Limit deviations of the outer diameter of the thread. Designation of accuracy and fit of metric threads

Thread accuracy class

According to GOST 9253-59, three accuracy classes are established for all metric threads, and as an exception 2a (only for fine pitch threads).

The most accurate carving of the 1st class. In tractors and cars, threads of the 2nd and 3rd classes are used. In the drawings, the thread class is affixed after the step. 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 for fine threads, six degrees of accuracy were established, which are indicated by letters:

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

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

Degrees of accuracy with; d (C; D) roughly correspond to grade 1; e; f (E; F) - 2nd grade; h; k (H; K) - 3rd grade.

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

For an inch thread 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 by limiting thread gauges having two sides:

Checkpoint (denoted "PR");

Impassable (indicated by "NOT").

The pass side for all thread accuracy classes is the same. The impassable side corresponds to a certain class of thread accuracy, which is indicated by a corresponding stamp on the end of the caliber.

Degrees of accuracy of thread diameters GOST 16093-81

Thread type	Thread diameter	Degree of accuracy
Bolt	outer d
	average d 2	3, 4. 5, 6, 7, 8, 9, 10
screw	average D 2	4, 5, 6, 7, 8, 9*
	interior D 1
* Only for threads on plastic parts

Make-up lengths according to GOST 16093-81

carving Р, mm	Nominal thread diameterd according to GOST 8724-81, mm	MAKE-UP LENGTH, mm
		(small)	(normal)	(large)
	St. 2.8 to 5.6 St. 5.6 to 11.2 St. 11.2 to 22.4		St. 1.5 to 4.5 St. 1.6 to 4.7 St. 1.8 to 5.5
	St. 2.8 to 5.6 St. 5.6 to 11.2 St. 11.2 to 22.4 St. 22.4 to 45.0		St. 2.2 to 6.7 St. 2.4 to 7.1 St. 2.8 to 8.3 St. 3.1 to 9.5
	St. 5.6 to 11.2 St. 11.2 to 22.4 St. 22.4 to 45.0 Over 45.0 to 90.0		Over 3.0 to 9.0 Over 3.8 to 11.0 Over 4.0 to 12.0 St. 4.8 to 14.0
	St. 5.6 to 11.2 St. 11.2 to 22.4		Over 4.0 to 12.0 St. 4.5 to 13.0
	St. 5.6 to 11.2 St. 11.2 to 22.4 St. 22.4 to 45.0 Over 45.0 to 90.0		Over 5.0 to 15.0 St. 5.6 to 16.0 St. 6.3 to 19.0 St. 7.5 to 22.0
	St. 11.2 to 22.4		St. 6.0 to 18.0
	St. 11.2 to 22.4 St. 22.4 to 45.0 Over 45.0 to 90.0		St. 8.0 to 24.0 St. 8.5 to 25.0 St. 9.5 to 28.0
	St. 11.2 to 22.4		Over 10.0 to 30.0
	St. 22.4 to 45.0 Over 45.0 to 90.0 St. 90.0 to 180.0 St. 180 to 355.0		St. 12.0 to 36.0 St. 15.0 to 45.0 St. 18.0 to 53.0 Over 20.0 to 60.0

The concept of the reduced average thread diameter

Reduced average thread diameter called average diameter of an imaginary perfect 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 (without mutual displacement or interference) contact with the real thread on the flanks of the thread.

In short, reduced average thread diameter is the average diameter of an ideal threaded element that connects to a real thread. When talking about the reduced average thread diameter, do not think of it as the distance between two points. This is the diameter of a conditional ideal thread, which in reality does not exist as a material object and which could curl up 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 reduced average diameter, it is necessary to separately measure the values ​​of the thread parameters that prevent screwing and calculate this diameter.

In the manufacture of a thread, the deviations of individual thread elements depend on the errors of the individual components of the technological process. So, the error of the thread pitch, processed on threading machines, mainly depends on the error of the lead screw of the machine, the angle of the profile - on the inaccuracy of filling the angle of the tool and its installation relative to the thread axis.

It must be remembered that threaded surfaces of bolts and nuts never touch over the entire helical 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 official purpose. Therefore, it seems possible to change the average diameter of a bolt or nut and achieve make-up with errors in pitch and profile, while the thread contact will be, but not over the entire surface. For some profiles (with a pitch error) or in certain sections of the profile (with profile errors), as a result of compensating for these errors by changing the average diameter, there will be a gap in several junction points. Often only 2 - 3 turns are in contact along the threaded elements.

Step error compensation 5P. Thread pitch error is usually "in-pitch", and progressive error, sometimes referred to as "stretch" pitch. Error compensation is performed for progressive error. Two axial sections of the bolt and nut are superimposed on each other. These threaded elements do not have equal pitch values ​​along the make-up length, and therefore, make-up 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 the nut or decrease the average diameter of the bolt. After this, make-up will occur, although contact will occur only on the outermost profiles.

Thus, if there is a pitch error of 10 µm, 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 µm, and then the pitch errors will be compensated and the threaded elements of the parts will be screwed together.

Profile angle error compensation Sa/l. The error of the profile angle or the angle of inclination of the lateral side usually arises from the error of the profile of the cutting tool or the error of its installation on the machine relative to the axis of the workpiece. Thread profile error compensation is also performed by changing the value of the average diameter, i.e. an increase in the average diameter of the nut or a decrease in the average diameter of the bolt. If you remove the 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 on a limited section of the side of the profile. Such contact is sufficient for make-up to occur, i.e. fastening two parts. Thus, the requirement for thread accuracy in relation to the average diameter is normalized by the total tolerance, which limits both the reduced average diameter (the diameter of an ideal thread that provides screwing) and the average thread diameter (actual average diameter). The standard only mentions that the tolerance for the average diameter is total, but there is no decoding of this concept. For this admission, the following additional interpretations can be given.

1. For internal threads (nuts), the reduced average diameter Should not be less than the size corresponding to the maximum material limit (often said - the passage limit), and the largest average diameter (actual average diameter) should not be greater than the minimum material limit (often said - impassable limit). The value of the reduced average diameter for the internal thread is determined by the formula.

2. For an external thread (bolt), the reduced mean diameter must not be greater than the maximum mean material diameter limit, and the smallest mean mean diameter proper at any location must 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 original position can be thought of as a thread coaxial with the real thread, but for a bolt much larger in diameter. If now the ideal thread is gradually compressed (the average diameter decreases) until it is in 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 given in the standard for the average diameter of the bolt (Tch) and nut (TD2) actually include the tolerances for the average diameter itself (Tch), (TD2) and the value of the 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 maybe a new name, which will make it possible to distinguish this tolerance from the tolerance only for the average diameter.

When manufacturing a thread, a 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 for accuracy, the machines can be set with smaller tolerances per step and larger tolerances for the angle and average diameter, etc.

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

System of tolerances and landings of metric threads with a gap.

The most common, which has received the widest application, is a metric thread with a gap for a diameter range from 1 to 600 mm, the tolerance and fit system 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 for individual thread parameters, designation of accuracy and fit of metric threads in drawings, control of metric threads and other issues of the system are common to all varieties of metric threads, although each of them has its own characteristics, sometimes significant, which are reflected in the relevant GOSTs.

Degrees of accuracy and accuracy classes of threads. Metric thread is defined by five parameters: average, outer and inner diameters, pitch and angle of the thread profile.

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

In accordance with established practice, the degrees of accuracy are grouped into 3 classes of accuracy: exact, medium and coarse. The concept of accuracy class is conditional. When assigning degrees of accuracy to the accuracy class, the length of the make-up is taken into account, since in manufacturing the difficulty of ensuring the specified accuracy of the thread depends on the length of the make-up that it has. There are three groups of make-up lengths: S - short, N - normal and L - long.

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

Approximate correspondence of accuracy classes and degrees of accuracy is as follows: - the exact class corresponds to 3-5 degrees of accuracy; - the middle class corresponds to the 5-7th degree of accuracy; - rough class corresponds to 7-9th degrees of accuracy.

The initial degree of accuracy for calculating the numerical values ​​of the tolerances of the diameters of the 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 gears and gears are regulated by GOST 16531-83. According to the shape and arrangement of the gear teeth, spur gears are divided into the following types: rack and pinion, spur, helical, chevron, involute, cycloid, etc. Novikov gears, which have a high bearing capacity, are increasingly being used in industry. The profile of the teeth of the wheels of these gears is outlined by arcs of circles.

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

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

Speed ​​gears include gears of turbine gearboxes, engines of turboprop aircraft, kinematic chains of various gearboxes, etc. The circumferential 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 is smooth operation, i.e. noiselessness, absence of vibrations and cyclic errors, repeatedly repeated per wheel revolution. With an increase in the speed of rotation, 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 trains of gear stands of rolling mills, mechanical rollers, hoisting and transport mechanisms, gearboxes, gearboxes, rear axles, etc. The main requirement for them is the completeness of contact of the teeth. Wheels for such gears are made with a large module (over 10 mm) and a large tooth length.

A separate group is formed by general purpose gears that 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 occur when cutting gears can be reduced to four types: tangential, radial, axial processing errors and tool producing surface errors. The joint manifestation of these errors during gear processing causes inaccuracies in the size, shape and location of the teeth of the gears being machined. During the subsequent operation of the gear wheel as a transmission element, these inaccuracies lead to uneven rotation, incomplete fit of the tooth surfaces, uneven distribution of side 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 the errors in the manufacture and assembly of gears. To this end, tolerance systems were created that regulate not only the accuracy of an individual wheel, but also the accuracy of gears based on their official 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 spur gears, the tolerance system of which is presented in GOST 1643-81.

Thread accuracy classes

Make-up length

Thread Accuracy Degrees

The standard establishes eight degrees of thread accuracy, on which tolerances are set. The degrees of accuracy are indicated by the numbers 3, 4, 5, ..., 10 in descending order of accuracy. According to the diameters of the external and internal threads, the degrees of accuracy are set 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)

inner 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 length of the thread make-up and the requirements for accuracy, three groups of make-up lengths are established: S - small; N - normal; L - large 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 group (S), and more than 6.7Р ·d 0.2 belong to the group of large (L) make-up lengths. In the calculation formulas, the lengths of make-up P and d are in mm.

There are three accuracy classes installed on the threads: fine, medium and coarse. The division of threads into accuracy classes is conditional. The drawings and gauges do not indicate accuracy classes, but tolerance fields. Accuracy classes are used for comparative assessment of thread accuracy. Exact class recommended for critical threaded connections that are under static load, as well as in cases requiring small fluctuations in the nature of the fit. Middle class recommended for general purpose threads. Rough class it is used when threading hot-rolled blanks, in long blind holes, etc. With the same accuracy class, the tolerances of the average diameter with the make-up length L (large) must be increased, and with the make-up length S (small) reduced by one degree according to 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 length of the make-up N, you need to take the 6th degree of accuracy, and for the 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 the first place) and for d or D 1. For example, 7g6g (where 7g - tolerance field for the average diameter of the bolt, 6g - tolerance field for the outer diameter of the bolt d), 5Н6Н (5Н - tolerance field for the average diameter of the nut, 6Н - tolerance field for the inner diameter of the nut D 1). If the tolerance fields of the outer diameter of the bolt and the inner diameter of the nut coincide with the tolerance field of the average diameter, then they are not repeated (for example, 6g, 6H). The designation of the thread tolerance field is indicated after specifying the size of the part: M12 - 6g (for a bolt), M12 - 6H (for a nut). If the bolt or nut is made with a pitch that differs from the normal pitch, then the pitch is indicated in the thread designation: M12x1 - 6g; M12x1 - 6H.

Designation of landings of threaded parts is made by fraction. The numerator indicates the tolerance field of the nut (internal thread), and the denominator indicates the tolerance field of the bolt (external thread). For example, M12 x 1 - 6H / 6g. If the thread is left-handed, then the index LH (M12x1xLH - 6H / 6g) is entered into its designation. The make-up length is entered in the thread designation only if it differs from the normal one. In this case, indicate its value. For example, M12x1xLH - 6H / 6g - 30 (30 - make-up length, mm).

Landing threaded connections are with a gap, with interference And transitional. Please note that cylindrical joints also have clearance, interference and transition fits.

For the formation of an appropriate landing, the following tolerance fields are established by the standard, which are given in tables 42, 43 and 44. The same tables set out the features and areas of application of these landings.

Nominal thread profile- the profile of the external and internal thread, which is determined by the nominal dimensions of its linear and angular elements and to which the nominal dimensions of the external, middle and internal thread diameters refer.

The main means of control of threaded products

Threaded products are controlled mainly by limit gauges (complex method). The kit for the control of cylindrical threads includes working through and non-through limit gauges. Checkpoints threaded gauges must be screwed with a threaded product (Table 41). They control the given average and outer (for nuts) or inner (for bolts) thread diameters. impassable threaded gauges control the actual average diameter.

Element control threaded products (differentiated method) is used mainly for precise threads: plug gauges, threading tools, etc. At the same time, the average diameter, pitch and half of the profile angle α are checked separately using universal and specialized devices. For example, the average diameter is measured on a universal and instrumental microscope, using the method of three or two wires on contact devices, and a threaded micrometer.

The thread pitch and half the angle of the profile are measured on microscopes, projectors, etc.

Thread designations

(decoding of the thread symbol)

A specialist, deciphering the symbol of the thread, can get almost all the parameters of the thread or threaded connection. This section provides examples of deciphering the symbol for specific examples of threads and threaded connections.

1. Thread M12-6g. The thread is metric, since the letter M is in front. The thread is external, since the main deviation in is indicated by a line in Latin letters. Nominal (outer) diameter d=12 mm. A thread with a large pitch, since the thread pitch is not indicated in the symbol. The thread is single-start, since the number of starts is not indicated in the symbol. Right-handed thread, as the symbol is not indicated in the symbol LH. The thread has a normal make-up length, since the symbol does not specify the make-up length of the thread. The thread is made to form a clearance fit, since the main deviation g serves to form a fit with a gap (Table 41).

Tolerance field, medium diameter - T d 2 and outside diameter T d are the same and are 6 g. The fact is, if the tolerance field of the average and outer diameters are the same, then the tolerance field is indicated once in the symbol. The tolerances of the average and outer diameters are assigned according to the 7th degree of accuracy.

2. Thread M12-6N. Nominal (outer) thread diameter D=12 mm. The thread is internal, since, the main deviation H indicated by a capital Latin letter. Please note, according to the main deviation H it is not possible to determine which fit the thread was made to form, since the main deviation H used in the formation and landings with a gap, with an interference fit and transitional. If there were major deviations G And D, then it would be immediately clear that the thread is made to form a fit with a gap. Since these deviations are designed to form a fit with a gap.

Tolerance field of average - T D 2 and outdoor - T D diameters are the same and are 6H. The fact is, if the tolerance field of the average and outer diameters are the same, then the tolerance field is indicated once in the symbol. The tolerances of the average and outer diameters are assigned according to the 6th degree of accuracy. The remaining parameters are the same as in the first option.

3. Thread M12 - 7g6 g. The carving is external. 7 g- tolerance field of the middle diameter, 6g - tolerance field of the outer diameter. The fact is, if the tolerance field for the average and outer diameters of the thread is different, then each tolerance field in the symbol is shown separately.

4. Thread M12 - 5 H6 H. The thread is internal. 5 H- tolerance field of the middle diameter, 6H - tolerance field of the outer diameter.

5. Carving M12 x1 - 6 g. Fine pitch external thread P = 1 mm.

6. Thread M12 x1 - 6 H. Internal thread with fine pitch P = 1 mm.

7. Carving М12х1LH - 6 g. The thread is external with a fine pitch, left, as the symbol indicates a thread pitch of 1 mm and a sign LH.

8. Carving M12x1 LH - 6 g. The thread is internal with a small pitch, left-hand, as the thread pitch 1 mm and the sign LH are indicated in the symbol.

9. Carving M12 - 7 g6 g - 30. The thread is metric, external, with a make-up length that differs from the nominal. Since the thread symbol indicates the length of the screwing equal to 30 mm.

Landing in a threaded connection it is indicated by a fraction, in the numerator of which the designation of the tolerance field of the internal thread is indicated, and in the denominator the tolerance field of the external thread. Please note that the fit of a smooth cylindrical joint is also indicated in the same way.

1.M12 - 6H/6 g. Conventional designation of the fit of a threaded connection with a gap, with a large pitch, since the thread pitch is not specified.

2. М12х1 - 6H/6 g. Symbol for a threaded connection with a gap, with a fine pitch, since the thread pitch is 1 mm.

3. М12х1LH - 6 H/6 g. Symbol for threaded connection with fine pitch clearance and left hand rotation, as the sign LH is indicated.

Threaded connection according to GOST 11708-82 “Basic standards of interchangeability. Thread. Terms and definitions "is called the connection of two parts using a thread, in which one of the parts has an external thread and the other has an internal thread.

Threaded connections are one of the most common types of connections. In mechanical engineering, about 80% of parts either have threaded surfaces, or they are fastened using threaded products.

Main virtues threaded connections are relatively easy assembly and disassembly and a high level of interchangeability of products.

TO shortcomings threaded connections can be attributed to the complication of design and technology (the processing of threaded surfaces requires the use of special equipment and tools, the control of parts becomes more complicated).

Depending on the profile forms threads are divided into:

Metric (with a triangular profile, the initial for which is an equilateral triangle with an angle at the top of 60 °);

inch (with a symmetrical triangular profile and an angle at the top of 55 °), usually used for pipes, pipe;

rectangular (with a rectangular profile);

Trapezoidal (with a symmetrical trapezoidal profile);

Resistant (with asymmetric trapezoidal profile);

round (with a profile formed by arcs).

In addition, threads have been developed for parts made of certain materials, for example, for plastic parts, for ceramic parts, special threads for specific types of products, for example, ocular threads, etc.

According to the functional purpose, threaded connections should be distinguished fissile("reference") and power. The first ones are designed to ensure high accuracy of linear and angular movements in measuring instruments and process equipment. So, in micrometric instruments, the main measuring transducer is a micrometric pair of screw - nut, in dividing machines, the main mechanism is also a pair of screw - nut.

Power threaded connections are designed to create significant forces when moving parts (screw presses, jacks) or to prevent mutual movement of the connected parts (lid-body connections, threaded connections of pipeline parts, fastening bushings on the shaft, etc.). The division of threaded connections into “reference” and power ones is conditional and is carried out based on the main function of the mechanism.

Depending on the nature of the functioning, there are motionless(fixing) and mobile(kinematic) threaded connections. Movable threaded connections are formed due to the use of clearance fits. In fixed joints, all types of fits can be used - with an interference fit, transitional and with a gap. In order to ensure the immobility of the threaded connection when landing with a gap, artificial methods of its selection are used (up to the creation of tightness in the connection) or additional structural elements are used that protect parts from self-unscrewing (lock washers, lock nuts, wire locks, sealants, etc.). It follows from this that in fixed threaded connections obtained by using fit with a clearance, after final assembly, tensions are possible on the working sides of the thread profile while maintaining gaps on opposite sides of the profile. In those threaded connections where transitional fits are used, interference is created using special “wedging elements” (a flat shoulder or a cylindrical pin on a stud or wedging along an incompletely cut thread profile).

In practice, metric threads are most widely used.

For metric threads standardized:

Thread profile

· nominal diameters and steps;

standards of accuracy.

Metric thread profile is regulated
GOST 9150-2002 (ISO 68-1-98) “Basic norms of interchangeability. The thread is metric. Profile".

The thread profile is based on the original thread triangle (Fig. 30) with a profile angle of 60°, the height of the original triangle H and given step R.

Rice. 30. Nominal profile of metric thread

and the main dimensions of its elements

The main dimensions of metric thread elements include:

d, D- external diameter of the external thread (bolt), external diameter of the internal thread (nut);

d 2 ,D 2 – average diameter of external thread (bolt), average diameter of internal thread (nut);

d 1 ,D 1 – internal diameter of the external thread (bolt), internal diameter of the internal thread (nut);

d 3 – inner diameter of the bolt along the bottom of the cavity;

R - thread pitch;

H - the height of the original triangle;

α – thread profile angle;

R- nominal radius of the bolt root;

H 1 = 5/8H- working height of the profile.

GOST 8724-2002 (ISO 261-98) “Basic standards of interchangeability. The thread is metric. Diameters and pitches” sets metric thread diameters from 0.25 to 600 mm and pitches from 0.075 to 6 mm.

The standard establishes 3 rows of thread diameters (when choosing a diameter, preference is given to the first row). Corresponding pitches are defined for each nominal thread diameter, which may include a coarse pitch and one or more fine pitches.

The nominal values ​​of metric thread diameters are regulated by GOST 24705-81 “Basic standards of interchangeability. The thread is metric. Main dimensions.

Thread fits standardized with clearance, with interference and transitional, which determine the nature of the connection on the sides of the threaded profile.

The system of tolerances and fits of metric threads is standardized by the following standards:

GOST 16093-81 “Basic norms of interchangeability. The thread is metric. Tolerances. Landing with a gap ";

GOST 4608-81 “Basic norms of interchangeability. The thread is metric. Landing with an interference ";

GOST 24834-81 “Basic norms of interchangeability. The thread is metric. transitional landings.

To obtain threaded fits with a gap, the tolerances of thread diameters are normalized in degrees of accuracy from 3 to 10. To normalize the position of the tolerance fields of the internal thread (nut), four main deviations are provided - H, G, F, E(Fig. 31), and for the external thread (bolt), the five main deviations are h, g, f, e, d(Fig. 32).

Rice. 31. Schemes of tolerance fields for internal threads:

a - with major deviations E, F, G;b - with the main deviation H

Rice. 32. Schemes of tolerance fields for external threads:

a - with major deviations d, e, f, g, b - with the main deviation h

For external and internal threads, in addition to degrees of accuracy, three accuracy classes are also established, conventionally called fine, medium and coarse, which include tolerances of degrees of accuracy defined by the standard.

Threads of the exact class are recommended for critical statically loaded threaded connections and, if necessary, small fluctuations in the nature of the fit. Medium grade is recommended for general purpose threads. For threads cut on hot-rolled blanks, in long blind holes, etc., the coarse grade is preferred.

GOST 16093 also establishes three groups of make-up lengths: short S, normal N and long L.

With the same accuracy class, the tolerance for the average thread diameter with the make-up length L it is recommended to increase, and with the length of the make-up S– decrease by one degree of accuracy compared to the tolerances specified for the make-up length N. These recommendations allow you to choose the accuracy of the thread depending on the design and technological requirements.

The correspondence of the tolerance fields of external and internal threads to accuracy classes and make-up lengths is given in Table. 23.

Table 23

Tolerance classes of threaded surfaces

Metric thread is a screw thread on the outer or inner 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 the rise of the turns, the metric type thread is right or left. In addition to metric, as you know, there are other types of threads - inch, pitch, etc. A separate category is the modular thread, which is used for the manufacture of worm gear elements.

Key Parameters and Applications

The most common is the metric thread applied to the outer and inner surfaces of a cylindrical shape. It is she who is most often used in the manufacture of fasteners of various types:

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

Parts of a conical shape, on the surface of which a metric-type thread is applied, are required in cases where the connection being created needs to be given high tightness. The profile of the metric thread 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 various media are transported, as well as in the manufacture of plugs for containers containing liquid and gaseous substances. Keep in mind that the thread profile of the metric type is the same on cylindrical and conical surfaces.

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

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

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

Geometric parameters

Consider the geometric parameters that characterize the main elements of the metric type thread.

• The nominal thread diameter is denoted by the letters D and d. In this case, the letter D means the nominal diameter of the external thread, and the letter d means the same parameter of the internal thread.
• The average thread diameter, depending on its external or internal location, is indicated 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 inner diameter of the bolt is used to calculate the stresses generated in the structure of such a fastener.
• The thread pitch characterizes the distance between the tops or troughs of adjacent threaded turns. For a threaded element of the same diameter, a main 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 stroke is the distance between the tops or troughs of adjacent turns formed by one helical surface. The thread lead, which is created by one helical 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 in one revolution.
• A parameter such as the height of the triangle that forms the profile of the threaded elements is denoted by the letter H.

Table of values ​​for metric thread diameters (all parameters are in millimeters)

Metric Thread Diameter Values ​​(mm)

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

Complete table of metric threads according to GOST 24705-2004

The main parameters of the metric type thread are specified by several regulatory documents.

GOST 8724

This standard contains requirements for the parameters of the thread pitch and its diameter. GOST 8724, the current version of which entered 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). At the moment, the revision of GOST 8724 2002, which entered into force in 2004 instead of GOST 8724 81, is relevant. It should be borne in mind that GOST 8724 regulates certain parameters of a metric thread, the requirements for which are stipulated 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 that include metric threads with diameters that are in the above range. Both left-hand and right-hand metric type threads shall comply with the requirements of this standard.

GOST 24705 2004

This standard specifies what the main dimensions of the metric thread should be. GOST 24705 2004 applies to all threads, the requirements for which are regulated by GOST 8724 2002, as well as GOST 9150 2002.

GOST 9150

This is a regulatory document that specifies the requirements for a metric thread profile. GOST 9150, in particular, contains data on what geometric parameters the main threaded profile of various sizes should 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 up (right-hand type), and to those whose helix rises to the left (left-hand type). The provisions of this regulatory document are closely related to the requirements given by GOST 16093 (as well as GOSTs 24705 and 8724).

GOST 16093

This standard specifies tolerance requirements for metric threads. In addition, GOST 16093 prescribes how the designation of a metric type thread should be carried out. GOST 16093 in the latest edition, which came into force in 2005, includes the provisions of the international standards ISO 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 indicated in the metric type thread tables must correspond to the thread sizes 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 field of a particular diameter of a metric thread, a combination of a number is used that indicates the accuracy class of the thread, and a letter that determines the main deviation. The thread tolerance field must also be indicated by two alphanumeric elements: in the first place - the tolerance field d2 (medium diameter), in the second place - the tolerance field d (outer diameter). In the event that 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 designation of the tolerance field. It should be borne in mind that the thread pitch in the marking is not indicated. You can find out this parameter from special tables.

The designation of the thread also indicates which group it belongs to by the length of the make-up. 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 designation of the tolerance zone and are separated from it by a long horizontal line.

Be sure to indicate such an important parameter as the fit of the threaded connection. This is a fraction formed as follows: in the numerator, the designation of the internal thread is affixed, referring to the field of its tolerance, and in the denominator - the designation of the tolerance field for the thread of the external type.

Tolerance fields

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

• precise (with such tolerance fields, a thread is made, the accuracy of which is highly demanded);
• medium (a group of tolerance fields for general purpose threads);
• rough (with such tolerance fields, threading is performed on hot-rolled bars and in deep blind holes).

Tolerance fields for threads are selected from special tables, while the following recommendations must be followed:

• first of all, the tolerance fields in bold are selected;
• in the second - tolerance fields, the values ​​\u200b\u200bof which are entered in the table in light font;
• in the third - tolerance fields, the values ​​\u200b\u200bof which are indicated in parentheses;
• in the fourth (for commercial fasteners) - tolerance fields, the values ​​​​of which are contained in square brackets.

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