Machine design slide 14

# Machine design slide 14 - Week9,Lecture1 SpringDesign...

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ENGR 3220 – Machine Design ENGR 3220 – Machine Design Week 9, Lecture 1 Week 9, Lecture 1 Spring Design Spring Design

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Stresses in Helical Springs 2 3 max 4 8 d F d FD π π τ + = The maximum stress in the wire may be computed by superposition of the direct shear stress and the torsional stress: Or: A F J Tr + = max τ
Shear-Stress Correction Factor 3 8 d FD K s π τ = Shear equation in the last slide can be rewritten as: Where K s is called shear stress correction factor, which is given by: C C K s 2 1 2 + = Where C is called spring index, which is defined as C = D/d

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Curvature Effects of Springs 3 8 d FD K B π τ = The curvature of the wire increases the stress on the inside of the spring but decreases it only slightly on the outside, if curvature effects are considered, then the stress equation becomes: Where K B is given by: 3 4 2 4 - + = C C K B
Deflection of Helical Springs AG l F GJ l T U 2 2 2 2 + = G d FDN G d N FD F U y 2 4 3 4 8 + = = N D G d k 3 4 8 Spring rate is given by:

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Compression Springs Four basic types of spring ends are used: closed (squared) ends, closed (squared) ends ground, plain ends, and plane ends ground. Figure above illustrates the various ends conditions.
Compression Springs The number of coils is defined by either the total number of coils N t , or the number of active coils N a . The difference between N t and N a equals the number of inactive coils, which are those end coils that do not deflect during the service.

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Compression Spring Terminology Free Length Free length L 0 is the overall length measured parallel to the axis when the spring is in a free, or unloaded, state. If loads are not given, the free length should be specified. If they are given, then the free length should be a reference dimension which can be varied to meet the load requirements Solid Height The solid height L s is the length of the spring when it is loaded with enough force to close all the coils. For ground springs, L s = N t d. For unground springs, L s = (N t +1) d Direction of the Helix Springs can be made with the helix direction either right or left hand.
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