Types of screws

Types of screws:

1- Ordinary screws: Ordinary screws are made of mild or low carbon steel. Ordinary screws are marked in the ASTM standard with the A307 symbol, which is mainly engraved on the screw head and in the hexagonal part. This screw is used in light structures such as small trusses, temporary structures and secondary structural members such as braces. The breaking strength of the A307 screw is about 4200 kg/cm2. In the German DIN standard, the classification of screws is determined by two numbers separated by a dot and engraved on the screw head. The first number Fu is the ultimate strength of the screw and the second number Fy indicates the yield stress of the screw in kilograms per square centimeter. For example:

A 4.6 screw has the following specifications.

Fu=400 ton/cm2=4000 kg/cm2 Fy=0/6 Fu=2400 kg/cm2

3.6 – 4.6 – 4.8 – 5.6 bolts are considered ordinary bolts. Ordinary bolts are found in the market with diameters from 12 mm (M12) to 36 mm (M36).

2- High-strength bolts: The material of high-strength bolts is medium carbon steel whose bolts and nuts are heat treated. In the ASTM standard, high-strength bolts are classified as A325 and A449 with a yield stress of about 5600 to 6300 kg/cm2 and A490 is classified as alloy steel with a yield stress of 8000 to 9000 kg/cm2. In the DIN standard, 8.8 bolts are identified with an ultimate stress of Fu=9000kg/cm2. 12.9 and 14.9 screws, which are considered very high-strength screws, are rarely found in the Iranian market. The diameter of high-strength screws is about 12 to 38, and diameters of 20 and 22 mm are more commonly used in construction work. Ordinary screws may loosen under the influence of alternating loads such as forces from vibration, oscillation, impact, and dynamic forces, and the nuts may separate from the screw. To prevent the nuts from loosening, two nuts, grooved plates on the nut, a bent or spring-loaded bar on the back of the nut, or a metal washer can be used.