1. Introduction
1.1Background
Screws, as one of the most common fasteners, play a vital role in mechanical connections. Whether in large-scale industrial equipment, precision instruments, or daily necessities, screws play an important role in connection and fixation. The quality of screws directly affects the performance, safety ,and service life of the product. For example, under high stress or vibration environment, if the quality of screws is not up to standard, it may lead to loosening, fracture, and even cause safety accidents.
1.2The importance of quality inspection
In order to ensure that screws can work stably and reliably in various application scenarios, it is crucial to conduct comprehensive and strict quality inspections of screws. Quality inspection avoids product failure and safety accidents, improves product reliability, and enhances customer satisfaction and trust. Through the standardized quality inspection process, companies can find potential problems in the production process in a timely manner and continuously improve the process to enhance market competitiveness.
2. The screw quality inspection overview
2.1The purpose of quality inspection
Ensure that the product meets the standards and design requirements: through inspection, verify that the performance of the screws meets the relevant international standards and special requirements of customers.
Discover potential problems in the production process: timely identification and correction of deviations in the production, to prevent bulk unqualified products into the market.
2.2The basic principles of quality inspection
- Scientific: Adopt scientific methods and inspection instruments to ensure the results’ accuracy.
- Accuracy: Ensure the precision of measurement data and reduce errors.
- Objectivity: The inspection process should be fair and objective, without the influence of subjective factors.
2.3Applicable standards and norms
In the quality inspection of screws, the following international standards need to be referred to:
- ISO 898-1: Mechanical properties of bolts and studs. Read the pdf to learn more
- ISO 965: Tolerances for common threads. Read the pdf to learn more
- ISO 3269: Acceptance inspection of fasteners. Read the pdf to learn more
- ISO 4759-1: Fastener tolerances. Read the pdf to learn more
3. Material inspection
3.1Acceptance inspection of raw materials
Material supporting documents audit
Before producing screws, raw materials should be accepted first. Suppliers should provide material certificates and certificates of conformity, including information on the material’s chemical composition and mechanical properties.
3.2Material Performance Testing
3.2.1Mechanical properties testing
Tensile Strength and Yield Strength: Using a universal material testing machine, test the maximum capacity of the material to withstand the tensile process and the value of the stress at which permanent deformation occurs.
For example, the tensile strength of Grade 8.8 bolts should reach 800 MPa, and the yield strength should not be less than 640 MPa.
3.2.2Hardness test
Brinell hardness (HB) and Rockwell hardness (HRC): Assess a material’s hardness to ensure that it is sufficiently resistant to plastic deformation.
Commonly used 8.8 grade carbon steel bolts, its surface hardness is generally around 22 HRC.
4. Size and appearance inspection
4.1Size inspection
4.1.1Thread size measurement
Pitch and thread diameter:
- Use thread ring gauge to check the outer diameter of thread (major diameter).
- Use thread projector to measure the center diameter and small diameter to ensure the accuracy of thread processing.
4.1.2Head size measurement
Head diameter, head height, countersunk head angle:
- Use vernier calipers or micrometers to measure each dimension of the head to ensure compliance with design specifications.
4.1.3Length Measurement
Overall length and thread length:
- Use measuring instruments to check the overall length of the screw and the length of the threaded part, and the tolerance should be in accordance with the standard requirements.
4.1.4Tolerance requirements
Dimensional tolerances are determined in accordance with ISO 4759-1 or the corresponding national standards and are usually categorized into three accuracy levels: A, B, and C.
- Grade A: High precision, used for critical parts or occasions requiring high assembly precision.
- Class B: medium precision, widely used in general machinery.
- Grade C: low precision, used for occasions with low dimensional requirements.
4.2Appearance Inspection
4.2.1Surface roughness inspection
Measured by surface roughness meter to ensure that the surface meets the design requirements of finish and reduces stress concentration points.
- Typical Requirements:
Fastener surface roughness is generally required to be Ra no greater than 12.5 μm.
4.2.2Surface Defect Detection
Cracks, burrs, rust, indentations, etc:
Visual inspection or use of magnifying glass, timely detection, and treatment of surface defects, to prevent the impact of screw performance.
- Testing Requirements:
Surface defects should comply with the provisions of ISO 6157 series standards.
Cracks, obvious bursts, and rust are not allowed.
4.2.3Plating integrity and uniformity
Ensure the uniformity of the surface finish (e.g., galvanizing) without leakage of plating, blistering, etc.
- Testing Requirements:
Plating quality assessment in accordance with ISO 4042 or ASTM B633 (Standard Specification for Electro-galvanizing of Steel Parts).
4.2.4High Volume Automated Inspection
In mass production, manual inspection is inefficient and has a high rate of misjudgment, so automated inspection equipment is used:
- Fully automated optical inspection machine:
Detection principle: through the high-precision camera and image processing software, automatically measure the various dimensions of the screw.
Detection items: length, diameter, head size, thread integrity, etc. - Advantage:
High efficiency: fast full inspection for a large number of products.
High precision: avoid human error.
Data recording: test results can be saved for easy traceability and analysis.
5.Mechanical Performance Test
5.1Tensile Test
- Test purpose: To verify the tensile strength and yield strength of screws under tensile load.
- Test method: Use a universal material testing machine, according to the specified tensile speed, apply tensile force to the screws, and record the stress-strain curve during the tensile process.
- Test Requirements: The test method shall be in accordance with ASTM F606/F606M (Mechanical Test Methods for Fasteners).
Specimens shall be prepared and mounted in accordance with the standard, paying attention to the selection of fixtures to prevent deviations during the Test.
Tensile speed should be controlled below 25 mm/min.
5.2Torque Test
- Test Objective: To evaluate the torque performance of screws during tightening to ensure that they meet the installation requirements.
- Relationship between torque and preload:
Through the torque test, the relationship between the tightening torque and the resulting preload force can be determined to prevent over-tightening or over-loosening. - Test Requirements:
The test method shall be in accordance with ISO 16047 (Fastener Torque/Clamping Force Test) or ASTM F606/F606M.
The torque-clamping force curve shall be within the specified range, e.g., the recommended torque value is 70%-80% of the yield torque of the thread specification.
5.3Hardness Testing
- Evaluate surface and core hardness:
Using a microhardness tester or Rockwell hardness tester, test the hardness distribution on the surface and in the cross-section of the screw. - Ensure heat treatment effectiveness:
The hardness test helps to verify that the heat treatment process meets the expected requirements.
6. Surface treatment and coating inspection
6.1Coating thickness measurement
Use coating thickness gauge:
- Magnetic method: For non-magnetic coatings on ferromagnetic substrates.
- Eddy current method: For non-conductive coatings on non-ferromagnetic metals.
6.2Corrosion resistance test
Salt spray test (neutral salt spray test):
- In accordance with ISO 9227, samples are placed in a salt spray chamber to simulate the corrosion of coatings in a marine environment.
Test cycle and judging criteria:
Test Requirements:
- Salt solution concentration of 5% (mass fraction) sodium chloride solution, pH between 6.5-7.2.
- The temperature is maintained at 35°C ± 2°C.
- The spray volume is 1-2 mL/h (80 cm² horizontal area).
- General galvanized screws require a neutral salt spray test of 48/72 hours without red rust.
For screws with high corrosion resistance requirements (e.g., for use in marine environments), it may be necessary to achieve more than 500 hours.
Judging Criteria:
- Red rust or substrate corrosion is not allowed in accordance with ASTM B537 (Standard Method for Appearance of Plating after Salt Spray Test).
7 Thread Inspection
7.1 Thread gauge inspection
Through and stop gauge method
- Use through gauge and stop gauge:
The through gauge should be able to be screwed into the thread smoothly and pass through the whole length.
The stop gauge should not be able to be screwed in to ensure that the thread size is within the allowable tolerance. - Testing Requirements:
The through and stop gauges shall conform to ASME B1.2 (Unified Inch Thread Gauge) or ASME B1.16M (Metric Thread Gauge).
Inspection should be performed without lubrication to prevent influencing the results.
7.2Thread Accuracy Test
- Thread Profile Projector Test:
Magnify the thread cross-section and measure parameters such as tooth angle and pitch. - Thread Combined Error Measurement:
Evaluates the cumulative error of threads to ensure smooth assembly.
7.3Thread Damage Inspection
- Thread integrity:
Check threads for missing teeth, deformation, etc. - Thread surface defects:
Visually inspect the thread surface for scratches, burrs, and other defects.
8. Sampling program and statistical quality control
Sampling standard
- ISO 2859 sampling program:
Internationally recognized sampling inspection standard.
Sampling method
- Random sampling:
Random selection of samples from the batch to prevent human bias. - Stratified sampling:
For multiple production batches, proportionally sampling from each batch is used to improve representativeness.
9. Quality records and document management
Inspection report preparation
- Records of test items, methods, and results:
Detailed records of the results of each test item as proof of quality. - Judgment conclusion and treatment measures:
Suggest treatment for unqualified items, such as rework, downgrade, or scrap.
10. Common quality problems and solutions
10.1Size exceeds the difference
- Reason:
Insufficient machining precision, equipment wear and tear. - Solution:
Regularly calibrate the equipment, adjust the process parameters, and strengthen the equipment maintenance.
10.2Hardness failure
- Cause:
Improper heat treatment process, unstable furnace temperature. - Solution:
Optimize the heat treatment process, ensure furnace temperature uniformity, and calibrate the equipment regularly.
10.3Poor adhesion of plating
- Reason:
Surface pretreatment is not sufficient; the plating solution composition is not qualified. - Solution:
Improve the surface cleaning and activation process and strictly control the plating solution formula and process parameters.
10.4Thread failure
- Reason:
Problems in the rolling process, mold wear. - Solution:
Check and adjust the rolling process, and regularly replace or repair the mold.
11 Conclusion
The key role of quality inspection
Through comprehensive and systematic quality inspection, we can ensure the high quality of screw products so as to enhance the market competitiveness and brand image of the enterprise.
