Because the valve spring is subjected to torsion when it works, the stress distribution on its circular section is uneven. From the origin near the center to each point on the edge, the stress gradually increases, and the stress on the surface is greater. In terms of surface points, the positive inner surface is subjected to greater stress and is subject to plane stress. For this reason, once the valve spring surface is defective, it is possible to generate a greater stress concentration at the defect, which leads to the early fracture of the spring.
The reasons for valve springs to break, in addition to manufacturing defects, may also cause them to suffer early damage due to improper use. The most common reasons are the following:
①There are pits and corrosion pits on the surface of the spring. Due to improper storage, corrosion pits will be formed on the surface of the spring. When the spring is subjected to a large torsion force, it is easy to cause stress concentration at the corrosion pit, which will eventually lead to fatigue fracture of the spring.
The quality inspection method of the new valve spring: compress the spring to a small length on a bench vise, so that there is no gap between the rings as much as possible, and keep it for 48 hours. If there are defects on the surface of the spring, after this compression It will break after handling. This is because the internal stress of the spring is highly concentrated near the flaw, causing the spring to break.
The strength of the valve spring elasticity can be identified by the comparison method. The specific method is: first, connect an old valve spring to be checked with a new valve spring in series, and separate them with a steel washer. Then add a certain amount of pressure to a valve spring and observe the degree of compression of the old and new springs. If the elasticity of the old spring is insufficient, it must be pressed down first.
②The center line of the spring is skewed. If the two end faces of the valve spring are not perpendicular to the center line of the spring, the spring will work at high speed for a long time, and the metal material will be easily broken due to fatigue. The inspection method for the verticality of the valve spring is: first place the spring vertically on the flat plate, use a square to lean on the bottom of the spring, and then rotate the spring for a circle, and measure the greater value of the distance between the upper circle of the spring and the square. . Usually, the inclined distance of the valve spring to the vertical line is 1.0-1.5mm. If it exceeds this value, it is better to replace it with a new one.
③The valve guide is moving or the camshaft bearing is loose. If the valve guide moves during use, it may cause the valve spring to break due to the bending stress when it is compressed. Loose camshaft bearings can cause valve springs to resonate and cause them to break.
④ Improper operation or installation. During the operation of the diesel engine, if the rotational speed frequently changes suddenly, the frequency of the valve spring being compressed and stretched will suddenly increase sharply, resulting in its fatigue fracture.
⑤ The valve spring is not assembled as required. When assembling valve springs, some models have special requirements. For example, the Isuzu 6BBl diesel engine requires that the blue-coated side of the spring face the upper plane of the cylinder head. Otherwise, the spring is easy to break.
