Threads and threaded connections - Engineering graphics

Thread and thread connections

Many parts of machines and devices are threaded. The thread surface forms a flat contour when screwing along a cylindrical or conical surface. In this case, different sections of the plane contour can form different co-axial helical surfaces - straight lines (see Figures 8.8, 8.9), oblique (see Figure 8.10) or other shapes. The most common are cylindrical and tapered threads, that is, threads formed on cylindrical or conical surfaces (details).

Fig. 14.2

Threaded connection is the joining of parts by thread, ensuring their relative immobility or the movement of one part relative to the other. In the threaded connection, one of the parts has an external thread, the other - an internal thread.

Outer thread is a thread formed on an outer cylindrical or conical surface. In a threaded connection, the external thread is a male surface, and the part bearing it is called the "bolt" (screw, etc.).

The formation of the external thread, for example, by cutting the cutter, is illustrated in Fig. 14.2. If the cutter, which moves uniformly along the generatrix, is deepened into a uniformly rotating workpiece, a helical surface is formed on its surface; The appearance of this surface depends on the shape of the tool. For example, in Fig. 14.2, a the thread has a trapezoidal profile, and in Fig. 14.2, b is a triangular one.

In the parts drawings, the external thread is shown conditionally: by solid main lines along the outer diameter of the thread and by continuous thin lines along the inner diameter - along the border of the depressions - Fig. 14.2, in with the notation considered below. In the plane of projections perpendicular to the axis of the thread, a thin line along the border of the valleys is made open at any place on the section near & frac14; circle, but one should not start and end a break on the center lines.

Fig. 14.3

In addition to threading with thread-cutting tools on screw-cutting lathes, it can be cut into dies (Figure 14.3), rolled with rolling rolls or dies.

If, at the end of the thread, the cutter is smoothly retracted from the workpiece, a section of the incomplete profile in the thread transition zone to the smooth part of the workpiece is obtained (Figure 14.4). Such a section is called a thread run (in Figure 14.4 thread run off at the length x). At the points of transition from the threaded section to the end of the part, when cutting the thread, thread profile (see Figure 14.3, left of the die). This section is called undercut . It must be taken into account when designing connections.

In those cases where the tight fitting of the ends of the parts in the threaded joint is necessary, a cylindrical groove is made on one of the parts (their shape and dimensions are discussed below).

Internal thread is a thread formed on an inner cylindrical or conical surface. In the threaded connection, the female thread is the female surface and is called the "nuts" (nest, etc.).

Tapping the internal thread The internal thread is cut with a chisel or with a special tapping tool (Figure 14.5, c, d). Thread cutting in through holes is relatively simple . It is more difficult to thread in blind blind holes. Such a threaded hole is called a socket .

Fig. 14.4

Fig. 14.5

The threading sequence in the socket is shown in Fig. 14.5:

a - drilling a hole (nest) and chamfering;

b - a hole ready for threading;

in - thread tapping;

r - threaded socket (cut);

- a threaded socket (cut), depicted in the drawing.

The diameter d, of the drill is selected according to the technological standards, depending on the size of the thread, approximately corresponds to the internal diameter of the thread. The length/is the total length of the cylindrical part of the hole. The bottom of the nest, formed by the cutting part of the drill, is conditionally represented as a cone with an apex angle of 120 °. The depth of the hole to be drilled depends on the length of the thread with the full profile (to be cut) and the amount of thread run. In turn, on the details, the length of thread with a full profile is selected depending on the material of the part (steel, aluminum, bronze, etc.).

The sharp edge at the end of the hole is machined onto a cone with an apex angle of 90 (this conical indentation is called a chamfer). The size of the chamfer is shown in Fig. 14.5, 6. The presence of a chamfer makes it easier to insert the tap. For the gradual insertion into the metal, the taps have a tapered conical part that, when machined at the end of the hole, forms a thread run-the thread of the incomplete profile (see Figure 14.4, d). The drawing shows the length of the thread with the total profile l 1 and the length of the cylindrical hole l (Figure 14.5, ∂). Practically the quantity a must be at least 0.5 times the thread diameter.

Threaded socket with the conventional thread designation by solid main lines along the internal diameter of the thread and continuous thin lines along the external diameter is shown in Fig. 14.5, ∂. The dimensions l 1 of the thread length with the full profile indicate in the working drawings of the parts, the dimension l of the length of the cylindrical hole, , but this size and diameter d indicate the operational technological sketches for drilling the nest (Figure 14.5, b). The dimension a in the drawings does not indicate, and the diameter sign d is replaced by the standard thread designation discussed below.

Thread can be either right or left . Turning clockwise, the right-handed parts move the part along the axis in the direction from the observer. To move the part with the left thread in the direction from the observer, the piece is rotated counter-clockwise.

Depending on the number of visits (i.e., protrusions or grooves), the threads are divided into single-pass and multi-pass (two-pass, three-pass, etc.). In the production of the thread, the thread is also called a screw thread. An example of a two-thread helix is ​​shown above (see Figure 7.17).

Basic threading parameters. In Fig. 14.6 depicts the thread profile (conjugate, screwed internal and external threads) and outlines its basic parameters.

Thread axis - a straight line, relative to which the screw movement of the flat contour forming the thread occurs.

Thread Profile is the thread cross-section in the plane passing through its axis. In industry, as a rule, standard thread profiles are used, some of which are discussed below. Details with external threads of trapezoidal and triangular profiles are shown above - see Fig. 14.2, a, b.

The sides of the profile are the straight sections of the profile that belong to the screw surfaces.

The sections of the profile that connect the sides of the protrusions or grooves are called the apex or valley of the profile, respectively.

Fig. 14.6

From the number of basic quantitative parameters of the thread, we note: profile angle a - the angle between the sides of the profile; angles of inclination of the lateral sides of the profile β, y are the angles between the lateral sides of the profile and the perpendicular to the thread axis; for threads with a symmetrical profile, the slopes are equal to half the angle of the profile a/2 ; working profile height h - the contact height of the sides of the profile of the external and internal threads in a direction perpendicular to the axis of the thread; length of screwing - the length of contact of the screw surfaces of the external and internal threads in the axial direction.

The parameters related only to cylindrical threads are as follows: height of the initial profile H - the height of the acute-angled profile obtained by continuing the sides of the profile until they intersect (if the profile is constructed from a triangle); profile height h 1; thread pitch P - the distance between the adjacent side faces of the profile in the direction parallel to the thread axis; thread path P h is the distance between the closest sides of the profile that belong to the same screw surface in a direction parallel to the thread axis; thread stroke is the value of the relative axial movement of the screw (nut) per revolution; in single-thread threads, the stroke is equal to the pitch, in multi-thread threads, the product of the number of inputs n per step: P h = P • n; the angle formed by the tangent to the helical line at a point lying on the average thread diameter and the plane perpendicular to the thread axis, the angle ψ is determined by the

outer thread diameter d is the diameter of an imaginary cylinder described around the vertices of the external thread or hollows of the internal thread; inner diameter d 1 - diameter of an imaginary cylinder inscribed in the cavities of the external thread or the top of the internal thread; average thread diameter d 2 - diameter of an imaginary coaxial with a threaded cylinder, where the groove width is equal to half the nominal thread pitch.

Specific values ​​of parameters such as profile shape, outer diameter, pitch, direction of the helical surface (right or left thread), number of approaches, are reflected in the conventional alphanumeric thread designation. The corresponding examples are discussed below.

For operational purposes, threads are divided into fasteners (metric, inch), fastening and sealing (pipe, conical), running (trapezoidal, persistent), special .

All threads used in practice can be divided into two groups:

first - standard (all threads with standard settings: profile, pitch, diameter and relationships between them). Standard threads make up the majority of the threads used;

Second - non-standard or special, such as rectangular and square threads.

Standard metric thread. Metric thread is the main type of triangular thread fixing thread (see Figure 14.6) with a profile angle α = 60 °. It is also used in the details of devices. The dimensions of the elements of the metric thread are given in millimeters. For metric thread in GOST 8724-2002 the following step values ​​are set, mm: 0,075; 0.08; 0.09; 0.1; 0.125; 0,15; 0.175; 0.2; 0.225; 0.25; 0.3; 0.35; 0.4; 0.45; 0.5; 0.6; 0.7; 0.75; 0.8; 1.0; 1.25; 1.5; 1.75; 2 and further to 6 through 0,5 mm. For metric threads of general purpose, the standard sets diameters in the range from 0.25 to 600 mm and steps in the above range.

In accordance with GOST 8724-2002, the metric thread diameter from 1 to 600 mm is divided into two types: with a large step (for diameters from 1 to 68 mm) and

Fig. 14.7

with small steps (for diameters from 1 to 600 mm). Each thread diameter corresponds to certain steps (large and small).

All standard thread diameters are divided into 1st, 2nd and 3rd rows. Each of them has carvings with large and small steps. In this case, only one row corresponds to each thread diameter (the thread diameters in the rows are not repeated).

The standard recommends that when choosing threads, the first row should be preferred - the second row, the second - the third. So, for example, if for design reasons it is permissible to use threads with a diameter of 14, 15 or 16 mm, a thread with a diameter of 16 mm must be used.

Pipe cylindrical thread. This thread is used for connections in pipelines, cylindrical threaded connections. The profile of this thread (Figure 14.7) is an isosceles triangle with an angle α = 55 °, the tops and hollows of the profile are rounded, and there are no gaps in the joint between the vertices and valleys of the external and internal threads. The carving thread is designed in inch system (1 inch (1 = 25.4 mm) and has small steps. The pitch of the pipe thread is specified indirectly - indicate the number of thread threads that fit on 1 & quot ;. This number of threads is standardized in the range from 28 to 11.

The designation of the pipe thread size has a special feature, which is that the thread size is set not by the outer diameter of the pipe on which the thread is cut, but by the value of the inner diameter of the pipe. It is called the diameter of the tube "in the light" and is defined as the conditional pass-through size of the pipe. The explanation for this conditionality is that the design calculation of pipelines

Fig. 14.8

is carried out along conditional passages of pipelines, fittings and connecting parts.

For example, pipe thread in 1 is cut from the outside on a pipe that has an internal diameter of 1 (25.4 mm), the size of the outer diameter is always greater than the diameter in the light on two thicknesses of a wall of a pipe.

Pipe conical thread. In the connections of the fuel, oil, water and air piping of machines, a conical pipe thread is used widely, ensuring good tightness of joints without the use of special seals. Pipe conical threads (Figure 14.8) have two variants of the thread profile (with the initial profile in the shape of an isosceles triangle): rounded profile with a profile angle α = 55 ° (the dimensions of this tubular thread are standard GOST 6211-81 );

inch with a profile angle a = 60 ° (the dimensions of this conical inch thread are GOST 6111-52).

The conicity of the surfaces on which the conical thread is cut is usually 1:16 (for the taper design, see Figure 14.8, top left). The profile angle bisector is perpendicular to the thread axis.

Fig. 14.9

The diametrical dimensions of the tapered threads are established in the main plane, which is perpendicular to the axis and is separated from the end of the part with the external thread at a distance l , regulated by the standards for taper threads. In the main plane, the diameters of the threads are equal to the nominal diameters of the tubular thread.

Thread trapezoidal (GOST 9484-81). Thread profile is an equilateral trapezoid with a profile angle of 30 ° between the sides (Figure 14.9, a). Standardized for diameters from 10 to 640 mm with steps from 2 to 48 mm. For each diameter, the standard provides three different steps.

Thread resistant (Figure 14.9, b). Standardized for diameters from 10 to 600 mm with steps from 2 to 24 mm. For each thread diameter, three different steps are provided. Has an asymmetrical profile and is designed for lead screws with a large one-sided load (vises, jacks, presses, etc.).

The image of the threads. Examples of the image of the outer cylindrical and tapered threads on the entire length of the workpiece are shown in Fig. 14.10, a, b (see also fig. 14.2, c), of the inner cylindrical and tapered threads - in Fig. 14.11, a, b (see also Figure 14.5, ∂).

The chamfer, which does not have a special constructional purpose, is not represented in the projection on a plane perpendicular to the axis of the thread. Therefore, in Fig. 14.10, a and 14.11, and in the left view the chamfer is not shown, and in the right view the chamfer is shown, since it has a special constructive purpose.

The invisible thread is represented by dashed lines of the same thickness in the outer and inner diameters.

The line defining the thread boundary is applied at the end of the complete thread profile (before the start of the run). The thread boundary is always drawn up to

Fig. 14.10

Fig. 14.11

lines of the outer diameter of the thread and are represented, when visible, by a solid main line (Figure 14.12).

The hatching in sections and sections is brought to a continuous thick line, i.e., up to the outer diameter of the external thread (Figure 14.13) and up to the internal diameter of the internal thread (see Figure 14.12).

The thread run, if necessary, is represented by a solid thin line. Examples of the thread run-off image are shown in Fig. 14.14 for the outer ( a) and internal (b) threads. A threadless section, called a non-threaded thread, may remain behind the thread run off when cutting the thread at the stop (Fig. 14.14, a) . As a result of thread and nibble runoff, there is a shortage of threads (Figure 14.14, a). The presence of a thread undercut must be taken into account when designing threaded connections,

Fig. 14.12

Fig. 14.13

Fig. 14.14

and specify the length of the full profile in the parts drawings, taking into account the standard length of the undercut.

If a blind hole with a thread is made in the wall of a part that is sealed or vacuum-dense (Figure 14.15), then the depth L of the thread hole to the top of the socket, along with the length l threads of the full profile.

When the end of the thread in the blind hole is located close to the bottom of the hole, it is allowed to depict the threads on the drawings, which do not thread (for example, in assembly drawings), to the end of the hole.

The thread profiles, if necessary (for example, with a non-standard profile) are shown in the detail image (Figure 14.16) or as a remote element (see Figure 14.19, and).

Only those parts of the female thread that are not covered by the external thread (Figure 14.17) are shown on the threads of the threaded joint.

If a hole or a slot passes through the thread, then it is shown conditionally, interrupting the solid thin line at the locations of the hole or slot (Figure 14.18, a). When it is necessary to show the presence of threads in the area of ​​this hole or slits, it is fully represented (Figure 14.18, b).

Thread designations. Examples of thread designations in the drawings are shown in Fig. 14.19. The conventional image of a metric thread with a large pitch consists of the letters M and the nominal diameter, for the thread with a small step, the step size is added.

For example, in Fig. 14.19, a shows the designation of a metric thread with a nominal diameter of 24 mm with a large step of 3 mm on the rod, and Fig. 14.19, b - the metric thread with nominal diameters -

Fig. 14.15

Fig. 14.16

Fig. 14.17

Fig. 14.18

The meter is 24 mm with a fine step of 2 mm in the hole. For the left-hand thread, after the legend, put LH, for example M24 × 2 LH.

Multi-thread threads are denoted by the letter M, with the nominal diameter, the numerical value of the stroke and in parentheses with the letter P and the numerical value of the step. Examples of designations: for three-thread threads with a pitch of 1 mm, stroke value 3 mm

Fig. 14.19

for the same left-hand thread

The conventional designation of a pipe cylindrical thread consists of the letter G and the nominal size of the inner diameter of the pipe in inches. Examples of designation are shown in Fig. 14.19, in - on the pipe, in Fig. 14.19, d - in the hole.

The symbol for the trapezoidal thread consists of the letters Tr, of the outer diameter and thread pitch, for example Tr36 × 6 in Fig. 14.19, ∂.

The conventional designation of the stop thread consists of the letters S, of the outer diameter and thread pitch, for example S 80 × 16 in Fig. 14.19, e.

In Fig. 14.19, x, s are examples of the designation of a conical inch thread of the left direction (R 3/4 LH) on the rod and conical right-handed thread - right hole (Rc1).

To indicate the non-standard thread parameters, all its main dimensions are shown. For example, in Fig. 14.19, and shows the thread of a rectangular profile. It is recommended to show the scale of the increase in the profile of this thread and all its dimensions: d - the thread diameter along the protrusions; d , is the thread diameter along the troughs, P is the thread pitch, a is the protrusion size.

Constructive and technological elements of thread - groove, chamfers, runaways, undercuts. The duct - the annular groove on the rod or in the hole - is necessary for the exit of the thread-forming tool (Figure 14.20). The dimensions of the grooves are standardized in GOST 10549-80.

The radius value R of the roundings is taken to be approximately half the pitch of the thread.

For the external thread, the chamfer height with is conventionally taken to be equal to the pitch P of the thread, the angle of the chamfer to the thread axis is 45 °. The chamfer for the internal thread is installed, as shown in Fig. 14.20.

Fig. 14.20

The maximum values ​​of the thread run-off value in relation to the pitch of the thread P are assumed to be equal to: normal run - about 2.5 P, short run - about 1.25 R.

The maximum values ​​of the undercut value: normal - about 3 P, short - about 2 P, long - about 4 P.

Threaded connections and their parts. Threaded connections have become very widespread in engineering. Usually they are divided into two types:

a) joints performed by direct screwing of the connected parts, without the use of special connecting parts;

b) joints performed by means of special connecting parts, such as bolts, screws, studs, fittings, etc.

In many cases, a threaded joint is used as an element of another joint in which a threaded joint creates a large axial force. Thus, in the design of a vacuum-tight flange connection (see Fig. 14.1, a) , four bolted joints compress the flanges along the ends along the axis, and the vacuum density of the joint is ensured by the design of the end parts of the flanges I and 2 in the form of an acute tooth and groove with a 3 gasket clamped between them from a ductile metal (copper, aluminum).

Bolt connection. The bolt connection set (Figure 14.21) includes the following fasteners (fasteners): 1 - bolt, 2 - the nut, 3 - the washer. These fasteners have different shapes and sizes. When designing devices and machines, as a rule, only standardized fasteners are used.

Bolt (Figure 14.22) is a cylindrical rod with a thread on one end and a head on the other - most often in the form of a hexagonal prism. When connecting the fasteners to the threads of the bolt, a nut is fitted. The head of the bolt is processed from the end to the cone (this element is called a chamfer). The chamfer is also carried out on the rod for the convenience of threading and eliminating the fragile part of the final turn. The indicated chamfers in Fig. 14.22 are given by a diameter D 1 and an angle of 15 ... 30 ° on the head and a designation with × 45 " on the rod (c is the chamfer, usually equal to the pitch P of the thread). The diameter designation d in Fig. 14.22 on the bolt drawings are replaced by the thread designation.

Usually, bolts are used to connect parts of not very large thickness (flanges, etc.) and when frequent connections and disassembly of parts are necessary in terms of their operation.

Fig. 14.21

Fig. 14.22

Industry-issued bolts are distinguished by the shape and size of the head, the shape of the rod, the thread pitch, the nature of the design, the accuracy of production.

Depending on the purpose and working conditions, the bolts are made with hexagonal, semicircular and countersunk heads. Different standards of bolts have developed and approved their standards.

Hexagon head bolts are the most widely used. They are made of normal, high and coarse precision, they have from three to four options. Version 1 is shown in Fig. 14.22. These bolts are standardized in GOST 7798-70. Usually they are recommended for use in the learning process.

The standard symbol for the bolt, which is written in the technical documentation and used in the literature, contains the basic design dimensions. For example, the record Bolt M12 × 60 GOST 7798-70 means that the bolt has a metric thread with a diameter of 12 mm with a large step, a rod length of 60 mm, a hex head, version 1. The bolt image of the same design in conjunction with other parts, with a diameter thread 36 mm with a large step and length of the rod 120 mm see in Fig. 14.21.

Nut is a part that has a threaded hole for screwing onto a bolt or pin (Figure 14.23). Nuts are distinguished: according to the shape of the outer surface, the type of execution, the type of thread, the accuracy of manufacture.

The shape of the outer surface of the nut is made of hexagonal, hexagonal, slotted, crown, round, lamb, etc. In height, hex nuts distinguish normal height, low, high and particularly high. In addition, the nuts are released with a reduced size turnkey & quot ;.

Nuts are made of normal, elevated and coarse precision.

By the type of thread, the nuts are distinguished with a metric thread with a large or small pitch.

The chamfer is made to cut sharp edges of the corners of the hexagonal prism, which can cause cuts.

The choice of the nut type depends on the purpose of the structure and the operating conditions.

The symbol for the nut contains the thread size and the number of the standard that establishes the structure. For example, the record Nut M12 GOST 5915-70 means that the nut has a diameter of 12 mm metric thread with a large pitch, hexagonal, normal accuracy. Picture of nut with metric thread diameter of 36 mm in connection with other parts - see Fig. 14.21.

Fig. 14.23

Fig. 14.24

These abbreviated notations of bolt and nut symbols are used when making drawings in the learning process. Their standard designations also contain information on accuracy and strength classes, performance, thread tolerance, type and thickness of the coating, grade of steel or alloy.

The image of the bevels on the bolt heads and the nuts. On hexagon head bolts and nuts, the chamfer crossing line with the face plane is a hyperbola. The hyperbola projections in the drawings of these parts are replaced with images of arcs of circles, as shown in Fig. 14.24.

In the drawings of assembly units, hexagonal nuts and bolt heads with chamfers are allowed to be shown without bevels in standards. These images are less time consuming, but less obvious. Therefore, in the drawings performed in the training process, they are usually not used. With this simplified image, the presence of chamfers is judged by the designation of the nut or bolt.

Washer is a part placed under the nut or bolt head (screw) and designed to transfer and distribute forces to the parts to be connected or to prevent them from unscrewing (locking). A drawing of standard round washers with the notation of the main dimensions is shown in Fig. 14.25.

Washers are divided into washers round, spring, lock, etc.

Round washers have several types: normal normal washers in accordance with GOST 11371-78, washers increased, washers reduced. The washers of the normal row have two versions - version 1 without bevel, version 2 with bevels (Figure 14.25). They have dimensions that are compatible with fasteners with threads from 1 to 48 mm.

Example of the symbol for a fastener for a fastener of version 1 with a diameter of 12 mm of the specified thickness, of material of group 01, with a coating of thickness 9 μm:

The washer is 12.01.019 GOST 11371-78.

For a similar washer, but for Execution 2, the notation will be:

The washer is 2.12.01.019 GOST 11371-78.

Fig. 14.25

Fig. 14.26

Washers spring (GOST 6402-70) protect the nut from self-unscrewing during shocks and shocks (Figure 14.26).

Spring washers are divided into types: light (L), normal (H), heavy (T) and especially heavy (OT).

The designation of spring washers after the thread diameter contains a type designation (the designation H does not indicate). For example, a record Spring washer 12GOST 6402- 70 indicates that the washer is spring-loaded, normal for a screw with a diameter of 12 mm.

Development of a bolted joint drawing. The bolt joint drawing (see Figure 14.21) is usually developed based on the specified thread diameter and thicknesses B 1 and B 2 parts to be joined. In this case, the length l of the bolt is calculated from the formula, mm (Figure 14.27):

Fig. 14.27

or

where B 1 and B 2 - the thickness of the parts; Sш - washer thickness; H - the height of the nut; a is the thread margin, which is assumed to be the chamfer height, usually equal to P, or , c - by the approximate formula

Set the bolt length l in accordance with the standard (see GOST 7798-70) and the length l 0 of the chopped part, mm:

The diameter of the bolt hole is usually taken to be 1 mm larger than the diameter of the bolt pin.

Example. Set d = 36 mm, mm. For threads with a diameter of 36 mm, we find: according to GOST 7798-70, the pitch P = 4 mm, according to GOST 5915-70 the height of the nut H = 29 mm, according to GOST 11371-78 thickness washers mm.

Bolt length l , mm:

or

According to GOST 7798-70, we take l = 160 mm.

The length of the chopped part l 0, mm:

According to GOST 7798-70 we accept l 0 = 84 mm.

The dimensions of the bolt joint (see Figure 14.21) are the bolt threads, bolt length and cut length, the diameter of the circumscribed circumference of the hex nut of the nut, the size of the key.

Screws . On purpose, the metal screws are divided into fastening (connecting) and mounting.

Fastening screw is a part that serves for a detachable connection and is a cylindrical threaded rod for screwing into one of the parts to be connected and a head of various shapes turnkey or with a slot "under the screwdriver". Screw drawings with a different head shape with a slotted screwdriver are shown in Fig. 14.28: cylindrical (a), semicircular (b), hidden (c) and half-secret (r).

Fig. 14.28

Fastening screws are used when assembling machines and mechanisms when an auxiliary is attached to the main part, for example: the cover to the gearbox housing, the key to the shaft, the panel to the chassis or body, etc.

Countersunk and semi-hollow (conical) screws are often used in place of bolts when protruding heads interfere with the operation of the mechanism.

The mounting screw differs from the fastener in that its rod is completely cut to its full length and has a pressure end of a special shape (flat, conical, spherical) entering the special recess of the mating part.

When assembling instruments, machines, the setscrews are used to fix one part relative to the other. On rotating parts, use setscrews with a slot for a screwdriver without a head.

The standard sets for the screws four versions with a certain length of the cut part, depending on the diameter and length of the screw. Screws are made with metric thread with large and small steps.

Example of the symbol conventions for screws: Screw M 12 × (Screw connecting with a semicircular head, version 1, normal accuracy, with a thread diameter of 12 mm, with a large step, a rod length of 50 mm) or A screw of 2M 12 x 10 mm. 1,25 GOST 17473-80 (screw with a semicircular head, version 2, normal -

Fig. 14.29

accuracy, with a diameter of 12 mm, with a fine thread pitch of 1.25 mm).

Screw connections. Variants of the structural arrangement of the screw heads with respect to the screwed parts are quite diverse. Some typical examples are shown in Fig. 14.29. In the constructions in Fig. 14.29, a-в the screw is prevented from self-unscrewing by a spring washer located under a cylindrical or spherical head. In designs r and ∂ screws with a secret and semi-blind heads, this kind of locking is not provided.

Examples of screw connections of parts of the same thickness are shown in Fig. 14.30, a-g. Both screws - with a cylindrical head (c) and with a conical (blind) head (d) have the same thread diameter of 16 mm. To the left of the screw connections in Fig. 14.30, a, b show operational technological sketches for drilling holes for threading and threading.

Development of the screw connection drawing. The drawing of the screw joint (Figure 14.30) is developed based on the specified thread diameter, the thickness B of the screwed part, the material grade

Fig. 14.30

with the threaded socket and the received head type and its location relative to the screwed part (see, for example, Figure 14.29).

When drawing a drawing, the thread P , the dimensions of the diameter D and the head height to , the radius R under the head, the width n and the depth/slot, the radius of the head (for semicircular), apply these dimensions on the sketch.

Determine (Figure 14.31):

the screw screwing depth L , depending on the material of the part with the threaded socket - for steel and bronze L = d, for cast iron L = 1 , 25 d, for aluminum L = 2 d;

the depth l 1 threads with a full profile (see figure 14.5, ∂), mm:

the depth I r of drilling a threaded socket (see l in Figure 14.5, b), mm:

length/screw, mm:

or (see Figure 14.29, 6)

Fig. 14.31

Nominal diameter d , holes for threading, mm:

diameter d 1 through hole in the screwed part, usually for screws

Draw a drawing, for example, according to the type shown in Fig. 14.30.

Example. Task - to develop a drawing of a screw connection according to the type shown in Fig. 14.29, b, for the screw with thread M 16; β = 40 mm, D = 25 mm, A1 = 12 mm, material of the part with threaded socket is steel. We write out:

of GOST 1491-80 for a screw with a thread L/16: UjarZi = 2 mm, diameter D = 24 mm head, width n = 4 mm and depth t = 4 mm of the slot, height to = 9 mm of the head, radius R = 1.6 mm; of GOST 6402-70 for washers with a spring diameter d = of 16.3, a thickness of Sul = A = 3.5 mm. Define:

the depth L of screwing into the steel socket - L = d = 16 mm; depth /, hole drilling -

length/screw -

set in accordance with GOST 1491-80 standard length/= 50 mm; we specify the drilling depth of the socket I r = 30 mm; the length of the cut portion b & gt; L = 16 - according to GOST 1491-80 b = = 38 mm;

nominal diameter <7, threaded holes for P = 2 mm -

diameter ⅛ through hole -

chamfer diameter 1.05 t = 1.05 ∙ 16 = 16.8 mm.

The selected screw is M16 x 50 GOST 1491-80.

Fig. 14.32

Fig. 14.33

Joints with union nuts. Variants for the connections with union nuts are shown in Fig. 14.32,14.33. In the construction in Fig. 14.32 The plug connector 3 is fixed in the casing 1 with the union nut 2. The tightening of the nut is done manually, for which the nuts are mesh corrugated on the outer cylindrical surface. In the construction in Fig. 14.33 a pipe 3 from a plastic material, for example a copper one, is attached to a piece 1 using a union nut 2. Contact surfaces A, B, B - conical, which ensures a good tightness of the connection. Between the tube 3 and an intermediate piece 4 (nipple) rotating from the nut 2 is laid out of a stronger material than the tube material. It protects the surface of the tube from damage. In the construction in Fig. Figure 14.34 shows a type of demountable vacuum connection of a copper tube (sthengel) 3 with a casing 1. Such a design has a very high airtightness - vacuum density. It is used to connect electrovacuum devices to the vacuum system when pumping gases

Fig. 14.34

from the internal cavity of the instruments. In this construction, the nut 2 , when tightened, creates a large frictional torque on the surface A of the washer 4 . To prevent rotation of the washer 4, of the tube 3 and the device connected to it relative to the body 1 , two projections are performed on the washer 4 , which enter the slots on the body 1. Their shape is seen in the left view, in which only the nut is shown in the section.

In all the constructions considered, the union nut 2, rotates along the thread with respect to the fixed part 1, moves along the axis. In this movement, it presses the fixed part 3 to the surface B of the fixed part 1. This surface, when applied intermediate part 4 (see Figure 14.33, 14.34) is carried out through the intermediate contact surface B.

In the connection images in Fig. 14.32, 14.33 the conventionality allowed by the standard is applied - the attached details 3, 4, not included in the design of this device with a union nut are shown by a solid thin line used for the image of boundary details ("furnishings").

Screw mechanisms. As already mentioned, screw surfaces, and in particular threads, are used as screw mechanisms that convert rotary motion into translational motion. When rotating for one revolution, the relative movement of the part with the external thread (screw) relative to the part with internal thread (nuts) is equal to the thread travel. With single-thread threads, the stroke is equal to the thread pitch. To reduce axial movements by one revolution, it is necessary to reduce the thread pitch, which can lead to almost impossible to implement the mechanism. In this regard, to obtain small axial movements, threaded joints with two large threads with different steps are used, differing from each other by the amount of the required axial displacement per revolution.

The design of such a movable joint, called connection with a differential screw , is given above - see Fig. 14.1, in - and consists of an immovable bushing 6 with internal thread, a differential screw 4 and a nonrotating rod 5 with external thread. The differential screw 4 has two single-thread threads with a large step in one direction: the outer thread with a pitch P 1 and an internal one with step P 1 (P ,> P 2 ). If you rotate the differential screw 4 clockwise one revolution, it will move in the axial direction to the axis of the device relative to the fixed bushing 6 by the step size P r The non-rotating screw 5 is screwed into the differential screw by its internal thread in the direction from the axis of the device by the step size of this thread, P 2.

The total axial displacement of the nonrotating screw 5 is equal to

Thus, practically this displacement can be made arbitrarily small at large thread steps in the details of the screw mechanism.

thematic pictures

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