Performance grades of bolts for steel structure connections include 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9. Bolts of Grade 8.8 and above are manufactured from medium carbon steel or low-alloy steel and subjected to quenching and tempering heat treatment, which are collectively defined as high-strength bolts. The rest are ordinary bolts. A bolt performance grade consists of two sets of figures, representing the nominal tensile strength and yield-to-tensile ratio of the material respectively.
For a Grade 4.6 bolt: its nominal tensile strength is 400 MPa, and the yield-to-tensile ratio is 0.6. The nominal yield strength is calculated as 400×0.6=240 MPa.
For a Grade 10.9 high-strength bolt: after heat treatment, the nominal tensile strength reaches 1000 MPa with a yield-to-tensile ratio of 0.9. The nominal yield strength is 1000×0.9=900 MPa.
Bolt performance grades follow international general standards. Bolts with the same performance grade deliver identical mechanical properties regardless of material and origin. Designers may select bolts simply according to performance grades.
A Grade 8.8 bolt has a nominal tensile strength of 800 N/mm2 and a nominal yield strength of640 N/mm2.
General calculation rule for bolt strength marked as X.Y:
The nominal tensile strength equals X×100.
The nominal yield strength = Nominal tensile strength ×(Y÷10).
Example: A Grade 4.8 bolt has a tensile strength of 400 MPa and a yield strength of 400×8÷10=320 MPa.
Stainless steel bolts are usually marked as A2-70, A4-70, etc., which adopt independent criteria different from the above metric grade rules.
1 Units of Measurement
Two major length measurement systems are widely used worldwide: metric system and imperial system. The metric system uses meters, centimeters and millimeters, which are popular in China, Europe, Japan and Southeast Asia. The imperial system is based on inches and is mainly applied in the United States, the United Kingdom and other European and American countries.
Metric conversion (decimal system):
1 m=100 cm=1000 mm
Imperial conversion:
1 inch = 8 eighths; 1 inch = 25.4 mm
Example: 83 inch×25.4=9.52 mm
Thread fasteners with nominal diameter less than 1/4 inch are designated by screw numbers, commonly including 4#, 5#, 6#, 7#, 8#, 10# and 12#.
2 Thread Classification
A thread is a continuous helical ridge formed on the inner or outer surface of a component. According to structural features and applications, threads are divided into three categories:
- General-purpose thread: Triangular thread profile, used for connecting and fastening components. It is classified into coarse thread and fine thread. Fine thread provides higher joint strength.
- Power transmission thread: Profiles include trapezoidal, rectangular and saw-tooth shapes, mainly used for motion and power transmission.
- Sealing thread: Designed for sealed connections, mainly including pipe thread, taper thread and taper pipe thread.
3 Thread Fit Classes
Thread fit defines the tightness between mating internal and external threads. Fit classes are specified by the combination of limit deviations and tolerances for internal and external threads.
3.1 Unified Imperial Thread
External thread classes: 1A, 2A, 3A
Internal thread classes: 1B, 2B, 3B
All are clearance fits. A higher class number means a tighter fit. Limit deviations are specified for Class 1A and 2A only; Class 3A has zero fundamental deviation. Class 1A and 2A share the same limit deviation value. A higher class number indicates a smaller tolerance zone.
- Class 1A & 1B: Largest tolerance zone, for low-precision connections.
- Class 2A & 2B: The most commonly used tolerance classes for imperial mechanical fasteners.
- Class 3A & 3B: Tightest fit with strict tolerance requirements, applied to critical structures with high safety requirements.
Tolerance comparison: The tolerance of Class 1A external thread is 50% larger than Class 2A and 75% larger than Class 3A. For internal threads, Class 1B tolerance is 50% larger than Class 2B and 75% larger than Class 3B.
3.2 Metric Thread
Common tolerance zones for external threads: 4h, 6h, 6g
Common tolerance zones for internal threads: 5H, 6H, 7H
Japanese standards divide thread precision into Class Ⅰ, Ⅱ and Ⅲ. Class Ⅱ is adopted for general working conditions.
For metric threads: Tolerance zones H and h have zero fundamental deviation; G has positive fundamental deviation, while e, f and g have negative fundamental deviation.
H is the most widely used tolerance zone for internal threads, suitable for uncoated threads or threads with thin phosphating coating. G is rarely used and only applied to special occasions with thick coatings.
Tolerance zone g is generally used for threads with thin coating of 6~9 μm. If the finished bolt requires 6h tolerance, the thread shall be machined to 6g to reserve coating allowance.
Recommended thread fit combinations: H/g, H/h, G/h. For precision fasteners such as bolts and nuts, 6H/6g is the preferred standard fit.
4 Thread Parameters & Self-Tapping Thread Specifications
4.1 Main Geometric Parameters
- Major diameter: Diameter of the imaginary cylinder coinciding with the thread crests, approximately equal to the nominal thread diameter.
- Minor diameter: Diameter of the imaginary cylinder coinciding with the thread roots.
- Pitch: Axial distance between corresponding points of two adjacent thread crests measured along the pitch line. Pitch of imperial threads is expressed by the number of threads per inch.
4.2 Specifications of Common Self-Tapping Threads
(1) Metric Self-Tapping Threads
| Size | ST1.5 | ST1.9 | ST2.2 | ST2.6 | ST2.9 | ST3.3 | ST3.5 | ST3.9 | ST4.2 | ST4.8 | ST5.5 | ST6.3 | ST8.0 | ST9.5 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pitch | 0.5 | 0.6 | 0.8 | 0.9 | 1.1 | 1.3 | 1.3 | 1.3 | 1.4 | 1.6 | 1.8 | 1.8 | 2.1 | 2.1 |
(2) Imperial Self-Tapping Threads
| Size | 4# | 5# | 6# | 7# | 8# | 10# | 12# | 14# |
|---|---|---|---|---|---|---|---|---|
| Threads per inch (AB type) | 24 | 20 | 20 | 19 | 18 | 16 | 14 | 14 |
| Threads per inch (A type) | 24 | 20 | 18 | 16 | 15 | 12 | 11 | 10 |
