Automotive Coil Spring - Assignment Example

AUTOMOTIVE COIL SPRING by Section One. Function of a coil spring in context of an automotive suspension systemThe coil spring in context of an automotive suspension system is an essential addition and performs an important function. The front and rear suspension systems must perform several important functions to maintain vehicle safety and owner satisfaction when driving. The suspension system must supply the steering control for the driver under all road conditions, irrespective of how bad they are. A vehicle owner or driver expects the suspension system to offer a comfortable driving experience. The suspension along with the frame must maintain proper directional stability when the automobile is in motion (Craig, 2011:1). The coil spring is part of the suspension system and aids in the above functions. Coil springs absorb most of the shock that come from the tire impacts from the road surface. For example, when the front wheel of a vehicle strikes a road irregular road surface and moves upward, the coil spring compresses and absorbs energy during the movement. It immediately dissipates the energy as the spring goes back to its original form. The shock absorbers are put in the front suspension system to inhibit the oscillations of the coil springs. Figure 1 in the appendix section shows an example of a coil spring. Any spring, including torsion, leaf or coil spring must compensate for the irregularities found in the road surface. There is no road surface that is regular no matter how it is smooth or well tarmacked. The coil spring also maintains the suspension system at a certain height that is already predetermined and supports the added weight in the automotive without excessive sagging that could cause damage to the car or the tires. Each of the above functions is extremely important in providing comfort for the occupants of the automotive as it moves. They also aid in precise handling and load bearing capability of the vehicle. These are three key areas that any person who considers buying a vehicle puts into consideration in terms of the automotive suspension system in a vehicle of choice. Historically, the steel multi-leaf spring is the oldest and most widely used spring designs. However, the coil spring has become a widely used spring design in the modern world. Its function can be better understood when visualized as a long, thin torsion bar that is made into a coil shape. The coil spring operates with the same principle as a torsion bar because the bar twists during the spring’s compression and or extension cycles. Because it occupies a small space in the suspension system, the coil spring can be used in a variety of suspension designs. Generally, the number gauge of the wire, overall diameter, length and the number of coils determine the strength of the coil spring as well as its characteristics that make its work more efficient or less efficient. The coil spring cushions the ride of an automobile according to the principle of sprung-to-unsprung weight ratios. A vehicle without any coil spring in the suspension has a 100 percent unsprung weight. Therefore, when the coil spring is put between the chassis and axles of this vehicle, the sprung-to-unsprung weight ratio becomes about 90 percent on the chassis weight and 10 percent the axle and wheel weight. When a vehicle gains speed, the coil spring absorbs the impact of striking irregularities that the vehicle meets on the road surface. A stiffer spring rate is essential in ensuring that the wheels and axles are in contact with the road surface as the vehicle moves. Figure 2 in the appendix shows how the calculation of spring stiffness is done. A compressed coil spring can extend in a violent way and lead the vehicle to lose control especially in a rough road. Shock absorbers must, therefore, be used to dampen the spring’s compression. High speed vehicles use stiffer suspension systems and coil springs compared to common passenger vehicles because suspension is directly related to speed (Romlay, Ouyang & Nizam, 2011:21). The service conditions that the coil spring must withstand There are several conditions that an automotive coil spring must withstand. The fact that a coil spring is part of the automotive suspension system, it is essential that it withstands different conditions because it is exposed to a wide variety of conditions and things. First, a coil spring must withstand a hot condition. When an automobile’s engine is on, it is bound to generate heat that may get to the coil springs because they are part of the vehicle especially the front ones because they are near the engine. In addition, when the wheels move at a first speed, the heat they emit could reach the coil spring. The coil spring should be able to withstand this type of heat. Secondly, the car is exposed to different weather conditions. It could be too hot, too cold, rainy or dusty. All these are conditions that the coil spring must withstand. For example, the coil spring should not have a difficulty to compress because of the stiffness caused by extreme cold. In addition, the coil spring should be rust resistant because of water that the car is meant to pass through when running or when being washed. It should also be able to withstand winter seasons when snow creeps in. Rust can destroy coil springs if they are made from poor quality metals and cause accidents, injury and/ or discomfort for the car owner, driver or passengers (Das, Mukhopadhyay, Kumar & Bhattacharya, 2007:158). The coil spring should also be able to withstand dust and mud. These are some of the things that the coil spring must come into contact with especially in roads that are not tarmacked. Therefore, it should be able to withstand such conditions and still be strong. The coil spring should also withstand corrosion from things such as battery acid, oil spills and other properties. This is essential to ensure the safety of the vehicle owner and occupants when driving. The coil spring should also be able to withstand the loads of weight recommended for it. It should be able to withstand the compressions that the road surface might bring about when the vehicle is loaded with the recommended weight or slightly higher. This is the most important aspect because a coil spring should support the weight of the car as well as the additional weight. Additional weight as long as it is within the recommended weight should not feel the occupants of the vehicle to feel uncomfortable in an irregular road surface. Section Two. The material property requirements for the coil spring Because of the numerous conditions that an automotive coil spring should withstand, the material property requirements for the coil spring is very important because proper material requirements provide resistance to harsh conditions and unfriendly weather conditions. The actual operating environment has a significant role to play in the material specifications of an automotive coil spring. The first material property requirement for a coil spring is that the material should be non-corrosive. Many coatings are available that can provide adequate corrosion resistance for coil springs that would themselves lack the capability to resist corrosion. Therefore, coil springs should include coatings such as powder coating, phosphating with spray or oil dip and plating. However, stainless steel is also one of the best material properties because it is highly corrosion resistant and can be used in a variety of environments without any problem. Temperature also determines the material properties to be used in coil springs. For instance, higher temperatures require different material specifications depending on the amount. For instance, music wire and hard drawn carbon materials are used in a maximum temperature of 250 degrees. Oil tempered carbon is used in a temperature that should not go over 300 degrees. 302 stainless materials should be used in temperatures not more than 500 degrees and Inconel X750 should be used in very high temperatures, which is 1100, and below. For fatigue applications, for instance torsional stresses. Music wire and chrome-silicon valve spring quality materials are the best. Steel alloys are the best materials for use. Other metals that can also serve this purpose include beryllium copper alloy, phosphor bronze and titanium. Ceramic material can also be used for coil springs to be used in high-temperature areas (Robert Bosch GmbH, 2007:32). Material selection (using CES software) to justify the material used in the coil spring Material selection is a significant task in product development especially in coil spring manufacturing. Depending on the design approach, material selection is done at the advanced stage for specific materials used in automotive coil springs. The material selection in this report was done using CES Edupack 2011 software to fine materials to be used for automotive coil spring. The parameters and values considered were density 100kg/m^3, fatigue strength- 400 mpa, fracture toughness- 15 mpa.m^ 0.5 Stage 1: this stage involved limit selection that was done on materials with high fatigue strength, high yield strength and high fracture toughness. The software eliminated 90 materials and the possible materials for a coil spring included low carbon steel, stainless steel, high carbon steel, steel alloy, beryllium copper alloy, phosphor bronze and titanium. Stage 2: this stage involved the comparison of fatigue strength against density and fracture toughness. From the analysis, it was found that high carbon steel (music wire) was the best material for coil springs followed by steel alloys (Yamada & Kuwabara, 2007:72). Section Three. Discuss potential suitable manufacturing routes for the coil spring in the selected material Here, the coil spring is formed in a cold state from the highest tensile strength music spring wire. The cold forming process ensures that a high degree of flexibility is provided to the sprong design to allow for adaptation of geometry to the suspension axle without making it necessary to use any special devices. In step one of the cold coiling step, predetermination of the required diameter is done. After this, the coil rod is coiled to the diameter that is required. This step is followed by another step, which is known as the annealing step. Here, the coil is annealed at a low temperature. Thirdly, the ends of the coil are ground in a step known as end-grinding step. Next, the coil is given a complete residual stress. This step is known as the shot peening step. The next step involves coating the coil with paint in a process called paint coating step. What follows here is the second low temperature annealing step where the earlier step is repeated. Lastly, a predetermined load is put on the annealed coil in a step called hot setting step, which lasts for about 15 seconds and at a temperature of 250 degrees Celsius. Hardening is also an important route and is done through heat treating. This is meant to relieve the stress and ensure that the coil spring maintains its resilience. In the coiling process, both CNC and mechanical machines are used. When using the machine for instance, there are grooved coiling points (vertical and horizontal) that are positioned with large wire feeders that force the wire through the coiling points that force the wire round the mandrel to give the required spring coils. Stress relief is an important aspect in the manufacturing process. Manufacturing coil springs must include bending stress relief through heat treatment. The process of baking out the stress in the music wire will change the dimension of the coil spring. In other coil springs such as stainless steel, the coil will expand slightly. However, in music wires coil springs, the coil will contract slightly. A common requirement for coil springs is that the ends should be grinded to make them flat. This is done by using an automatic grinding machine. The main purpose of grinding the ends of the coil springs is to make them safe to handle for persons who fit them on cars and also enable easy and safe fitting on the axle and chassis of the vehicle to ensure that they do not slide out when the automobile is in motion (Knowles & Erjavec, 2002:28). Shot peening is also done as a process that is meant to strengthen the steel to resist issues of metal fatigue or cracking. This is done by hammering the spring and compressing it. After peening, setting is also done to come up with a permanent length and pitch desired for the spring. This involves compressing the coils to the extent that they touch each other. Lastly, coating is done to prevent erosion. This can be done through a variety of ways that include painting, dipping in liquid rubber and coating/ plating with other metals such as chromium. Section 4 Review & explain the selected manufacturing route in detail Coiling in the above method can be done using different machines that include a lathe, a spring-winding machine, a winding machine or an electric hand drill. Hot winding or heating can also be used to coil the springs. After coiling, it is put into oil for cooling and hardening purposes. The spring can be used but it requires tempering. The manufacturing process is not over without quality control and testing. Various methods can be used to check for compliance with the required specifications. The testing devices measure hardness of the metal, the spring’s deformation under a certain force and also the ability of the coil spring to withstand pressure. Computer-operated coiling machines are the best for quality because they ensure that the pitch and the diameter of the spring are of the required quality. Manufacturing process influences the material microstructure The manufacturing process influences the material microstructure in the sense that it makes it stronger. Most often coil spring failure is caused by issues of corrosion accelerated by adverse weather conditions. The corrosion exposes the microstructure of the material to the outside atmosphere and makes it highly susceptible to damages, such as hydrogen embrittlement. However, this manufacturing method makes it difficult to tamper with the materials microstructure. First, through heat treatment, heating and cooling the music wire helps to obtain the desired conditions and properties needed. Annealing also helps to produce the desired changes in the microstructure of the coil spring by heating the music wire to the required temperature, for instance, 250 degrees Celsius and holding it to that temperature, which is then followed by cooling at a suitable rate to soften it. This ensures that the coil spring’s microstructure is not exposed to damage. Stress relieving also contributes to the improvement of the material’s microstructure in the sense that it enables the residual stress to be removed and also ensure that the development of residual stress when in use is minimized considerably. This is important for maintaining a quality microstructure. Aging is also essential because it increases the strength of the material used to manufacture the coil spring. This is done through cold working and aging to produce a high tensile material and produce strengths that can withstand the harsh conditions of the environment where the coil spring is to be used. Therefore, it is clear that this manufacturing method is important for ensuring that the microstructure of the coil spring is developed to the expected standards and that it is not exposed to harsh conditions, for instance through issues such as corrosion. The steps involved are all important and should not be ignored in the manufacturing process. References Craig, K. (2011). Automotive suspension systems. Presentation: Motivation for the study of mechanical system physical & mathematical modeling, 1-72. Das, S. K., Mukhopadhyay, N. K., Kumar, B. R., & Bhattacharya, D. K. (2007). Failure analysis of a passenger car coil spring. Engineering Failure Analysis, 14(1), 158-163. Knowles, D., & Erjavec, J. (2002). Automotive suspension and steering systems. Clifton Park, NY: Thomson Delmar Learning. Robert Bosch GmbH. (2007). Automotive handbook. Plochingen: Robert Bosch. Romlay, F. R. M., Ouyang, H., & Nizam, M. K. (2011). Failure prediction for automotive suspension springs using Gaussian and Monte Carlo method. International Journal of Vehicle Design, 55(1), 23-34. Yamada, Y. & Kuwabara, T (2007). Materials for Springs. New York: Springer Science & Business Media Appendices Fig 1 Fig 2 Fig 3 Read More