which after some time return to their previous dimensions, thereby forming the clearance fit we need.
• Transitional landings. Designed for fixed connections that are often subject to disassembly and assembly (for example, during repairs). In terms of their density, they occupy an intermediate position among planting varieties. These fits have an optimal balance between accuracy and connection strength. In the drawing they are designated by the letters k, m, n, j. A striking example of their application is the fit of the inner rings of a bearing on a shaft.

Typically, the use of one or another landing is indicated in special technical literature. We simply determine the type of connection and select the type of fit and accuracy grade we need. But it is worth noting that in especially critical cases, the standard provides for individual selection of tolerances for mating parts. This is done using special calculations specified in the relevant methodological manuals.

Metrology is the science of measurements, means and methods of ensuring their unity, as well as methods of achieving the required accuracy. Its subject is the extraction of quantitative information about the parameters of objects with a given reliability and accuracy. for metrology these are standards. In this article we will consider the system of tolerances and landings, which is a subsection of this science.

The concept of interchangeability of parts

In modern factories, tractors, cars, machine tools and other machines are produced not in units or tens, but in hundreds and even thousands. With such production volumes, it is very important that each manufactured part or assembly fits exactly into its place during assembly without additional metalwork adjustments. After all, such operations are quite labor-intensive, expensive and time-consuming, which is not permissible in mass production. It is equally important that the parts supplied for assembly can be replaced with others of a common purpose, without any damage to the functioning of the entire finished unit. This interchangeability of parts, assemblies and mechanisms is called unification. This is a very important point in mechanical engineering, it allows you to save not only the cost of designing and manufacturing parts, but also production time, in addition, it simplifies the repair of the product as a result of its operation. Interchangeability is the property of components and mechanisms to take their place in products without prior selection and perform their main functions in accordance with

Mating parts

Two parts that are fixedly or movably connected to each other are called mating. And the value by which this articulation is carried out is usually called the mating size. An example is the diameter of the hole in the pulley and the corresponding shaft diameter. The value at which the connection does not occur is usually called the free size. For example, the outer diameter of a pulley. To ensure interchangeability, the mating dimensions of the parts must always be accurate. However, such processing is very complex and often impractical. Therefore, technology uses a method for obtaining interchangeable parts when working with so-called approximate accuracy. It lies in the fact that for different conditions work, components and parts determine the permissible deviations of their dimensions, at which the flawless functioning of these parts in the unit is possible. Such indents, calculated for a variety of working conditions, are built in a given specific scheme, its name is “a unified system of tolerances and landings.”

The concept of tolerances. Characteristics of quantities

The calculated data of the part, supplied in the drawing, from which deviations are calculated, are usually called the nominal size. Usually this value is expressed in whole millimeters. The size of the part that is actually obtained during processing is called the actual size. The values ​​between which this parameter fluctuates are usually called the limit. Of these, the maximum parameter is the largest limit size, and the minimum parameter is the smallest. Deviations are the difference between the nominal and limiting values ​​of a part. In the drawings, this parameter is usually indicated in numerical form at the nominal size (the upper value is indicated above, and the lower value below).

Example entry

If the drawing shows the value 40 +0.15 -0.1, then this means that the nominal size of the part is 40 mm, the largest limit is +0.15, the smallest is -0.1. The difference between the nominal and maximum limit values ​​is called the upper deviation, and between the minimum - the lower. From here the actual values ​​can be easily determined. From this example it follows that the largest limit value will be equal to 40 + 0.15 = 40.15 mm, and the smallest: 40-0.1 = 39.9 mm. The difference between the smallest and largest limit sizes is called tolerance. It is calculated as follows: 40.15-39.9 = 0.25 mm.

Clearances and interference

Let's consider a specific example where tolerances and fits have key value. Let's assume that we need to place a part with a hole of 40 +0.1 on a shaft with dimensions of 40 -0.1 -0.2. From the condition it is clear that the diameter in all options will be less than the hole, which means that with such a connection there will definitely be a gap. This type of fit is usually called movable, since the shaft will rotate freely in the hole. If the part size is 40 +0.2 +0.15, then under any condition it will be larger than the diameter of the hole. In this case, the shaft must be pressed in, and tension will arise in the connection.

Conclusions

Based on the above examples, the following conclusions can be drawn:

• The gap is the difference between the actual dimensions of the shaft and the hole, when the latter are larger than the former. With this connection, the parts have free rotation.
• Preference is usually called the difference between the actual dimensions of the hole and the shaft, when the latter is larger than the former. With this connection, the parts are pressed into place.

Landings and accuracy classes

Plantings are usually divided into stationary (hot, pressed, light-pressed, blind, tight, dense, tense) and movable (sliding, running, moving, easy-running, wide-running). In mechanical and instrument engineering there are certain rules that regulate tolerances and fits. GOST provides for certain accuracy classes in the manufacture of components using specified dimensional deviations. It is known from practice that parts of road and agricultural machines, without harm to their functioning, can be manufactured with less accuracy than for lathes, measuring instruments, cars. In this regard, tolerances and fits in mechanical engineering have ten different accuracy classes. The most accurate of them are the first five: 1, 2, 2a, 3, 3a; the next two refer to average accuracy: 4 and 5; and the last three are considered rough: 7, 8 and 9.

In order to find out what accuracy class the part should be manufactured in, on the drawing next to the letter indicating the fit, put a number indicating this parameter. For example, marking C4 means that the type is sliding, class 4; X3 - running type, class 3. For all second class landings, a digital designation is not given, since it is the most common. Get detailed information You can learn about this parameter from the two-volume reference book “Tolerances and Landings” (Myagkov V.D., published in 1982).

Shaft and hole system

Tolerance and fit are usually considered as two systems: hole and shaft. The first of them is characterized by the fact that in it all types with the same degree of accuracy and class belong to the same nominal diameter. The holes have constant maximum deviation values. The variety of landings in such a system is obtained as a result of changing the maximum shaft deviation.

The second of them is characterized by the fact that all types with the same degree of accuracy and class belong to the same nominal diameter. The shaft has constant maximum deviation values. A variety of landings is carried out as a result of changing the values ​​of the maximum deviations of the holes. In drawings, the hole system is usually designated by the letter A, and the shaft by the letter B. An accuracy class sign is placed next to the letter.

Examples of notation

If “30A3” is indicated on the drawing, this means that the part in question must be machined with a hole system of the third class of accuracy; if “30A” is indicated, it means according to the same system, but of the second class. If tolerances and fits are made according to the shaft principle, then the required type is indicated at the nominal size. For example, a part with the designation “30B3” corresponds to processing using a third-class accuracy shaft system.

In his book, M. A. Paley (“Tolerances and Fit”) explains that in mechanical engineering the principle of a hole is used more often than a shaft. This is due to the fact that it requires less equipment and tooling costs. For example, in order to process a hole of a given nominal diameter using this system, only one reamer is needed for all fits of a given class; to change the diameter, one limit plug is needed. With a shaft system, a separate reamer and a separate plug are required to ensure each fit within one class.

Tolerances and fits: table of deviations

To determine and select accuracy classes, it is customary to use special reference literature. Thus, tolerances and fits (a table with an example is given in this article) are, as a rule, very small values. In order to avoid writing extra zeros, in the literature they are designated in microns (thousandths of a millimeter). One micron corresponds to 0.001 mm. Typically, the first column of such a table indicates the nominal diameters, and the second column indicates the hole deviations. The remaining columns show various landing values ​​with their corresponding deviations. The plus sign next to this value indicates that it should be added to the nominal size, and the minus sign indicates that it should be subtracted.

Threads

The tolerance and fit of threaded connections must take into account the fact that the threads are mated only on the sides of the profile, with the exception of vapor-tight types. Therefore, the main parameter that determines the nature of the deviations is the average diameter. Tolerances and fits for the outer and inner diameters are set so as to completely eliminate the possibility of pinching along the recesses and crests of the thread. Errors in reducing the external size and increasing the internal size will not affect the make-up process. However, deviations in the profile angle will lead to jamming of the fastener.

Thread tolerances with clearance

The most common are tolerance and clearance fits. In such connections, the nominal value of the average diameter is equal to the largest average value of the nut thread. Deviations are usually measured from the profile line perpendicular to the thread axis. This is determined by GOST 16093-81. Tolerances for the thread diameter of nuts and bolts are assigned depending on the specified degree of accuracy (indicated by a number). The following series of values ​​for this parameter is accepted: d1=4, 6, 8; d2=4, 6, 7, 8; D1=4, 6, 7, 8; D2=4, 5, 6, 7. Tolerances are not established for them. Placing the thread diameter fields relative to the nominal profile value helps determine the main deviations: upper for external values ​​of bolts and lower for internal values ​​of nuts. These parameters directly depend on the accuracy and pitch of the connection.

Tolerances, fits and technical measurements

To produce and process parts and mechanisms with given parameters, a turner has to use a variety of tools. Typically, rulers, calipers and bore gauges are used for rough measurements and checking the dimensions of products. For more accurate measurements - calipers, micrometers, gauges, etc. Everyone knows what a ruler is, so we won’t dwell on it.

A caliper is a simple tool for measuring the external dimensions of workpieces. It consists of a pair of rotating curved legs fixed on one axis. There is also a spring type of caliper; it is adjusted to the required size using a screw and nut. Such a tool is a little more convenient than a simple one, since it saves a given value.

The bore gauge is designed to take internal measurements. Available in regular and spring types. The design of this tool is similar to a caliper. The accuracy of the devices is 0.25 mm.

A caliper is a more precise device. It can measure both external and internal surfaces of workpieces. When working on a lathe, a lathe operator uses a caliper to take measurements of the depth of a groove or shoulder. This measuring instrument consists of a rod with divisions and jaws and a frame with a second pair of jaws. Using a screw, the frame is fixed on the rod in the required position. is 0.02 mm.

Vernier depth gauge - this device is designed for measuring the depth of grooves and recesses. In addition, the tool allows you to determine the correct position of the shoulders along the length of the shaft. The design of this device is similar to a caliper.

Micrometers are used to accurately determine the diameter, thickness and length of the workpiece. They give a reading with an accuracy of 0.01 mm. The object to be measured is located between the micrometer screw and the fixed heel, adjustment is carried out by rotating the drum.

Bore gauges are used to carry out precise measurements of internal surfaces. There are permanent and sliding devices. These instruments are rods with measuring ball ends. The distance between them corresponds to the diameter of the hole being determined. The measurement limits for the bore gauge are 54-63 mm; with an additional head, diameters up to 1500 mm can be determined.