What is the tolerance range of precision screws?
What is the tolerance range of precision screws?
Thread Code Broadcast Coarse Thread Series UNC Fine Thread Series UNF Extra Fine Thread Series UNEF Fixed Pitch Series UN Marking Method Broadcast Marking Method: Thread Diameter - Number of Threads per Inch Serial Code - Accuracy Grade Example: Coarse Thread Series 3/8—16 UNC— 2A Fine pitch series 3/8—24 UNF—2A Extra fine pitch series 3/8—32 UNEF—2A Fixed pitch series 3/8—20 UN—2A The first digit 3/8 indicates the outer diameter of the thread, in inches , to convert to metric unit mm, multiply by 25.4, that is, 3/8×25.4=9.525mm; the second and third digits 16, 24, 32, 20 are the number of teeth per inch (the number of teeth on the 25.4mm length); The text code UNC, UNF, UNEF, UN after the third digit is the series code, and the last two digits 2A are the precision grade. UNC: Unified Coarse Thread.
Screws are commonly found in machinery, electrical appliances and buildings. They are generally made of metal or plastic, cylindrical in shape, and grooves engraved on the surface are called screw threads. The main function of the screw is to join two objects, or to fix the position of an object. Screws can often be removed or re-tightened at will without compromising their efficiency. Generally, the diameter of the top of the screw is larger, and the common ones are round, square or regular hexagon. If the top is regular hexagon, you can turn the screw with a wrench. If the top is round, there will also be grooves on the front of the top, which is convenient for use screwdriver to turn the screw. *The common grooves are one-shaped, cross-shaped and square, and there are other shapes. The grooves of the existing screws are often easy to wear, which makes the screws inconvenient to use and causes the screws to be scrapped. In addition, because the screws have no chip removal The structure makes it difficult for the screw to penetrate other objects, which affects the efficiency of the screw.
From the perspective of use: The bolted connection of the main components of the building structure is generally connected by high-strength bolts. Ordinary bolts can be reused, but high-strength bolts cannot be reused. High-strength bolts are generally used for permanent connections. The high-strength bolts are prestressed bolts. The friction type uses a torque wrench to apply the specified prestress, and the pressure type screw off the torx head. Ordinary bolts have poor shear performance and can be used in secondary structural parts. Ordinary bolts just need to be tightened. Ordinary bolts are generally 4.4, 4.8, 5.6 and 8.8. High-strength bolts are generally 8.8 and 10.9, of which 10.9 is mostly. Level 8.8 is the same level as 8.8S. The mechanical properties and calculation methods of ordinary bolts and high-strength bolts are different. The stress of high-strength bolts is firstly by applying a pre-tension force P inside, and then generating frictional resistance on the contact surface between the connected parts to bear the external load, while ordinary bolts directly bear the external load.
A hand screw is a screw with a plastic head, and the user fixes the hand screw by turning the plastic head by hand. At present, a hand screw is disclosed in the market publication number CN202203253U, which includes a plastic head and a screw rod, the plastic head and the screw rod are connected together, and the height of the plastic head is higher than that of an ordinary screw, and the height is 13 cm. The screw of the hand screw is fixed in the plastic head by glue. After a period of use, the aging of the glue leads to the loosening of the screw and the plastic head. When the plastic head is stressed, the screw rod comes out of the plastic head, which affects the normal use of the hand screw.
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