Design of helical spring

This is a power point presentation on the design of Helical springs subjected to Static and Fluctuating load. It is part of Design of Machine elements subject.

1. DESIGN OF HELICAL SPRING

2. SPRING SUBJECTED TO STATIC LOAD OBJECTIVES FOR DESIGN It should possess sufficient strength to withstand the external load . It should have the required load deflection characteristic’s. It should not buckle under the external load.

3. DESIGN PROCEDURE For given application, estimate the maximum spring force (P) and the corresponding required deflection (δ) of the spring. In some case max. spring force and stiffness k , which is (P/δ) are specified. Select f s i.e. factor of safety as 1.5 or less if not specified. Select a suitable spring material and find out ultimate tensile strength S ut from the data. Calculate the Permissible shear stress in following manner : Permissible shear stress (τ) τ = , Assume, S yt = 0.75S ut and S sy = 0.577S yt τ = τ ≈ 0.3S ut In general, τ 0.3S ut - 0.5S ut  

4. Assume the suitable value of spring index (C). For industrial applications, the spring index varies from 8 to 10. A spring index of 8 is considered good value. C can be taken as 5 in valves and clutches. C should never be less than 3. Calculate the Wahl correction factor by following relation : K = + Determine wire diameter by following relation : τ = K Determine mean coil diameter by following relation : D = Cd Determine the no. of active coils (N) by following relation : δ= G = 81370 N/mm 2  

5. Determine the style end and find out no. of active coils. Adding total no. of active and inactive coils find total no. of coils ( N t ). Determine the solid length by following relation : L s = N t d Determine the actual deflection of the spring by following relation: δ = Assume gap of 0.5 to 2 mm between adjacent coils, when the spring is under the action of maximum load, the total axial gap between the coils is given by : Total gap = ( N t – 1) * gap between two adjacent coils Determine free length of the spring by following relation : Free length = solid length = total gap = δ Determine rate of spring by following relation : k =  

6. Prepare list of spring specifications.

7. SPRING SUBJECTED TO FLUCTUATING LOAD In many applications the spring is subjected to fluctuating load. In such cases spring is designed on the basis of two criteria - design for infinite life. - design for finite life. Let us consider a spring subjected to an external fluctuating force, which changes its magnitude P max . to P min in the load cycle. The mean force P m and the force amplitude P a are given by, P m = ( P max – P min ) P a = ( P max + P min )  

8. The mean stress is calculated from mean force (P m ) by using shear stress correction factor (K s ). It is given by, τ = K s ( ), where K s = K s is the correction factor for direct shear stress only, it is applicable only to mean stress only. For torsional shear amplitude ( τ a ), it is necessary to also consider the effect of stress concentration but to curvature in addition to direct shear stress. Therefore, τ a = K s K c τ a = K Where K is the Wahl correction, which takes into account the effect of direct shear stress as well as of stress correction due to curvature.  

9. In general the spring wires are subjected to pulsating shear stresses, which vary from 0 to S’ se (endurance limit). For Patented and cold-drawn steel wires : S’ se = 0.21S ut S sy = 0.42S ut For oil-hardened and tempered steel wires: S’ se = 0.22S ut S sy = 0.45S ut The general equation used for spring design is : =  

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