1. Structural Characteristics of Various Fasteners
1.1 Head Structure
The primary functions of bolt heads and screw heads are load bearing and wrenching. Common head types include hexagon head, double hexagon head, spline head, pan head, countersunk head, cylindrical head and other special heads. Each structure corresponds to specific application scenarios and assembly requirements.
1.2 Wrenching Structure
The wrenching structures of bolts and screws are divided into external wrenching structures and internal wrenching structures. Common external wrenching forms cover hexagon head, twelve-point head, spline head, ten-point head, octagon head, square head and pentagon head, among which hexagon head and twelve-point head are the most widely used. Internal wrenching structures are mostly adopted for revolving heads such as countersunk heads, pan heads and cylindrical heads. Typical internal wrenching types include slotted drive, Phillips drive, internal hexagon and internal spline (Torx). Besides, there are patented foreign slot types: high-torque Phillips drive and high-torque slotted drive.
Under normal circumstances, external wrenching structures feature better anti-slip and torsion resistance than internal ones. Hence, high-strength fasteners mostly adopt external wrenching structures. Nevertheless, external wrenching requires larger assembly space and is subject to more restrictions on installation environments. Among internal wrenching types, internal hexagon and internal Torx deliver remarkably superior torsion performance and can be applied to high-strength working conditions. Internal hexagon holes are generally formed by pre-drilling pilot holes followed by punching with hexagonal punches. Metal debris produced by punching tends to accumulate at the hole bottom and may fall off during service, generating foreign contaminants. For applications with strict control over foreign objects, all residues inside holes must be thoroughly cleaned.
1.3 Shank Structure
The bolt shank refers to the smooth unthreaded section. According to shank diameter, it is classified into major-diameter shank, pitch-diameter shank, stepped shank and enlarged shank.
Major-diameter shank (standard shank): Its nominal diameter equals the thread major diameter with two tolerance grades. Loose tolerances (h12, h14) are mainly used for tension bolts; tight tolerances (f9, f7, r6) apply to shear bolts.
Pitch-diameter shank (thin shank): Its nominal diameter approximates the thread pitch diameter, normally with h12 tolerance or no special tolerance requirements. It is suitable for tension bolts.
Stepped shank (waisted shank): Its diameter is less than or equal to the thread minor diameter, designed to reduce bolt stiffness and relieve stress concentration at thread roots.
Enlarged shank (reinforced shank): Its diameter exceeds the thread major diameter. It can form interference fit with assembly holes and is commonly used for equipment maintenance and reinforcement.
1.4 Thread Structure
Thread parameters include effective thread length, thread accuracy, thread profile, thread root radius, thread runout and shoulder distance. For bolts of the same standard and nominal diameter, the effective thread length is fixed. Changes in overall bolt length are achieved by adjusting the length of the smooth shank. Threads are categorized into long thread, medium thread and short thread:
Long thread: Matched with tension bolts to bear large tensile loads;
Short thread: Matched with shear bolts, relying on surface contact for anti-drop positioning with minimal tensile load;
Medium thread: Applied to bolts under combined tension and shear loads, capable of bearing both tension and shear.
2. Exclusive Structure of Studs
Studs have no bolt-style heads and are fully threaded at both ends. Threads on two ends can be manufactured with identical or different diameters. During installation, one end is screwed into the base component for permanent fixing with infrequent disassembly; the other end mates with nuts for repeated assembly and disassembly. To guarantee tight fastening of the embedded end, transition-fit threads are adopted for interference locking. Retaining keys and other structures can also be used for anti-loosening fixation.
3. Overall Properties of Bolts, Screws and Studs
Fastener properties fall into mechanical properties and metallurgical properties. Mechanical properties evaluate the capacity of fasteners to withstand various external loads from a macroscopic perspective, with core indicators including tensile property, shear property and fatigue resistance. Metallurgical properties assess the microscopic quality of material structures, covering metal flow lines, metallographic structure, and material discontinuities such as porosity and cracks.
4. Classification of Fastener Performance Grades
Two major fastener standard systems are adopted domestically: the general-purpose national standard system (GB system) and the aerospace-specific fastener system. Differences in service environments and technical indicators lead to distinct grading rules for mechanical properties between the two systems.
4.1 Performance Grades under the GB National Standard System
There are 12 current national standards specifying mechanical properties for bolt-series fasteners in China.
(1) Performance Grades of Carbon Steel and Alloy Steel Bolts, Screws and Studs
Nine grades are defined based on tensile strength and yield strength: 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 and 12.9. Conventionally, grades below 8.8 are made of carbon steel, while grades 8.8 and above adopt alloy steel with dedicated heat treatment.
Marking format: X.Y. Rules are defined as follows:
The number before the decimal point equals nominal tensile strength (MPa) divided by 100;
The number after the decimal point equals yield ratio multiplied by 10;
Nominal yield strength = (product of the two numbers) ÷ 10.
Example: Grade 9.8 bolt. The digit 9 means nominal tensile strength = 9 × 100 = 900 MPa; the digit 8 means yield ratio = 8 ÷ 10 = 0.8.
(2) Performance Grades of Stainless Steel Bolts, Screws and Studs
Stainless steel fasteners are divided into three categories (austenitic, martensitic, ferritic) and nine material groups: A1, A2, A3, A4, A5, C1, C3, C4, F1. Combined with forming processes, 23 complete performance grades are formed.
Marking rule: material group + two-digit strength number. Letters stand for material categories: A = austenitic stainless steel, C = martensitic stainless steel, F = ferritic stainless steel. The two digits represent one-tenth of the tensile strength value.
Examples: A2-70 refers to Group 2 austenitic stainless steel formed by cold working with minimum tensile strength of 700 MPa; C3-80 refers to Group 3 martensitic stainless steel treated by quenching and tempering with minimum tensile strength of 800 MPa.
(3) Performance Grades of Non-Ferrous Metal Fasteners
Common non-ferrous materials include copper alloy and aluminum alloy. Grading is determined by tensile strength, and these fasteners are generally classified as low-strength types. Fasteners of identical grade and specification share the same minimum tensile load. Grades correspond to material grades and tensile indicators. Copper alloys have seven grades: CU1, CU2, CU3, CU4, CU5, CU6, CU7; Aluminum alloys have six grades: AL1, AL2, AL3, AL4, AL5, AL6.
Example: CU2 stands for Grade 2 copper alloy corresponding to grade H63 with tensile strength of 370 MPa. Marking consists of two letters plus one digit. Letters correspond to chemical element symbols (CU for copper Cu, AL for aluminum Al); digits are serial numbers only, not arranged in ascending strength order, and match fixed material grades and mechanical parameters.
(4) Performance Grades of Set Screws
Set screws are also externally threaded fasteners but adopt different grading criteria from ordinary bolts. They are classified by hardness rather than tensile strength.
Carbon steel and alloy steel set screws: four grades, namely 14H, 22H, 33H, 45H. The number denotes one-tenth of the minimum Vickers hardness, and H represents hardness. For instance, 14H means the minimum Vickers hardness = 14 × 10 = 140 HV.
Stainless steel set screws: only austenitic stainless steel is used, divided into five groups consistent with stainless steel bolts: A1, A2, A3, A4, A5 with identical chemical composition. Two hardness grades are specified: 12H and 21H. 12H applies to soft austenitic set screws with minimum hardness of 125 HV; 21H applies to cold-worked hardened set screws with minimum hardness of 210 HV.
