Introduction
The tightening apparatus for weld nuts and weld screws necessitates assurance of quality during the installation process. Besides, to evaluate the weld quality and process reliability, installation strength requires monitoring throughout the procedure.
To substantiate the reliability of the installation process, other quality assurance measurements may also be integrated into the production process, potentially obviating the need for subsequent inspections of weld nuts and weld screws.
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, such as how to identify defects in the test piece.
7.2 Visual Inspection
7.2.1 Inspection Process
Visual inspections must comply with evaluation standards. They must be conducted by trained inspectors under suitable distance and lighting conditions.
7.2.2 Inspection Records
Visual inspections must be recorded in a checklist.
Confirmed defects, such as weld points trending towards the edge, must immediately be corrected in the production system or welding equipment.
7.2.3 Evaluation Standards
The inspection of weld nuts must comply with the standards in the table below.
| Serial Number | Evaluation Criteria: | Example | |
| 1 | Missed welds of studs/nuts |  | |
| 2 | Damage or contamination of studs/nuts (including weld spatter and thread damage) |  |  |
| 3 | The gap is inappropriate h > 0.1m |  |  |
| 4 | Deviation 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. | |||
7.3 Torque Testing
7.3.1 Welded Nuts
Prior to conducting torque testing on welded nuts, an external inspection is required. Inspection standards are as per 7.2.3. Torque testing must be carried out utilizing a torque wrench that falls within the appropriate testing range.
During the testing process, a torque is applied to the nut. If the weld seam is sheared or cracked before reaching the minimum torque, the strength is deemed insufficient.
The testing standards are detailed in 7.3.3.
| Torque Measurement | |
| M4 | 6 Nm |
| M5 | 8 Nm |
| M6 | 14 Nm |
| M8 | 32 Nm |
| M10 | 70 Nm |
| M12 | 100 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).
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.
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 Number | Assessment Criteria | Example | |
| 1 | Screws or nuts must not be loose |  |  |
| 2 | Weld seams cannot exhibit separation |  |  |
| 3 | 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 | |
| M4 | 4 Nm |
| M5 | 5 Nm |
| M6 | 8 Nm |
| M8 | 20 Nm |
| M10 | 50 Nm |
| M12 | 80 Nm |
Note: Enhanced screws may be used if necessary.
8. Destructive Testing
Destructive testing is a special inspection performed on the vehicle frame for quality improvement and quality issue investigation.
8.1 Destructive Testing of Grounding Nuts
Before inspecting the grounding nuts, a visual inspection must be performed, which is standardized in 7.3.3.
Use the appropriate tools to strip the welded steel plate of the grounding nut.
Check if 80% or more of the welding circumference is welded.
Note: 80% welding is sufficient for grounding.
8.2 Destructive Torque Testing
Use a torque wrench with an appropriate torque range to determine how much torque can 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)
The destructive testing of round nuts is similar to non-destructive testing (see 7.2.3).
However, a torque must be applied to the nut by the screw until the weld fails. The same as the torque wrench in Section 9.
8.2.2 Welding Stud Destructive Torque Testing
The destructive torque testing of the screw is similar to non-destructive testing (see 7.3.2).
However, a torque must be applied to the nut by the screw until the weld fails. The same as the torque wrench in Section 9.
8.3 Compression Test
8.3.1 Test Sequence
The compressive strength of the appropriate testing equipment must be able to measure the separated weld.
The force FA must be compared with the minimum force value in Table 8.4. Additionally, the fracture surface must be evaluated to determine if a complete weld point was formed.
8.3.2 Testing Equipment
| Hexagonal Nut | Square Nut | Diameter (mm) | Plate Thickness (mm) | Length (mm) |
| M3 | 10 | 2 | 40 | |
| M4 | M4 | 12 | ||
| M5 | M5 | 13 | ||
| M6 | M6 | 14 | ||
| M8 | 18 | |||
| M8 | 21 | |||
| M10 | 23 | |||
| M12 | M10 | 27 | ||
| M14 | M12 | 31 | ||
| M16 | M14 | 33 |
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 Diameter | Plate Thickness | Compressive Force |
| M4 | 0.75 1.0 1.5 | >1.3kN |
| M5 | 0.75 1.0 1.5 | >2.0kN |
| M6 | 1.0 1.5 2.5 | >2.5kN |
| M8 | 1.0 2.0 3.0 | >3.0kN |
| M10 | 1.25 2.0 3.0 | >4.0kN |
| 7/16’’ | 1.25 2.0 3.0 | >5.0kN |
| M12 | 1.5 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
For welded steel plate nuts, a peeling method can be employed for inspection.
Using suitable tools, the nut is peeled off from the steel plate, such as with a hammer, chisel, or tension testing equipment.
Each weld point needs to be checked for whether the dimensions of the weld point on the peeled steel plate are the same as before welding (for instance, the minimum dimension of a 24mm diameter weld point is 24mm; the minimum dimension of a pre-welded 3x8mm weld point is 3x8mm).
Upon inspection of all weld seams, if they meet the following conditions, they are deemed acceptable:
- 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 special cases, a metal cross-section can be used to inspect the fastening state of bolts and nuts.
Special metal cross-section inspection requires operation by specially trained personnel.
Training guidelines must be issued by an authorized welding engineer or a certified welding expert.
9. Torque Test Table
As mentioned in Section 6, for the parts.
| Destructive Testing | Non-Destructive Testing | ||
| Thread Diameter | Plate Thickness | The torque setting in the welding system. | Monitoring the torque during the part inspection process, which is related to the thickness of the plate. |
| M4 | 0.7 1.25 1.5 | 13 Nm 13 Nm 16 Nm | 6 Nm 8 Nm 8 Nm |
| M5 | 0.7 1.25 1.5 | 20 Nm 29 Nm 29 Nm | 8 Nm 10 Nm 10 Nm |
| M6 | 0.8 1.5 2.0 | 24 Nm 33 Nm 34 Nm | 14 Nm 20 Nm 20 Nm |
| M8 | 1.0 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 | >1.5 | 140 Nm | 100 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 period of time.
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 and EP/CSV.
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
