Everything About Fasteners You Should Know

As a common part, fasteners should be familiar to everyone.

Today, we introduce fasteners from four aspects: the classification of fasteners, the identification and inspection of threads, the material requirements, heat treatment requirements and mechanical performance requirements of bolts, screws and studs, and the types and structure of steel structure bolts.

Fasteners

1. Classification of fasteners

1. What are fasteners?

Fastener is the general name of a kind of mechanical parts used to fasten and connect two or more parts (or components) into a whole.

Also known as standard parts on the market.

2. It usually includes the following 12 types of parts:

Bolts, studs, screws, nuts, self tapping screws, wood screws, washers, retaining rings, pins, rivets, assemblies and connecting pairs, welding nails.

(1) Bolt:

A kind of fastener composed of head and screw (cylinder with external thread), which needs to be matched with nut to fasten and connect two parts with through holes.

This type of connection is called bolt connection.

If the nut is unscrewed from the bolt, the two parts can be separated, so the bolt connection is a removable connection.

As shown below:

full thread of outer hexagon head bolt

Fig. 2-1-1 full thread of outer hexagon head bolt

half tooth of hexagon socket bolt with cylindrical head

Fig. 2-1-2 half tooth of hexagon socket bolt with cylindrical head

(2) Stud:

There is no head, only a kind of fastener with external threads on both ends.

When connecting, one end must be screwed into the part with internal thread hole, the other end must pass through the part with through hole, and then screw on the nut, even if the two parts are firmly connected as a whole.

This connection form is called stud connection, which is also a detachable connection.

It is mainly used when one of the connected parts is thick, requires compact structure, or is not suitable for bolt connection due to frequent disassembly.

As shown below:

double head stud

Fig. 2-2-3 double head stud

full thread stud

Fig. 2-2-4 full thread stud

(3) Screw:

It is also a kind of fastener composed of head and screw.

According to the purpose, it can be divided into three categories: steel structure screw, set screw and special purpose screw.

Machine screws are mainly used for the fastening connection between a part with a fixed threaded hole and a part with a through hole, without nut matching (this connection form is called screw connection, which also belongs to removable connection; it can also be matched with nuts, which is used for the fastening connection between two parts with a through hole).

The set screw is mainly used to fix the relative position between two parts.

Special purpose screws, such as eyebolt, are used for hoisting parts.

As shown below:

pan head screw

Fig. 2-3-5 pan head screw

hexagon socket set screw

Fig. 2-3-6 hexagon socket set screw

eyebolt

Fig. 2-3-7 eyebolt

(4) Nut:

With internal thread hole, the shape is generally flat hexagonal column, or flat square column or flat cylindrical.

It is used to fasten and connect two parts into a whole with bolts, studs or steel structure screws.

As shown below:

hex nut

Fig. 2-4-8 hex nut

(5) Self tapping screw:

Similar to the screw, but the thread on the screw is a special thread for self tapping screw.

It is used to fasten and connect two thin metal components into a whole.

Small holes need to be made on the component in advance.

Because the screw has high hardness, it can be directly screwed into the hole of the component to form corresponding internal threads in the component.

This connection form also belongs to removable connection.

As shown below:

self tapping screw

Fig. 2-5-9 self tapping screw

(6) Wood screws:

It is also similar to the screw, but the thread on the screw is a special thread for wood screw, which can be directly screwed into the wooden component (or part) to firmly connect a metal (or non-metal) part with a through hole with a wooden component.

This connection is also a detachable connection.

As shown below:

hexagon head wood screw

Fig. 2-7-10 hexagon head wood screw

(7) Washer:

A kind of fastener with flat circular shape.

It is placed between the support surface of bolts, screws or nuts and the surface of connecting parts to increase the contact surface area of connected parts, reduce the pressure per unit area and protect the surface of connected parts from damage;

Another kind of elastic washer can also prevent the nut from loosening.

As shown below:

flat washer

Fig. 2-7-11 flat washer

elastic washer

2-7-12 elastic washer

(8) Retaining ring:

It is installed in the shaft groove or hole groove of steel structure and equipment to prevent the parts on the shaft or hole from moving left and right.

As shown below:

retaining ring

Fig. 2-8-13 retaining ring

(9) Pin:

It is mainly used for positioning parts, and some can also be used for connecting parts, fixing parts, transmitting power or locking other fasteners.

As shown below:

Everything About Fasteners You Should Know 1

Fig. 2-9-14 pin

(10) Rivets:

A kind of fastener composed of head and nail rod, which is used to fasten and connect two parts (or components) with through holes to make them a whole.

This connection form is called rivet connection, which is called riveting for short.

It is a non detachable connection.

Because to separate the two parts connected together, the rivets on the parts must be destroyed.

As shown below:

half round head rivet

Fig. 2-10-15 half round head rivet

(11) Assembly and connecting pair:

Assembly refers to a kind of fastener supplied in combination, such as the combination of a certain machine screw (or bolt, self supplied screw) and flat washer (or spring washer, lock washer);

Connection pair refers to a kind of fastener that combines a special bolt, nut and washer, such as high-strength large hexagon bolt connection pair for steel structure.

As shown below:

Everything About Fasteners You Should Know 2

Fig. 2-11-16 machine screw assembly

Everything About Fasteners You Should Know 3

Fig. 2-11-17 torsion shear bolt connection of steel structure

(12) Tack:

Due to the dissimilar fastener composed of polished rod and nail head (or no nail head), it is fixedly connected to one part (or component) by welding, so as to connect with other parts.

As shown below:

Everything About Fasteners You Should Know 4

Fig. 2-12-18 welding nail

2. Identification and inspection of threads

1. Purpose and characteristics of thread:

Threads are widely used, from aircraft and cars to water pipes and gas used in our daily life.

In many occasions, most threads play the role of fastening connection, followed by the transmission of force and motion.

There are also some threads for special purposes. Although there are many kinds, their number is limited.

The use of thread can last for a long time because it has the characteristics of simple structure, reliable performance, convenient disassembly and convenient manufacturing, which makes it an indispensable structural element in all kinds of electromechanical products.

According to the purpose of the thread, all kinds of threaded parts should have the following two basic functions:

  • Good spinnability;
  • Sufficient strength.

2. Classification of threads

a. According to its structural characteristics and uses, it can be divided into four categories:

Ordinary thread (fastening thread): the tooth shape is triangular, which is used to connect or fasten parts. Ordinary thread can be divided into coarse thread and fine thread according to the pitch.

The connection strength of fine thread is high.

Transmission thread: the tooth shape includes trapezoid, rectangle, saw shape and triangle.

Sealing thread: used for sealing connection, mainly pipe thread, conical thread and conical pipe thread.

Special purpose thread, abbreviated as special purpose thread.

b. Threads can be divided into metric threads (metric threads), British threads and American threads according to regions (countries).

We are used to collectively refer to British thread and American thread as British thread, and its tooth profile angle is 60 ° and 55 °.

The diameter, pitch and other relevant thread parameters adopt inch size.

In our country, the tooth profile angle is unified as 60 °, and the diameter and pitch series in mm are used.

At the same time, this kind of thread is named ordinary thread.

3. Common thread profile

Common thread profile

4. Basic terms of thread

A continuous thread formed on a specified cylindrical or conical surface.

External thread: a thread formed on the outer surface of a cylinder or cone.

Internal thread: internal thread formed on the inner surface of a cylinder or cone.

Major diameter: the diameter of an imaginary cylinder or cone tangent to the top of an external thread or the bottom of an internal thread.

Small diameter: the diameter of an imaginary cylinder or cone tangent to the bottom of an external thread or the top of an internal thread.

Middle diameter: the diameter of an imaginary cylinder or cone whose generatrix passes through a groove on the tooth form where the width of the bulge is equal.

This imaginary cylinder or cone is called a pitch diameter cylinder or pitch diameter cone.

Basic terms of thread

Right hand thread: the thread screwed in when rotating clockwise.

Left hand thread: thread screwed in when rotating counterclockwise.

Profile angle: the included angle between two adjacent tooth sides on the thread profile.

Pitch: the axial distance between two points on the pitch line of two adjacent teeth.

5. Marking of threads

Metric thread marking:

Generally, a complete metric thread mark should include the following three aspects:

A represents the thread type code of thread features;

B refers to the size of thread: generally.

It shall be composed of diameter and pitch. For multi thread thread, it shall also include lead and number of threads;

C refers to the accuracy of threads: the accuracy of most threads is determined by the tolerance zone of each diameter (including the position and size of the tolerance zone) and the screwing length.

Metric thread marking

Inch thread marking:

Inch thread marking

6. Thread measurement

For general standard threads, thread ring gauge or plug gauge is used to measure.

Because there are many thread parameters, it is impossible to measure each parameter of the thread one by one. Usually, we use thread gauge (thread ring gauge and thread plug gauge) to comprehensively judge the thread.

This inspection method belongs to the acceptance method of simulated assembly, which is not only convenient and reliable, but also equivalent to the accuracy requirements of ordinary threads.

Therefore, it has become the most common acceptance method used in actual production.

Everything About Fasteners You Should Know 5

7. Thread measurement (pitch diameter)

In threaded connection, only the pitch diameter determines the nature of thread fit, so how to correctly judge whether the pitch diameter is qualified is very key.

Based on the pitch diameter size, the most basic service performance of the thread should be ensured to achieve this purpose.

The standard specifies the judgment principle of pitch diameter qualification, that is: “the actual pitch diameter of the thread cannot exceed the pitch diameter of the maximum solid tooth profile.

The single pitch diameter of any part of the actual thread shall not exceed the pitch diameter of the minimum solid profile. “

At present, there are two convenient methods for measuring single pitch diameter: one is to measure pitch diameter with thread pitch diameter micrometer, and the other is to measure pitch diameter with three needle method (our company adopts three needle method).

Thread measurement

8. Thread fit grade:

Thread fit is the loose or tight size between screw threads.

The grade of fit is the specified combination of deviation and tolerance on internal and external threads.

(1) For unified inch threads,

External thread has three thread grades: 1A, 2A and 3A;

There are three grades of internal threads: 1B, 2B and 3B, all of which are clearance fit.

The higher the grade number, the tighter the fit.

In English thread, the deviation only specifies class 1A and 2A, the deviation of class 3A is zero, and the grade deviation of class 1A and 2A is equal.

The larger the number of grades, the smaller the tolerance, as shown in the figure:

Thread fit grade

1) Class 1A and 1B, very loose tolerance class, which is applicable to the tolerance fit of internal and external threads.

2) Class 2A and 2B are the most common thread tolerance grades specified for British series mechanical fasteners.

3) Grade 3A and 3B, the tightest fit is formed by screwing.

It is suitable for fasteners with tight tolerance and is used for key design of safety.

4) For external threads, there is a fit deviation for class 1A and 2A, but not for class 3A.

Class 1A tolerance is 50% greater than class 2A tolerance, 75% greater than class 3A tolerance, and class 2B tolerance is 30% greater than class 2A tolerance for internal threads.

Class 1B is 50% larger than class 2B and 75% larger than class 3B.

(2) Metric thread, external thread has common thread grades: 4h, 6e, 6g and 6h, and internal thread has common thread grades: 6G, 6H and 7H.

(The accuracy grade of Japanese standard thread is divided into three grades I, II and III, which is usually grade II).

In metric thread, the basic deviation of H and h is zero.

The basic deviation of G is positive and the basic deviation of e, f and g is negative.

As shown in the figure:

thread grades

1) H is the commonly used tolerance zone position of internal thread, which is generally not used as surface coating or very thin phosphating layer.

The basic deviation of G position is rarely used for special occasions, such as thicker coating.

2) G is often used for plating 6-9um thin coating.

For example, for bolts requiring 6h in product drawings, 6g tolerance zone shall be adopted for their threads before plating.

3) The thread fit is best combined into H / g, H / h or G / h.

For the threads of refined fasteners such as bolts and nuts, 6H / 6g is recommended.

Medium accuracy grade of ordinary thread

Nut: 6H

Bolt: 6g

Medium accuracy grade of threads with thick covering

Nut: 6G

Bolt: 6e

High precision grade

Nut: 4H

Bolt: 4h, 6h

Thread Dia.
M6-P1. 0external diameterEffective diameter
6e5.76-5.945.178-5.29
8g5.694-5.9745.144-5.324
6g5.794-5.9745.212-5.324
6h5.82-6.005.238-5.350
4h5.868-6.005.275-5.350

9. Common special thread

1). Self tapping thread: it is a kind of wide thread with large lead.

GB/T5280 JIS B1007

Self tapping thread
SpecificationsTooth pitch
ST 1.50.5
ST 1.90.6
ST 2.20.8
ST 2.60.9
ST 2.911
ST 3.31.3
ST 3.51.3
ST 3.91.3
ST 4.214
ST 4.81.6
ST 5.51.8
ST 6.31.8
ST 82.1
ST 9.52.1
Self tapping thread
Specifications22.5335445568
Number of teethAB teeth4028242018161412
A teeth  2418161412109

Specifications

Number of teeth

AB teeth

A teeth

2

40

2.5

28

3

24

24

35

20

18

4

18

16

45

16

14

5

12

6

14

10

8

12

9

2) Self tapping locking thread (triangular thread)

GB6559

Self tapping locking thread

3.) Wall panel nail thread (Fast thread)

GB/T14210

Wall panel nail thread (Fast thread)

4) Wood thread:

See Fig. 1-1-32 for the thread profile and size of wood screw (CB / T922-1986)

thread profile for wood screw

Fig. 1-1-32 thread profile for wood screw

3. Material requirements, heat treatment requirements and mechanical performance requirements of bolts, screws and studs

Material requirements for bolts, screws and studs

Performance level

Materials and heat treatment

Chemical composition /%

Tempering temperature ℃ min

C

P

max

S

max

B

max

min

max

4.6

Carbon steel or carbon steel with added elements

0.55

0.05

0.06

nothing

4.8

5.6

0.13

0.55

0.05

0.06

5.8

0.55

0.05

0.06

6.8

0.15

0.55

0.05

0.06

8.8

Carbon alloy copper (e.g., copper, manganese or chromium) with added elements is quenched and tempered

0.15

0.40

0.025

0.025

0.003

425

Medium carbon steel, Quenched and tempered

0.25

0.55

0.025

0.025

Alloy steel rate and tempering

0.20

0.55

0.025

0.025

Performance level

Materials and heat treatment

Chemical composition

/%

Tempering temperature ℃ min

C

P

max

S

max

B

max

min

max

9.8

Carbon alloy steels with added elements (such as boron, manganese or chromium) are ignited and tempered

0.15

0.40

0.025

0.025

0.003

425

Medium carbon steel, Quenched and tempered

0.25

0.55

0.025

0.025

Alloy steel quenched and tempered

0.20

0.55

0.025

0.025

10.9

Carbon alloy steel with added elements (such as boron, manganese or chromium), Quenched and tempered

0.20

0.55

0.025

0.025

0.003

425 (340 cancelled)

Medium carbon steel, Quenched and tempered

0.25

0.55

0.025

0.025

Alloy steel quenched and tempered

0.20

0.55

0.025

0.025

12.9

Alloy steel, rate of ignition and tempering

0.30

0.50

0.025

0.025

0.003

425

12.9

Carbon alloy steels with added elements (such as boron, manganese, chromium or molybdenum) are quenched and tempered

0.28

0.50

0.025

0.025

0.003

380

Mechanical and physical properties of bolts, screws and studs

Sub item

Mechanical and physical properties

4.6

4.8

5.6

5.8

6.8

8.8

9.8

10.9

12.9/12.9

d≤M6

d≥

M16

1

Nominal tensile strength rmpA

nominal

400

500

600

800

900

1000

1200

min

400

420

500

520

600

800

830

900

1040

1220

2

Lower yield strength rmpA

nominal

240

300

min

240

300

3

The stress of 0.2% of non proportional elongation is specified for the machined test piece.

RP0.2  Mpa

nominal

640

640

720

900

1080

min

640

660

720

940

1100

4

The specified non proportional elongation stress of the fastener is 0.0048d.

RXY  Mpa

nominal

320

400

480

min

340

420

480

5

Guaranteed stress MPa

225

310

280

380

440

580

600

650

830

970

Certified stress ratio

0.94

0.91

0.93

0.90

0.92

0.91

0.91

0.90

0.88

0.88

Sub item

Mechanical and physical properties

4.6

4.8

5.6

5.8

6.8

8.8

9.8

10.9

12.9/12.9

d≤M6

d≥

M16

6

Elongation after fracture of machined test piece Af%

22

20

12

12

10

9

8

7

Reduction of area of machined test piece Z% min

52

48

48

44

8

Elongation after fracture of fastener A1% min

one

0.24

one

0.22

0.2

9

Head firmness

Terminal new crack

10

Vickers hardness HVF ≥ 98N

min

120

130

155

160

190

250

255

290

320

385

220

250

320

335

360

380

435

11

Brinell hardness HRB F=30D2

min

114

124

147

152

181

238

242

276

304

366

max

209

238

304

318

342

361

414

12

Rockwell hardness

HRB

min

67

71

79

82

89

max

95.0

99.5

Rockwell hardness

min

22

23

28

32

39

4. Types and structure of steel structure bolts

1. Overview of bolted connection of steel structure

Bolt connecting for steel structure is a connection method that connects more than two steel structure parts or components into one by bolts.

Bolt connection is the simplest connection method in component pre assembly and structural installation.

Everything About Fasteners You Should Know 6

Full bolt connection of beam column joints

Bolt connection was first used in metal structure installation.

In the late 1930s, bolt connection was gradually replaced by rivet connection, which was only used as a temporary fixing measure in component assembly.

High strength bolt connection method appeared in the 1950s.

High strength bolts are made of medium carbon steel or medium carbon alloy steel, and their strength is 2 ~ 3 times higher than that of ordinary bolts.

High strength bolt connection has the advantages of convenient construction, safety and reliability.

It has been applied in the manufacturing and installation of steel structures in some metallurgical factories since the 1960s.

Everything About Fasteners You Should Know 7

Full bolt connection of beam splicing

2. Specification of bolt

Bolt specifications commonly used in steel structures include M12, M16, M20, M24 and M30.

M is the bolt symbol and the number is the nominal diameter.

According to the performance grade, bolts are divided into 10 grades: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 and 12.9.

Bolts above grade 8.8 are made of low-carbon alloy steel or medium carbon steel and undergo heat treatment (quenching and tempering), which are generally referred to as high-strength bolts, and bolts below grade 8.8 (excluding grade 8.8, refined ordinary bolts also include grade 8.8) are generally referred to as ordinary bolts.

The following table shows the performance grade and mechanical properties of bolts.

mechanical property

Performance level

3.6

4.6

4.8

5.6

5.8

6.8

88

9.8

10.9

12.9

≤M16

≥M16

Tensile strength, MPa

Nominal value

300

400

500

600

800

800

900

1000

1200

Small value

330

400

420

500

520

830

Rockwell hardness

HRB

HRC

minimum value

52

67

70

80

83

89

22

25

28

34

39

Maximum

95

99

32

35

37

41

44

Yield point, MPa

Nominal value

180

240

320

300

400

480

minimum value

190

340

420

yield strength, MPa

Nominal value

640

640

720

900

1080

Minimum stress

660

940

1000

Guarantee stress

Sp.Mpa

180

230

310

280

380

440

580

600

660

830

970

The bolt performance grade is composed of two parts of numbers, which respectively represent the nominal tensile strength of the bolt and the yield ratio of the material.

For example, the meaning of bolts with performance grade of 4.6 is: the number in the first part (4 in 4.6) is 1 / 100 of the nominal tensile strength (n / mm2) of bolt material, that is, fu ≥ 400N / mm2;

The number in the second part (6 in 4.6) is 10 times of the yield ratio of bolt material, that is, fy / fu = 0.6;

Product of two numbers (4) × 6 = “24”) is 1 / 10 of the nominal yield point (or yield strength) (n / mm2) of bolt material, which means fy ≥ 240n / mm2.

According to the manufacturing accuracy, ordinary bolts of steel structure can be divided into three levels: A, B and C.

A. Grade B is refined bolt, which is generally used for mechanical products, and grade C is rough bolt.

Unless otherwise specified, ordinary bolts of steel structure are generally ordinary coarse grade C bolts with performance grade of 4.6 or 4.8.

The strength design value of bolted connection shall be taken from table 3.4.1-4 of GB50017-2003 code for design of steel structures.

Table 3.4.1-4 strength design value of bolted connection (n / mm2)

Performance grade of bolt, grade of anchor bolt and component steel

Common bolt

Radium suppository

Pressure bearing connection high strength bolt

Grade C bolt

Grade A and B bolts

Tensile

Shear resistance 

Pressure bearing

tensile

Shear resistance

Pressure bearing

tensile

tensile

Shear resistance

Pressure bearing

Common bolt

Level 4.6 and 4.8

170

140

Level 5.6

210

190

Grade 8.8

400

320

Anchor bolt

Q235 steel

one

140

Q345 steel

180

Pressure bearing connection high strength butterfly bolt

Grade 8.8

400

250

Level 10.9

500

310

component

Q235 steel

one

305

405

470

Q345 steel

385

510

590

Q390 steel

400

530

615

Q420 steel

425

560

615

Table 3.4.1-5 strength design value of rivet connection (n / mm2)

Willow nail steel grade and component steel grade

Pull off nail

Shear resistance

Pressure bearing

Type I hole

Class II hole

Type I hole

Class II hole

rivet

BL2 or BL3

120

185

155

component

Q235 steel

450

365

Q345 steel

565

460

3. Classification of bolts

There are many names for bolts, such as screws, bolt nails, standard parts, fasteners, etc.

In a broad sense, bolts include ordinary bolts, high-strength bolts, anchor bolts, expansion bolts, chemical anchors, screws, studs, etc;

Bolts in the narrow sense are divided into ordinary bolt connection and high-strength bolt.

(1) Common bolt connection

Ordinary bolts are divided into rough bolts and refined bolts according to the manufacturing accuracy.

Ordinary bolts can be divided into hexagon head bolts, stud bolts, countersunk head bolts, etc;

countersunk head bolts

The above picture shows countersunk head bolts

Rough bolt

Class C bolts are generally rough bolts made of carbon structural steel.

In order to make the bolts penetrate into the screw holes smoothly, the hole diameter shall be 1.0 ~ 2.0mm larger than the nominal diameter d of the bolts, which is a class II hole.

The bolt hole spacing shall be arranged to facilitate the wrench to tighten the nut.

When the rough bolts are used to connect the components of columns, beams and roof trusses, the connection structure with supporting plates shall be adopted.

At this time, the bolt is in tension, and its shear force is borne by the supporting plate (as shown in the figure below).

Rough bolt

The low strength grade of materials used in rough bolts limits its application range in structural connection, but it is still widely used in the connection of working platform secondary beam, wall skin beam, roof beam, support and hinged support with small shear force.

ordinary bolts

The figure above shows ordinary bolts

Rough bolts are also widely used for pre assembly of steel structures in workshops, pre fastening of riveted components before riveting, assembly before high-strength bolt connection and temporary fastening before node welding.

When the rough bolt is used as a permanent fixing bolt, it shall be tightened after alignment and anti loosening measures shall be taken.

double nut locking measures of the column base bolt

The above figure shows the double nut locking measures of the column base bolt

Refined bolt

Grade A and B bolts are refined bolts, and the holes are generally class I holes, and the hole diameter shall be 0.3 ~ 0.5mm larger than the nominal diameter d of the bolt.

Refined bolt connection is used for some structural connections that are often disassembled and riveted.

Refined bolts are generally used in mechanical products and rarely used in building steel structures.

(2) High strength bolt connection

Bolts made of high-strength steel or requiring large preload can be called high-strength bolts.

High strength bolts exert pretension and transmit external force by friction.

Ordinary bolt connection transfers shear force by the shear resistance of bolt rod and the bearing pressure of hole wall.

When tightening the nut, the pretension is very small, and its influence can be ignored.

In addition to its high material strength, high-strength bolt also applies a large pretension to the bolt, resulting in extrusion pressure between connecting members, so that there is a great friction perpendicular to the screw direction.

Moreover, pretension, anti slip coefficient and steel type directly affect the bearing capacity of high-strength bolt.

Working principle of high strength bolt

Working principle of high strength bolt

High strength bolts are mainly divided into friction type and pressure type according to their stress conditions.

According to the construction process, high-strength bolts are divided into two types: torsional shear high-strength bolts and large hexagon high-strength bolts.

Torsional shear type high-strength bolt and large hexagon high-strength bolt

Torsional shear type high-strength bolt and large hexagon high-strength bolt

Friction type high-strength bolt connection transmits external force by the friction generated on the contact surface of the steel plate after the connecting plate layer is tightly adhered by the bolt tightening pressure.

The red rust surface is generated after sand blasting on the component surface, which can obtain a large friction coefficient and reduce the number of connecting bolts.

The hole diameter of friction type high-strength bolt shall be 1.5 ~ 2.0mm larger than the nominal diameter d of the bolt.

Pressure bearing high-strength bolt connection is to transfer the stress by making the friction between components and the shear force of the central axis of the bolt act simultaneously with the bearing pressure of the component, and its hole diameter shall be 1.0 ~ 1.5mm larger than the nominal diameter d of the bolt.

CNC drilling machine and drilling jig are used for drilling holes.

In short, friction type high-strength bolt and pressure type high-strength bolt are actually the same bolt, and the difference is whether sliding is considered in the design.

In design, the friction surface of friction type high-strength bolt cannot slide, and the screw does not bear shear.

Once the friction surface slides, it is considered to reach the design failure state, which is relatively mature and reliable in technology;

The friction surface of pressure bearing high-strength bolt can slide, and the screw also bears shear.

The final failure is the same as that of ordinary bolt (bolt shear failure or steel plate compression failure).

The large hexagon high-strength bolt consists of a high-strength bolt, a nut and two washers, which can form a high-strength bolt connection pair.

During construction, the structure shall be temporarily fixed with rough bolts.

After the structure is installed and aligned, the rough bolts shall be replaced with high-strength bolts one by one from the middle of the bolt group, and the initial tightening shall be carried out. After the initial tightening, the re tightening and final tightening shall be carried out in sequence.

the large hexagon head high-strength bolt connection pairs with different lengths

The figure above shows the large hexagon head high-strength bolt connection pairs with different lengths

During the installation of large hexagon head high-strength bolt connection pair, a washer shall be added on both sides of the bolt.

The initial tightening torque value is 50% of the final tightening torque value, and the re tightening torque value is equal to the final tightening torque value.

The calculation formula of the final tightening torque value is:

TC = k * Pc * d

Where

  • Tc is the final tightening torque value, in n · m;
  • k is the torque coefficient;
  • Pc is the construction pretension, in kN;
  • d is the thread diameter of high-strength bolt, in mm.

Torque wrench shall be used for tightening, and torque correction shall be carried out before each use.

Torsional shear type high-strength bolt, a high-strength bolt, a nut and a washer form a torsional shear type high-strength bolt connection pair.

Torsional shear type high strength bolt

Torsional shear type high strength bolt

Torsion shear electric wrench

Torsion shear electric wrench

Installation principle of torsional shear high strength bolt

Installation principle of torsional shear high strength bolt

When installing the torsion shear type high-strength bolt connection pair, only one washer shall be added on one side of the nut.

The calculation formula of initial tightening torque value is:

Tc = 0.065 * Pc * d

Where

  • Tc is the initial tightening torque value, in n · m;
  • Pc is the construction pretension, in kn;
  • d is the thread diameter of high-strength bolt, in mm.

Finally, use a special wrench to unscrew the tail plum blossom head until it is broken.

The quality inspection shall focus on the supervision and inspection of the construction process.

(3) Anchor bolt

Anchor bolt, also known as anchor bolt, anchor screw, anchor wire, etc., is used for the connection between steel structure column base and concrete foundation.

Q235 and Q345 round steel are generally used.

Different types of anchor bolts

Different types of anchor bolts (anchor plate shall be used if the diameter is greater than 24mm)

During installation, the anchor bolt group shall be fixed by the steel frame, installed together with the binding reinforcement cage, and then the concrete shall be poured.

The bolt head shall be exposed to the concrete surface for a certain length.

After the concrete reaches a certain strength, install the steel column base, and finally conduct secondary grouting at the column bottom.

Steel frame fixed anchor bolt group

Steel frame fixed anchor bolt group

Everything About Fasteners You Should Know 8

Schematic diagram of anchor bolt

steel structure column base before secondary grouting

Picture of steel structure column base before secondary grouting (rubber sleeve is used to protect anchor bolt top from thread)

(4) Chemical anchor bolt

Chemical anchor bolt is a new type of fastening material, which is composed of chemical agent and metal rod.

It is used to install connectors of other structures on the built concrete structure.

It can be used for the installation of post embedded parts in the construction of various steel structures, curtain walls and marble dry hanging.

It can also be used for equipment installation, highway and bridge guardrail installation, building reinforcement and transformation and other occasions.

Screw and agent of chemical anchor bolt

Screw and agent of chemical anchor bolt

Chemical anchor bolt is a new type of anchor bolt after expansion anchor bolt.

It is a composite part that is cemented and fixed in the drilling of concrete substrate through special chemical adhesive and screw, so as to realize the anchoring of fixed parts.

Because the chemical anchor bolt has large pull-out bearing capacity, it can replace the embedded anchor bar.

It is often used to forget to install the embedded parts of steel structure on the construction site, but the concrete has been poured, and the later embedded parts of chemical anchor bolt can be used to remedy it.

The construction steps of chemical anchor bolt are as follows:

1) According to the engineering design requirements, drill holes at corresponding positions in the base material (such as concrete), and the hole diameter, hole depth and bolt diameter shall be determined by professional technicians or field tests.

2) Drill holes with percussion drill or water drill.

3) Clean the dust in the drilling hole with a special air cylinder, brush or compressed air machine. It is recommended to repeat it for no less than 3 times.

There should be no dust and open water in the hole.

4) Ensure that the bolt surface is clean, dry and free of oil masonry.

5) Confirm that the glass tube anchor package has no abnormal phenomena such as appearance damage and agent solidification, put its round head outward into the anchor hole and push it to the bottom of the hole.

6) Use the electric drill and special installation fixture to insert the screw into the hole bottom by strong rotation, and the impact method shall not be adopted.

7) When it is screwed to the hole bottom or the marked position on the bolt, stop the rotation immediately, remove the installation fixture, and avoid disturbance after the gel is completely cured.

Overtime rotation will lead to the loss of glue and affect the anchoring force.

(the rotation time shall not exceed 30 seconds, the rotation speed shall not be less than 300 rpm, not more than 750 rpm, the bolt propulsion speed is about 2cm / s, and the impact method is not allowed)

(5) Expansion bolt

The function of expansion bolt is the same as that of chemical anchor bolt, which is used for anchors with less stress.

Expansion bolts of different specifications

Expansion bolts of different specifications

Expansion bolts shall not be used for the parts with cracks and parts prone to cracks in the concrete structure.

At the same time, the expansion bolts used in the design of main load-bearing structures, important pipelines, high-speed operation, bearing impact load and large vibration shall be selected according to the calculated design tensile force and design shear force.

4. Arrangement and construction requirements of bolts

Bolt arrangement can be divided into parallel and staggered:

Juxtaposition – simple, neat and compact.

The size of the connecting plate used is small, but the weakening of the component section is large;

Staggered arrangement – the arrangement is not compact, the size of the connecting plate used is large, but the weakening of the member section is small.

Arrangement and construction requirements of bolts

Stress requirements

Vertical stress direction: in order to prevent the mutual influence of bolt stress concentration and excessive weakening of section and reduce the bearing capacity, the edge distance and end distance of bolt shall not be too small;

Direction of force action: in order to prevent the plate from being broken or sheared, the end distance should not be too small;

For compression members: in order to prevent buckling of connecting plates, the middle distance should not be too large.

Construction requirements

The edge distance and middle distance of bolts should not be too large, so as to avoid loose fitting between plates, moisture intrusion and corrosion of steel.

Construction requirements

In order to facilitate the wrench to tighten the nut, the bolt pitch shall not be less than 3do;

According to the above requirements, the allowable spacing of bolts and relevant design values are given in GB50017-2017 code for design of steel structures.

Table 8.3.4 maximum and minimum allowable distances of bolts or rivets

name

Position and direction

Maximum allowable distance (whichever is smaller)

Minimum allowable distance

Center spacing

Outer row (vertical or along the direction of internal force)

8d。 Or 12t

34d。

Middle row

Vertical internal force direction

16d。 Or 24t

Along the direction of internal force

Member under pressure

12d。 or 18t

Member tension

16d。 or 24d。

Along the direction of diagonal

Distance from center to component edge

Along the direction of internal force

4d。 or 8t

2d。

Vertical internal force direction

Cutting edge or manual gas cutting edge

1.5d。

Rolling edge, automatic gas cutting or sawing edge

High strength bolt

Other bolts or nails

1.2d。

Note:

1. d0 Is the hole diameter of the bolt or nail, and t is the thickness of the outer sheet.

2. The maximum distance between the steel plate edge and the bolt or rivet connected to the rigid member (such as angle steel, channel steel, etc.) can be adopted according to the value of the middle row.

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