[Professional Knowledge] Principles for Selecting Common Fasteners—Essential for Mechanical Design Professionals

　　I. Determine the Category
　　
　　Standard fasteners are classified into ten major categories; when selecting them, one should base the choice on the application environment and intended function of the fastener.
　　
　　Bolts: Bolts are typically used in conjunction with nuts—often accompanied by one or two washers—to join and fasten components.
　　
　　Nut: A nut is used in conjunction with a bolt.
　　
　　Screws: Screws are typically used individually (sometimes with washers) and generally serve to fasten or secure components.
　　
　　Stud: A stud typically has threads on both ends (a single‑ended stud has threads on only one end). One threaded end is securely screwed into the component’s body, while the other end mates with a nut, serving to connect and fasten the parts; moreover, it often functions as a spacer to maintain a fixed distance.
　　
　　Washers: Washers are placed between the bearing surfaces of bolts, screws, nuts, and similar fasteners and the mating bearing surfaces, serving to prevent loosening and reduce stress on the bearing surfaces.
　　
　　Self-tapping screws: The threaded holes in the workpiece, which are compatible with self-tapping screws, do not require prior tapping; as the self-tapping screw is driven in, an internal thread is formed.
　　
　　Rivet: A rivet has a head at one end and a shank without threads. During installation, the shank is inserted into the holes of the joined components, and the end of the shank is then riveted to secure the joint or provide fastening.
　　
　　Pin: When in use, the pin is pressed into the workpiece and typically serves to connect or locate components.
　　
　　Retaining ring: Retaining rings are typically mounted on a shaft or within a hole, serving to restrict the axial movement of the workpiece.
　　
　　Wood screws: Wood screws are used to be screwed into wood, serving to connect or fasten components.
　　
　　II. Determining Standards and Varieties
　　
　　1. Principles for Variety Selection:
　　
　　From the perspective of processing and assembly efficiency, within the same machine or engineering project, the number of standard part types should be minimized.
　　
　　From an economic standpoint, priority should be given to selecting commercially available standard components.
　　
　　Based on the intended service requirements of standard fasteners, select the appropriate type by considering factors such as design, mechanical properties, accuracy, and thread specifications.
　　
　　2. Type:
　　
　　Bolt:
　　
　　General-purpose bolts: There are many types, distinguished by hexagonal or square heads; see national standards GB 5780–GB 5790, among others.
　　
　　Bolts for reamed holes: When in use, the bolt is tightly press-fitted into the reamed hole to prevent misalignment of the workpiece; see GB 27, etc.
　　
　　Set screws: Classified into square-necked and tapered types, as specified in standards such as GB 12–15.
　　
　　Special-purpose bolts: such as T-slot bolts, see GB 37; toggle bolts, see GB 798; anchor bolts, see GB 799, and so on.
　　
　　High-strength bolted connections for steel structures: Typically used in friction‑type connections of steel structures in applications such as buildings, bridges, towers, pipe supports, and lifting machinery; see GB 3632 and other relevant standards.
　　
　　Nut:
　　
　　General-purpose nuts: Primarily refer to hexagonal nuts, with square nuts also available; a wide variety exists, as specified in standards such as GB 41 and GB 6170–6177.
　　
　　Slotted nut: Primarily refers to the hexagonal slotted nut, which has a slot machined into its top. It is used in conjunction with stud bolts and split pins to prevent relative rotation between the bolt and the nut; see standards such as GB 6178–6181.
　　
　　Locking nuts: Nuts equipped with a locking mechanism, including nylon-insert hexagonal locking nuts—see GB 889, GB 6182, and GB 6183—and all-metal hexagonal locking nuts—see GB 6184 through GB 6187.
　　
　　Special-purpose nuts: such as washer-type nuts, see GB 62; ring nuts, see GB 63; cap nuts, see GB 802 and GB 923; knurled nuts, see GB 806 and GB 807; and insert nuts, see GB 809, among others.
　　
　　Screw:
　　
　　Machine screws are classified into numerous types according to head shape and drive type. Head styles include cylindrical, pan, countersunk, and semi-countersunk; drive types comprise slotted (flat‑head) and Phillips. See standards such as GB 65, GB 67–69, and GB 818–820.
　　
　　Set screws: Set screws, by means of their shank ends, serve to prevent relative displacement between workpieces and to transmit relatively small torques. See GB 71, GB 73–75, GB 77, GB 78, etc.
　　
　　Hex socket screws: Hex socket screws are suitable for applications where installation space is limited or where the screw head needs to be countersunk. See standards such as GB 70, GB 6190, GB 6191, and GB 2672–2674.
　　
　　Special-purpose screws: such as set screws, see GB 72, GB 828, and GB 829; non‑stripping screws, see GB 827–839 and GB 948; and eye bolts, see GB 825, among others.
　　
　　Stud:
　　
　　Unequal-length double-ended studs: Suitable for applications where one end is screwed into the component body to provide connection or fastening. See GB 897–900, among others.
　　
　　Equal-length double-ended studs: Suitable for use with end caps and nuts, serving to connect or maintain a fixed spacing. See GB 901, GB 953, etc.
　　
　　Washer:
　　
　　Flat washers: Used to compensate for unevenness on the workpiece’s bearing surface and to increase the stress-bearing area of the bearing surface. See GB 848, GB 95–97, and GB 5287.
　　
　　Spring (elastic) washers: Spring washers are specified in GB 93, GB 859, and others; elastic washers are specified in GB 860, GB 955, and others.
　　
　　Lock washers: internal-tooth lock washers, see GB 861; external-tooth lock washers, see GB 862; single‑tab lock washers, see GB 854; double‑tab lock washers, see GB 855; and lock washers for round nuts, see GB 858, among others.
　　
　　Bevel washers: To accommodate the slope of the workpiece support surface, bevel washers may be used. For I-beams, use square bevel washers; see GB 852. For channel sections, use square bevel washers; see GB 853.
　　
　　Self-tapping screws:
　　
　　Plain self-tapping screws: Their threads comply with GB 5280 (threads for self-tapping screws), have a coarse pitch, and are suitable for use in thin steel sheets as well as in copper, aluminum, and plastics. See standards such as GB 845–847 and GB 5282–5284.
　　
　　Self-tapping locking screws: Their threads conform to standard metric coarse‑thread specifications and are suitable for applications requiring vibration resistance. See GB 6560–6564.
　　
　　Rivet:
　　
　　Hot-forged rivets: Typically available in larger sizes, they are commonly used in locomotives, ships, and boilers, and their heads are usually formed by hot forging. See GB 863–866.
　　
　　Cold‑heading rivets: Typically have a nominal diameter of ≤16 mm and are usually formed by cold heading. See GB 867–870, GB 109, and other relevant standards.
　　
　　Hollow and semi‑hollow rivets: Hollow rivets are specified in GB 976; semi‑hollow rivets are specified in GB 873–875, among others.
　　
　　Special-purpose rivets: headless rivets, see GB 1016; tubular rivets, see GB 975; nameplate rivets, see GB 827, etc.
　　
　　Pop rivets are single‑sided fasteners that have become widely used in recent years. They offer speed, safety, and convenience, while also providing waterproofing and leak‑proofing benefits.
　　
　　Sales:
　　
　　Cylindrical pins: threaded and unthreaded cylindrical pins, see GB 119; internally threaded cylindrical pins, see GB 120; externally threaded cylindrical pins, see GB 878; spring-type cylindrical pins, see GB 879; slotted cylindrical pins, see GB 880, etc.
　　
　　Taper pins: cylindrical pins with or without threads, see GB 117; cylindrical pins with internal threads, see GB 118; taper pins with threaded shanks, see GB 881; split taper pins, see GB 877.
　　
　　Split pin: Typically used in conjunction with slotted bolts and slotted nuts to prevent loosening of the bolt or nut. See GB 91.
　　
　　Retaining ring:
　　
　　Elastic retaining rings: slotted elastic retaining rings—see GB 893; shaft-type elastic retaining rings—see GB 894; and split shaft-type retaining rings—see GB 896.
　　
　　Wire retaining rings: slotted wire retaining rings, see GB 895.1; shaft wire retaining rings, see GB 895.2; and wire snap rings, see GB 921.
　　
　　Retaining rings for shafts: there are conical-pin‑type retaining rings, as specified in GB 883, and screw‑type retaining rings, as specified in GB 884, GB 885, and others.
　　
　　Shaft end retaining rings: shaft end retaining rings secured with screws, as specified in GB 891, and shaft end retaining rings secured with bolts, as specified in GB 892.
　　
　　Wood screw:
　　
　　They are classified into numerous types according to head shape and slot configuration. Head shapes include round heads, countersunk heads, and semi-countersunk heads, while the head slots come in two forms: slotted (flat‑head) and cross‑recessed. See GB 99–101, GB 950–952, and other relevant standards.
　　
　　III. Mechanical Property Grades
　　
　　Bolts, screws, studs, and nuts
　　
　　Table 1–1: Property Grades of Bolts, Screws, Studs, and Nuts
　　
　　Set screw
　　
　　Table 1–2
　　
　　Self-tapping screws and self-tapping locking screws
　　
　　Plain self-tapping screws are not graded according to mechanical properties; as long as an appropriate specification is selected—refer to Clause 3.6 and Table 3 of GB 3098.5—they can meet general service requirements.
　　
　　The mechanical property grades of self-tapping locking screws are classified into two levels: A and B. Grade A denotes a high-strength class, while Grade B denotes a low-strength class.
　　
　　Stainless steel bolts, screws, studs, and nuts
　　
　　Table 1–3
　　
　　IV. Accuracy
　　
　　The precision of standard parts (fasteners) is determined by the product grade.
　　
　　Table 1–4: Product Grades and Accuracy Levels of Standard Parts (Fasteners)
　　
　　V. Threads
　　
　　Thread tolerance grade
　　
　　Thread Tolerance Grade for Standard Parts (Fasteners)
　　
　　Table 1–5
　　
　　Note: 1) Only the thread tolerances of set screws with a mechanical property class of 45H and hex socket head cap screws with a property class of 12.9 may be specified as 5g. (2) Selection of pitch.
　　
　　Comparison of Coarse-Thread and Fine-Thread Screws
　　
　　Table 1–6
　　
　　Confirm specifications
　　
　　When selecting diameter specifications, prioritize values from the first series; avoid, as much as possible, the values enclosed in parentheses in the product standard size table and those listed in the “not recommended” column of the specification table.
　　
　　Length specifications generally do not adopt dimensions other than those prescribed in the product standard. When bolts and nuts are mated for connection, it is advisable to leave a thread length of (0.2–0.3)d exposed at the end of the thread.
　　
　　For diameters and lengths that match, preference should be given to specifications within the commercially available range.