English
In connectors, particularly rectangular connector products, wire thread inserts are adopted for their vibration resistance, impact resistance, and corrosion protection. These properties significantly enhance connection strength, interchangeability, and overall performance. However, challenges such as incomplete screw installation and thread jamming during usage have been observed in rectangular connectors.
This study investigates the root causes of these issues through systematic failure analysis and machining experiments. It further evaluates quality control gaps in production and inspection processes, proposing an optimized manufacturing and testing methodology for threaded holes equipped with coated wire thread inserts. The goal is to ensure precision, reliability, and compliance with stringent standards.
Technical Issue Report: Thread Insert Misalignment and Screw Jamming in Rectangular Socket Product
During field installation of the rectangular socket product, customers reported issues with screws failing to seat properly and thread inserts (wire inserts) ejecting from the assembly. A schematic of the failure is provided in Figure 1.
Root Cause Investigation:
Analysis of the defective units revealed that the non-mating ends of certain wire thread inserts exhibited incomplete adhesion to the threaded base holes, with visible gaps/uneven contact (lifting) observed.
Validation Testing:
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Screw Engagement Test:
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Both customer-returned M3 screws and in-house M3 screws were used to validate thread functionality.
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During insertion, screws became jammed near the bottom of the threaded hole and could not be fully seated.
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Attempts to reverse the jammed screws resulted in unintended screw ejection.
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Key Observations:
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The jamming and subsequent ejection correlate with the misalignment of the wire thread inserts, particularly at the non-mating ends.
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The incomplete contact between the inserts and base holes likely created localized stress points, contributing to the thread deformation and screw seizure.
This issue requires further evaluation of the thread insert installation process and dimensional tolerances of the base holes to ensure proper insert seating and screw engagement.
Professional English Translation:
Analysis of Potential Causes for Inadequate Installation and Egress of Rectangular Connector Screws Based on Product Structure and Production Process
A fault tree analysis was conducted to identify root causes of screw installation inadequacy and disengagement in rectangular connectors, as illustrated in Figure 2. Each basic event in the fault tree was systematically investigated, and the results of the root cause analysis are summarized below.
2.1 Investigation of Basic Event A1 (Design Issue)
The design specifications for the threaded base hole in the housing and its coordination with the wire thread insert were verified against the standard GJB119.3A-2001. The selected thread specification (ST3×0.5-5H) was confirmed to align with established design requirements. As the design basis is well-substantiated, Basic Event A1 (design issue) is excluded.
2.2 Investigation of Basic Event A2 (Wire Thread Insert Dimensional Deviation)
Per design requirements, the M3 wire thread insert must exhibit 10 turns in its free state and an outer diameter within the range of 3.90–4.10 mm. A review of the inventory confirmed that the wire thread inserts meet these criteria: 10 turns in the free state and an outer diameter of 3.97 mm. Since all dimensions comply with specifications, Basic Event A2 (dimensional deviation) is excluded.
2.3 Investigation of Basic Event A3 - Shell Thread Hole Dimension Deviation
The drawing specifies the thread pilot hole dimension for wire thread insert engagement as 4×ST3×0.5-5H. Measurement of returned defective parts revealed that the GO end of the ST3×0.5-5H thread gauge failed to pass through the shell thread pilot holes, indicating non-conforming hole dimensions. Consequently, the basic event A3 (Shell Thread Hole Dimension Deviation) cannot be excluded.
2.4 Investigation of Basic Event A4 - Shell Plating Coating Issue
Per drawing requirements, the surface treatment consists of nickel plating with a specified coating thickness of 18-25μm. Actual measurements of the threaded hole nickel coating thickness showed values of 18.3μm/24.1μm, which comply with the drawing specifications. A review of inspection records for the batch confirmed that all plating thickness measurements met requirements.
2.5 Investigation of Basic Event A5 – Improper Installation of Wire Thread Inserts
A review of the product assembly process documentation confirmed that the procedures provide detailed and actionable assembly instructions. Inspection of the wire thread insert installation status in the defective units revealed compliance with assembly specifications. Consequently, the basic event A5 (Improper Installation of Wire Thread Inserts) can be excluded.
The investigation determined that the screw dislodgment occurred during customer use: when inserting the screw, it became jammed, and upon backing out the screw, the wire thread insert was inadvertently extracted. Analysis of the failure mechanism indicates that the root cause of the incomplete screw installation and subsequent dislodgment in the rectangular connector lies in undersized threaded holes in the shell (relative to standard requirements), which compromised the engagement integrity of the wire thread insert.
3 In-depth Analysis of Thread Hole Size for Wire Thread Inserts Being Smaller Than Standard Requirements
3.1 Analysis of Coating Thickness Impact on Thread Geometric Parameters
Assuming uniform distribution of coating thickness on the thread, the influence on pitch diameter dimensions is illustrated in Figure 4. Taking the commonly used 60° thread angle as an example: d₁ represents the pre-plating pitch diameter; d denotes the post-plating pitch diameter; t indicates coating thickness; Δd shows the unilateral reduction of pitch diameter after plating. According to geometric relationships:
Δd = t/(sin(α/2))
Where α represents the thread angle. For a standard 60° thread:
Δd = t/(sin30°) = 2t
This demonstrates that the unilateral reduction of pitch diameter equals twice the coating thickness, meaning the total pitch diameter reduction after plating will be 4t (2Δd).
Thus, the total reduction in pitch diameter after plating is:
d1-d=2Δd=4t
This indicates that the change in pitch diameter is four times the coating thickness.
3.2 Analysis of Thread Engagement Issues in Wire Thread Inserts
Minimum pitch diameter occurs when the part is machined to its minimum allowable dimensions and the coating thickness is at maximum.
Maximum pitch diameter occurs when the part is machined to its maximum allowable dimensions and the coating thickness is at minimum.
After plating, the pitch diameter tolerance range becomes Φ3.313–Φ3.225 mm. If the threaded hole is not designed with a reserved coating allowance, the post-plating hole dimensions will exceed allowable tolerances.
During assembly, the wire thread insert is installed into the threaded hole using a specialized tool, forming an M3 thread through dimensional interference. The cross-section of the wire thread insert is shown in Figure 5.
Case 1: If the insert hole is oversized and the insert dimensions are within the mid-to-lower tolerance range, the assembled thread hole remains compliant, allowing full screw engagement.
Case 2: If the insert hole is oversized but the insert dimensions are within the mid-to-upper tolerance range, the assembled thread hole becomes non-compliant. Attempting to force the screw into the hole may cause jamming. Upon unscrewing, the insert may be pulled out, resulting in screw disengagement.
The dimensions of the wire thread insert hole ST3×0.5-5H are listed in Table 1. The standard pitch diameter range without reserved coating allowance is Φ3.325–Φ3.385 mm.