The door spring is a small tool that ensures timely seating and tight fit of the valve, preventing the valve from jumping and damaging its sealing during engine vibration.
The valve spring is located between the cylinder head and the spring seat at the end of the valve stem. The function of the valve spring is to ensure that the valve can tightly fit with the valve seat or valve seat ring when the valve is closed, and to overcome the inertial force generated by the valve mechanism when the valve is opened, so that the transmission parts are always controlled by the cam and do not detach from each other.
Valve springs are often made of high-quality alloy steel wire and undergo heat treatment to improve their fatigue strength. To avoid spring corrosion, the surface of the spring should be galvanized and phosphated. The two ends of the spring must be ground flat and perpendicular to the spring axis to prevent the spring from tilting during operation.
Valve springs are mostly cylindrical spiral springs. When the working frequency of the valve spring is equal to or a multiple of its natural frequency, the valve spring will resonate and the probability of breakage will increase. To prevent resonance, variable pitch springs can be used, and currently most engines use concentric dual springs. The rotation direction of the inner and outer springs is opposite, and the stiffness of the outer spring is greater than that of the inner spring. Double springs not only prevent resonance, but also shorten the length of the spring. In addition, when one spring breaks, the other spring can continue to work, preventing the valve from falling into the cylinder.
Design method
Valve spring design, like cam design, has equal importance to engine system performance. The function of a valve spring includes preventing the valve from jumping off the valve seat under pressure load, as well as controlling valve movement to avoid valve mechanism separation. The design of valve springs affects cam stress, valve mechanism friction, and spring vibration. The valve spring of an engine is usually an open coil compression spring with closed ends. Most engines use fixed stiffness springs, although some use variable stiffness springs. For low-speed diesel engines, using a single spring design is usually sufficient to meet the requirements, but sometimes it is also necessary to use a dual spring design with a damping spring or inner spring to reduce the severity of valve spring flutter. Valve spring design is a very complex task. It can serve as an example to illustrate the principles of engine system design for two or three reasons. Firstly, the analytical spring design method demonstrates the link between component parameters and system design parameters. Secondly, the analytical spring design method demonstrates that for the same design problem, there can be two different mathematical construction methods: one is to treat it as a deterministic solution, and the other is to solve it as an optimization problem. In the mathematical construction of optimization problems, both the objective function and constraint function are listed as explicit functions as examples. It should be noted that in other areas of engine system design, such as cycle performance, cam design, and valve train dynamics. The functions used for optimizing construction are usually more complex implicit functions. Thirdly, the analytical spring design method provides an example of using graphic design to construct a parametric sweep design diagram. These typical parameter diagrams can be used to address multidimensional design problems commonly encountered in diesel engine system design.
In valve spring design, known input data includes the following: ① maximum valve lift; ② Given spring installation length; ③ The required spring preload force; ④ The required spring stiffness. It should be noted that the preload force and stiffness of the spring are design parameters at the engine system level, which need to meet the maximum allowable spring force and cam stress, prevent the exhaust valve from jumping, and ensure that the valve train does not fly off. There is a strong interaction between valve spring design and cam design. If it is difficult to find a solution in spring design, it is necessary to modify these input data.
In valve spring design, the following parameters are calculated as output data: ① Basic or independent spring design parameters (i.e. average spring diameter, spring coil wire diameter, number of working coils); ② The exported design parameters (such as the free length of the spring, maximum compression length, compression length, free gap between coils, solid gap between coils at maximum compression, natural frequency and flutter order of the spring, maximum spring load, maximum spring torsion force). The basic spring design parameters determine the stiffness of the spring.
Some output parameters are limited by design constraints. For example, installation length and average spring diameter are limited by packaging space. The maximum spring compression and the torsional stress of the spring at the compacted length are limited by the fatigue life, strength, and maximum allowable stress limit of the spring. The constraint conditions for spring flutter protection are achieved by controlling the physical clearance and the natural frequency of the spring. The order of spring flutter refers to the ratio of the natural frequency of the spring to the operating frequency of the engine. To ensure that the spring does not experience strong vibration during operation. The natural frequency of the valve spring should usually be at least 13 times the operating frequency of the engine, meaning that the order of spring vibration is expected to be higher than 13. The analysis of the natural frequency of the spring shows that if the spring responds very sensitively to one of the dominant harmonics of the cam profile, the trend of vibration is definitely present. In this case, it is necessary to modify the design of the cam or spring. Sometimes variable stiffness or nested springs can be used to change the frequency of the spring to help alleviate vibration issues.
Spring design is a multidimensional parameter problem that can be handled through a graphical approach to check parameter sensitivity trends. The purpose of optimizing valve spring design is to maximize the natural frequency of the spring to reduce spring vibration, while meeting the following constraints: ① the required spring preload and valve spring stiffness in the engine system; ② Maximum allowable spring stress; ③ Appropriate physical clearance to control spring vibration
Design Steps
The calculation of valve springs is a complex system design issue. A well-designed spring can minimize friction and wear in the valve mechanism. The analysis formula method for valve spring design based on constructing parameter sensitivity design diagram is summarized as follows:
(1) Step 1: By analyzing the downhill driving performance of the vehicle and engine braking, determine the design target of the valve train's take-off speed in order to determine the required valve spring preload and spring stiffness;
(2) Step 2: Establish a dynamic model of the valve train to accurately predict runaway and evaluate the impact of cylinder recompression pressure on runaway;
(3) Step 3: Perform parameter sweep calculations on different values of spring preload and spring stiffness to construct a parameter diagram of the valve train dynamics, in order to examine their impact on the vibration of the valve train. It is necessary to draw the curves of push rod force, valve train acceleration, and spring deceleration relative to the crankshaft angle in the figure to display the design margin for runaway, so as to conveniently and wisely select the target values of spring preload and spring stiffness required in step 4;
(4) Step 4: Based on the static force balance of the exhaust valve head, calculate the required spring preload to prevent the exhaust valve from jumping. Select the exhaust valve spring preload for engines with and without exhaust brakes, and use the design parameter diagram in step 3 to choose the matching spring stiffness;
(5) Step 5: Perform parameter sweep calculation on the design parameters and use graphical design methods to construct a parameter sensitivity design diagram for the spring design. Choose the average diameter of the spring, the diameter of the coil wire, and the number of coils, while satisfying design constraints such as spring torsional stress, natural frequency, and coil clearance. Alternatively, analytical optimization methods can be used to directly solve the equation.
The method of breaking
Because the valve spring bears torque during operation, the stress distribution on its circular cross-section is uneven. The stress gradually increases from the origin near the center to each point on the edge, and the surface experiences the highest stress. In terms of surface points, the inner surface bears the highest stress and is subjected to plane stress. Therefore, once there is a defect on the surface of the valve spring, it is possible to generate the maximum stress concentration at the defect location, leading to early fracture of the spring.
Reason for breakage
The reason for valve spring fracture, in addition to manufacturing defects, improper use may also cause early damage. The common reasons are as follows:
① There are pitting and corrosion pits on the surface of the spring. Improper storage can cause corrosion pits on the surface of the spring. When the spring is subjected to high torque, stress concentration can easily occur at the corrosion pits, ultimately leading to fatigue fracture of the spring.
The quality inspection method for new valve springs: clamp the spring on a vise and compress it to the minimum length, 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, it will break after this compression treatment. This is because the internal stress of the spring is highly concentrated near the defect, causing the spring to break.
The strength of valve spring elasticity can be identified by comparison method. The specific method is to first connect the old valve spring being inspected in series with a new valve spring, and separate them with a steel washer in the middle. Then apply a certain amount of pressure on a valve spring and observe the degree of compression of the new and old springs. If the elasticity of the old spring is insufficient, it must be pressed down first.
② The centerline of the spring is skewed. If the two end faces of the valve spring are not perpendicular to the centerline of the spring, the spring will work at high speed for a long time, and its metal material is also prone to fracture due to fatigue. The method for checking the verticality of valve springs is to first place the spring vertically on a flat plate, use a square ruler to rest on the bottom circle of the spring, then rotate the spring once and measure the maximum distance between the top circle of the spring and the square ruler. Normally, the inclination distance of the valve spring to the vertical line is 1.0-1.5mm. If it exceeds this value, it is best to replace it with a new one.
③ Valve guide movement or loose camshaft bearings. If the valve guide moves during use, it may cause the valve spring to break due to bending stress when compressed. Loose camshaft bearings can cause resonance in valve springs and also lead to their breakage.
④ Improper operation or installation. During the operation of a diesel engine, if the speed suddenly changes frequently, the frequency of compression and extension of the valve spring will suddenly increase, leading to fatigue fracture.
⑤ The valve spring was not assembled as required. When assembling valve springs, some models have special requirements. For example, the Isuzu 6BBl diesel engine requires the blue side of the spring to face the flat surface of the cylinder head. Otherwise, the spring is prone to breakage.
Emergency Management
If the valve spring of a diesel engine is found to be broken while driving, the broken spring can be removed first, and then the working surfaces at both ends of the spring can be reinstalled for temporary use. If the spring breaks into several sections, the adjustment bolts of the intake and exhaust valves of the cylinder can be removed to keep the valves closed. Then, the high-pressure oil pipe of the fuel injection pump leading to the cylinder can be removed to prevent it from injecting fuel into the cylinder, allowing the car to continue driving to the destination.
Inspection steps
(1) Check the free length of the valve spring. Measure the free length of the valve spring with a caliper, and its value should meet the standard value. If it does not meet the requirements, it should be replaced.
(2) Check the verticality of the valve spring. Use a square ruler and a flat plate to check the verticality of the valve spring. Its value should meet the standard value, otherwise it must be replaced.
(3) Check the preload of the valve spring. Use a force gauge to detect the preload force of the valve spring, and its value should meet the standard. If the preload force is lower than the standard value, the valve spring should be replaced.
(4) To prevent damage, the spring should be compressed frequently.
