Weld Nut and Weld Stud Tightening Checks (Testing & Inspection)

Ever wondered how the integrity of welded nuts and screws in your car is ensured? This article reveals the meticulous process of quality checks and inspections that keep your vehicle safe and reliable. Learn how various tests and standards are applied to guarantee the strength and durability of these crucial components.

Expert Guide to Weld Nut and Weld Stud Tightening Checks

Table Of Contents

Introduction

The tightening apparatus for weld nuts and weld screws necessitates assurance of quality during the installation process. Ensuring the integrity of these components is crucial to maintaining the overall reliability and safety of the assembled structure. To evaluate the weld quality and process reliability, it is essential to monitor the installation strength throughout the procedure. This continuous monitoring helps in detecting any deviations or anomalies that could compromise the weld’s integrity.

To further substantiate the reliability of the installation process, additional quality assurance measurements can be integrated into the production workflow. These measures may include real-time data collection and analysis, automated inspection systems, and statistical process control (SPC) techniques. By incorporating these advanced quality assurance methods, manufacturers can potentially obviate the need for subsequent inspections of weld nuts and weld screws, thereby streamlining the production process and ensuring consistent quality.

1. Overview

This standard document addresses the tightening of welded nuts and studs on steel plates and outlines the installation process for bolts. It details the welding installation conditions for the entire vehicle. Appropriate departments are responsible for these inspections. Methods not mentioned in the document should not be used.The manufacturing department requires process inspection. In the event of quality issues, the Quality Department can increase random inspections. For improvements in quality and system, and in response to quality issues, destructive testing of the vehicle frame is necessary.

2. Other applicable documents

  • MBN 73B – Hexagonal Nuts
  • MBN 73C – Square Nuts
  • MBN 75 – Threaded Weld Studs
  • MBN 10176 – Hexagonal Nuts with Flange
  • MBN 10369 – Round Nuts
  • MBN 10390 – Dome-shaped Round Nuts
  • MBN 10391 – Weld Studs with Welding Ring
  • N13008 – Flanged Nuts
  • DIN EN ISO 14270 – Sample Sizes and Testing Procedures for Mechanical Stripping of Weld Spots and Seams
  • DIN EN ISO 14272 – Sample Sizes and Testing Procedures for Cross Tensile Testing of Weld Spots
  • DIN EN ISO 14273 – Sample Sizes and Testing Procedures for Shear Testing of Weld Spots and Seams

3. Use of Abbreviations, Definitions, and Symbols

Boundary Weld: A joining weld is an incomplete fusion weld, where the stud merely adheres to the metal component without the required strength.

4. Material and Cycle Specifications

To control materials and cycles, all materials, methods, processes, parts, and systems should comply with the applicable legal specifications.

5. Description

The following content can only be applied to steel plate welding.

6. Installation Categories

Steel Plate Nuts

Category A

Category B

Styles

  • Square Nut (MBN 73C or DIN 928)
  • Hexagonal Nut (MBN 73B or DIN 929)
  • Round Weld Nut (MBN 10369)

Style A                      Style B

  • Hexagonal Nut with Flange (MBN 10176)
  • Round Nut

Weld Stud

  • Class A
  • Class B

Example: MBN 75        MBN 10391

7. Non-Destructive Testing

7.1 Description of Procedure Sequence

All welds identified as defective through parameter monitoring (such as color marking) must be repaired. Additional test samples must be separated from the specified random test pieces being produced. Relevant process documents must be referred to when inspecting weld studs and weld nuts. The inspection department needs to record the inspection process in detail, including the methods used to identify defects in the test pieces.

7.2 Visual Inspection

7.2.1 Inspection Process

Visual inspections must comply with established evaluation standards. These inspections should be conducted by trained inspectors under appropriate distance and lighting conditions to ensure accuracy and consistency.

7.2.2 Inspection Records

Visual inspections must be meticulously recorded in a checklist. Any confirmed defects, such as weld points trending towards the edge, must be immediately addressed and corrected within the production system or welding equipment to prevent recurrence.

7.2.3 Evaluation Standards

The inspection of weld nuts must adhere to the standards outlined in the table below. These standards ensure that all welds meet the required quality and safety criteria.

Serial NumberEvaluation Criteria:Example
1Missed welds of studs/nuts  
2Damage or contamination of studs/nuts (including weld spatter and thread damage)  
3The gap is inappropriate
h > 0.1m
  
4Deviation from the center position
Nuts must not obstruct the installation of bolts.
  
Reference values:
For nuts with M ≤ 5, S should be ≤ 1mm.
For nuts with M ≥ 6, S should be ≤ 2mm.
For arched round nuts, S should be < 0.8mm.
 

By adhering to these standards, the quality and reliability of the welds can be maintained, ensuring the safety and performance of the final product.

7.3 Torque Testing

7.3.1 Welded Nuts

External Inspection

Before conducting torque testing on welded nuts, an external inspection is mandatory. This inspection must adhere to the standards specified in section 7.2.3. The purpose of this inspection is to identify any visible defects or irregularities that might affect the integrity of the weld or the performance of the nut during torque testing.

Torque Testing Procedure

Torque testing is a critical step to ensure the strength and reliability of welded nuts. The following steps outline the proper procedure:

  1. Selection of Torque Wrench: Utilize a torque wrench that is calibrated and falls within the appropriate testing range for the specific type of welded nut being tested.
  2. Application of Torque: Gradually apply torque to the nut. It is essential to increase the torque steadily to avoid sudden stress that could lead to premature failure.
  3. Observation of Weld Seam: Carefully monitor the weld seam during the application of torque. The primary focus is to detect any signs of shearing or cracking.
  4. Assessment of Strength: If the weld seam shears or cracks before reaching the minimum specified torque, the welded nut is considered to have insufficient strength and fails the test.
Testing Standards

The detailed standards and procedures for torque testing of welded nuts are provided in section 7.3.3. These standards outline the specific torque values, testing conditions, and acceptance criteria that must be met to ensure the welded nuts are fit for use.

Torque Measurement
M46 Nm
M58 Nm
M614 Nm
M832 Nm
M1070 Nm
M12100 Nm

Note: Torque inspection must be conducted on square and hexagonal nuts, while the thickness of the steel plate need not be considered for arch-shaped and round nuts.

7.3.2 Welding Studs

During the torque inspection process, first tighten the two nuts on the welding stud (as shown in Figure 1), then apply a predetermined Mtest inspection torque with a suitable torque wrench, thereby subjecting the nut to a torsional load (as shown in Figure 2).

Figure 1: Side View of the Weld Stud
Figure 2: Inspection Equipment (Welding Stud, Two Nuts, Torque Wrench)

Subsequently, remove the two nuts. It’s plausible to employ an appropriate nut (as shown in Figure 3). Install the nut fully, then perform a torque check.

Figure 3: Selectable Nut (h: Depending on Actual Conditions)

7.3.3 Evaluation Criteria

Following torque inspection, the installation of screws and nuts must be assessed based on the descriptions provided in the table below.

Serial NumberAssessment Criteria Example 
1Screws or nuts must not be looseScrews or nuts must not be loose  
2Weld seams cannot exhibit separation  
3Weld seams must not be damaged (cracked)Weld seams must not be damaged (cracked)  

7.3.4 Torque Testing of Circular Nuts MBN 10369 and MBN 10390 (Arch Nuts)

Before conducting torque tests on the circular nuts, visual inspections must be performed first as specified in section 7.2.3. Start by screwing a bolt into the circular nut using a torque wrench with an appropriate torque range.

During the test, the nut is subjected to a torque by the screw. If a crack appears at the seam before reaching the minimum torque, it is deemed insufficient in strength. The testing standard is in section 7.3.3.

  • Minimum torque for M5 circular nut: 8Nm
  • Minimum torque for M6 circular nut: 14Nm
  • Minimum torque for M8 circular nut: 32Nm
  • Minimum torque for M10 circular nut: 70Nm
  • Minimum torque for M20 circular nut: 100Nm

7.3.5 Torque Testing of Grounding Nuts

Before conducting torque tests, visual inspections need to be performed first as dictated in section 7.2.3. Begin by screwing a bolt into the ground nut using a torque wrench with an appropriate torque range.

During the test, the nut is subjected to a torque by the bolt. If a crack appears at the seam before reaching the minimum torque, the strength is deemed insufficient. The testing standard is in section 7.3.3.

  • Torque for M6 grounding nut: 14Nm
  • Torque for M8 grounding nut: 27Nm

7.3.6 Torque Testing of Inaccessible Nuts

For nuts installed in cavities that do not require regular testing, they can be tested using headless screws. Screw the headless bolt into the nut, and then use a torque wrench with an appropriate torque range to test the nut’s torque.

If a crack appears in the nut before reaching the minimum torque, the strength is deemed insufficient. The testing standard is in section 7.3.3.

Torque Measurement
M44 Nm
M55 Nm
M68 Nm
M820 Nm
M1050 Nm
M1280 Nm

Note: Enhanced screws may be used if necessary.

8. Destructive Testing

Destructive testing is a specialized inspection method performed on vehicle frames to enhance quality and investigate quality issues. This type of testing involves intentionally damaging or destroying the component to assess its performance and integrity under stress.

8.1 Destructive Testing of Grounding Nuts

Before conducting destructive testing on grounding nuts, a visual inspection must be performed as standardized in Section 7.3.3.

  1. Preparation: Use appropriate tools to strip the welded steel plate from the grounding nut.
  2. Inspection: Check if 80% or more of the welding circumference is welded.
    • Note: 80% welding is considered sufficient for grounding purposes.

8.2 Destructive Torque Testing

Destructive torque testing involves using a torque wrench with an appropriate torque range to determine the torque required to unscrew the nut. The minimum separation torque of the weld is listed in Table 6.6.

8.2.1 Destructive Torque Testing of Round Nuts MBN 10369 and MBN 10390 (Arch Nuts)

  1. Procedure: The testing method for round nuts is similar to non-destructive testing (see Section 7.2.3).
  2. Application: Apply torque to the nut using a screw until the weld fails.
    • Tool: Use the same torque wrench as specified in Section 9.

8.2.2 Welding Stud Destructive Torque Testing

  1. Procedure: The testing method for welding studs is similar to non-destructive testing (see Section 7.3.2).
  2. Application: Apply torque to the nut using a screw until the weld fails.
    • Tool: Use the same torque wrench as specified in Section 9.

8.3 Compression Test

8.3.1 Test Sequence

  1. Equipment: The compressive strength of the testing equipment must be adequate to measure the separated weld.
  2. Comparison: The force $$ F_A $$ must be compared with the minimum force value listed in Table 8.4.
  3. Evaluation: Additionally, the fracture surface must be evaluated to determine if a complete weld point was formed.

8.3.2 Testing Equipment

Hexagonal NutSquare NutDiameter (mm)Plate Thickness (mm)Length (mm)
M3 10240
M4M412
M5M513
M6M614
M8 18
 M821
M10 23
M12M1027
M14M1231
M16M1433

For components not listed in the table, such as round screws or nuts, the inspection equipment must be similar to the above.

8.4  Force Inspection List

The components mentioned in section six.

Thread DiameterPlate ThicknessCompressive Force
M40.75
1.0
1.5
>1.3kN
M50.75
1.0
1.5
>2.0kN
M61.0
1.5
2.5
>2.5kN
M81.0
2.0
3.0
>3.0kN
M101.25
2.0
3.0
>4.0kN
7/16’’1.25
2.0
3.0
>5.0kN
M121.5
2.0
3.0
>6.0kN

Excessive pressure that extends beyond the scope needs to be agreed upon with the relevant responsible departments.

8.5 Peel Test Inspection

The peel test inspection is a crucial method for evaluating the integrity of welded steel plate nuts. This method involves peeling the nut off the steel plate using appropriate tools such as a hammer, chisel, or tension testing equipment. The objective is to ensure that the weld points maintain their dimensions and integrity post-welding.

Procedure:

  1. Peeling the Nut: Using suitable tools, carefully peel the nut from the steel plate.
  2. Inspection of Weld Points: Examine each weld point to verify that the dimensions of the weld point on the peeled steel plate match the pre-weld dimensions. For example:
    • A weld point with a pre-weld diameter of 24mm should maintain a minimum diameter of 24mm post-weld.
    • A weld point with pre-weld dimensions of 3x8mm should maintain a minimum dimension of 3x8mm post-weld.

Acceptance Criteria:

  • Spot Welds: The weld seams are deemed acceptable if they meet the following conditions:
    • 3 out of 4 spot welds meet the requirements.
    • 2 out of 3 spot welds meet the requirements.

8.6 Special Metal Cross-Section Inspection

In certain special cases, a metal cross-section inspection is utilized to assess the fastening state of bolts and nuts. This method provides a detailed view of the internal structure and integrity of the fastening.

Procedure:

  1. Special Training: This inspection must be conducted by personnel who have received specialized training.
  2. Training Guidelines: The training must be provided by an authorized welding engineer or a certified welding expert.

Importance:

  • This inspection method is critical for ensuring the reliability and safety of the fastening in applications where standard inspection methods may not suffice.

9. Torque Test Table

As mentioned in Section 6, for the parts.

  Destructive TestingNon-Destructive Testing
Thread DiameterPlate ThicknessThe torque setting in the welding system.Monitoring the torque during the part inspection process, which is related to the thickness of the plate.
M40.7
1.25
1.5
13 Nm
13 Nm
16 Nm
6 Nm
8 Nm
8 Nm
M50.7
1.25
1.5
20 Nm
29 Nm
29 Nm
8 Nm
10 Nm
10 Nm
M60.8
1.5
2.0
24 Nm
33 Nm
34 Nm
14 Nm
20 Nm
20 Nm
M81.0
2.0
3.0
58 Nm
61 Nm
60 Nm
32 Nm
38 Nm
38 Nm
M107/16’’1.25
2.0
3.0
112 Nm
133 Nm
125 Nm
70 Nm
90 Nm
90 Nm
M12>1.5140 Nm100 Nm

Note: The inspection standard in 6.2.3 is specifically for thin steel plates.

10. Inspection Document

The inspection of random samples must be recorded. The results of random inspections must be preserved for a specified period.

10.1 Remedial Measures for Defects

If defects are discovered during the inspection process, they must be immediately rectified. Additionally, the relevant systems must be inspected or corrected.

All vehicles currently experiencing the same issues must be repaired. Defective rivet nuts must be removed, and to secure new screws or nuts, the surface of the plate must be kept clean and flat.

For individual cases where screws and nuts cannot be replaced, suitable repair methods must be established through QPQ (Quench-Polish-Quench) and EP/CSV (Electropolishing/Chemical Surface Treatment).

11. Inspection Tools

The inspection department also needs to check the inspection tools.

The torque wrenches used must meet the following conditions:

  • Torque work difference within 10% of the inspection range
  • Visible inspection results
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Shane
Author

Shane

Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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