II. Standardized Installation Method: Five-Core Operational Specifications

(I) Tool and Part Pretreatment

• For manual installation: Use a Phillips screwdriver matching the screw slot type (Type H or Type Z). If pre-tightening force control is required, a torque wrench must also be prepared.
• For automated assembly: Electric screwdrivers should be used, with the rotation speed set to no more than 300 r/min to avoid thread overheating and wear caused by high-speed rotation.

• Whether there is rust or burrs on the screw surface, deformation of the washer, or collision damage to the thread.
• Measuring the diameter of the installation hole, which should be 0.1-0.2mm larger than the thread outer diameter (e.g., an M4 screw corresponds to a φ4.1-4.2mm hole) to prevent thread stripping due to an overly tight hole.

(II) Positioning and Preliminary Fixation

(III) Torque Control and Tightening Sequence

• For 4.8-grade carbon steel combination screws: The recommended torque for M4 specification is 2.5-3.5 N·m, and for M6 specification is 6-8 N·m.
• For stainless steel combination screws (A2-70): Due to the high friction coefficient, the torque should be reduced by 10%-15% (e.g., the M4 specification should be controlled at 2.2-3.2 N·m) to avoid bolt breakage caused by stress concentration.

(IV) Anti-Loosening Verification and Environment-Adapted Treatment

• Gently shake the connected part manually to confirm no obvious loosening.
• Or draw a line with a marker at the junction of the screw and the base, then check for line displacement after 24 hours.

• Humid environments: Carbon steel screws require supplementary application of anti-rust paint or zinc coating repair agent within 24 hours; stainless steel screws can be coated with molybdenum disulfide anti-seize agent.
• High-temperature scenarios: Replace with high-temperature-resistant washers (e.g., copper washers) to prevent spring washer failure due to annealing.

(V) Completion Acceptance and Recording

• Conduct a visual inspection to ensure the washer is fully flattened, and the screw head is tightly attached to the surface without tilting or gaps.
• For critical connections, use a torque wrench to conduct a 10% sampling re-inspection, with an error ≤ ±5%.

III. Typical Risks and Cases of Incorrect Installation

1. Over-tightening: This can cause thread stripping or bolt breakage. In one equipment assembly, an M3 stainless steel screw was tightened with a torque of 5 N·m (standard ≤ 3 N·m), leading to screw rod breakage that pierced the main board and caused a short circuit. The maintenance cost for a single device exceeded 800 yuan.
2. Washer misalignment: This results in anti-loosening failure. During the installation of a motor, the spring washer did not cover the edge of the screw hole; after 3 months of operation, the screw loosened due to vibration, causing the motor housing to fall off and halting the production line for 2 hours, resulting in losses of approximately 30,000 yuan.
3. Forced installation with hole misalignment: This causes deformation of the connected part and accelerated thread wear. In one case, a chassis with a 0.5mm hole misalignment still had the screw forced in, leading to panel sealing failure; rainwater seeped in and damaged internal components, resulting in batch rework costs as high as 100,000 yuan.

IV. Core Importance of Correct Installation

• Mechanical performance: Proper torque control enables the screw to generate appropriate clamping force. Correctly installed combination screws can withstand vibration of 30-50 times per minute without failure, while connections with uncontrolled torque loosen within 1 hour under vibration of 10 times per minute. The arc design of the pan head and washer disperses surface pressure; if installed at an angle, local stress increases by more than 40%, accelerating fatigue fracture.
• Lifecycle cost: Correct installation extends the service life of fasteners by 2-5 times. For stainless steel screws in high-temperature working conditions, the disassembly qualification rate increases from over 60% to over 95%.
• Safety compliance: Industry standards for food machinery and medical equipment mandate torque value recording. A case involving a chemical reaction kettle: loosening of one screw caused medium leakage, leading to production suspension and losses exceeding 500,000 yuan—proving the practical significance of "small screws determining major safety".

V. Differential Installation Points for Screws of Different Materials

• Carbon steel combination screws: In humid environments, anti-rust treatment must be completed within 24 hours; the torque tolerance can be relaxed to ±10%. They are suitable for ordinary household appliances.
• Stainless steel combination screws: At high temperatures (>200℃), nickel-based grease must be used for anti-seizure; mixing with carbon steel tools is strictly prohibited to avoid iron ion contamination. They are often used in food equipment.
• Copper alloy combination screws: Due to the soft material, the torque should be reduced by 20%; contact with strong oxidants should be avoided. They are suitable for electrical connection scenarios.

Supplementary Notes on Key Terms

1. Spring washer/Flat washer: Translated as "spring washer/flat washer"—standard washer types. Spring washers use elastic deformation to prevent loosening; flat washers mainly disperse pressure, and their combination with pan head screws forms the core of the "combination screw" structure.
2. Thread stripping: "Thread stripping" refers to the damage of thread teeth (either on the screw or in the hole) due to excessive force, resulting in loss of fastening function. It is a common failure caused by overly tight holes or excessive torque.
3. Molybdenum disulfide anti-seize agent: A lubricating agent containing molybdenum disulfide, which reduces friction between threads and prevents seizing (thread jamming due to high temperature or corrosion), widely used in stainless steel fastener installation.
4. Annealing (of washers): "Annealing" is a heat treatment process where the washer is heated and cooled slowly, reducing its hardness and elasticity. In high-temperature environments, spring washers lose their anti-loosening effect after annealing, hence the need for high-temperature-resistant alternatives like copper washers.