| General machinery assembly Indoor, moderate temperature, mainly axial clamping | Preload must remain stable under normal vibration and service loads. | Carbon or alloy steel, property class 8.8 | M8 × 1.25 to M16 × 2.0, selected after calculating the required preload. | Minimum tensile strength: approximately 800 MPa.
Nominal yield strength: approximately 640 MPa.
Proof stress: approximately 580 MPa or higher, depending on the applicable standard. | Use a compatible nut with a suitable strength class. Check thread engagement, washer seating, tightening method, and access for installation. |
| High-load structural or industrial equipment Repeated axial loading or higher clamping force | High tensile capacity with controlled preload and resistance to fatigue. | Alloy steel, property class 10.9 | M10 × 1.5 to M20 × 2.5, subject to the calculated tensile-stress area. | Minimum tensile strength: approximately 1,000 MPa.
Nominal yield strength: approximately 900 MPa.
Proof stress: approximately 830 MPa. | Avoid over-tightening. Verify joint stiffness, fatigue loading, thread run-out, nut compatibility, and whether the mating component can withstand the higher clamp load. |
| Outdoor equipment in normal atmospheric exposure Moisture, rain, and intermittent corrosion risk | Adequate tensile strength combined with improved corrosion resistance. | Austenitic stainless steel, A2-70 | M8 × 1.25 to M16 × 2.0 for general equipment; use larger sizes when required by the calculated load. | Minimum tensile strength: approximately 700 MPa.
Minimum proof stress: approximately 450 MPa. | Stainless steel can gall during tightening. Use suitable assembly practice, avoid mixing incompatible metals where galvanic corrosion is possible, and account for lower proof strength than class 8.8 steel. |
| Marine, chemical, or high-corrosion environment Salt spray, chlorides, or frequent chemical exposure | Corrosion resistance is the primary requirement, but preload must still be maintained. | Austenitic stainless steel, A4-80, where suitable for the environment | M10 × 1.5 to M20 × 2.5, with size confirmed by load and corrosion calculations. | Minimum tensile strength: approximately 800 MPa.
Minimum proof stress: approximately 600 MPa. | Confirm resistance to the specific chemicals and chloride concentration. Check crevice corrosion, galling, temperature limits, and compatibility with the mating material. |
| Elevated-temperature service Engines, exhaust systems, boilers, or heated tooling | Retention of strength and preload at the actual operating temperature. | Heat-resistant alloy steel or a material specifically qualified for the service temperature | Commonly M8 × 1.25 to M20 × 2.5, but thread size alone does not establish temperature suitability. | Room-temperature strength values are not sufficient for design. Use certified elevated-temperature yield, creep, and relaxation data. | Check thermal expansion, relaxation, oxidation, thread lubrication, thermal cycling, and the temperature rating of the nut and washer materials. |
| Vibration-prone machinery Rotating equipment, pumps, motors, or cyclic operation | Fatigue resistance and resistance to loosening are more important than static tensile strength alone. | Alloy steel, commonly property class 8.8 or 10.9, selected for fatigue requirements | M10 × 1.5 or larger may be preferred when space permits, because larger thread areas reduce tensile stress. | Static proof and tensile values do not define fatigue life. Fatigue performance depends on preload, stress amplitude, surface condition, and joint geometry. | Use positive locking or a qualified prevailing-torque solution when appropriate. Check joint separation, bending, transverse slip, thread fit, and tightening consistency. |
| Flange, pipe, or pressure-containing joint Axial clamping with sealing-gasket requirements | The bolt set must provide the required gasket seating load without exceeding bolt or flange limits. | Material and property class selected from the equipment design code and service temperature | Size and quantity must be determined from required gasket load, flange geometry, and allowable stresses. | Do not select solely from nominal tensile strength. The design must consider allowable stress, preload scatter, relaxation, and pressure-temperature conditions. | Verify bolt circle, grip length, flange stiffness, gasket type, tightening sequence, hydraulic or thermal effects, and applicable pressure-equipment requirements. |
| Limited installation space or blind assembly Studded connection with one end permanently installed | Correct engagement at both ends and sufficient usable length for the nut and washer. | Match the material and strength class to the connected components and service environment. | Specify both thread lengths separately, for example: M12 × 1.75 on one end and M12 × 1.75 on the other, with a defined overall length. | Strength is governed by the smallest effective tensile area and the weakest engaged thread or component. | Check installation-end and nut-end engagement, unthreaded grip length, thread run-out, insertion depth, protrusion, and the possibility of bottoming in a blind hole. |
| Lightweight or non-metallic mating component Aluminum, polymer, or thin-wall housing | Prevent pull-out, crushing, thread stripping, and excessive local deformation. | Bolt material may be carbon steel, alloy steel, or stainless steel; the mating component usually controls the design. | Use a larger diameter, longer engagement, insert, sleeve, or through-bolt arrangement when needed. | Bolt tensile strength alone is not enough. Check internal-thread shear area, bearing stress, pull-out strength, and local component stiffness. | Use a controlled tightening torque and suitable washers or load-spreading features. Avoid selecting a high-strength bolt that can damage the weaker component. |
| Basic sizing reference: Required tensile-stress area can be estimated as As ≥ F × S / σallow, where F is the maximum service tensile load, S is the design safety factor, and σallow is the permitted tensile stress. Final selection must also check shear, bending, fatigue, thread stripping, preload loss, temperature, corrosion, and the strength of the connected parts. |
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