Fundamental Stress Circulation - Assignment Example

Compression Coil Springs By Lecturer’s and SECTION This paper is generally a review of the fundamental stress circulation, features of helical coil springs. There is an in depth discussion regarding the parameters that influences the coil spring’s design and quality. Factors that tend to affect the coil spring strength have also been totally analysed. Therefore entire manufacturing process should involve greater designs that encompass the entire ability to curb with higher stresses no matter the dimensions. This hence requires critical coil springs designs especially using the laser melting manufacturing process (SPRING MANUFACTURERS INSTITUTE, 1997).  The most widely used types of springs around the globe are usually the helical compression springs. It can be applied in various practical areas. Due to this fact, appropriate and highly valued designs should be put into dire considerations in due course of the manufacturing process. The basic factors that should be duly looked at include spring’s stability, spring relaxation, and surge in spring, strain energy, fatigue loading and other basic design processes. Laser melting refers to an additive manufacturing process that utilizes 3D CAD data as the ultimate source of digital information as well as the energy that results in form of high-powered laser beams; towards creation of 3-dimensional metal segments through fusing together of fine metal particles (MOHAMED A.E. SALEH and ADHAM E. RAGAB, 2013). The manufacturing process usually starts by first slicing the file data into a number of layers that often revolves between twenty to a hundred micrometres in terms of thickness, hence establishing a 2D image for each layer (MOHAMED A.E. SALEH and ADHAM E. RAGAB, 2013).The file is then put into a package of file preparation software that hence assigns values, parameters and supports that will allow for file interpretation and built by various types of manufacturing machines. This kind of spring manufacturing process tends to melt the thin layers of the fine metal particles that are distributed evenly by use of a coating mechanism. This often involves the act of fastening a substrate plate that usually exists in a metallic nature, into an indexing table moving in a vertical axis. The entire process takes place within a chamber that contains a tightly controlled atmospheric nature of an inert gas. After the constant distribution of each and every layer, each geometric slice of 2D layer is fused through selective application of a highly-powered laser beam. The applied laser energy is often more intense to allow for full melting or welding of the given particles, hence forming a solid metal. The all process is repeatedly done from one layer to the other until the all process is complete. Examples of different types of springs: Concept of spring design: The entire spring design involves various considerations that include: the space at which the spring should operate and fit into, the values of deflections and working forces, the needed reliability and Accuracy, permissible variations and tolerances with regards to specifications, environmental factors such as temperature, existence of a more corrosive atmosphere, as well as the aspects of quality and cost (SPRING MANUFACTURERS INSTITUTE, 1997).  The designers often use these ultimate factors while selecting a material and specifying suitable wire size value, the average number of coil turns, the diameter of coils and length, the type of endings, as well as the needed spring rate for satisfaction of the operational force deflection conditions. The main constraints that tend to emerge with regards to various designs are that, there should be a commercial availability of the wire sizes and the solid length stress should no longer be greater than the ultimate torsion strength. This means that, further spring operations should be entirely stable. Buckling (Stability of the spring): This is a very common phenomenon when it comes to the aspect of coil spring. Buckling often occurs when the load exerted to a slender column tend to go beyond the critical value. Likewise, the compression springs will also buckle in a situation whereby the spring’s free length is a little bit larger, while the ultimate conditions are not properly set for even load distribution along the coil’s circumference. The coil springs often have greater buckling tendency when there is greater deflection. This aspect can be entirely prevented through limitation of the spring’s deflection or the spring’s free length. Such kind of behaviour can be characterized by use of critical length, critical deflection and two dimensionless parameters. Critical deflection refers to the rate of proportion of deflection against the spring’s free length. Critical length on the other hand refers to the free length ratio to that of mean coil diameter. It is often a function of the critical length and hence has to be entirely below a specific limit. Springs function can be evaluated with accordance to the main course as well as the extent of its ultimate deformation based on the applied load. With regards to the pattern of deformation, springs can be sub-divided into three main types that include; springs with either linear, degresssive or the ones with progressive properties. Under the spring’s characteristic curve as shown above, the W region is representing the spring’s deformation work performed during the loading process. Spring Surge and Critical Frequency: Generally, a compressive wave travelling back and forth can be created especially if a single compression spring end is held on a flat surface while the other loose end is essentially disturbed. Under specific circumstances, a resonance might occur hence resulting in a violent motion that makes the spring to detach from contact with the entire end plates, thus resulting in a number of destructive stresses. This practically occurs if the damping of the internal spring’s material is a bit low. This incident is termed as the spring surge. When a helical spring is used in an application that requires a fast reciprocating motion, the designer should be assured that the spring’s physical dimensions are not in such a position of creating a vibratory frequency that is much closer to the frequency involving the applied force. Fatigue Loading: The springs usually have to be in a position of sustaining millions of operation cycles without failure. So, this means that the design should be meant for an infinite life. Helical springs cannot be used as either extension or compression springs. They are generally assembled together with a preload such that there is an additional working load. This can be expressed as follows: Fa = (Fmax - Fmin)/2 Fa = (Fmax + Fmin)/2 This can be illustrated as shown in the figure below: (SPRING MANUFACTURERS INSTITUTE, 1997).  Strain energy: Springs are typically made up of various material components. The key factor that should hence be put into consideration in spring design is the quantity of strain energy with regards to the used material. It can be expressed as follows: U = б2 / (ρ×g) This shows that any given material with lower density (ρ) will have a relatively much higher strain energy levels under same ratio of stress (б). Spring Relaxation: All types of springs are often expected to function over longer time periods without any significant changes in terms of dimensions, spring rates, or displacements, often under the fluctuating weights. If a given spring is entirely deflected under the application of full load or the situation when the induced stress exceeds the material’s yield strength, the resultant deformation might prevent such a spring from provision of the needed force or the delivery of the accumulated energy for later operations. Various springs are usually subject to some sort of relaxation within their span of life even under gentle conditions. The level of spring relaxation is usually the role of the ultimate spring material as well as the time frame at which the spring is subjected to the higher temperatures or stresses. Static springs on the other hand can be utilized in constant load applications or constant deflections. A steady deflection spring can be cycled through a specific deflection range, a relaxation that often lowers the rate of applied stress or even the spring load that causes some set. With time the spring will ultimately relax, hence reducing the applied mass (SPRING MANUFACTURERS INSTITUTE, 1997). The elevated temperatures as well can lead to occurrence of thermal relaxation, excessive changes with regards to the spring’s dimensions or even reduction in the load quantity that supports capability. Often, there is no change on the load that is applied to a coil spring especially under constant conditions. Stable load springs might creep or set, but the amount of applied stress remains constant. Moreover, the constant stress might result in shorter fatigue lives as compared to those found within a constant deflection. In most applications, extension and compression springs are often subjected to the elevated temperatures at higher stresses that can result into relaxation of load. This kind of condition is usually termed as set. After determination of the operating conditions, set predictions and allowances are made within the spring design. When a set is not allowed to operate within an application, the coil spring manufacturer might have the ability of pre-setting the spring at stresses and temperatures higher than the ones bumped into in the operating field. Relaxation is often an operation of a moderately higher stress over a given time period. The spring’s creep might also lead to the unacceptable changes with regards to the dimensions even under the static set. A spring that is held at a definite stress level will essentially relax more at a given time period as compared to a spring that has been cycled between a higher and a lower stress level. This is simply because it tends to spend more time within high stress levels. The estimation of the rate and level of spring relaxation within a given time frame can be done through initial determination of the applied temperature, the maximum quantity of stress on the spring as well as the ultimate spring’s life span when exposed into the greatest stress levels and the higher temperatures. Design considerations: The design of a compression spring involves the aspect of over simplifying the stress distribution within a wire. It is usually sets its basis on the basic assumption that; a component of a helical spring that is axially loaded behaves significantly as a straight piece in a pure torsion. The mostly used notations during coil spring computations include: the applied load, pitch angle, shear stress, coil radius, and the wire diameter. Traditionally, the compression stress and the bending stresses are often neglected in situations when the ultimate pitch angle happens to be less than ten. Application: The form of applications that are mostly suitable for the compression springs that are produced through the laser melting process are often the complex geometries and structures with hidden channels or voids. Greater advantage can be observed during production of hybrid forms whereby solid and lattice variety of geometries can be mutually produced so as to establish a distinct object. This can be taken to include objects such as cetabular cup, hip stems or other forms of orthopaedic implants whereby the aspect of oseointegration can be enhanced through surface geometry (BERTI GUIDO and MONTI MANUEL, 2011). With laser melting technology, much pioneering work is usually on lightweight aerospace parts whereby traditional manufacturing limitations, including physical and tooling accessibility to machinery surfaces, tend to restrict the component’s design. It generally allows for additive building of various parts towards formation of near net shaped sections rather than removal of waste materials. Conclusion: It is generally viewed that main factors that tend to impact on the springs’ strengths are often the aspects of Design parameters, Raw material defect, Material selection, surface imperfection, as well as the spring geometry. The design parameters that includes the operating modes, shot peening, Operating temperature, and the imperfections that occurs within the coil that directly affects the spring’s fatigue life. This is due to the fact that, as the increase in temperature results into proportionate increase in the tensional and modulus yield strength hence decreasing the spring’s material strength. Shot peening of the inner coil spring tends to duly reduce coil surface stress while increasing the coil’s fatigue. It can also be viewed that, presence of any form of impurity within the raw materials also reduces coil spring strength. Consideration of all these factors can hence lead to the production of high quality compression coil springs. SECTION 2: A spring refers to a device that significantly changes with regards to its shape in response to the applied external force, and returns back to its normal shape after removal of the applied force. The expended energy in spring deformation is often stored in the entire spring and thus recovered when it returns to its initial shape. Generally, the amount of the exerted force is directly proportional to the amount of spring deformation. The spring can hence deform permanently without any sign of return to the original shape if there is application of an extreme force. There are various categories with regards to the coil springs. The most common amongst them all consists of a wire that is wound into a conical shape. An extension coil or spring refers to a coiled spring with coils that are usually in contact with each other after a specific force is applied towards stretching of the spring that leads to coils separation. Contrary, a compression coil spring is characterized with spaces in-between consecutive coils; especially when a specific force that shortens the spring is applied hence leading to the aspect of mutual pushing of coils. The third type that is often referred to as the coiled spring or the torsion spring is usually designed in such a manner that the applied force has the ability of twisting the coil into a tighter spiral. The common instances of the torsion springs are often found in butterfly hair clips and clipboards. Watch spring refers to another coiled spring variation is that is coiled into an even spiral rather than a cone or a cylinder. One spring end is usually at the mid of the coil, while the other end is often at its ultimate outer edge Design of a conical helical spring: Some of the springs are normally fashioned without any kind of coils. The most common instance is the aspect of leaf spring that is often shaped with the resemblance of a shallow segment; it is generally used for the automobile suspension structures (ARMAMENT DESIGN ESTABLISHMENT, 1951).  Another main type refers to a disc spring that is comprised of a washer-like instrument that is normally shaped just like a condensed cone. Open-core solid cylinders as well as the elastic materials can also operate as springs. Non-coil springs on the other hand generally operate as the compression springs. Throughout history, there has been massive utilization of very simple, non-coil springs. More classy springs dates back to the Bronze Age. Ever since, there have been a number of progressive developments with regards to the coil production process. Coiled springs were entirely developed even within the early 15th century. This was done through replacement of the weight systems that powered clocks commonly with a coiled spring mechanism. The diagram shown below depicts the aspect of spring coiling that was done by use of a CNC machine. This sort of advancement was responsible for offering celestial ship navigations. In the 18th century, the aspect of Industrial Revolution pushed for the establishment of mass-production methodologies for manufacturing springs. The machine that was used by then which was apparently a lathe adaptation carried a reel of wire rather than of a cutting head. The wire from the entire reel was hence wrapped up around a rod that was secured within the lathe. The lead screw’s speed, which was comprised of the reel that was parallel to the entire spinning rod, could be entirely be adjusted in such a manner that it offered variations in terms of the spring coil spacing. The most common examples of the latest spring usage usually range from small coils that offer support to the keys on cellular phone’s touch pads to the larger coils that offers support to the entire buildings and protection from the earthquake vibrations. Raw materials: The most popularly used alloys include High-carbon, chrome silicon; oil-tempered low-carbon, stainless steel and chrome vanadium are some of the most popular alloys that are usually used (GAYER & STONE, 1955).  Other form of metals that are at times utilized in spring production includes beryllium, copper alloy, bronze, phosphor and titanium. Urethane or Rubber may also be utilized for manufacture of the cylindrical, non-coil springs. On the other hand, ceramic materials have been established for the coiled springs in high-temperature conditions. In line with this, one-directional glass fibre compound materials are entirely being tested for any possible usage in manufacture of springs. PARAMETERS AND THEIR EFFECTS: Springs often tend to be extremely stressed simply because of their subsequent designs that are aimed towards ensuring that they fit into very small spaces with the lowest material cost and the least possible weight. They are also required to distribute the needed force over a longer time period. The spring reliability is therefore highly related to its entire material strength, operating environment and the design characteristics (GAYER & STONE, 1955). There are various parameters that tend to influence the spring strength. Some of these parameters include the aspect of material selection, raw materials defect, heat treatment, spring geometry, as well as the design parameters. Material selection: This is always one of the most essential decisions towards obtaining the best product quality in any perspective, including the coil springs. The raw materials’ selection normally includes the aspect of enforcing cleanliness, decarburization inspection and microstructure. The major sub parameters in each and every aspect of material selection are the inappropriate heating patterns, decarburization, microstructure as well as enforcement. Raw material defect: A distinctive raw material defect refers to the existence of any foreign material within the steel metal, for instance the non-metallic elements. This specific coil usually fails early in spite of all the other essential parameters being standard Heat treatment: Improper form of heat treatment can easily be overlooked since a slight difference with regards to temperature in due course of the heating process does not directly relate to the material’s level of hardness. There is thus a greater need for extensive evaluations towards identification of such a problem. On the other hand, prolonged heating rate can also lead to prior to the significant growth of austenite grain. This aspect of inappropriate heat treatment can end up with transformation of the microstructure into pearlite as opposed to the needed martensite (ARMAMENT DESIGN ESTABLISHMENT, 1951). Identification of this defect is often much easier because of the clear variations in hardness. Spring geometry (wire diameter): This parameter works towards offering clear descriptions on the wire’s diameter that is utilized as an ultimate spring’s material. The spring’s internal diameter can be computed through subtraction of the doubled diameter from the external spring’s diameter. External diameter on the other hand can be computed by addition of the doubled diameter to the internal spring’s diameter. There is also the aspect of solid length. This refers to the maximal spring’s length after total blockage. Free compression spring’s length can on the other hand be measured within its uncompressed nature. Pitch as well refers to the distance between subsequent spring coils. All these factors are very essential when it comes to the aspects of coil springs. Most often, the strength of a spring is mostly affected by its height in most circumstances. This is the basic reason why height variations should be put into dire consideration during the process of spring designs based on the geometric valuations. This can hence be illustrated graphically as shown below: Design parameters: The metal spring’s development has progressed within the past few years and the ultimate concentration has been on the aspect of reducing the spring’s operational weight. This often leads to a small mounting space whereby the particular spring stresses increases continuously. Hence a lot of care needs to be undertaken for careful processing and manufacture of the entire spring. With respect to the surface layer, shot peening and hot pre-setting should be taken into dire consideration. The surface quality often plays a very important role for the spring’s operational durability rather than the material properties. There are various design parameters that tend to affect the overall designs off the helical compression springs CHAO ZHANG, WIESLAW K. & GREGORY N, 2012). They include the operation mode, internal diameter’s imperfections, stress peening, and the operating temperatures. Operating mode Based on the ultimate application, a spring might be in a cyclic, dynamic or a static operating mode. A spring is generally considered as being static especially if a change in load or deflection occurs for a few times. A static coil spring might remain loaded for longer time periods. For the static springs, the failure interest modes include set, spring relaxation and creep. Cyclic springs are repeatedly flexed and are normally expected to display a higher rate of failure due to the ultimate fatigue. Cyclic springs can be operated in either a reversed stress or a unidirectional mode. The stress is usually applied within similar directions in some of the springs, while on the others; stress is first applied in one direction before applying it in the ultimate opposite direction. For the similar deflection and maximum stress between a reversed and unidirectional stress spring, the range of stress for the reversed stress spring is often twice the stress of the ultimate unidirectional spring, therefore leading to the expectation of a shorter fatigue life. Moreover, dynamic loading is taken to mean the intermittent load surge occurrences including shock absorbers that induce higher stresses on the entire spring. Dynamic spring loading falls into 3 major categories: shock, the entire spring’s resonance, as well as the resonance of the mass system. Shock loading usually occurs especially when a load with sufficient speed is applied in the sense that the first coils springs assume more of the weight than the one that would be intended for cyclic a static situation. This aspect of loading is often due to the spring’s force of inertia. Resonance occurs especially when there is equivalence between the operating speed and the spring’s natural frequency or the natural frequencies’ harmonic. Resonance on the other hand can cause highly elevated stresses as well as the possible coil clash that might result in a sort of premature failure. a) Imperfections on inside diameter of spring: Helical compression springs usually respond to the external force with torsion stress that is caused by the entire torsion of active spring coils. Since the entire shear angle is however greater on the internal coil surface other than the outer surface, the marginal inner coil’s torsion stress is often higher than that of the external surface. This situation can be described using a rectification factor k that is entirely dependent on the wire’s curvature (ARMAMENT DESIGN ESTABLISHMENT, 1951). Curvature can hence be exemplified by the mean wire diameter and spring diameter quotient, as well as the coil ratio. This hence means that the maximum stress on the helical spring usually occurs on the internal coil surface. Accordingly, the helical compression spring’s fatigue fractures usually start off from this area. The spring’s internal coil surface therefore has to be entirely shot peened with specific care, which according to the spring geometry, it constitutes of a very fastidious task. A good example of such a recommended design is as shown below: b) Stress peening: Shot peening refers to a standard technical procedure. Peening is generally termed as the interaction between the working piece surface and the particle. If the entire particles have round shapes, it is regarded as shot peening. Compressive residual stresses are often induced within the surface layer. At lower working piece hardness, an extra hardening is usually attained. Stress peening is mostly used for obtaining of much better outcomes through this process. c) Operating temperatures: Compression springs that is subjected to a much higher temperature needs special attention towards spring design and spring material selection. Advanced alloys are often required to offer stable spring load properties in the increased temperature service. Temperature increase impacts on the elastic limit and modulus of most springs. Maximum working temperatures for a spring material simply refers to the ultimate temperature whereby the metallurgical transformation begins (MOHAMED A.E. SALEH and ADHAM E. RAGAB, 2013).The time dependent spring changes occur especially when a persistent stress is subjected at an enhanced temperature ratio. Such changes often occur at room temperature especially if the subjected stress is much higher. Increase in temperature merely leads to increase in the rate of transformation. The change normally occurs simply as a reduction with regards to the helical spring length or spring load reduction at a fixed span. Reference List ARMAMENT DESIGN ESTABLISHMENT. (1951). Design of helical compression springs: design data sheets. H.M.S.O. BERTI GUIDO and MONTI MANUEL. (2011). Experimental and numerical analysis of the cold forming process of automotive suspension springs; University of Padua. CHAO ZHANG, WIESLAW K. & GREGORY N. (2012). Experimental and FEM study of thermal cycling induced micro cracking in carbon/epoxy triaxial braided composites. GAYER, J. D., & STONE, P. H. (1955). Helical spring tables; an easy-to-use index of ready-designed compression and tension springs from which selections may be made with minimum calculations. New York, Industrial Press. SPRING MANUFACTURERS INSTITUTE. (1997). Handbook of spring design. Oak Brook, IL, The Institute. MOHAMED A.E. SALEH and ADHAM E. RAGAB. (2013). Helical Spring Manufacturing via SLM: Effect of Geometry on Shear Modulus, Hong Kong. Read More