STUDY OF UNCOILING IN SUSPENSION SPRINGS ITS EFFECTS

Size: px

Start display at page:

Download "STUDY OF UNCOILING IN SUSPENSION SPRINGS ITS EFFECTS"

Shannon Allen
5 years ago
Views:

STUDY OF UNCOILING IN SUSPENSION SPRINGS ITS EFFECTS Kushal A Jolapara 1*, Adhip Puttaraj 2, Abhishek Chatterjee 3 1* Kushal A Jolapara, B.E (Mech). Production Manager, KushalPolycoats, Bangalore.

1 STUDY OF UNCOILING IN SUSPENSION SPRINGS ITS EFFECTS Kushal A Jolapara 1*, Adhip Puttaraj 2, Abhishek Chatterjee 3 1* Kushal A Jolapara, B.E (Mech). Production Manager, KushalPolycoats, Bangalore. PH: E- mail: bollerophon@gmail.com 2 AdhipPuttaraj, B.E (Mech) Administrator, Manasa Hospital, Bangalore. PH: adhipputtaraj@gmail.com 3 AbhishekChatterjee, B.E (Mech), Project Co-ordinator, CBRE Pvt Ltd, Bangalore, PH: E- mail: abhishek.chatterjee23@gmail.com Abstract The uncoiling effect is evident when motion of one end of the spring is completely constrained and the other end is allowed to rotate freely during compression.the free rotation results in a change in spring attributes like rate of the spring and stresses generated within the spring. This has an obvious impact on the working and life of the spring. This paper showcases the work in a project, carried out to determine the extent of variation in load rates between helical suspension springs operating under both the cases (coiling restricted and coiling unrestricted). The experimental and FEM data showed a variation in the load rate as well as rotational movement in the end coil. Based on the study, it was concluded that, firstly, there is a definite change in spring performance under the two conditions. Secondly, during the design process and failure analysis, uncoiling is not considered. Lastly, testing processes need to be standardized as certain load machines have a rotating bottom table to allow uncoiling and some do not, resulting in different readings in a test for the same spring in different machines. Keywords Uncoiling effect, helical suspension springs, static loading, load, deflection, shear stress, uncoiling angle. 1. INTRODUCTION Sixteenth century wagons and carriages tried to solve the problem of feeling every bump on the road by slinging the carriage body from leather straps attached to four posts of a chassis that looked like an upturned table. Because the carriage body was suspended from the chassis, the system became to be known as a suspension. This term is still used to describe the mechanisms built for this purpose. The slung body suspension was not a true spring system but it did enable the body and wheels of the carriage to move independently. By the time powered vehicles hit the road, more efficient spring systems were developed so that passengers could experience a smooth ride. The suspension system is a setup that supports the weight, absorbs and dampens shock and helps maintain tire contact with the road. FIG. 1 SUSPENSION STRUT A spring is a member used to convert and store mechanical energy, i.e.; Kinetic energy to Potential energy and vice versa. Theworking of a spring is based on the elastic properties of the material used[1, 2]. This means that within the elastic limit, a material will try to withstand any change in its shape and dimensions and try to regain its original shape. There are stresses created within the body of the object which tends to counter act the force causing deformation. The manner in which a helical spring structure behaves under forces acting on it is also complex. When a compressive force (compression load) is applied on helical springs, they undergo plastic deformation and store the energy. This deformation is a decrease in the pitch distance and hence the length of the spring. 312

Most commonly used suspension springs are the open coil helical springs [3] (Fig. 1). They offer resistance to compressive force. Round wire is the material is commonly used to make the springs.

2 Most commonly used suspension springs are the open coil helical springs [3] (Fig. 1). They offer resistance to compressive force. Round wire is the material is commonly used to make the springs. This is because it is readily available and adaptable to standard manufacturing methods. Square, rectangular or special shaped wires are used in some cases. When the diameter of the spring is constant throughout, it is called straight helical. There are types where the diameter of the spring varies. These are tapered or conical springs. 2. THE UNCOILING PHENOMENON In the design of helical compression springs, the coiling/uncoiling of active coils during compression of the spring is not accounted for [4, 5, 6]. Although this concept forms the basis for the function of torsional springs, it is ignored during the design of helical compression springs. The uncoiling effect is evident when motion of one end of the spring is completely constrained and the other end is allowed to rotate freely during compression. This also results in torsional forces on supporting structures of the spring. The free rotation results in a change in spring attributes like rate of the spring and stresses generated within the spring.this has an obvious impact on the working and life of the spring. FIG. 2 UNCOILING ANGLE θ Green line: Position of free end with rotation constrained Pink line: Position of free end without constraintment This paper showcases the work in a project [7], carried out to determine the extent of variation in load rates between helical suspension springs operating under both the cases (coiling restricted and coiling unrestricted). The work involved FEM analysis and experimental testing on springs, carried out at a leading spring manufacturer s premises. A fixture was designed and built to allow and measure the extent of uncoiling in springs. The experimental and FEM data showed a variation in the load rate as well as rotational movement in the end coil. Uncoiling angles ranging between 10º ~ 14º and load variation of up to 10% were noted at same compression height experimentally. When a compressive force (compression load) is applied on helical springs, they undergo plastic deformation and store the energy. This deformation is a decrease in the pitch distance and hence the length of the spring. It also causes a little observed angular movement of the coils around the axis of the spring or uncoiling of the helix (Fig. 2). In automobiles, this effect is given a free reign by providing a thrust bearing at one of the ends to allow free rotation of the spring end [8]. By comparing the same suspension spring under different conditions uncoiling restricted and uncoiling unrestricted, it was determined that uncoiling has a significant effect on the functioning of the spring. Currently, manufacturers ignore this effect while designing the spring and testing it. The project work has been to determine: The variation in load between free and restricted springs The variation in stresses produced The extent to which the end coil turns during free uncoiling compression Five different springs were chosen. Experimental analysis was done on all 5 springs. And FEM analysis was done on one spring. The experimental analysis was carried on a Larson testing machine. The FEM analysis was carried out on software called NASTRAN. The 3D modeling was done on SOLID WORKS

3 FIG. 3 DESIGN SOFTWARE MATH PAGE 3 SUSPENSION SPRING DESIGN Manufacture of an automobile suspension springs involves a high level of designing and testing [9, 10]. This is needed because the spring is a critical component in the working of an automobile and its failure can be disastrous. The spring is a member which has to undergo constant load as well as fatigue loading. Hence it is essential to be certain about the performance of the spring before manufacturing it on a large scale. With advances in testing machines and software tools like 3D drawing packages (SOLID WORKS 2006) and FEM analysis packages (ABACUS, NASTRAN), spring design has become more precise, reliable and accurate. The design process followed at Stumpp, Schuele&Somappa Pvt. Ltd. is at par with international standards [11]. The steps involved are as follows: 1. Initially the customer drawings for the suspension setup are studied. Data such as inner diameter, length, outer diameter, no. of fatigue cycles, bump load and height, unladen load and height etc. are collected from the customer. 2. This collected data is input into the SSS design tool kit. SSS tool kit is software built specifically for springs. The data is entered in the MATH page (Fig. 3) and unknown parameters are filled in using standard or approximate values. This generates a Load vs. Deflection graph which is based on the customer requirement. Wire diameter is calculated. 314

4 3. Then the data is entered into the testing software and one-dimensional results are acquired. The tests involve load conditions at Full Bump, Laden (Fig. 4), Unladen, Rebound and Solid height. The calculations are solved and stored as a file known as a iterate. 4. The iterate is imported to the tool kit again. And the Load vs. Deflection graph of the designed spring is plotted. This graph should meet with the customer requirement graph. It is seen whether the spring satisfies the safety conditions for a safe design. 5. Along with the Load vs. Deflection graph, we get the axis and gap information from which we get the coordinates of the spring. These co-ordinates are entered into SOLID WORKS 2006, 3D designing software, to create a 3D model of the spring. FIG. 4 SPRING PARAMETERS AT LADEN CONDITION 6. After completion, this model is sent for 3D FEM analysis to a company called Wave Axis. There the analysis is done to see the stresses acting on the spring along with the deformation undergone in 3D and results are sent back to Stumpp, Schuele&Somappa Pvt. Ltd. where the results are checked. 7. Upon approval, the production designs are made and sent to the production department to manufacture samples. 8. These samples are put through rigorous tests which include Fatigue testing and Static load testing. If the spring passes, the customer is given all the details. 9. After receiving the go ahead from the customer and making any changes requested by the customer, the spring is put into production. 4 STUDY METHODOLOGY 4.1 FEM Analysis FEM is done for 3-dimensional models to analyse it in a more detailed manner [11, 12]. Firstly the entire model is discretised into small elements and later it as meshed into the actual model. FEM uses a complex system of points called nodes which make a grid called a mesh. This mesh is programmed to contain material and structural properties which define how the structure will react to different loading conditions. Nodes are assigned at a certain density throughout the material depending on the anticipated stress levels of a particular area. Steps involved in FEM analysis- 315

FIG. 5 FULL BUMP HEIGHT COMPRESSION STRESS (WITH UNCOILING RESTRICTED) 1. Creating the 3-dimensional model in SOLID WORKS 2006 using co-ordinates of the spring acquired from Math page. 2. Meshing of the model using Hyper Mesh.

Analysis and experimental verification of the data. 4.2 Experimental Testing The Larson testing machine (Fig. 6) in Stumpp, Schuele&Somappa Pvt. Ltd is computerized.

5 FIG. 5 FULL BUMP HEIGHT COMPRESSION STRESS (WITH UNCOILING RESTRICTED) 1. Creating the 3-dimensional model in SOLID WORKS 2006 using co-ordinates of the spring acquired from Math page. 2. Meshing of the model using Hyper Mesh. 3. FEM analysis using NASTRAN. 4. Generation of stress distribution, load reaction and rotational displacement under different loading conditions (Fig. 5) 5. Capturing a video of the analysis. 6. Analysis and experimental verification of the data. 4.2 Experimental Testing The Larson testing machine (Fig. 6) in Stumpp, Schuele&Somappa Pvt. Ltd is computerized.it is used to test the behavior of springs under the application of compressive loads. The Larson testing machine has two test beds at the top and the bottom. The top test bed is supported by four columns.it has the computer on the right side. The entire machine has a protective casing to ensure a dust free environment which is perfect for testing conditions. Periodic calibration is done to ensure the accuracy of the machine. The specifications of the Larson testing machine are as follows- FIG. 6 LARSEN TESTING MACHINE 316

6 1. Compression range-635mm 2. Extension range-533mm 3. Platform diameter-305mm(equipped with three load cells for maximum off centre load capacity) 4. Operating system-ms Windows XP 5. Software-FLASH Pro Spring 6. Load cell capacity-1361 kg, 3000lb 7. Power-240VAC, 50Hz 8. Emergency stop button is provided in case of overloading 9. Least count of the machine is 1 N 4.3 Experimental testing procedure The operation of the Larson testing machine is as follows- 1. As the machine is powered on an automatic self check takes place to ensure that the machine is working properly. 2. The top and the bottom surfaces touch and thus zero calibration is automatically done so that the machine knows the exact location of the top surface. 3. The end seats are fixed on both the top and bottom surfaces of the testing machine. 4. The spring is now placed on the bottom end seat and the top end seat is made to just come in contact with the top part of the spring. 5. The shackle height is calculated and the total offset height is fed into the computer. 6. Now the load is gradually applied on the spring and values are directly read off the screen. 7. The spring height is decreased in steps of 25mm and the loads values are taken. 8. When the shackle is placed on the machine along with the load readings the turn in Degrees is also noted down from the laminated circular scale on the shackle. 4.4 Shackle Design Under normal testing conditions in the Larson testing machine, the coiling and uncoiling motion of a compression spring is restricted. The end seats which are used under normal testing conditions fits snugly into the end coil profile. The top plate and the bottom test bed of the machine are fixed, thus preventing the uncoiling of the spring. To allow the coiling and uncoiling motion we have designed a shackle [7]. It also measures the degree to which the spring coils and uncoils under normal compressive load. The shackle has been designed with a thrust bearing. The thrust bearing takes the load on the spring but allows free rotation along the axis of the spring. The shackle consists of a base plate, the thrust bearing and a top plate. The bottom plate is fixed to the bottom test bed of the Larson testing machine with the help of a bolt made to specification. The thrust bearing itself is made up of two hardened steel circular plates, both of which have grooves to account for the ball bearings which are in the middle of the two circular plates. The two circular plates of the thrust bearing rotate because of the ball bearings. The bottom circular plate of the thrust bearing is press fitted onto the base plate of the shackle and the top circular plate of the thrust bearing is press fitted onto the top plate of the shackle. A laminated circular measuring scale is fixed onto the base plate of the shackle which measures the degree of rotation of the compression spring being tested in Degrees. The top plate of the shackle has a pointer attached to it. When the bottom end of the spring rotates the top plate of the shackle also rotates and the pointer gives the reading on the circular scale. The top and bottom plates of the shackle are made of mild steel. The top and the bottom plates of the shackle have holes with M20 screw threads which fit firmly into the top and bottom test beds of the Larson testing machine. The set up of the shackle is as follows- 1. The bottom plate of the shackle along with the laminated circular scale are placed on the bottom test bed of the Larson testing machine and is bolted using M20 bolt to make sure that it is fixed firmly. 2. The ball bearing ring is then placed on top of the bottom circular plate of the thrust bearing. Oil is then applied to ensure smooth rotation. 3. Now the top circular plate of the thrust bearing which is press fitted onto the top part of the shackle is put on top of the ball bearing ring. The top part of the shackle also has a pointer attached to it. 317

7 FIG. 7 SHACKLE PLACED DURING TESTING 4. The pointer is made to coincide with the zero of the laminated circular scale. 5. The end seat is then placed on top of the shackle and is bolted firmly using a M20 nut. 6. The notch in the end seat is then aligned with the pointer. 7. The spring is then placed on top of the entire shackle arrangement and is ready to be tested in the Larson testing machine. 5 RESULT 5.1 FEM analysis results FEM analysis was carried out using NASTRAN. The 3D model was created on SOLID WORKS 2006 and the model was meshed and imported into NASTRAN. This FEM analysis is for SPRING-1. Fig. 8 shows the data presented in a tabular form. Keeping the deflection as constant between the two conditions, a variation of 0.4% was seen in load, 0.35 % in shear stress and 10º angular rotation of the free end [7]. 5.2 Experimental test results Experimental results (Table 1) for the 5 springs tested showed a much higher variation in load: upto 9% in one spring. Angular rotation of upto 15º was observed. Graphs (Fig. 9 & 10) were plotted with the available data to compare the performance of the spring under the two different conditions as well as to further analyse the behavior of uncoiling, which seemed more or less linear [7]. 318

8 TABLE 1. SHOWING FEM ANALYSIS RESULTS FOR SPRING -1 SL NO. SPECIFICA- TION HEIGHT (mm) UNCOILING UNRESTRICTED LOAD (N) SHEAR STRESS (N/mm2) TURN (Rad) UNCOILING RE- STRICTED LOAD (N) SHEAR STRESS (N/mm2) LOAD % DIFF (N) SHEAR STRESS % DIFF (N/mm2) 1 REBOUND UNLADEN FULL BUMP FIG. 9 LOAD vs DEFLECTION GRAPH 7. CONCLUSION Based on the work, it is established that there is a clear difference between the two cases of spring loading under static conditions. The load, load rates and stress values are higher for restricted uncoiling springs compared to unrestricted uncoiling. Both the experimental and FEM analysis results support this claim [7]. The inference is: 1. Failure of the bearing in the suspension strut results in increased load on the spring, which increases the chances of failure. We also found that the bearing failure is not considered during spring failure analysis. 2. Some testing machines at the facility had a turning test bed already placed on a bearing, while others did not. Thus, standardization of these testing machines will result in more accurate results on all the machines. 3. During the course of the project, a shackle was designed that can be mounted on the Larson testing machine and the uncoiling in any compression spring can be measured. 319

9 FIG. 10 LOAD vs DEFLECTION GRAPH The parameters of the spring can be varied individually and a detailed study can be made on this phenomenon. The result will be an improvement in the spring design process where theoretical values will be closer to the actual values depending on the spring working conditions. The parameters that should be varied for a better understanding of this phenomenon are: 1. Material of the spring 2. Wire diameter 3. Spring diameter 4. Number of active turns 5. Spring end type The scope of future work would be: 1. Arriving at a formula for the uncoiling of the helical spring 2. Determining whether a bearing should be used for a particular spring application 3. Determination of exact stress values and load values based on restricted or unrestricted uncoiling of the spring 4. Determination of torsional stresses on the support structure TABLE 2. SHOWING EXPERIMENTAL RESULTS FOR ALL 5 SPRINGS SL NO. SPRING NAME 1 SPRING-1 2 SPRING-2 3 SPRING-3 4 SPRING-4 5 SPRING-5 SPRING TYPE STRAIGHT HELICAL STRAIGHT HELICAL (MULTI RATE) STRAIGHT HELICAL DOUBLE PIG TAIL DOUBLE PIG TAIL MANUFAC- TURING PROCESS COLD COILED COLD COILED HOT COILED COLD COILED HOT COILED APPLICATION REAR SUSPEN- SION FRONT SUS- PENSION FRONT SUS- PENSION FRONT SUS- PENSION REAR SUSPEN- SION UNCOIL- ING RE- STRICTED MAX LOAD (N) UNCOIL COIL- ING TURN ( ) UNCOILING UNRE- STRICTED MAX LOAD (N) % DIFFE- FE- RENC E ACKNOWLEDGEMENT The authors wish to thank Mr. J. SharanaBasavaraja (Senior Lecturer, Dept of Mechanical Engineering, B.M.S.C.E.), Mr. Deepak Hiremath (R&D Dept, Stumpp, Schuele and SomappaPvt Ltd.) & Mr. Prakash (Wave Axis Pvt. Ltd.). Without their guidance and support, the research would not have been possible. 320

10 REFERENCES [1] Machine Design - Robert L. Norton, Pearson Education Asia [2] Design of Machine Elements - V.B. Bhandari, Tata McGraw Hill [3] Suspension Spring Design Manual - Trainee Handbook, Stumpp, Schuele&Somappa Pvt. Ltd. [4] Design Data Hand Book- K. Mahadevan and Balaveera Reddy, CBS Publication [5] Design Data Hand Book - K. Lingaiah, McGraw Hill [6] Design of Helical Compression Springs - Course material, Institute of Spring Technology, UK [7] Study of uncoiling in suspension springs and its effect - Kushal A Jolapara, AdhipPuttaraj, AbhishekChatterjee (Visvesvaraya Technological University, Final project report) [8] Suspension Coil Spring and Rubber Insulators: Towards a Methodology of Global Design - Michel Langa and AbderrahmanOuakka [9] A Textbook of Machine Design Dr. RajendraKarwa, LP Publishers [10] Design Study for MSIL YV Deepak Hiremath, R&D paper, Stumpp, Schuele&Somappa Pvt. Ltd. [11] Manufacturing Process Flowchart - Production department, Stumpp, Schuele&Somappa Pvt. Ltd [12] Internet 321

Finite Element Modeling and Analysis of Vehicle Space Frame with Experimental Validation

Finite Element Modeling and Analysis of Vehicle Space Frame with Experimental Validation Assoc. Prof Dr. Mohammed A.Elhaddad Mechanical Engineering Department Higher Technological Institute, Town of 6