導入

GB/T 3098.4 is a Chinese national standard that specifies the mechanical properties of fine pitch thread nuts for fasteners. This standard covers marking systems, materials, and mechanical performance requirements for nuts with fine threads, ensuring compatibility with bolts, screws, and studs. It applies to nuts with nominal heights and effective thread lengths as defined, focusing on performance grades to guarantee reliability in mechanical assemblies. The standard emphasizes preventing thread stripping failures by prioritizing bolt fracture in over-torquing scenarios, enhancing safety in applications.

Marking System

The marking system in GB/T 3098.4 distinguishes nuts based on their nominal height and effective thread length.

3.1 Nuts with Nominal Height ≥ 0.8D (Effective Thread Length ≥ 0.6D)

These nuts are marked using the first digit of the bolt performance grade they are compatible with, selecting the highest compatible grade from Table 2. Higher performance grade nuts can generally replace lower ones. Bolt-nut assemblies can sustain stresses above the bolt’s yield or proof stress.

Over-tightening may lead to failures such as bolt fracture, bolt thread stripping, nut thread stripping, or combined stripping. Bolt fracture is preferred as it is sudden and detectable, unlike gradual stripping. For threads from 8 to 39 mm, proper nut selection per Table 2 ensures no stripping at the bolt’s proof load. Designs should ensure bolt fracture at 10% over-tightening to warn of improper assembly.

For detailed information on assembly strength and nut types, refer to GB/T 3098.2 Appendix A.

3.2 Nuts with Nominal Height ≥ 0.5D but < 0.8D (Effective Thread Length ≥ 0.4D but < 0.6D)

These nuts are marked with two digits: the second digit represents 1/100 of the nominal proof stress (in N/mm²) measured with a hardened test mandrel; the first digit “0” indicates reduced load capacity compared to 3.1 nuts. Actual capacity depends on nut hardness, effective length, and mating bolt tensile strength. See Table 3 for marking and proof stress.

Materials

Materials for each performance grade are specified in Table 4, with chemical compositions compliant to relevant standards. Grades 05, 8 (Type 1), 10, and 12 require quenching and tempering. Alloy elements may be added for improved properties.

Note 1: Performance grades 5 and 6 can use free-cutting steel (unless otherwise agreed), with max S 0.34%, P 0.11%, Pb 0.35%.

Note 2: Alloy additions are optional for grades 8, 10, and 12 to enhance mechanical properties.

機械的特性

Mechanical properties must meet Table 5 when tested per Chapter 8. Surface defects comply with GB/T 5779.2.

Proof Load and Failure Load

Table 6 provides proof load values for fine pitch nuts. Stress area A_s is calculated as A_s = (π/4) * (d₂ + d₃)/2², where d₂ is pitch diameter, d₃ = d₁ – H/6, H = 0.866025P.

Table 7 provides ratios for minimum stripping strength relative to bolt proof loads for nuts with nominal height ≥ 0.5D but < 0.8D.

よくある質問

What is the purpose of preferring bolt fracture over thread stripping in GB/T 3098.4?

Bolt fracture is sudden and easily detectable, reducing accident risks from undetected gradual stripping failures.

How does the marking system differ for thin nuts in this standard?

For nuts with height < 0.8D, marking uses “0” followed by 1/100 of nominal proof stress, indicating reduced capacity compared to standard height nuts.

Can higher grade nuts replace lower ones?

Yes, generally, but compatibility with bolt grades and thread sizes must be verified per Table 2 to ensure performance.

What heat treatment is required for grade 10 nuts?

Grade 10 nuts require quenching and tempering to achieve specified hardness and proof stress.

How is the stress area calculated for proof load?

Using the formula A_s = (π/4) [(d₂ + d₃)/2]², where d₂ and d₃ are derived from thread dimensions.

What surface defect standards apply?

Surface defects must comply with GB/T 5779.2 to ensure integrity.