tailieunhanh - mechanical design an integrated approach Part 11

Tham khảo tài liệu 'mechanical design an integrated approach part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 580 PART II o Applications Comment If this corresponds to operating speeds for equipment mounted on this . spring it may be necessary to redesign the spring. d Check for buckling for extreme case of deflection 5 Ạ í 411 hf 115 D Since is inside of the stable region of curve A in Figure the spring will not buckle. I HELICAL EXTENSION SPRINGS Figure illustrates a round-wire helical extension spring. Observe that a hook and loop are provided to permit a pull force to be applied. The significant dimensions of a standard end hook or loop are shown in Figure . Most of the preceding discussion of compression springs applies equally to helical extension springs. The natural frequency of a helical extension spring with both ends fixed against axial deflection is the same as that for a helical spring in compression. In extension springs however the coils are usually close wound so that there is an initial tension or so-termed preload p. No deflection therefore occurs until the initial tension built into the spring is overcome that is the applied load p becomes larger than initial tension P pị . It is recommended that 1 the preload be built so that the resulting stress Tjnitiai calculated from Eq. equals about C. Here st and c present ultimate strength and spring index respectively. b Figure Points of maximum stress in hook in conventional extension springs a stress at the cross section through A is due to axial force and bending b stress at the cross section through B ỈS due primarily to torsion. 581 CHAPTER 14 Springs Coil deflection of helical extension springs through the use of Eq. with p p Pị is given as follows o 8AIaC3 P-Pi 5 -. .-------i- uG The reduced coil diameter results in a lower stress because of the shorter moment arm. Hence hook stresses can be reduced by winding the last few coils with a decreasing diameter D. No stress concentration factor is needed for the axial component of