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A brief analysis of the hard-looped bolts of Q's latest modality
This standard is specifically designed for aerospace applications, with a primary focus on the connection of carbon fiber reinforced resin matrix composite structures. Its main objective is to prevent severe galvanic corrosion that may occur between fasteners and these composite materials. Additionally, it enables the use of high-lock Bolts and Nuts to effectively control the pre-tightening force of joints, thereby ensuring proper compressive strength in the connected composite components.
The standard consists of two key components: QJ2825-965MJ threaded titanium small countersunk head tight tolerance high lock bolt and QJ2826-965MJ threaded titanium alloy flat head tight tolerance high lock bolt. These bolts are manufactured with tight tolerances (f9), making them ideal for shear-loaded structural connections. Flat-headed bolts are typically used, while small countersunk bolts are preferred when the bolt heads do not need to be raised or when one of the joined parts is thin.
The materials used for manufacturing high-lock bolts must comply with GJB22195 for titanium and titanium alloy rods (wire) and GJB2189-94 for fastener specifications. The most commonly used titanium alloys in China are TB3 and TC4. While TC4 offers good mechanical properties, its machining process is complex and costly, limiting its use in space systems. Therefore, TB3 is often chosen due to its superior cold heading performance and acceptable thermal stability. With a tensile strength ranging from 840 MPa to 940 MPa in the solid solution state and up to 1100 MPa after aging treatment, TB3 meets all design requirements for aerospace applications.
The threads of high-lock bolts follow the MJ thread standard, which is widely adopted in the aerospace industry. This thread type provides excellent fatigue resistance and high precision (external thread: 4h6h), significantly improving anti-loosening performance under dynamic loads.
According to ISO/TC20/SC4 standards, MJ fastener specifications are indicated by codes such as 050024, where "050" represents an MJ5 diameter and "024" indicates a 24mm length. This ensures full compliance with international aerospace standards.
The mechanical properties of high-lock bolts are similar to those of standard bolts, so this standard references QJ2602-945MJ thread 1100MPa titanium alloy bolts. However, due to the smaller and thinner heads of the bolts, fatigue tests should be conducted using data from the no-slot small countersunk head specification in QJ2602 to ensure safety.
When the surface slope of the structural member exceeds 0.15b, a sloped washer must be used under the bolt head to prevent excessive stress concentration and potential failure. To determine the correct bolt length, the thickness of the composite material should be measured using an interlayer thickness gauge. If the measurement point lies between two lines, the maximum reading should be taken. The final polished rod length is calculated by adding the measured thickness to the gasket thickness.