In a traditional fuel vehicle, fasteners mainly carry out structural assembly. In a new energy vehicle, however, the battery, motor and electronic control system push fasteners into the position of safety-critical components. The battery pack must be waterproof and dustproof, the motor must run continuously at high temperature, and the electronic control unit must withstand sustained vibration and voltage transients. These working conditions place demands on bolt property class, corrosion resistance, anti-loosening performance and torque consistency that are far higher than those of ordinary bodywork parts. OEMs generally bind suppliers through the IATF 16949 automotive quality management system, and separately specify torque, anti-loosening and salt-spray indicators for critical connection points in the part technical agreement.

Why the Three Electric Systems Treat Fasteners Differently

The essential difference between three-electric connections and ordinary body joints is that the consequences of failure are amplified. A loose interior bolt produces at most an abnormal noise, whereas a loosened high-voltage connection in the battery pack can cause arcing, overheating and even thermal runaway. For this reason, three-electric fasteners must answer three questions at the design stage: will the joint loosen within the service life, can the corrosion protection survive the vehicle's 15-year life, and can torque remain consistent during mass production. This is why critical three-electric bolts generally use 10.9-class and even 12.9-class alloy steel, paired with anti-loosening and anti-corrosion processes, rather than ordinary 8.8-class standard parts.

Battery System: The Connection with the Highest Safety Redundancy

The battery pack is the assembly with the most demanding fastener requirements among the three electric systems. According to GB 38031-2020 Safety Requirements for Traction Batteries of Electric Vehicles and GB/T 31467.3, the pack must pass a series of safety tests including vibration, shock, and temperature-humidity cycling; these tests are essentially assessing the long-term reliability of the connection system. Engineering practice usually adopts triple redundancy: selecting bolts with anti-loosening structures or lock washers, applying a specified tightening torque to high-voltage joints with witness marks, and using an integrated sealing and anti-corrosion design for the cover and housing. Busbar connections must also control contact resistance; insufficient torque increases contact resistance and heat, while excessive torque crushes flexible shunts. The torque window must therefore be fixed on the part drawing.

Motor and Electronic Control: Fastening under High Temperature and Vibration

Under high load, the winding and housing of a drive motor can exceed 120 °C. End-shield and stator bolts must maintain clamping force at this temperature without stress relaxation. Engineering practice selects alloy-steel bolts with good high-temperature stability and verifies clamping-force retention through thermal aging tests; anti-galling coatings are also applied to thread mating surfaces to avoid cold welding of stainless or yellow-zinc bolts during repeated disassembly. The electronic-control housing, meanwhile, endures whole-vehicle road vibration, so bolted joints need anti-loosening measures and must guarantee housing grounding continuity, preventing contact-resistance changes in the shielding layer caused by vibration.

Material and Corrosion Protection: Zinc-Nickel, Galvanic Corrosion and Hydrogen Embrittlement

Three-electric housings use large amounts of aluminum die castings and aluminum covers. If carbon-steel zinc-plated bolts are used directly, a significant potential difference exists between steel and aluminum, and galvanic corrosion readily occurs in hot and humid environments. The mainstream approach is zinc-nickel alloy electroplating on high-strength bolts, achieving neutral salt-spray resistance of several hundred hours and further improved by passivation sealing. At joints contacting magnesium alloy, aluminum bolts or insulating isolation are preferred, eliminating galvanic corrosion from the potential-matching side. Note especially that high-strength steel above 10.9 class carries a risk of hydrogen-embrittlement delayed fracture during plating; hydrogen-baking must follow process specifications, and coating thickness and hydrogen-embrittlement testing must be verified.

Tightening and Process Control: Torque, Torque-Angle and the K Factor

The same torque applied to bolts with different friction coefficients can yield preload differing by a factor of two. Critical three-electric joints generally use torque-angle or yield-point control rather than simple torque control, while batch management of the torque coefficient K keeps friction-coefficient fluctuation within the design window. Before mass production, torque-clamp-force tests confirm that preload falls within the design band across the specified torque range, and tightening tools on the line are periodically calibrated. This step looks tedious, yet it is the key to truly realizing the "designed preload" on the product.

Selection Recommendations and Acceptance Points

Narrowing three-electric fastener selection into executable checklist items:

• Property class: high-voltage battery connections and motor end shields use 10.9 class or above; ordinary covers and brackets may use 8.8 class.
• Corrosion protection: outdoor and three-electric housings prefer zinc-nickel plating + seal; joints contacting magnesium switch to aluminum bolts or insulating isolation.
• Anti-loosening: add lock washers or prevailing-torque lock nuts at vibrating positions, and verify torque decay.
• Process: critical bolts use torque-angle tightening, retain torque curves, and apply K-factor batch management plus torque-clamp-force tests.
• Validation: complete vibration and temperature-humidity cycling per GB 38031 and GB/T 31467.3, then re-inspect torque and appearance.