Types and markings of threaded fasteners
The parts that play a role of tightening through threads are called threaded fasteners. Commonly used threaded fasteners are bolts, studs, screws, nuts and washers.
The structure and dimensions of threaded fasteners have been standardized. In mechanical design, it is not necessary to draw their drawings separately. You can find the required structural dimensions from the corresponding national standards according to the design requirements.
1. Types of commonly used threaded fasteners
The bolt is composed of a bolt head and a bolt rod. The shape of the head is most used in the hexagonal shape, and the commonly used grades are grade A and grade B. At the same time, there are full threads and partial threads and thick and thin rods. The size of the bolt specification is the nominal diameter d of the bolt and the length l of the bolt. After selecting a bolt, the dimensions of other parts can be checked according to the relevant national standards (see the attached table).
(2) Double-headed stud
Double-ended studs are threaded at both ends, one end is screwed into the prefabricated screw hole of the connected piece, called the screw-in end; the other end is screwed with the nut, and the other connected piece is fastened, called the tightening end. Structurally, it is divided into two types: A grade and B grade. Class A must be marked "A" before the specification, class B can not be marked. The screw-in length bm of the studs is related to the material of the connected parts with screw holes:
(1) Bronze and steel, take bm = d
(2) Cast iron, take bm = 1.25d
(3) Aluminum alloy, bm = 1.5d
(4) For non-metallic materials, take bm = 2d
The size of the double-headed stud is determined as the stud diameter d and the length of the fastening end l. After selecting a double-headed stud, the dimensions of other parts can be checked according to the relevant standards.
According to the nature of use, screws can be divided into two types: connecting screws and set screws. The former is used for connection. According to the shape of the head, there are slotted cylindrical head screws, slotted countersunk head screws, slotted pan head screws, etc. The latter is mainly used to prevent relative movement between the two matching parts, including slotted flat-end set screws, slotted tapered set screws, hexagon socket flat set screws, etc. The size of the screw specification is the screw diameter d and length l, and the other parts can be checked according to the relevant standards.
Nuts are used in conjunction with bolts or studs for connection. The form of the nut is hexagonal, round, square, etc., and the hexagonal head nut is most commonly used. Round nuts are often used for axial fixing of parts on the shaft. Hexagon head nuts are divided into types I, II, A, B, and C, as well as thickness and thickness. The size of the nut specification is the nominal diameter D of the thread. After selecting a nut, the size of other parts can be checked according to the relevant standards.
The washer is installed between the nut and the connected piece, and its purpose is to increase the contact area between the nut and the connected piece, and protect the surface of the connected piece from being scratched when the nut is tightened. In addition to flat washers, there are spring washers and stop washers, etc., which can prevent nuts from loosening due to vibration.