The professional knowledge of fasteners, the mechanical worthy collection

As a mechanical engineer, fasteners are the most commonly used parts and must be understood for their foundation and expertise.

Today we mainly talk about:

1, the classification of fasteners

2, the identification and inspection of the thread

3, material requirements, heat treatment requirements and mechanical properties of bolts, screws and studs

Classification of fasteners

What is a fastener and what is the meaning of the fastener?

1. What is a fastener? A fastener is a general term for a type of mechanical part used to securely join two or more parts (or components) into one unit. The market is also known as standard parts.

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 joints, welding studs

2.1) Bolt: A type of fastener consisting of a head and a screw (a cylinder with an external thread) that is to be fitted with a nut for fastening two parts with through holes. This type of connection is called a bolted connection. If the nut is unscrewed from the bolt, the two parts can be separated, so the bolt connection is a detachable connection.

As shown below:

2.2) Stud: A type of fastener that has no head and only has threads on both ends. When connecting, one end must be screwed into the part with the internally threaded hole, the other end is passed through the part with the through hole, and then the nut is screwed, even if the two parts are fastened together. This type of connection is called a stud connection and is also a detachable connection. It is mainly used for the case where one of the connected parts has a large thickness, requires a compact structure, or is not suitable for bolting due to frequent disassembly.

As shown below:

2.3) Screw: A type of fastener that is also composed of two parts, the head and the screw. It can be divided into three categories according to the purpose: machine screws, set screws and special purpose screws. The machine screw is mainly used for a tightly threaded part and a tight connection between a part with a through hole and does not require a nut fit (this type of connection is called a screw connection and is also a detachable connection; Fitted with a nut for a secure connection between two parts with through holes.) The set screw is primarily used to secure the relative position between the two parts. Special purpose screws such as eyebolts are used for lifting parts.

As shown below:

2.4) Nut: With internal threaded hole, the shape is generally flat hexagonal column shape, but also has a flat square column shape or a flat cylindrical shape. It is equipped with bolts, studs or machine screws for fastening two parts. It becomes a whole.

As shown below:

2.5) Self-tapping screws: Similar to machine screws, but the threads on the screw are dedicated self-tapping screws. It is used to fasten and connect two thin metal members to make them a whole. The components need to be made into small holes in advance. Because of the high hardness of the screws, they can be directly screwed into the holes of the components. Form the corresponding internal thread. This type of connection is also a detachable connection.

As shown below:

2.6) Wood screws: Also similar to machine screws, but the threads on the screw are special wood screw threads that can be screwed directly into the wood component (or part) for a metal (or non-metal) with a through hole. The parts are fastened together with a wooden component. This connection is also a detachable connection.

As shown below:

2.7) Washer: A type of fastener that is oblate in shape. Between the support surface of the bolt, the screw or the nut and the surface of the connecting part, it increases the contact surface area of â€‹â€‹the connected part, reduces the pressure per unit area and protects the surface of the connected part from damage; another type of elastic washer, It also acts to prevent the nut from loosening. As shown below:

2.8) Retaining ring: It is installed in the shaft groove or hole of the machine and equipment, and plays the role of preventing the left and right parts of the shaft or the hole from moving. As shown below

2.9) Pin: It is mainly used for part positioning, and some can also be used for connecting parts, fixing parts, transmitting power or locking other fasteners. As shown below:

2.10) Rivet: A type of fastener consisting of two parts, the head and the shank, used to fasten the two parts (or components) with through holes to make them integral. This type of connection is called a rivet connection, referred to as riveting. Dependent and non-detachable connection. Because the two parts that are joined together must be separated, the rivets on the part must be destroyed. As shown below:

2.11) Assembly and connection pair: The assembly refers to a type of fastener supplied in combination, such as a combination of a certain machine screw (or bolt, self-supply screw) and a flat washer (or spring washer, lock washer); A pair of fasteners that supply a combination of special bolts, nuts, and washers, such as high-strength large hex head bolts for steel structures.

As shown below:

2.12) Solder Stud: Due to the heterogeneous fasteners consisting of the polished rod and the nail head (or the nailless head), it is fixedly attached to a part (or component) by welding to connect with other parts.

As shown below:

Thread identification and inspection

1, the use and characteristics of the thread:

Threads are used in a wide range of applications, from airplanes and cars to water pipes and gas used in our daily lives. Most of the threads play the role of fastening connections, followed by the transmission of force and motion. There are also some special-purpose threads, although there are many types, but the number is limited.

The use of the thread can last for a long time because of its simple structure, reliable performance, easy disassembly, and easy manufacturing. It has become an indispensable structural element in various electromechanical products.

According to the use of the thread, all kinds of threaded parts should have the following two basic functions: one is good screwing; the other is sufficient strength.

2, the classification of the thread

a, according to its structural characteristics and uses can be divided into four categories:

Ordinary thread (fastening thread): The tooth shape is a triangle for connecting or fastening parts. Ordinary threads are divided into coarse teeth and fine thread according to the pitch, and the connection strength of the fine thread is high.

Drive thread: The shape of the tooth is trapezoidal, rectangular, saw-shaped and triangular.

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

Special purpose thread, referred to as special thread

b. According to the region (country), the thread can be divided into: metric thread (metric thread) inch thread, American thread, etc. We used to refer to the inch thread and the US thread collectively as the inch thread, and its tooth angle is 60Â°, 55Â°. Etc., the relevant thread parameters such as diameter and pitch are in inch sizes. In our country, the angle of the tooth is unified to 60Â°, and the diameter and pitch series in millimeters (mm) are used to name the thread at the same time: ordinary thread

4, the basic terminology of the thread

Thread: a continuous protrusion with a defined tooth shape formed along a spiral on a 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

Large diameter: diameter of an imaginary cylinder or cone tangent to the root of an external thread or internal thread

Trail diameter: diameter of an imaginary cylinder or cone tangent to the crest of an external thread or internal thread

Middle: The diameter of an imaginary cylinder or cone that passes through the groove and the width of the projection on the tooth. The imaginary cylinder or cone is called a medium diameter cylinder or a medium diameter cone.

Right-hand thread: thread that is screwed in when clockwise

Left-hand thread: thread that is screwed in when rotated counterclockwise

Tooth angle: the angle between two adjacent flank on the thread profile

Pitch: the axial distance between two points on the middle diameter line of two adjacent teeth

5, the mark of the thread

Metric thread mark:

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

a indicates the thread type code of the thread feature;

b Thread size: generally consists of diameter and pitch, for multi-thread threads, should also include lead and line number;

c Thread accuracy: The accuracy of most threads is determined by the tolerance zone of each diameter (including the tolerance band position and size) and the screw length.

Inch thread marking:

6, thread measurement

For standard standard threads, threaded ring gauges or plug gauges are used for measurement.

Due to the large number of thread parameters, it is impossible to measure each parameter of the thread one by one. Usually, we use a thread gauge (thread ring gauge, thread plug gauge) to comprehensively determine the thread. This kind of inspection method belongs to the analog assembly type acceptance method. It is not only convenient and reliable, but also has the same precision requirements as ordinary threads. Therefore, it has become the most common acceptance method in actual production.

7, thread measurement (medium diameter)

In the threaded connection, only the medium diameter size determines the nature of the thread fit, so how to correctly judge whether the middle diameter is qualified is very important. Based on the size of the medium diameter, it should be ensured that the most basic performance of the thread can be achieved. The standard specifies the principle of judging the qualification of the medium diameter, that is: "The actual diameter of the actual thread cannot exceed the diameter of the largest solid tooth type. Actually The single median diameter of any part of the thread must not exceed the median diameter of the smallest solid tooth."

There are two convenient methods for measuring a single medium diameter. One is to measure the medium diameter with a threaded medium diameter micrometer, and the other is to measure with a three-needle method (we use the three-needle method).

8, thread matching level:

The thread fit is the loose or tight size between the threaded threads, and the level of fit is the specified combination of deviations and tolerances on the inner and outer threads.

(1) For unified inch thread, the external thread has three thread grades: 1A, 2A and 3A, and the internal thread has three grades: 1B, 2B and 3B, all of which are clearance fit. The higher the level number, the tighter the fit. In the inch thread, the deviation only specifies the 1A and 2A levels, the 3A level deviation is zero, and the 1A and 2A level deviations are equal.

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

1, 1A and 1B grade, very loose tolerance class, which is suitable for tolerance fit of internal and external threads.

2, 2A and 2B, is the most common thread tolerance class specified by the British series of mechanical fasteners.

3, 3A and 3B, screwing to form the tightest fit, suitable for tight tolerance fasteners, for critical design of safety.

4. For external threads, 1A and 2A have a matching deviation, and 3A does not. The 1A tolerance is 50% larger than the 2A tolerance and 75% larger than the 3A. For the internal thread, the 2B tolerance is 30% greater than the 2A tolerance. Level 1B is 50% larger than 2B and 75% larger than 3B.

(2) metric thread, external thread has common thread grade: 4h, 6e, 6g and 6h, internal thread has common thread grade: 6G, 6 H, 7H. (The Japanese standard thread accuracy grade is divided into three grades I, II, and III, and the normal grade is grade II.) In the metric thread, the basic deviation of H and h is zero. The basic deviation of G is a positive value, and the basic deviation of e, f, and g is a negative value. as the picture shows:

1. H is the common tolerance zone position for internal threads. It is generally not used as a surface coating or an extremely thin phosphating layer. The basic deviation of the G position is used for special occasions, such as thicker coatings, which are rarely used.

2, g is commonly used to plate 6-9um thin coating, such as the product drawing requirements of 6h bolts, the pre-plated thread uses a 6g tolerance band.

3, the thread fit is best combined into H / g, H / h or G / h, for bolts, nuts and other refined fastener threads, the standard is recommended to use 6H / 6g.