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In-depth analysis of motorcycle movement and system
When the natural frequency of the valve spring matches or is an integer multiple of the valve opening and closing cycle, resonance occurs. In motorcycle engines, this phenomenon is extremely dangerous. It not only increases engine noise but also causes the torsional shear stress in the valve spring to rise significantly, often several times the normal level. The fluctuating spring force can lead to gaps in the valve mechanism, resulting in impact and abnormal valve movement. This may cause the spring to break or even lead to serious safety issues. To prevent resonance, most modern valve springs are designed with a high natural frequency. Additionally, many engines use a dual-spring system, where two springs are placed inside and outside the valve. This distributes the load according to the principle of equal strength, reduces the overall height of the spring, and ensures that if one spring breaks, it does not immediately damage other components, thus improving safety.
Valve springs must have sufficient rigidity to ensure that the spring force exceeds the maximum inertial force generated by the valve train. This force is calculated as the product of the mass of the valve train and its maximum acceleration. However, during actual operation, vibrations caused by the cam's harmonic motion can distort the valve's motion pattern, leading to continuous spring vibration. As engine speed increases, the amplitude of these vibrations changes, sometimes causing resonance. Experience shows that the actual load on the spring can be twice the inertial force of the valve. Therefore, when designing the spring, there should be a safety margin, especially for high-speed engines. Key dimensions include the mean diameter, wire diameter, pitch, number of active coils, and total number of coils. Based on the valve’s position when closed and fully open, the pre-load height and maximum tension height are determined, allowing for initial load calculations.
At high speeds, the valve opens and closes rapidly, causing the spring to vibrate at a high frequency. Although the spring moves quickly, it doesn't compress or extend uniformly. Instead, one end compresses first, creating a need for the spring’s natural frequency to be at least twice the engine speed, effectively reducing vibration. Another solution is to use variable-pitch springs, which adjust stiffness based on valve lift, changing the natural frequency and preventing resonance. Using dual springs with different frequencies can also help dampen vibrations. Some engines incorporate external damping plates to further reduce vibration.
Valve springs are typically made from high-quality materials like chrome-vanadium or chromium-silicon alloy steel wire, which is oil-quenched and tempered. These materials offer excellent mechanical properties, including high fatigue resistance and good anti-relaxation characteristics. During production, the wire undergoes two rolling processes to improve surface quality, making it suitable for high-stress applications. After shot peening, soft nitriding is often applied to enhance fatigue life. Recent advancements include composite treatments combining soft nitriding and reheating, which have proven effective in real-world applications. Finally, to protect against corrosion from moisture in engine oil, the spring’s outer surface is usually coated with oil.