Compression Spring Design and CAE Analysis for Fatigue life improvement

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ISSN 2395-1621 Compression Spring Design and CAE Analysis for Fatigue life improvement #1 Nilam N. Tati, #2 Prof. S. L. Shinde 1 nilamtati@gmail.com

1 ISSN Compression Spring Design and CAE Analysis for Fatigue life improvement #1 Nilam N. Tati, #2 Prof. S. L. Shinde 1 nilamtati@gmail.com 2 samirlshinde@yahoo.co.in 1-2 Department of Mechanical Engineering, SKN Sinhagad Institute of Technology & Science, Lonavla, Pune, Maharashtra, India. ABSTRACT Compression Springs plays vital role in industrial field. Compression spring or coil spring is used to resist axial compressive force. These are available in different shapes (cylindrical conical, tapered, concave, convex). By changing spring parameters such as spring constant, spring ends, material & material finish allows meeting industrial requirement. The Sheet metal industry needs 'Die' for manufacturing stamped components. The stripper for the Die uses a set of coiled springs sandwiched between the top plate and the stripper. Its operate under high frequency of cyclic operation, typically 30 to 60 cycles per min (with a maximum of about 300 strokes per minute) along with high spring rate is the characteristic feature of this application. This poses a challenge for designing an appropriate spring for the given application & its predefined period. For designing proper compression spring, it s need to be considering all load, movement and available space for spring. Though this is an option of selecting the nearest available specification for the standard variety of spring. The spring is designed and modelled using CAD software. This further evaluated using FEA software for fatigue analysis for cyclic loading. Find out best suitable compression spring using FEA. ARTICLE INFO Article History Received :18 th November 2015 Received in revised form : 19 th November 2015 Accepted : 21 st November, 2015 Published online : 22 nd November 2015 Keywords Coil springs, fatigue life, Low cycle fatigue test. I. INTRODUCTION A spring is an elastic object used to store mechanical energy. Springs are elastic bodies (generally metal) that can be twisted, pulled, or stretched by some force. They can return to their original shape when the force is released. In other words it is also termed as a resilient member. A spring is a flexible element used to exert a force or a torque and, at the same time, to store energy. The force can be a linear push or pull, or it can be radial, acting similarly to a rubber band around a roll of drawings. The torque can be used to cause a rotation, for example, to close a door on a cabinet or to provide a counterbalance force for a machine element pivoting on a hinge. A compression spring can also be explained as an open-coil helical spring that offers resistance to a compressive force applied axially. Compression Springs are the most common metal spring configuration and are in fact one of the most efficient energy storage devices available. Other than the common cylindrical shape, many shapes are utilized, including conical, barrel and hourglass. Generally, these coil springs are either placed over a rod or fitted inside a hole. When you put a load on a compression coil spring, making it shorter, it pushes back against the load and tries to get back to its original length. 2015, IERJ All Rights Reserved Page 1

2 Fig. 1 Spring Dimensional Parameters Common Applications: Compression springs are found in a wide variety of applications ranging from automotive engines and large stamping presses to major appliances and lawn mowers to medical devices, cell phones, electronics and sensitive instrumentation devices. Cone shape metal springs are generally used in applications requiring low solid height and increased resistance to surging. The objectives of the spring based on the application could be: To provide Cushioning - to absorb, or to control the energy due to shock and vibration. Car springs or railway buffers to control energy, springs-supports and vibration dampers To Control motion - Maintaining contact between two elements (cam and its follower) Creation of the necessary pressure in a friction device (a brake or a clutch) To Measure forces - Spring balances, gages II. OBJECTIVE OF THE PROJECT "The Sheet metal industry needs 'Die' for manufacturing stamped components. The stripper for the Die uses a set of coiled springs sandwiched between the top plate and the stripper. The high frequency of cyclic operation, typically 30 to 60 cycles per min (with a maximum of about 300 strokes per minute) along with high spring rate is the characteristic feature of this application. This poses a challenge for designing an appropriate spring for the given application. Though these is an option of selecting the nearest available specification for the standard variety of spring, it is recommended to ascertain the design specs through 'Fatigue Analysis' (using FEA methodology and validate the same using low cycle short duration test for determining permanent set and/ or fatigue life." From given problem statement here we are going to design and analyze new design for die spring to withstand high loads in stamping operation of metal sheet according to stamping operation the given tonnage of die is specified as 7.68 Tone (8T approx). As 1Tone = 1000 kg. Therefore, the tonnage will in the Newton form is given as follows, 8 Tone = 8 x 1000 kg 8 Tone = 8000 kg Therefore, this conversion into Newton is =8000 x 9.81 =78480 N But during the operation of stamping this required some force to create blank in to sheet called as striping force. This is considered to be as 15 % to 20 % of tonnage load. So here we are considering the 20 % of striping force which help to calculate blank holding force acting on the die spring. This is given as, Total BHF = 20 % of tonnage in Newton Total BHF = N 16 KN Stripping force required = 25% of total tonnage = 1920 kg =2T approx Compressed length = 40% of Free Length (FL) = 27mm (as required by application/ design) Force per mm= 42.42N/mm = 4.32kg/mm (as per catalog) Therefore, Force exerted during stripping per spring = 4.32X28=117.5 = 118kg Since, Total stripping force = 1920kg (2T approx); Number of springs = 1920/118= 16nos During Stamping operation the number of spring holds the sheet to create blank. Therefore total blank holding force will distributed to respective number of springs. Hence we can calculate the force acting on single spring during operation as there is 16 numbers of springs. Therefore force acting on single spring is as, Load (force) on single spring = 16000N 16 =1 x 10ᶾ N. Therefore the force acting single spring is 1 x 10ᶾ N. So here we consider the single spring as a standard specimen of spring as 1st variant & find out the desired results for given boundary conditions. Obtained results are considered as a bench mark for further design modification for die springs. Dimensional Specification of 1st Variant (standard) spring: 1st Variant OD ID FL Pitch Thickness Dimensions 31mm 17mm 68mm 8mm 4mm Material Specification of 1st Variant (standard) spring: Material: Steel Spring Young s Modulus: 2.1 X 10^5 N/mm2 Density: 7.9 x 10^6 kg/mm3 Poisson s Ratio: 03 Yield Stress: 1680N/mm2 III. FINITE ELEMENT ANALYSIS 1) Element type: Selecting element types for FEA model is the most important decision in analysis, because element represents the actual properties of the material. We select Hex8 2015, IERJ All Rights Reserved Page 2

(Hexahedral shaped, 3Degree of freedom, linear shape) element for analysis. The load and constraints are applied on the mater node which is connected to the spring by ID rigid RBE1 and RBE2 element.

2) Material properties 3) Element Input Fig. 5 imum shear stresses in the spring Fig. 2 Boundary Conditions Analysis post preferences reports: Fig. 6 Fatigue Life IV.

3 (Hexahedral shaped, 3Degree of freedom, linear shape) element for analysis. The load and constraints are applied on the mater node which is connected to the spring by ID rigid RBE1 and RBE2 element. A load of 1.3x103 N is applied on the top rigid element RBE2 at the master node, whereas the boundary condition is applied at the top bottom rigid element RBE1 at the mater node. 2) Material properties 3) Element Input Fig. 5 imum shear stresses in the spring Fig. 2 Boundary Conditions Analysis post preferences reports: Fig. 6 Fatigue Life IV. MODIFIED DESIGN OF SPRING: Here we have considering three different variations in spring de Here we took the three different variant of springs as according to change dimension, Tapered and changing pitch. Specification for variants are shown in following table Fig. 3 Displacement Plot Dimensional Parameters Case 1: By changing Dimension Case 2 : By changing the pitch Case 3 : By applying Tapered OD 31mm 31mm 31mm ID 17mm 17mm 9mm FL 68mm 68mm 68mm Pitch 8mm 6mm 8mm Thickness 3mm 4mm 2mm Fig. 4 imum Von Misses stress in the spring 2015, IERJ All Rights Reserved Page 3

Displacement: For changing thickness (from 4mm to 3mm) in Case I, we are getting displacement greater than standard spring by 2.

18mm For Taper spring in Case 3, we are getting displacement less than standard spring by 1.1mm Fig.

But in Case 3 Variant shows the less stresses as compare to stress generated in other two.

4 Displacement: For changing thickness (from 4mm to 3mm) in Case I, we are getting displacement greater than standard spring by 2.48mm For changing Pitch (from 8mm to 6mm) in Case 2, we are getting displacement greater than standard spring by 1.18mm For Taper spring in Case 3, we are getting displacement less than standard spring by 1.1mm Fig. 6 Fatigue life for Case 1 Spring Stress (Von misses stress & shear stress): For Case I, Case 2 & Case 3, we are getting stress are greater than standard spring. But in Case 3 Variant shows the less stresses as compare to stress generated in other two. Stress (Von misses stress & shear stress): For fatigue life calculation we are considering min obtained result for component life. For Case 1 & Case 2: Minimum Fatigue Life is coming less as compared to standard spring (less than 3lac cycle) Case 3: we are Minimum Fatigue Life greater than standard spring (more than 3lac cycle) V. EXPERIMENTAL VALIDITY Fig. 6 Fatigue life for Case 2 Spring Fig. 6 Fatigue life for Case 3 Spring Result summary: Variants Displace Von Misses Shear Fatigue Life Stress Stress Standard x10^6 Case x 10^1 Case x 10^4 Case x 10^7 From above result for three different springs by considering standard spring as a bench mark at provided boundary conditions, we can summarized with help of following design criterion: Fig. 7 Experimental set up Through experimental set up we have estimated analysis report benchmark results to certify modified design spring for their satisfactory performance within required life time. VI. CONCLUSION There is scope of improvement in the performance of the standard spring with modification in geometry parameters. From the variants for this work, the tapered spring gives the best performance in terms of fatigue life & also the stresses are moderate. REFRENCES [1] B. Kaiser, B. Pyttel, C. Berger, VHCF-behavior of helical compression springs made of different materials, International Journal of Fatigue, (2011), pp [2] B. Pyttel, I. Brunner, B. Kaiser, C. Berger, M. Mahendran, Fatigue behaviour of helical compression 2015, IERJ All Rights Reserved Page 4

5 springs at a very high number of cycles Investigation of various influences, International Journal of Fatigue, (2013) [3] D.E. Moulton, T. Lessinnes, A. Goriely, Morphoelastic rods. Part I: A single growing elastic rod, Journal of the Mechanics and Physics of Solids, (2013), pp [4] I. Pöllänen, H. Martikka, Optimal re-design of helical springs using fuzzy design and FEM, Journal of Advances in Engineering Software, (2010), pp [5] Ji-Eun Choi, Gyoung-Dek Ko, Ki-Ju Kang, Taguchi method-based sensitivity study of design parameters representing specific strength of wire-woven bulk Kagome under compression, Journal of Composite Structures, (2010), pp [6] K. Michalczyk, Analysis of the influence of elastomeric layer on helical spring stresses in longitudinal resonance vibration conditions, SciVerse Science Direct,(2013),pp , IERJ All Rights Reserved Page 5

Belleville Spring. The relation between the load F and the axial deflection y of each disc. Maximum stress induced at the inner edge

Belleville Spring. The relation between the load F and the axial deflection y of each disc. Maximum stress induced at the inner edge Belleville Spring Disc spring, also called Belleville spring are used where high capacity compression springs must fit into small spaces. Each spring consists of several annular discs that are dished to