Design, analysis and mounting implementation of lateral leaf spring in double wishbone suspension system

Transcription

1 Design, analysis and mounting implementation of lateral leaf spring in double wishbone suspension system Rahul D. Sawant 1, Gaurav S. Jape 2, Pratap D. Jambhulkar 3 ABSTRACT Suspension system of an All-TerrainVehicle is one of the important systems for the stability and handling of the vehicle. In this paper, the front suspension of the ATV is designed as a lateral leaf suspension system for all terrain environments. As the leaf is mounted centrally each side acts as independent, thus acting as an independent suspension system. Also the leaf is analyzed in ANSYS software under appropriate loading conditions. The study of mounting of the leaf is done by Subtract and Operate method to optimize the system. KEYWORDS:- Leaf spring, lateral, mounting, Subtract and operate method 1. INTRODUCTION An All-Terrain Vehicle has to sustain the rugged nature of track. This demands the vehicle to be stable with ease for handling. The suspension system is that system of the vehicle which provides stability and comfort to the vehicle driver. Generally used suspension systems are double wishbone with helical spring, Macpherson strut suspension, swing axle suspension, trailing arm suspension and so on [1]. Here, the front suspension system is designed as a lateral leaf suspension. 2. LATERAL LEAF SUSPENSION SYSTEM This suspension system is somewhat different from other suspension systems because it is incorporating independent double wishbone suspension with lateral leaf spring. With dependent systems, when one wheel moves, the other is forced to move too. In current system, as center of the spring is fixed on the roll cage, it is not transferring the force from one side to another. The center of the leaf spring is mounted on the chassis at the center of the nose-cone. Along with the leaf spring, two shock absorbers are also mounted, one on each side on lower wishbone to the roll cage. 2.1 Advantages Over Coil Spring Less un-sprung weight Lowers the center of gravity Can handle much higher loads with less deflection Better serviceability Offers a very compact design enabling a Low front profile 3. DESIGN OF SPRING 3.1 Kinematic Analysis The suspension system is designed for a vehicle with a track of 60 inches. In this kinematic analysis the extreme position in bump condition is analyzed. The wishbones rotate according to the bump conditions. The leverage is found by appropriate and feasible mounting positions on the upper wishbone. The upper wishbone is connected to the leaf by shackle which deflects the leaf spring according to the height of the bump. From this analysis, we also come to know about the amount of deflection of the eye of the leaf. The considered parameters for this analysis are as follows: Table 1: Considerations for kinematic analysis PARAMETERS VALUES (in inches) Track 60 Width of tire 6 Volume 2, Issue 10, October 2014 Page 27

2 Ground clearance 8 Bump height 6 Fig.1: Kinematic analysis of the leaf spring In the above kinematic diagram, the thin lines indicate the condition at no bump. The thick lines indicate condition at 6 inch bump. From this kinematic analysis, we come to know that the deflection of the eye of the leaf is 3 inches. 3.2 Design consideration of the spring: Assumed weight of the vehicle Assumed weight of the driver Total weight = 270 kg Motion ratio = 0.7 = 200 kg = 70 kg While designing, both the static and dynamic loads were considered.therefore, the loading condition was taken as 2.5G. The load on each wheel is calculated. It is assumed that the load sharing between the spring and the damper is 40:60. Under these conditions, the force applied on the spring is rounded off to 950N. 3.3 Design Calculations: As the leaf is centrally mounted the calculations are done considering leaf spring acts as a cantilever beam. The leaf spring usually made up of spring steel material having yield stress values in the range of MPa [3]. For this leaf spring: Stiffness factor(s.f.) = 1.1 Spring thickness (t) = 8 mm Length of the spring (L) = 435 mm Width of cross-section (W) =35 mm Hence, area moment of inertia (I) = mm 4 Deflection of the spring (from kinematic analysis) = 76.2 mm Notations: I= Mass moment of inertia S.F. = Stiffening Factor l= Cantilever length of leaf spring Volume 2, Issue 10, October 2014 Page 28

3 t = maximum thickness of leaf spring W= width of leaf spring E= Modulus of elasticity f= Maximum deflection of eye K= Load rate As the leaf is mounted at the center, it can be considered as a cantilever beam. For a cantilever beam- Stress from deflection [2], = MPa Stress from load at spring seat [2], = MPa The maximum stress obtained = MPa 4. ANALYSIS OF THE SPRING The half-length spring was considered for analysis as it is centrally mounted. The section at the center of the leaf spring was fixed as it is bolted and load of 950N was applied to the eye in the direction of loading. This simulation is performed in ANSYS and the corresponding stress and maximum deflection was found. Fig.2: Equivalent stress distribution Volume 2, Issue 10, October 2014 Page 29

4 Fig.3: Total deformation analysis Table 2: ANSYS Results PARAMETERS Maximum stress Maximum deflection VALUES N/mm mm 5. MOUNTING FOR THE SUSPENSION SYSTEM For designing the mounting of this system it is first important to understand the force flow diagram to analyze the transfer of forces on various components. 5.1 Force Flow Diagram The major components with notations for force flow diagram: 1) Leafspring (L) 2) Tire + Knuckle assembly (T+K) 3) Wishbone (W) 4) Fixed mounting plate (M) To simplify the diagram, bolts acting as pinions are not considered in this analysis as they are to facilitate the rotational movement of the elements. However it is to be noted that the force is also transmitted through these bolts in the form of crushing and bearing. Here this force flow diagram is for half-length spring as each half acts as independent suspension. The elements in the diagram are symbolic representations. Fig.4:Force Flow Diagram Volume 2, Issue 10, October 2014 Page 30

5 From the above force flow diagram, it can be seen that the force (F1) acting on the tire is transmitted by knuckle on the wishbone (W). This wishbone which rotates about its pivot, transfers the force (F3) on the leaf. There is also a reaction force (F4) that the leaf exerts at the mounting point. 5.2 Subtract And Operate Method: The mounting of the conventional leaf spring is such that the leaf is held between the fixed points by the U-bolts. In this case, as this system is not mounted on axle, a separate plate needs to be welded to the roll-cage. It becomes a matter of concern whether to bolt the leaf over the fixed plate or under the fixed plate by the U-bolt. Subtract and operate method helps to solve this concern. In subtract and operate method, a component of the system is removed from the system either in working or is just studied if the system is not safe to operate. Here the system is studied on a half scale model shown in fig.6 and fig.8. Case 1: Mounting of leaf spring above fixed plate Fig.5: Mounting of leaf spring above fixed plate Fig.6:Study on half scale model with only mounted leaf spring (left) and with force applied (right) Now to start the study, it is the U-bolt that is subtracted from the whole assembly. The plate is fixed to the position. Now from the force flow diagram there is a force (F3) that is acting on the leaf at its ends. As there is no U-bolt that tightens the leaf with the plates, the leaf will move away from the fixed plate as shown in fig.6. This means that the U bolt is absorbing the force from the leaf. As the leaf is having fluctuating loads the U-bolt is to be designed for the same. Considering the factor of safety for such a critical part, the U-bolt needs to be designed according, making it heavy. Case 2: Mounting of leaf spring below fixed plate In this case, the leaf is mounted below the plate and bolted accordingly. Like the previous case the U-bolt is removed from the assembly for the purpose of analysis. Again the system is studied with the same force (F3) from the force flow diagram. Volume 2, Issue 10, October 2014 Page 31

6 Fig.7:Mounting of leaf spring below fixed plate Fig.8: Studyon half scale model with only mounted leaf spring (left) and with force applied (right) It can be clearly seen that as the plate is fixed, the force does not allow the leaf to move. Hence the force is transmitted to the mounting plate. Thus the force is distributed on a greater area of mounting plate than just bolt. This means that in this case, the function of the bolt is just to guide the leaf rather than take load as in previous case. The advantage of this assembly is that the system does not become heavier as the forces are uniformly distributed. 5.3 Damping at Mounting ofthe Leaf: If the leaf is immediate contact of the mounting plate, there can be damage to both parts because of metal contact under cyclic loading of the system. To avoid this, a piece of rubber sheet can be assembled so that there is no metal to metal contact as this rubber sheet will act as a damper. Fig.9:Mounting of leaf spring with rubber sheet Volume 2, Issue 10, October 2014 Page 32

7 The system is laterally mounted leaf with damper on both sides in parallel combination as shown in CAD model assembly in (Fig.10) and actual produced model in(fig.11) Fig.10:CAD model of Lateral Leaf Suspension System assembly Fig.11:Actual Produced Model 6.RESULTS The stress at the seat of the spring calculated is MPa. Analyzing the 3D model of halfspring under same loading conditions. The max stress obtained in the same region is MPa. There is a 1.013% difference in the difference of both the values and thus are quite comparable. As the stress in the leaf spring is below the range of the material (i.e MPa), the spring is safe. 7.CONCLUSION The leaf spring does not need any extra mounting cup or feature as required in helical spring. Two different springs are not required in this system and yet this system is independent. Because of these reasons the weight of this system is less than compared to a system with helical spring. The assembly of the system is relatively simple than helical spring system. As the damper is attached to lower wishbone, there is no need for removal of leaf while servicing of the damper. Thus serviceability of the system is simplified. As this is a planar 4-bar mechanism, the leaf has to be mounted in such a way that the eye of the leaf should move in the plane as that of the rotation of the upper wishbone. Thus the leaf has to be mounted at the same angle as the approach angle for this system. 8. FUTURE SCOPE This system can be analyzed in simulation software for studying various parameters like camber gain, response etc. The spring can also be replaced by a composite leaf spring. Composite leaf springs are more than 50% lighter in weight than the steel ones. Also the maximum yield stress of these materials is very high. Thus under the loading conditions they give better performance. Volume 2, Issue 10, October 2014 Page 33

8 REFERENCES [1]. Jack Erjavec, A Systems Approach to Automotive Technology, 1 st Edition-2009, Publisher: Cenage Learning [2]. Spring Design Manual (Standards search) Publisher: SAE Inc. [3]. Design Data Book, PSG College, Coimbatore, 2011 [4]. Kevin Otto and Kristin Wood, Product Design-techniques in Reverse Engineering and New Product evelopment, Fourth impression, 2009, Publisher: Pearson Education, Inc. Volume 2, Issue 10, October 2014 Page 34