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Basic specification description
Screw head groove type+head type+tooth type+body diameter * tooth distance * body length
2
Other description requirements
1, material requirements
2. Hardness requirements
3. Surface requirements
4. Anti-corrosion ability
Note: The description must include specifications, materials, screw heads and screw types.
Let’s look at the head shape first.
Screw head type
So many head types are divided into the following four categories:
1. Expansion bolts
2, the first machine thread bolts
3. Headed machine wire bolts
4, self-tapping screws
The following categories are common in our life.
Countersunk screw
Countersunk screw, also known as flat machine screw and flat head screw, has a 90-degree cone at its head, and a 90-degree cone-shaped round socket is also machined on the surface of the mounting hole on the connector, thus ensuring that the screw head is parallel to the surface of the connector, and is used in places where the surface can be slightly protruded.
Countersunk screws are commonly represented by KM, FM and KA.
Why can’t it always be tightened?
1. The thickness of the fastened part is greater than the thickness of the head of the countersunk head screw. After the screw is tightened, some threads of the screw do not enter the threaded hole. In this case, countersunk head screws can definitely be tightened.
2. The thickness of the fastened parts is less than the height of the head of countersunk screws, which is common in sheet metal parts in mechanical equipment, such as the hinge of the chassis and the connection between the door and the box; The connection between the sheet metal cover of the equipment and the equipment, etc. Due to the small thickness of the part, the screw hole of the fastened sheet metal part completely becomes a conical hole. In this case, when the countersunk head screw is tightened, the head of the screw is not pressed by the conical surface, but the bottom of the screw head and the high part of the threaded hole are squeezed to death. Although the screw is tightened, the sheet metal part is stuck rather than pressed. In this case, although the screw is tightened, the sheet metal part is not tightened. This is a very common situation.
3. The cone at the head of countersunk head screw has a 90-degree cone angle, and the height angle of the newly bought drill is usually 118-120. Some untrained workers simply don’t know the angle difference, and often use a 120-degree drill to bore holes directly, which causes that it is not the cone at the head but a line at the bottom of the countersunk head screw that is hard to tighten, which is one of the reasons why the so-called countersunk head screw can’t be tightened.
matters need attention
1. The reaming taper should be 90. To ensure it, it is better to be less than 90, but not more than 90. This is a key trick.
2. If the thickness of the sheet metal is less than the thickness of the head of the countersunk head screw, you can change the screw to a smaller size, or you would rather enlarge the hole smaller.
If countersunk head screws can be sunk and hidden in thin materials, then cylindrical head screws can only be hidden in thick materials with deep holes.
Cylindrical head screw
It is mainly used in occasions where the screw head is not allowed to be exposed, which has a certain aesthetic effect and is convenient for installing other accessories.
Semi-circular head screw
(the most important thing)
It is often used in some circular or curved surfaces. After installation, the deep hole is in the same plane as the product, and it is often installed on the exposed plane.
Scope of application: Because the head shape is spherical and has no sharp point, it plays a safe and beautiful role. Example: it is installed on the movable connector of wire drawing.
Semi-circular head plastic screws has such qualitative characteristics as insulation, magnetism, corrosion resistance, beauty and never rusting, while modified engineering plastics are comparable to metals in strength and impact resistance. We often say that the mechanical properties of plastic screws screw, commonly known as Nye screw, are far better than those of ordinary Nye. The materials used for semi-circular head plastic screws are increasingly diversified, the performance is constantly improved, and the application fields are becoming wider and wider.
1, medical equipment industry (insulation, magnetic, environmental protection, anti-interference, making the use of medical machinery and equipment safer)
2. Wind power industry (isolation and insulation of PCB board of chassis circuit)
3, aerospace industry (insulation, anti-interference number on electronic equipment)
4, office equipment industry (never rust, beautiful)
5, petrochemical industry (high temperature resistance, chemical resistance, corrosion resistance, prolong the service life of equipment)
6, electronics industry (insulation, anti-interference, light weight)
7. Communication industry (insulation, magnetism and safety)
8, shipbuilding industry (acid, alkali and corrosion resistance, prolong service life) and so on.
External hexagon screw
It is often used for connection installation of some planes.
Example: stair bone connection or wall installation.
There are A, B and C grades for external hexagon screws, with Grade A and B being more important, requiring high assembly accuracy, and being widely used in situations of heavy impact, vibration or intersection load.
The C-class external hexagon screw is used in the occasions with rough surface and low assembly accuracy. Class C is commonly used in some temporary connections and connections that need to be dismantled.
Other types of screws
A. Expansion bolts
Generally speaking, the expansion screw is a metal expansion screw, and the fixation of the expansion screw is to use the wedge slope to promote the expansion to produce a friction grip force to achieve the fixation effect. The screw is threaded at one end and tapered at the other. The outer bread is made of an iron sheet (some are steel pipes), and half of the iron sheet cylinder (steel pipes) has several cuts. Put them into the holes punched in the wall together, and then lock the nuts, which pull the screws outward and pull the taper into the iron sheet cylinder. The iron sheet cylinder is expanded, so it is tightly fixed on the wall. It is generally used for fastening protective fences, awnings, air conditioners and other materials on cement and bricks. However, its fixation is not very reliable, and it may loosen if the load vibrates greatly, so it is not recommended to install ceiling fans.
When in use, a hole with a corresponding size must be drilled on the fixed body with an impact electric drill (hammer), and then the bolt and the expansion pipe are put into the hole, and the bolt, the expansion pipe, the mounting piece and the fixed body can be tightened into a whole by screwing the nut.
Expansion screws are widely used in various decoration occasions because of their small drilling, large tension and flat exposure after use. If they are not used, they can be removed at will to keep the wall surface smooth. Round high nuts are generally used in combination with drawing and blasting. After using round high nuts, there is no need to cover the bottom of the column with decorative covers.
Difference between tension-explosion bolt and expansion bolt
The expansion bolt is wrapped with a ring cylinder, which has a gap. When in use, a hole is punched in the wall, and the expansion bolt is put in the hole. When the bolt is tightened, the ring cylinder is squeezed and spread, so that the bolt is stuck in the hole and plays a fixed role.
When the screw is exploded, a section of the nut structure can be deformed. When the screw tightens the nut, the nut is deformed by pulling the nut, and at the same time, the nut is riveted on the workpiece, and then the nut is fixed on the workpiece.