Definition and classification of bolts

Bolt: A type of fastener consisting of a head and a screw (a cylindrical body with external threads), which needs to be matched with a nut to securely connect two parts with through holes. This type of connection is called bolted connection. If the nut is unscrewed from the bolt, the two parts can be separated, so the bolt connection is a detachable connection

General classification and special bolts:

Divided by the force distribution method of the connection: ordinary and those with hinged holes. Ordinary main load-bearing axial forces can also bear lateral forces with low requirements. The bolts used for reaming holes should match the size of the hole and be used when subjected to lateral forces

According to the shape of the head, there are hexagonal, round, square, countersunk, and so on. Hexagonal heads are commonly used among them. Generally, countersunks are used in areas where smooth surfaces without protrusions are required after connection, as countersunks can be screwed into parts. Round heads can also be screwed into parts. The tightening force of the square head can be greater, but the size is quite large

In addition, to meet the needs of locking after installation, there are holes in the head and rod, which can prevent the bolts from loosening when subjected to vibration

Some bolts without threads need to be made thin, called slim waist bolts. This type of bolt is beneficial for connecting under variable forces

There are one-to-one high-strength bolts on the steel structure, with larger heads and varying sizes

Additionally, there are special uses for T-slot bolts, which are commonly used on machine tool fixtures and have a unique shape that requires cutting off both sides of the head. Anchor bolts, used for connecting and fixing machines to the ground, come in many shapes. U-shaped bolt, as mentioned earlier. Wait a minute

There are also one-to-one bolts for welding, with threads on one end and no threads on the other, which can be welded to the part, and the other side can be directly screwed with a nut

The meaning of bolt performance grades

The performance grades of bolts used for steel structure connections are divided into more than 10 grades, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc. Among them, bolts with grade 8.8 and above are made of low-carbon alloy steel or medium carbon steel and undergo heat treatment (quenching, tempering), generally referred to as higher strength bolts, while others are commonly referred to as ordinary bolts.

. The bolt performance grade number consists of two parts of numbers, which respectively represent the nominal tensile strength value and yield strength ratio of the bolt material. For example:

The meaning of a bolt with a performance level of 4.6 is:

1、 The nominal tensile strength of the bolt material reaches 400MPa level

2、 The yield strength ratio of bolt material is 0.6

3、 The nominal yield strength of the bolt material reaches 400 × 0.6=240MPa grade

Performance grade 10.9, which is a relatively high strength bolt. After heat treatment, the material can achieve:

1、 The nominal tensile strength of the bolt material reaches 1000MPa level

2、 The yield strength ratio of bolt material is 0.9

3、 The nominal yield strength of the bolt material reaches 1000 × 0.9=900MPa grade

The meaning of the bolt performance grade is a widely recognized standard. Bolts with the same performance grade, regardless of the difference in material and origin, have the same performance, and only the performance grade should be selected for design

Strength grades 8.8 and 10.9 refer to the shear stress resistance grades of bolts being 8.8GPa and 10.9GPa.

8.8 has a nominal tensile strength of 800N/MM2 and a nominal yield strength of 640N/MM2.

Generally, bolts are designed with...

; X.Y" Representing intensity

X * 100=the tensile strength of this bolt

X * 100 * (Y/10)=yield strength of this bolt

(Because according to the labeling regulations, yield strength/tensile strength=Y/10)

If the grade is 4.8

, then the tensile strength of this bolt

is 400MPa

The yield strength is 400 * 8/10=320MPa