K w = 1.2 is generally a suitable estimate at this stage. 7.Ĭalculate a trial wire diameter, d, by rearranging Eqn (15.6) or Eqn (15.9) and assuming a value for K s or K w, which is unknown at this stage. An estimate for the initial design stress can be made using Table 15.5 and Figure 15.12. Specify an initial estimate for the mean diameter. 4.Ĭalculate the free length, L f = L i + ( F i/ k). 3.ĭetermine the spring rate, k = ( F o − F i)/( L i − L o). Identify the operating force, F o, operating length, L o, the installed force, F i, and the installed length, L i. Select a material and identify its shear modulus of elasticity G. The design procedure requires access to tables of data for material properties and wire diameters. This approach is outlined below and in the following example. One approach, knowing the force and length of the spring, is to specify a material, guess a trial diameter for the spring considering the space available, check the values calculated for spring rate and free length, and if necessary try a new wire diameter. There are a number of strategies that can be followed in designing helical compression springs (see Mott, 1999). For a particular desired final length, the spring's dimensional changes must be accounted for by the manufacturer. During prestressing, the spring's dimensions will alter. This process is repeated a number of times, typically no less than three. It involves compressing the spring to its solid length or a fixed position that is greater than its maximum working length. Prestressing takes place after the spring has been coiled, stress-relieved, and ground. With prestressing it can be loaded to 70% of its ultimate tensile strength. For example, compression springs manufactured from BS EN10270-1 cold drawn carbon steel without prestressing can to be loaded to 49% of the material's ultimate tensile strength. Prestressing, also known as presetting, of a spring can be used to improve a spring's ability to withstand stress, increasing its load-carrying capability and fatigue resistance. Nonferrous and austenitic stainless with presetting. Nonferrous and austenitic stainless without presetting. Probably Figure 7.16 is the first record of tribonucleation that has been observed in an oil hydraulic device with a movable part. The emergence and disappearance of a cavity repeatedly occurs at an identical location on the valve seat with the same frequency as the poppet vibration. On the other hand, the cavity generation is more evident in Figure 7.16 a long thin cavity extending in the circumferential direction emerges in the third frame and disappears in the eighth. However, if you watch the original video movie of the phenomenon (Movie 7.16, ), you can surely recognize the motion. The lateral motion of the poppet comes out as such slight differences of the poppet-seat clearance among the images in Figure 7.16 that it is hard to recognize. Figure 7.16 demonstrates such a phenomenon with some images taken with the high-speed camera at 13 500 F/s. A little while later, the poppet starts to make a self-excited vibration in the lateral direction and a cavity emerges at the point where the poppet contacts the seat. Read moreĪs the upstream pressure is gradually raised to exceed the initial spring compression, the poppet is detached from the seat and the valve opens. Generally speaking, the spring coefficients should be taken with a larger value to make the design safer. Currently, there are few test data about joints, and the test methods still have shortcomings. However, in fact, the mechanical property of the structure is very complicated, and various spring coefficients are nonlinear, such that these spring coefficients must be determined on the basis of abundant joint loading test data.įor underground structures such as circular linings, the section shear force is small, owing to the support of the surrounding ground, and the section strength is mainly controlled by the bending moment and the axial force. At present, most of calculations based on joint test data are conducted by assuming that these spring coefficients are constants. K N, K S, and K θ represent the compression spring coefficient, the shear spring coefficient, and the rotary spring coefficient of the joint, respectively. Junsheng Yang, in Shield Tunnel Engineering, 2021 5.5.5.1 Determination of spring coefficient Structure type and design of shield tunnel lining
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