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When designing a valve spring, minimizing vibration is crucial, and the natural frequency of the spring plays a key role in its performance. During operation, the transition from positive to negative inertial forces causes the greatest fluctuation, which can lead to system detachment. To prevent this, the spring force must be greater than the inertial force generated by the system's vibration, with a certain margin of safety. The point where the spring's length corresponds to the vertical line on the inertial force diagram indicates the critical load. The spring’s load characteristic curve should be steep enough so that the damping force from vibration remains higher than the inertial load. At maximum engine speed, the valve system must not separate, and the spring must have sufficient force to close the valve without causing valve rebound. Ideally, the elastic force should be at least 30% greater than the inertial force.
To address these challenges, most automotive and motorcycle engine valve springs use variable pitch designs. These springs come in two main types: one end variable pitch springs, where the pitch increases from one end to a certain point, and symmetrical variable pitch springs, where the pitch increases from both ends. This design helps reduce stress concentrations and improves durability under high-speed conditions.
As vehicle performance continues to rise, the materials used for valve springs must offer high tensile strength, excellent heat resistance, and strong anti-relaxation properties. Oil-quenched alloy spring steel wire is commonly used, typically made from low-silicon, chromium-containing carbon steel that is cold-drawn and then continuously quenched and tempered. Surface imperfections such as dents, cracks, decarburization, and non-metallic inclusions can significantly reduce the spring’s lifespan. Japan has established strict standards for surface quality. Modern oil-hardened steel wires have slightly adjusted compositions of silicon and chromium, allowing tensile strengths up to 2100 N/mm². A small amount of nickel can also be added to slow crack propagation.
Commonly used materials include 50CrVA, 60SiZCIA, 60SiZCrVA, and 55Crsi. Due to fatigue concerns, profiled valve spring wires are now being developed. These include oval sections (A), circular sections (B), multi-arc ovals (C), and elliptical sections (D). Japanese engines increasingly use special-shaped cross-section valve springs, such as elliptical or egg-shaped ones, made from oil-quenched alloy steel. When manufacturing these springs, special tooling like adjustable guide plates and coil pins is required to ensure precision. Additionally, dedicated tools are used to check the cross-sectional accuracy. The surface hardness after soft nitriding should exceed 90 HV. If internal hardness is below 0.05, shot peening and heat setting techniques are applied post-molding to enhance the spring’s overall durability.