The degree of accuracy is called basic in threaded connections. Metric thread tolerance fields

There are three accuracy classes for metric threads

*Tolerance fields of preferred application.

For trapezoidal threads, two accuracy classes are established - medium and coarse; for persistent threads - class 1 and class 2; for cylindrical pipe threads, accuracy classes A and B.

1. Main types of fasteners.

The main types of threaded fasteners are bolts, screws, studs, and nuts.

To connect parts, bolts, screws with nuts (P6a), screws (P6b), studs with nuts (P6v) are used.

Bolts and screws are divided into three groups according to the shape of their heads:

with a head grasped by the tool from the outside;

with a head grasped by the tool from the end;

with a head that prevents the bolt from turning.

The most common are hexagonal heads (there are square ones, etc.), grasped with a key from the outside (P7a, b, c).

Heads with end grips are made with an internal hexagon or square, a slot for a regular screwdriver (P7g) and a Phillips slot.

To fix the position of the parts and prevent their mutual displacement, set screws are used (P7. 2B). By the nature of the action, these are pressure screws that work on compression; they are made short with threads along the entire length.

Special bolts include:

Hinged (P7. 2v), bolts with a machined groove (P7. 2a), conical (GOST 15163 - 69), cargo (P7. 3v eye bolt), etc.

A stud is a cylindrical rod equipped with threads at both ends (P7. 2d). Studs are recommended for use in cases where the connection is subject to frequent disassembly and reassembly, and the threads in the part due to the properties of the material (cast iron, light alloys, etc.) do not have strength and wear resistance.

The main type of nuts are hexagonal (P7. 4b, c, d).

The height of normal nuts is Н=0.8d; for frequent screwing and unscrewing and high forces, high nuts Н=(1.2…1.6)d are used.

Nuts to be locked using cotter pins are made of crown or slotted nuts (P7. 4g) with an increased height.

Nuts, which are often screwed in and out with low tightening force, are knurled (P7.5b) or wing-shaped (P7.5b) for manual screwing.

To protect fasteners from corrosion or improve their appearance, they are subjected to various coatings.

Types and symbols of coatings of bolts, screws, studs and nuts.

table 2

Designations

Types of coatings

Without cover

Zinc with chromating

Cadmium with chromate

Nickel: multilayer – copper – nickel

Multilayer – copper – nickel – chrome

Phosphate with oiling

Tin

Zinc

Oxide anodized with chromate plating

Passive

Silver

Bolts, screws, studs and nuts are designated according to the following scheme:

Bolt 2M12x1,25.6 dx60.58.S.029 GOST.

Tolerance range 8q, 8H, coarse thread pitch, version 1, type of coating 00 (without coating) is not indicated in the designation.

Strength classes of bolts and materials of threaded parts.

Table 3

30ХГСА

35ХГСА;

40ХН2МА

Note:

The strength class is indicated by two numbers.

The first number, multiplied by 100, determines the value of the minimum tensile strength in MPa; the second number multiplied by 10 is the ratio σ y /σ u in %; the product of numbers multiplied by 10 determines the value of the yield strength σ y in MPa.

Length Determination: (See Lab Book).

Bolta l = ∑δ + S + H + a + c

Screw l = δ + l 1 + S

Studs l = δ + S + H + a + c

∑δ – total thickness of fastened parts;

δ – part thickness;

l 1 – depth of screwing of the screw (stud);

S – washer thickness;

Chamfer with mm

Drilling depth reserve L, mm

Exit of the end of the screw from the nut a mm

Chamfer with mm

The depth of screwing of screws and studs is:

In a steel case l 1 = (0.8...1.0)d

In a cast iron body l 1 = (1.3...1.4)d

In aluminum alloy l 1 = (2.0...2.5)d

where d is the thread diameter.

30ХГСА

Determining the drilling depth for screw and stud

where L – drilling depth;

l 1 – depth of screwing of a screw or stud;

L 1 – drilling depth reserve.

The most accurate 1st class thread. Threads of classes 2 and 3 are used in tractors and cars. In the drawings, the thread class is indicated after the pitch. For example: M10x1 – class. 3; M18 – class. 2, which means: metric thread 10, pitch 1, thread accuracy class - 3; metric thread 18 (large), thread accuracy class - 2nd.

According to the noted metric thread standards, six degrees of accuracy were established for small threads, which are designated by letters:

With; d; e; f; h; k – for external threads;

C;D; E; F; H; K – for internal threads.

Degrees of accuracy c; d (C; D) approximately correspond to class 1; e; f (E; F) – 2nd class; h; k (H; K) – 3rd class.

For cylindrical pipe threads, 2 accuracy classes are established: 2 and 3. Deviations in the dimensions of cylindrical pipe threads are given in GOST 6357 - 52.

For inch threads with a profile angle of 55, two accuracy classes are also established: 2 and 3 (OST/NKTP 1261 and 1262).

Measurement of thread accuracy classes is carried out using limiting thread gauges, which have two sides:

Checkpoint (designated “PR”);

Impassable (indicated by “NOT”).

The leading side is the same for all thread accuracy classes. The non-go side corresponds to a certain class of thread accuracy, which is indicated by a corresponding mark on the end of the caliber.

Degrees of accuracy of thread diameters GOST 16093-81

Make-up lengths according to GOST 16093-81

The concept of reduced average thread diameter

Given average thread diameter called average diameter of an imaginary ideal thread, which has the same pitch and flank angle as the main or nominal thread profile, and a length equal to the specified make-up length, and which is in close contact (without mutual displacement or interference) with the actual thread at the flanks of the thread.

In short, reduced mean thread diameter is the average diameter of the ideal threaded element that connects to the actual thread. When talking about the given average thread diameter, do not think of it as the distance between two points. This is the diameter of a conditional ideal thread, which does not exist in reality as a material object and which could curl with a real threaded element with all the errors in its parameters. This average diameter cannot be measured directly. It can be controlled, i.e. find out if it is within acceptable limits. And in order to find out the numerical value of the given average diameter, it is necessary to separately measure the values ​​of the thread parameters that prevent make-up and calculate this diameter.

When manufacturing threads, the deviations of individual thread elements depend on the errors of individual components of the technological process. Thus, the pitch error of a thread processed on thread-processing machines mainly depends on the pitch error of the machine lead screw; the profile angle depends on the inaccuracy of threading the angle of the tool and its installation relative to the thread axis.

It must be remembered that threaded surfaces of bolt and nut never touch over the entire screw surface, but touch only in certain areas. The main requirement, for example, for fastening threads is that the screwing of the bolt and nut is ensured - this is their main service purpose. Therefore, it seems possible to change the average diameter of a bolt or nut and achieve make-up in case of pitch and profile errors, while there will be contact between the threads, but not over the entire surface. On some profiles (in case of pitch errors) or in certain sections of the profile (in case of profile errors), as a result of compensation for these errors by changing the average diameter, there will be a gap in several mating places. Often there are only 2 - 3 turns in contact along the threaded elements.

Step 5P error compensation. The pitch error of a thread is usually “intra-pitch”, and there is a progressive error, sometimes called “stretch” of the pitch. Error compensation is carried out for progressive errors. Two axial sections of a bolt and nut are superimposed on each other. These threaded elements do not have equal pitches along the screwing length, and therefore screwing cannot occur, although their average diameter is the same. In order to ensure make-up, it is necessary to remove part of the material (shaded areas in the figure), i.e. increase the average diameter of a nut or decrease the average diameter of a bolt. After this, make-up will occur, although contact will only occur on the outer profiles.

Thus, if there is a pitch error of 10 microns, then to compensate for it, the average diameter of the bolt should be reduced or the average diameter of the nut should be increased by 17.32 microns, and then the pitch errors will be compensated and the screwing of the threaded elements of the parts will be ensured.

Compensation for profile angle error Sa/l. An error in the profile angle or side inclination angle usually arises from an error in the profile of the cutting tool or an error in its installation on the machine relative to the axis of the workpiece. Compensation for thread profile errors is also made by changing the value of the average diameter, i.e. an increase in the average diameter of a nut or a decrease in the average diameter of a bolt. If you remove part of the material where the profiles overlap each other (increase the average diameter of the nut or decrease the average diameter of the bolt), then make-up will occur, but contact will occur in a limited area of ​​​​the side of the profile. Such contact is sufficient for make-up to occur, i.e. fastening of two parts. Thus, the requirement for thread accuracy in relation to the average diameter is normalized by a total tolerance, which limits both the given average diameter (the diameter of the ideal thread that ensures screwing together) and the average thread diameter (the actual average diameter). The standard only mentions that the tolerance on the average diameter is total, but there is no explanation of this concept. The following additional interpretations can be given for this tolerance.

1. For an internal thread (nut), the given average diameter must not be less than the size corresponding to the maximum material limit (often said - the throughput limit), and the largest average diameter (the actual average diameter) must not be greater than the minimum material limit (often said - no-go limit). The value of the given average diameter for an internal thread is determined by the formula.

2. For external threads (bolts), the given average diameter should not be greater than the maximum material limit for the average diameter, and the smallest actual average diameter at any location should be less than the minimum material limit.

The concept of an ideal thread in contact with a real one can be imagined by analogy with the concept of an adjacent surface and, in particular, an adjacent cylinder, which were considered when normalizing the accuracy of shape deviations. The ideal thread in the initial position can be imagined as a thread coaxial with the real thread, but for the bolt it is much larger in diameter. If now the ideal thread gradually contracts (the average diameter decreases) until it comes into close contact with the real thread, then the average diameter of the ideal thread will be the reduced average diameter of the real thread.

The tolerances that are given in the standard for the average diameter of the bolt (Tch) and nut (TD2) actually include tolerances for the actual average diameter (Tch), (TD2) and the value of possible compensation f P + fa, i.e. Td 2 (TD 2) = TdifJVi + f P + fa.

It should be noted that when normalizing this parameter, it must be understood that the tolerance for the average diameter must also take into account the permissible deviations of the pitch and profile angle. It is possible that in the future this complex tolerance will receive a different designation, or perhaps a new name, which will make it possible to distinguish this tolerance from the tolerance only for the average diameter.

When making a thread, the technologist can distribute the total tolerance between three thread parameters - average diameter, pitch, profile angle. Often the tolerance is divided into three equal parts, but if there is a margin of accuracy on the machines, you can set smaller tolerances for the pitch and larger tolerances for the angle and average diameter, etc.

It is impossible to directly measure the given average diameter, since, as a diameter, i.e. the distance between two points, it does not exist, but represents, as it were, a conditional, effective diameter of the mating threaded surfaces. Therefore, to determine 198 the value of the reduced average thread diameter, it is necessary to measure the average diameter separately, measure the pitch and half of the profile angle separately, calculate the diametrical compensations based on the errors of these elements, and then by calculation determine the value of the reduced average thread diameter. The value of this average diameter must be within the tolerance established in the standard.

System of tolerances and fits of metric threads with clearance.

The most common, most widely used, is a metric thread with a gap for the diameter range from 1 to 600 mm, the system of tolerances and fits of which is presented in GOST 16093-81.

The basics of this system of tolerances and fits, including degrees of accuracy, accuracy classes of threads, normalization of make-up lengths, methods for calculating tolerances of individual thread parameters, designation of accuracy and fits of metric threads in drawings, control of metric threads and other issues of the system are common to all types of metric threads, although each of them has its own characteristics, sometimes significant, which are reflected in the relevant GOSTs.

Degrees of accuracy and classes of thread accuracy. A metric thread is determined by five parameters: average, outer and inner diameters, pitch and thread profile angle.

Tolerances are assigned only for two parameters of the external thread (bolt); middle and outer diameters and for two parameters of internal thread (nut); middle and inner diameters. For these parameters, accuracy degrees of 3... 10 are set for metric threads.

In accordance with established practice, degrees of accuracy are grouped into 3 accuracy classes: fine, medium and coarse. The concept of accuracy class is conditional. When assigning degrees of accuracy to an accuracy class, the make-up length is taken into account, since during manufacturing the difficulty of ensuring a given thread accuracy depends on the make-up length available to it. Three groups of make-up lengths have been established: S - short, N - normal and L - long.

With the same accuracy class, the tolerance of the average diameter at the make-up length L should be increased, and at the make-up length S - reduced by one degree compared to the tolerance established for the make-up length N.

The approximate correspondence between accuracy classes and degrees of accuracy is as follows: - exact class corresponds to 3-5 degrees of accuracy; - middle class corresponds to 5-7 degrees of accuracy; - rough class corresponds to 7-9 degrees of accuracy.

The initial degree of accuracy for calculating the numerical values ​​of the tolerances of the diameters of external and internal threads was taken to be the 6th degree of accuracy with a normal make-up length.

Cylindrical gears are most widely used in mechanical engineering. Terms, definitions and designations of cylindrical gear wheels and gears are regulated by GOST 16531-83. Cylindrical gears, based on the shape and arrangement of the gear teeth, are divided into the following types: rack, spur, helical, chevron, involute, cycloid, etc. Novikov gears, which have a high load-bearing capacity, are increasingly being used in industry. The profile of the teeth of the wheels of these gears is outlined by circular arcs.

According to their operational purpose, four main groups of cylindrical gears can be distinguished: reference, high-speed, power and general purpose.

Reference gears include gears of measuring instruments, dividing mechanisms of metal-cutting machines and dividing machines, servo systems, etc. In most cases, the wheels of these gears have a small modulus (up to 1 mm), a short tooth length and operate at low loads and speeds. The main operational requirement for these transmissions is high accuracy and consistency of the rotation angles of the driven and driving wheels, i.e. high kinematic accuracy. For reversible reference gears, the lateral gap in the gear and the fluctuation of this gap are very significant.

High-speed gears include gears of turbine gearboxes, engines of turboprop aircraft, kinematic chains of various gearboxes, etc. The peripheral speeds of the gears of such gears reach 90 m/s with a relatively large transmitted power. Under these conditions, the main requirement for a gear transmission is smooth operation, i.e. noiselessness, absence of vibrations and cyclic errors repeated many times per wheel revolution. As the rotation speed increases, the requirements for smooth operation increase. For heavily loaded high-speed gears, the completeness of tooth contact is also important. The wheels of such gears usually have medium modules (from 1 to 10 mm).

Power transmissions include gears that transmit significant torques at low speeds. These are gear drives of gear stands of rolling mills, mechanical rollers, hoisting and transport mechanisms, gearboxes, gearboxes, rear axles, etc. The main requirement for them is complete tooth contact. Wheels for such gears are made with a large module (over 10 mm) and a long tooth length.

A separate group is formed by general-purpose gears, which are not subject to increased operational requirements for kinematic accuracy, smooth operation and tooth contact (for example, towing winches, non-critical wheels of agricultural machines, etc.).

Errors that arise when cutting gears can be reduced to four types: tangential, radial, axial processing errors and errors of the tool’s producing surface. The combined manifestation of these errors during gear processing causes inaccuracies in the size, shape and location of the teeth of the processed gears. During the subsequent operation of the gear as a transmission element, these inaccuracies lead to uneven rotation, incomplete contact of tooth surfaces, uneven distribution of lateral clearances, which causes additional dynamic loads, heating, vibration and noise in the transmission.

To ensure the required transmission quality it is necessary to limit, i.e. normalize errors in the manufacture and assembly of gears. For this purpose, tolerance systems were created that regulate not only the accuracy of an individual wheel, but also the accuracy of gears based on their service purpose.

Tolerance systems for various types of gears (cylindrical, bevel, worm, rack and pinion) have much in common, but there are also features that are reflected in the relevant standards. The most common are cylindrical gears, the tolerance system of which is presented in GOST 1643-81.

Metric thread is a screw thread on the external or internal surfaces of products. The shape of the protrusions and depressions that form it is an isosceles triangle. This thread is called metric because all its geometric parameters are measured in millimeters. It can be applied to surfaces of both cylindrical and conical shapes and used for the manufacture of fasteners for various purposes. In addition, depending on the direction of rise of the turns, metric threads can be right-handed or left-handed. In addition to metric, as is known, there are other types of threads - inch, pitch, etc. A separate category is made up of modular threads, which are used for the manufacture of worm gear elements.

Main parameters and areas of application

The most common is metric thread, applied to external and internal surfaces cylindrical. This is what is most often used in the manufacture of various types of fasteners:

• anchor and regular bolts;
• nuts;
• hairpins;
• screws, etc.

Conical-shaped parts, on the surface of which a metric type thread is applied, are required in cases where the created connection must be given high tightness. The metric thread profile applied to the conical surfaces allows the formation of tight connections even without the use of additional sealing elements. That is why it is successfully used in the installation of pipelines through which transport different environments, as well as in the manufacture of stoppers for containers containing liquid and gaseous substances. It should be kept in mind that the metric thread profile is the same on cylindrical and conical surfaces.

Types of threads belonging to the metric type are distinguished according to a number of parameters, which include:

• dimensions (diameter and thread pitch);
• direction of rise of turns (left or right thread);
• location on the product (internal or external thread).

There are also additional parameters, depending on which metric threads are divided into different types.

Geometric parameters

Let's consider the geometric parameters that characterize the main elements of metric threads.

• The nominal thread diameter is designated by the letters D and d. In this case, the letter D refers to the nominal diameter of the external thread, and the letter d refers to a similar parameter of the internal thread.
• The average diameter of the thread, depending on its external or internal location, is designated by the letters D2 and d2.
• The internal diameter of the thread, depending on its external or internal location, is designated D1 and d1.
• The inside diameter of the bolt is used to calculate the stresses created in the structure of such a fastener.
• The thread pitch characterizes the distance between the crests or valleys of adjacent threaded turns. For a threaded element of the same diameter, a basic pitch is distinguished, as well as a thread pitch with reduced geometric parameters. The letter P is used to denote this important characteristic.
• The thread lead is the distance between the crests or valleys of adjacent threads formed by the same helical surface. The progress of the thread, which is created by one screw surface (single-start), is equal to its pitch. In addition, the value to which the thread stroke corresponds characterizes the amount of linear movement of the threaded element performed by it per revolution.
• A parameter such as the height of the triangle that forms the profile of the threaded elements is designated by the letter H.

Table of metric thread diameter values ​​(all parameters are indicated in millimeters)

Metric thread diameters (mm)

Complete table of metric threads according to GOST 24705-2004 (all parameters are indicated in millimeters)

Complete table of metric threads according to GOST 24705-2004

The main parameters of metric threads are specified in several regulatory documents.

This standard contains requirements for the parameters of thread pitch and diameter. GOST 8724, the current version of which came into force in 2004, is an analogue of the international standard ISO 261-98. The requirements of the latter apply to metric threads with a diameter of 1 to 300 mm. Compared to this document, GOST 8724 is valid for a wider range of diameters (0.25–600 mm). IN currently the current edition of GOST 8724 2002, which came into force in 2004 instead of GOST 8724 81. It should be borne in mind that GOST 8724 regulates certain parameters of metric threads, the requirements for which are also specified by other thread standards. The convenience of using GOST 8724 2002 (as well as other similar documents) is that all the information in it is contained in tables, which include metric threads with diameters within the above range. Both left-handed and right-handed metric threads must meet the requirements of this standard.

This standard stipulates what basic dimensions a metric thread should have. GOST 24705 2004 applies to all threads, the requirements for which are regulated by GOST 8724 2002, as well as GOST 9150 2002.

This is a regulatory document that specifies the requirements for the metric thread profile. GOST 9150, in particular, contains data on what geometric parameters the main threaded profile of various standard sizes must correspond to. The requirements of GOST 9150, developed in 2002, as well as the two previous standards, apply to metric threads, the turns of which rise from the left upward (right-handed type), and to those whose helical line rises to the left (left-handed type). The provisions of this normative document closely echo the requirements given by GOST 16093 (as well as GOSTs 24705 and 8724).

This standard specifies the tolerance requirements for metric threads. In addition, GOST 16093 prescribes how metric type threads should be designated. GOST 16093 in the latest edition, which came into force in 2005, includes the provisions of international ISO standards 965-1 and ISO 965-3. Both left-hand and right-hand threads fall under the requirements of such a regulatory document as GOST 16093.

The standardized parameters specified in the metric thread tables must correspond to the thread dimensions in the drawing of the future product. The choice of the tool with which it will be cut should be determined by these parameters.

Designation rules

To indicate the tolerance range of an individual metric thread diameter, a combination of a number is used, which indicates the accuracy class of the thread, and a letter, which determines the main deviation. The thread tolerance field should also be indicated by two alphanumeric elements: in the first place - tolerance field d2 (middle diameter), in the second place - tolerance field d (outer diameter). If the tolerance fields of the outer and middle diameters coincide, then they are not repeated in the designation.

According to the rules, the thread designation is affixed first, followed by the tolerance zone designation. It should be borne in mind that the thread pitch is not indicated in the markings. You can find out this parameter from special tables.

The thread designation also indicates which screw length group it belongs to. There are three such groups:

• N – normal, which is not indicated in the designation;
• S – short;
• L – long.

The letters S and L, if necessary, follow the tolerance zone designation and are separated from it by a long horizontal line.

It is also necessary to indicate such an important parameter as the fit of the threaded connection. This is a fraction formed as follows: the numerator contains the designation of the internal thread related to its tolerance field, and the denominator contains the designation of the tolerance field for external threads.

Tolerance fields

Tolerance fields for a metric threaded element can be one of three types:

• accurate (threads are made with such tolerance fields, the accuracy of which is required high requirements);
• medium (group of tolerance fields for thread general purpose);
• rough (with such tolerance fields, thread cutting is performed on hot-rolled rods and in deep blind holes).

Thread tolerance fields are selected from special tables, and the following recommendations must be adhered to:

• First of all, the tolerance fields highlighted in bold are selected;
• in the second – tolerance fields, the values ​​of which are written in the table in light font;
• in the third - tolerance fields, the values ​​​​of which are indicated in parentheses;
• the fourth (for commercial fasteners) contains tolerance fields, the values ​​of which are contained in square brackets.

In some cases, it is allowed to use tolerance fields formed by combinations d2 and d that are not in the tables. Tolerances and maximum deviations for the threads on which the coating will subsequently be applied are taken into account in relation to the dimensions of the threaded product not yet treated with such a coating.

Tolerances of metric threads with large and small pitches for diameters 1-600 mm are regulated by GOST 16093-2004.

The thread is completely determined by five parameters: three diameters, pitch and angle of inclination of the sides. However, only the average diameter (for a bolt and nut), outer diameter (1 (for external threads - bolt) and internal diameter /), (for internal threads - nuts) are standardized by tolerances.

Landings with clearance

The standard regulates the degrees of accuracy that determine the tolerance values ​​for the diameters of external and internal threads (Table 5.53), as well as the series of main deviations (upper for bolts and lower for nuts) (Table 5.54).

The main deviations that determine the position of the tolerance fields relative to the nominal profile depend only on the thread pitch (except I and H). For threads with a given pitch, the deviations of the same name for all diameters (external, middle, internal) are equal.

All deviations and tolerances are measured from the nominal profile in the direction perpendicular to the thread axis (Fig. 5.101). It is customary to indicate half values ​​on diagrams, assuming that the second halves are located on diametrically opposite profiles.

The magnitudes of the main deviations are determined by the formulas:

The second maximum deviation is determined by the accepted degree of thread accuracy (еі = ех - /Ті/; еі = ех - /Ті/,; £5 = £/ + /ТО,; £5 = ЕІ + /TTL). The combination of the main deviation, designated by a letter, with the tolerance for the accepted degree of accuracy forms the tolerance field.

In table 5.55 shows the tolerance fields provided for by GOST 16093-81.

Landings can be formed by a combination of any tolerance fields given in table. 5.55. It is preferable to combine tolerance fields of the same accuracy class.

Rice. 5.101.

Make-up lengths. To select the degree of accuracy depending on the thread make-up length, three groups of make-up lengths have been established: 5-small (less than 2.24L/0-2), L^-normal (2.24L/02< Ы< 6,74Л/Л2) и ^-большие (больше 6,74А/а2) УиР-в мм). Длина свинчивания зависит от шага и диаметра резьбы.

Thread accuracy classes. The concept of accuracy classes is relative. The drawings indicate only tolerance fields, and accuracy classes are used for comparative assessment thread accuracy. The exact class is recommended for critical statically loaded threaded connections; middle class - for threads general use and rough class - when cutting threads on hot-rolled workpieces, in long blind holes, etc.

Thread tolerances. A wide range of tolerances for all diameters has been adopted to the 6th degree of accuracy. Tolerances of thread diameters for the 6th degree of accuracy with a normal make-up length are determined by the formulas:

For average bolt thread diameter -

For bolt outer diameter

For nut inner diameter

For medium nut diameter

where /° is taken in mm; th - geometric mean of the extreme values ​​of the range of nominal diameters; G - in microns.

Tolerances of other degrees of accuracy are determined by multiplying the tolerance of the 6th degree of accuracy by the following coefficients:

Tolerances on internal diameter

Preference fits

Interference fits along the average diameter are used in cases where the design of the assembly does not allow the use of a bolt-nut threaded connection due to possible self-unscrewing during operation under the influence of external factors (vibrations, temperatures, etc.).

The location of the tolerance fields for the thread diameter with interference is shown in Fig. 5.102.

Interference fits are provided only in the hole system.

The tolerance for the average thread diameter of parts sorted into groups is the tolerance for the actual average diameter (in contrast to threads with a gap, where the tolerance for the average diameter is total), and those not sorted into groups are total.

Rice. 5.102.

Tolerances for the internal diameter of external threads are not established. It is limited by the maximum deviations of the shape of the thread cavities.

To form tolerance fields, the main deviations and degrees of accuracy are used. In threads with interference, the following main deviations are established, depending on the thread pitch and the degree of diameter accuracy (Table 5.56).

The tolerance fields for interference fits are given in Table. 5.57.

For threads with interference, permissible deviations in the shape of external and internal threads are also established, which are determined by the difference between the largest and smallest actual values ​​of the average diameter. Their value should not exceed 25% of the average diameter tolerance.

The standard also establishes deviations of the pitch and angle of inclination of the side of the profile, which relate to standard make-up lengths (Table 5.58).

Deviations in thread shape, pitch and angle deviations are not subject to mandatory control unless specifically stated.

Transitional landings

Tolerances of metric threads for transitional fits are established for steel parts with external threads with diameters from 5 to 45 mm, mating with internal threads in steel parts with a make-up length / = (I...1.25)4 cast iron with / = (1, 25...1.5)

Tolerance fields and their combinations for obtaining transitional fits are given in table. 5.59, and the layout of tolerance fields in Fig. 5.103.

Transitional fits are used for simultaneous jamming of threads (the most common method of jamming is tightening the threaded rod against the thread run in parts with internal threads). To avoid thread deformation, a conical countersink is provided in the hole.

The numerical values ​​of the main deviations of the average diameter of the external thread are calculated using the formulas:

In the formula, the value of P is substituted in mm, and the value of e/ is obtained in microns.

Rice. 5.103.

Calculated values ​​are rounded to the nearest preferred numbers in the Da40 series.

Tolerances of average diameters of external and internal threads are determined by the formulas:

where a1 is the geometric mean of individual values ​​of the intervals of nominal thread diameters according to GOST 16093-2004 in mm, P - in mm, T - in microns.

For threads in transitional fits, as well as in interference fits, permissible deviations in the shape of external and internal threads are established, determined by the difference between the largest and smallest actual values ​​of the average diameter. They should not exceed 25% of the average diameter tolerance. The standard also establishes deviations of the pitch and angle of inclination of the side of the profile, which relate to standard make-up lengths (see Table 5.58). Deviations of the thread shape, deviations of the pitch and angle of inclination are not subject to mandatory control, unless specifically stated.

Thread accuracy classes

Make-up length

Degrees of thread accuracy

The standard establishes eight degrees of thread accuracy, for which tolerances are established. The degrees of accuracy are designated by the numbers 3, 4, 5, ..., 10 in descending order of accuracy. For the diameters of external and internal threads, degrees of accuracy are established as follows.

Degree of accuracy

Bolt diameter (male thread) for make-up lengths

outer diameter, d…………4; 6; 8,

average diameter d 2 …………… 3; 4; 5; 6; 7; 8; 9; 10.

Nut diameter(internal thread)

internal diameter D 1 ……… 4; 5; 6; 7; 8,

average diameter D 2 ………….. 4; 5; 6; 7; 8; 9.

To determine the degree of accuracy depending on the thread make-up length and accuracy requirements, three groups of make-up lengths have been established: S – small; N – normal; L – long make-up lengths. Make-up lengths from 2.24Р d 0.2 to 6.7Р d 0.2 belong to the normal group N. Make-up lengths less than 2.24Р d 0.2 belong to the small (S) group, and more than 6.7Р ·d 0.2 belong to the group of large (L) make-up lengths. In the calculation formulas, the make-up lengths P and d are in mm.

There are three accuracy classes for threads: fine, medium and coarse. The division of threads into accuracy classes is arbitrary. The drawings and calibers indicate not accuracy classes, but tolerance fields. Accuracy classes are used for comparative assessment of thread accuracy. Exact class recommended for critical threaded connections experiencing static loads, as well as in cases requiring small fluctuations in the nature of the fit. Middle class Recommended for general threads. Rough class used when cutting threads on hot-rolled workpieces, in long blind holes, etc. With the same accuracy class, the average diameter tolerances for make-up length L (long) must be increased, and for make-up length S (small) reduced by one degree according compared to tolerances for normal make-up length. For example, for the make-up length S take the 5th degree of accuracy, then for the normal make-up length N it is necessary to take the 6th degree of accuracy, and for a long make-up length L - the 7th degree of accuracy.

The thread tolerance field consists of a number indicating the degree of accuracy and a letter indicating the main deviation (for example, 6g, 6H, 6G, etc.). When designating combinations of tolerance fields for the average diameter and for d or D 1, it consists of two tolerance fields for the average diameter (in first place) and for d or D 1. For example, 7g6g (where 7g – tolerance range for the average diameter of the bolt, 6g – tolerance range for the outer diameter of the bolt d), 5Н6Н (5Н – tolerance range for the average diameter of the nut, 6Н – tolerance range for the internal diameter of the nut D 1). If the tolerance fields of the outer diameter of the bolt and the internal diameter of the nut coincide with the tolerance field of the middle diameter, then they are not repeated (for example, 6g, 6H). The designation of the thread tolerance field is indicated after specifying the part size: M12 – 6g (for a bolt), M12 – 6H (for a nut). If a bolt or nut is made with a pitch different from the normal pitch, then the pitch is indicated in the thread designation: M12x1 - 6g; M12x1 – 6H.

The designation of landings of threaded parts is made with a fraction. The numerator indicates the tolerance range of the nut (internal thread), and the denominator indicates the tolerance range of the bolt (external thread). For example, M12 x 1 – 6H / 6g. If the thread is left-handed, then the index LH (М12х1хLH – 6H/6g) is entered into its designation. The make-up length is entered into the thread designation only if it differs from normal. In this case, indicate its value. For example, М12х1хLH – 6H/6g – 30 (30 – make-up length, mm).