Connections, Basic concepts, Threaded connections - Applied Mechanics

Connections

Basic concepts

In the mechanisms, individual parts for performing their functions are connected in a certain way to each other, forming mobile and fixed connections. Movable joints, for example, coupling the shaft to the housing, engaging the gears, hinges, etc. allow the relative movement of one part relative to the other. Thus, mobile compounds form kinematic pairs. Fixed connections do not allow mutual movement of the mating parts. In engineering, the term compounds refer only to fixed connections.

By virtue of the connector, the connections are divided into detachable and one-piece.

Detachable connections - those that allow assembly and disassembly without damaging the material of the parts. This type of connections include threaded, keyed, splined, pin, profile and terminal. It is possible to distinguish a group of connections of the shaft-bushing type, intended for transferring the torque. These include key, splined, pin and profile joints.

Non-detachable connections - those that can not be disassembled without destroying the details. These include riveted, welded, soldered, glued and joints with interference (press).

Connections are a critical element in the design of mechanisms. Often the cause of failure and destruction of mechanisms is poor quality of connections.

Threaded connections

The most common threaded connections due to their versatility, ease of manufacture, reliability, ease of assembly and disassembly, complete interchangeability. They are widely used in all branches of technology. For example, in an IL-62 aircraft, the number of threaded connections is more than 150,000.

The main parts of the threaded connections are bolts, screws, nuts and washers.

Bolt - is the rod with the head 1 on one end and the thread on the other, on which the nut is screwed on? (Figure 4.81, a).

Washers 2 are placed under the head of the bolt or screw to increase the bearing surface and reduce the collapsing stresses when tightening the threaded joint; for pre -

Fig. 4.81

storage from damage to protective coatings on the parts to be connected; for securing the threaded connections from self-unscrewing. Bolts are used to connect parts of relatively small thickness, as well as when the material of the parts does not provide the required thread reliability. When connecting with bolts, it is not necessary to cut the threads in the parts to be connected.

Disadvantages: In the parts to be connected, a place must be provided for the location of the bolt head and nut. Therefore, the mass of the bolted joint is somewhat larger than that of the joints with screws.

A screw is a rod with a head on one end and a thread on the other, which it is screwed into the threaded hole in one of the parts to be connected (Figure 4.81, b). The connection with screws is used in the absence of a place under the nut and in the event that one of the parts has a relatively large thickness.

The studs 4 are used instead of the screws (Figure 4.81, in), when the material of the part to be joined to the threaded hole does not provide the required strength and reliability Threads with frequent assembly and disassembly. For example, in details of aluminum alloys. They are also used in structures subject to variable loads, since there is no stress concentrator in the pin 4 at the point of transition from the rod to the head of the bolt or screw.

The main element of the threaded connection is the thread. The thread profile is determined by the section shape of the turns in the axial plane. By appointment, the threads are divided into fasteners, intended for joining parts, and threads for running gears. In the direction of the winding line, the thread can be right and left, in the number of visits - single-pass and multi-turn.

The main fixing thread is the metric thread of the triangular profile with cut off vertices and valleys (Figure 4.82, a). Geometric thread parameters: outer d , average d 2 and internal d 1 diameters, thread pitch p, profile angle a and number of hits n. The thread profile is also characterized by the height of the original triangle of the thread H and the working height of the profile h. Nominal diameters d and d 1 are the same for the nut and bolt, the gap is formed due to the limiting deviations. The standard provides for a thread with a large and small step (p/ 3, p/ 4, p/ 5). Their profiles are geometrically similar. Primary is

Fig. 4.82

thread with a large step. The fine-pitch thread has a shallower depth and a correspondingly lower stress concentration. It is used in structures subject to dynamic loads, in small-sized and hollow parts.

Pipe thread is designed for hermetic pipe jointing (Figure 4.82, b). It also has a triangular profile, with rounded vertices and valleys.

The round thread (Figure 4.82, c) is produced by knurling and extruding on thin-walled metal and plastic parts, as well as casting on cast-iron, glass, plastic and other products.

Trapezoidal thread is used as the primary thread for the running mechanisms (Figure 4.82, d). It has lower friction coefficients and, accordingly, higher efficiency of the mechanism. In the axial section, this thread has the shape of an isosceles trapezoid.

Thrust thread is used when large single-sided loads are applied (press mechanisms, jacks, etc.) (Figure 4.82, e). It is a type of trapezoidal thread with a cut edge on the one hand.

Rectangular thread (Figure 4.82, e) is rarely used in screw mechanisms, is not currently standardized.

The standard specifies the degree of accuracy of the thread (Table 4.25), which is assigned depending on the working conditions of the connections. The exact class is used for precision threads in running gears, intended for precise movements; middle class - for critical threaded connections. The threads are produced in mass production by knurling. As a result of plastic deformation of the surface layer, residual compressive stresses are created and the strength of the threaded part is increased. In a single and small batch production, the thread is manually cut by bolt dies and taps in nuts or on screwdrivers.

Table 4.25

Accuracy class

Thread Tolerance Fields

outer (bolts)

inner (nuts)

Exact

4h, 4g

4H.5H

Medium

6h, 6g, 6f, 6e, 6d

6H, 6G

Rough

8h, 8g

7H, 7G

Steel threaded parts produce 12 classes of strength, which are denoted by two digits (Table 4.26).

Table 4.26

Class

Strength

σв, MPа

σ "MPa

Steel grade

ini η

shah

3.6

300

490

200

St3; 10

4.6

400

550

240

20

5.6

500

700

300

30; 35

6.6

600

800

360

35; 45; 40G

8.8

800

1000

640

35X; 38HA; 45G

10.9

1000

1200

900

40G2; 40X; ZOHGSL; 16ХСН

Note. The first number in the designation of the strength class multiplied by 100 is , the second divided by 10 corresponds to approximately the ratio .

In the designs, which require special requirements for weight, heat resistance, corrosion resistance and other parameters, titanium alloys, corrosion-resistant and heat-resistant steels, plastics are used as the material of threaded parts.

All fixing threads for stationary loads are self-braking, i.e. Do not self-unscrew. However, with random or systematic vibrations, to which almost all mechanisms are subjected, self-inhibition is not ensured. Therefore, it is necessary to protect the threaded connections from self-unscrewing, i.e. introduce their additional locking. Locking is carried out on two principles: increased friction in the thread and special fixing elements.

With tightening of the locknut (Fig. 4.83, a) the bolt pin is stretched and additional friction is created in the thread under the action of elastic forces. This method of locking is used mainly in stationary structures, since the mass of the connection increases. More often for locking use spring washers (Figure 4.83, b). The elastic forces of the washer, when tightening the nut, create frictional forces in the thread. The clutch between the nut and the washer is also ensured by the insertion of the sharp edges of the washer into the nut. A disadvantage of this method is the additional bending of the bolt rod due to displacement of the axial force relative to the center of the bolt. To exclude the bending of the bolt, use stop washers with external or internal teeth (Figure 4.83, c). In this case, the total resultant axial force from the nut acts on the bolt without axial displacement and the bolt only works for tension. Locking with spring washers is not highly reliable and is used in non-essential connections.

Fig. 4.83

In air and transport engineering, self-locking nuts are used in which the friction in the thread arises from the radial interference. In the upper part, the nut has slits (Figure 4.83, d). After threading, the upper part of the nut is crimped, reducing the diameter. The other self-locking nut has a wound polyamide ring inside it (Figure 4.83, d), in which the thread is not cut, but it is formed by screwing the nut onto the bolt.

The locking by the cotter pin (Figure 4.83, e ) with the crown nut is used in connections without controlled tightening. A threaded connection with a controlled tightening when cramped with cotter pins may not be tight or tapered, since it is necessary to align the notch in the nut with the transverse hole in the bolt stud. Threaded parts located on the edge of the body are fixed with lock washers (Fig. 4.83, ж) with paws. One leg is bent to the edge of the body, and the other two to the edge of the nut. The group threaded joints can be locked with special washers in pairs (Figure 4.83, Z ) or a strapping wire, passed through the holes with a stretch in the direction of screwing the nut (Figure 4.83, i). Connections that are not disassembled can be stopped by plastic deformation (Figure 4.83, k). Threaded unloaded connections are stopped with paint, varnish, glue .

Consider the three most common cases of loading bolted connections.

1. The bolt is placed in the hole with a gap and is loaded with axial force (Figure 4.84, a). All load is perceived by a bolt rod, working on tension:

From the strength condition for allowable stresses σρ & lt; [σ | determine the internal diameter of the thread of the bolt:

The magnitude of the allowable stress for steels is taken equal to (0.2 ± 0.4) σт at constant load and

Calculating Bolt Connections

Fig. 4.84

when working on the zero-cycle (smaller values ​​are chosen for bolts with a diameter mm, large at mm). The found value of the diameter is aligned with the standard.

2. The bolt is put into the hole without a gap (the prismatic bolt), and the connection is loaded with transverse force (Figure 4.846). The diameter of the bolt rod is found from the condition of shear strength:

whence

Here - allowable shear stress, MPa; - the yield strength of the material.

Technological difficulties in manufacturing such compounds limit the scope of their application. They are mainly used for joining thin-walled structures in aircraft and shipbuilding.

3. The bolt is installed in the hole with a gap, and the connection is loaded with transverse force (Figure 4.84, in). The basic condition for the normal operation of the joint is the immovability of the joint , where < img src="images/image2661.jpg"> - the force of friction. To create a friction force, the threaded connection must be tightened. The pretensioning force is determined from the condition , where is the friction coefficient. The tightened bolted connection works on tension and torsion. The calculation is carried out for equivalent stresses

whence inner diameter of thread of bolt

The diameter of the bolt with a preliminary tightening is 2-3 times larger than the prismatic bolt, and the mass of such a connection is 4-5 times higher.

thematic pictures

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