Novel Design of Flat Spiral Spring based Regenerative Braking System

Transcription

1 Indian Journal of Science and Technology, Vol 10(36), DOI: /ijst/2017/v10i36/118993, September 2017 ISSN (Print) : ISSN (Online) : Novel Design of Flat Spiral Spring based Regenerative Braking System Ankit Raj* Mechanical Engineering, MANIT Bhopal, Bhopal , India; ankitrj54@gmail.com Abstract Objectives: In this paper, design of the mechanical system and the kinematic analysis of the regenerative braking have been presented. The mechanism has been proposed to store brake energy in a spiral spring and release it back to the vehicle transmission. Methods/Statistical Analysis: First, the mechanism was modelled in Autodesk Inventor software package and the model then imported to Solid-Works Motion for kinematic analysis. The kinematic simulation was carried out for all the three modes of operation, viz. the idle mode, braking mode and the energy recovery mode. The analysis results were as per the desired output. Once the model was established, the individual components were transferred to ANSYS Static Structural to evaluate and optimize the design. Finally, the spiral spring being the main energy storing device was analyzed for variation in moment and deflection with its thickness and width. Findings: This concept of low-cost Kinetic Energy Recovery System (KERS) can store energy that is lost during braking in a spiral spring and release it smoothly to the vehicle whenever required. The paper presents the simple and lucid design of the mechanism that uses the combinations of planetary gear and one-way clutch. The system can be implemented on bicycle where use of other energy recovery techniques is restricted due to their various limitations. A sample calculation on amount of energy to be stored has been performed to find out the spring parameters so that the calculation can be replicated and the system can be customized for each vehicle. Application/Improvements: The designed setup is flexible and can be installed with ease on a wheel hub. This system will assist the driver during acceleration and thus will improve the overall performance of the vehicle. Keywords: Energy Harvesting, Flat Spiral Spring, Mechanism Design, Mechanical KERS, Regenerative Braking System 1. Introduction The regenerative braking system is a technique of harvesting brake energy that would otherwise have gone waste. The Regenerative Braking System stores energy while braking and recycles it as the vehicle accelerates again. Restoring wasted energy will, in turn, improve fuel economy. Basically, there are two types of Regenerative Braking System based on the energy storage technique namely, Electrical and Mechanical. Electrical Kinetic Energy Recovery System (KERS) employs the use of battery or capacitor to store brake energy. Storing energy with batteries involves multiple energy conversions. The available energy is in the form of kinetic energy which is first converted to electrical energy and finally gets stored as chemical energy in the batteries. The energy conversion from kinetic energy to electrical energy and then to chemical energy leads to significant energy losses, the overall energy conversion efficiency being 31 34% 1. The poor conversion efficiency of electrical system cancels out most of the advantages of battery based regenerative braking. A mechanical system has the potential to solve this problem which is found to be almost three times more efficient. The distinctive advantage of mechanical KERS, which employs flywheel, is its high power density. Especially with the development of carbon fibre, small and light flywheels can store very high amount of energy safely 2. They can also have low charging time. Flywheels are a zero pollution method of storing energy and have little performance degradation over time compared to a battery. However, stored energy in flywheel dissipates *Author for correspondence

2 Novel Design of Flat Spiral Spring based Regenerative Braking System with time, due to various mechanical losses associated with it, and eventually loses all its energy. To avoid this, flywheel is mounted on a magnetic bearing in a vacuum chamber, which limits its application in F1 cars only. Flywheel also has concerns about fatigue and wear from vibration and repeated use, as well as vehicle safety. The main limitation on flywheel in vehicles is in the design of its transmission system 2. The cumbersome size and weight of the flywheel based KERS limit their utility in a low speed, lightweight human powered vehicle (for example bicycle). In order to meet the requirement and constraints for a light-weighted vehicle, regenerative braking system must be of low cost, light in weight and can easily be mounted on the existing transmission system of the vehicle. Moreover, the system must not dissipate stored energy with time. Use of spiral spring as an energy storing medium fulfils the above-stated needs and thus attracts much more attention than other KERS techniques. Storing energy in spiral spring eliminates the use of flywheel and complications associated with its design. Additionally, springs possess the ability to quickly expel energy, unlike batteries. Energy storing and releasing operations are done gradually and uniformly by the use of the combination of planetary gears and one-way clutch. The Flat Spiral Spring ensures the permanent storage of energy. The energy stored in spring can provide instant power boost that can be controlled by the driver. But the problem with the use of Spiral Spring as regenerative braking is the sense of direction in which it stores and releases energy. The spiral spring releases energy in the direction that is opposite to the sense of twisting. So a system needs to be developed that can rectify this problem. The paper presents the design of the mechanical system that will ensure energy transfer to and from the spiral spring. The proposed design can easily be mounted on a wheel hub adhering to their compact design and simple mechanism. the arbor which is mounted on the hub supported on a ball bearing (7). The planetary carrier (1) holds the planet gears and is coupled with the arbor. The conventional friction brake is provided to stop the ring gear (3) in order to activate the KERS. 1-Planet Carrier, 2-Casing, 3-Ring gear, 4-Sun gear, 5-Planet gear 6- Barrel, 7-Ball bearing 8-One-way clutch, 9-Wheelhub, 10-Axle, 11-Arbor Figure 1. Cross-sectional view of the conceptual mechanism 2. Design Model The cross-section and 3-D view of the proposed kinetic energy recuperation device is shown in Figure 1 and Figure 2 respectively. It consists of a wheel hub (9) mounted on a wheel axle (10) fixed to the vehicle frame. On the wheel hub, a sun gear (4), a barrel (6) and an arbor (11) is keyed to it. The barrel, mounted on the wheel hub via a one-way clutch, is attached to one end of the spiral spring. The other end of the spiral spring is attached to Figure 2. Isometric view of Spring Based Regenerative Braking s CAD model 3. Design Requisites When the regenerative braking system is activated, it must be able to slow the vehicle and store energy in the spiral 2 Indian Journal of Science and Technology

3 Ankit Raj spring by virtue of torsion force 3. The system must also be capable of releasing stored energy uniformly and gradually to the vehicle s transmission system when desired. This designed system can be mounted on the wheel hub of the vehicle demanding it to be light weighted. When the brake is applied on a wheel rotating in clockwise direction, the regenerative system gets activated and allows the energy storage in the spiral spring. The system must hold the stored energy in the spring until required to be released. When energy release mode is activated by the driver, spring must unwind and releases stored energy in the clockwise direction only. Thus the system is intended to release energy in the same direction of energy storage. Moreover, the mechanism must not interfere with the normal functioning of the vehicle. 4. Main Components 4.1 Energy Storing Unit In the proposed model of the regenerative braking system, the energy storing element that has been used is a Spiral Spring as shown in Figure 3. Figure 3. CAD model of Flat Spiral Spring with constant pitch We consider a uniform rectangular cross section spring initially in the shape of an Archimedean spiral. The mathematical equation for the Archimedean spiral curve is given by is the radius of the starting point of the spiral curve, is the rotation angle from the starting point to thecurrent point on the spiral curve in radian. The pitch is the (1) distance betweentwo successive points along any ray from the origin. The length of Archimedean spiral between two angles can be calculated using the relation: (2) and where are the numbers of turns of inner and outer end of the spring respectively. 5. Analysis and Behaviour of Torsion Springs The outer end of the spring is clamped to the spring housing while the inner end is clamped to the arbour (see Figure 1). The spiral spring is subjected to a torque applied in such a way that the relative shaft housing rotation causes bending of the turns of the spiral strip 4. It is assumed that the spring housing is fixed using a driver controlled lever and the inner shaft (arbour) is loaded by a torque (during vehicle braking) about the spring axis, the spiral spring will thus deform and stores the kinetic energy of the vehicle as elastic potential energy 5. This stored energy can then be utilized to provide an instant boost to the vehicle. The variation of Torque (T) with angular deflection ( ) is usually non-linear in an Archimedean spiral which is represented in Figure 4 6. The slope of the Torquedeflection curve is the spring stiffness, k. (3) But for the simplicity of design we consider a linear spiral spring having constant spring stiffness, k. So the strain energy, stored, W in linear spring is given by equation (4) 7 : (4) Where b, h and l is the width, thickness, and length of the strip forming the spring as shown in Figure 5. is the maximum bending stress induced in the spring material which is given by: (in radians) can be cal- Also, the angular deflection culated using the relation (6), (5) (6) The energy to be stored and the maximum moment required to be delivered are the main input parameters of Indian Journal of Science and Technology 3

4 Novel Design of Flat Spiral Spring based Regenerative Braking System a spiral spring design. These are likely to change for different vehicle having different loads. For the spring of 20 mm and 40 mm width, variation of maximum moment with spring strip thickness has been illustrated in Figure 6. Similarly, the Figure 7 illustrates the deflection of the spring with spring strip thickness and interestingly it is observed that the deflection is independent of the spring s width. In order to reduce volume and shear forces, we can use spiral having two starts with half the width for the same value of torque 4. Figure 4. The T-θ curve of a nonlinear stiffening spring 4 Figure 6. Influence of strip width on moment-strip thickness Figure 5. Cross section of spiral spring with b>h 5 We assume that the strip forming the spring has cross section area A, Young s modulus E, mass density per unit volume ρ, the section coefficient of bending and the bending stiffness about the neutral axis I.As the spring strip is mainly under pure bending stress, the limit torque of spiral spring can be obtained using the tensile strength limit value σ of spring s material (7) (8) (9) (10) In this study, material of spring is chosen to be 55CrMnA. Material Properties: Young s modulus, E = 197 GPa Density, ρ = 7800 Kg/ Allowable bending stress, We choose, inner shaft radius and p=13. Therefore the number of turns of the spiral spring is 8 and the total distance from end to end, l is 3.27 m. Figure 7. The deflection-spring thickness curve is independent of spring s width 5.1 Spring Housing (Barrel) The Barrel is mounted on the hub using a one-way clutch. On the interior wall of the barrel, one end of the flat spiral spring is attached. The use of one-way clutch will ensure that the hub will not rotate the barrel during braking or idle mode, but will allow the barrel to turn the hub during energy recovery. This energy transfer to the hub through the barrel is controlled by the driver via a lever. 6. Analysis Using equation (1) the spring housing radius can be calculated for, and p=13 to be 154 mm. The limiting moment of the spiral spring (Figure 5) 4 Indian Journal of Science and Technology

5 Ankit Raj for 20 mm width and 2 mm thickness is N-m. Thus the tangential force at the circumferential edge of the barrel, where the outer end of the spring is secured, is 143 N. The set-up has been created on ANSYS static structural as shown in Figure 8. We assume that the spring housing is 2mm thick sheet of AL The above boundary conditions are applied and FEA is performed. For spring housing, 2mm thick sheet of AL-6061, the factor of safety comes out to be 1.5 as illustrated in Figure 9. Figure 8. Boundary conditions on spring housing. The housing is fixed at point A using lever (not shown), spring outer end is attached on the circumference at point B which will apply force when tries to unwind and the housing is mounted on a one-way clutch and is free to rotate the clutch The sun gear is mounted on the wheel hub and the planetary gears are mounted in between the ring gear and the sun gear. The planetary gears have ball bearings fitted into it and they are attached to the modified planetary carrier using studs. The planetary carrier cum one-way clutch will thus rotate about the axis of the hub only when planet gears rotate about the sun gear. 6.2 One-way Clutch The one-way clutch is a ratcheting device onto which the barrel is mounted. The one-way clutch ensures that the motion or energy is not transmitted to the barrel during idling or braking but the barrel, when activated can turn the hub. 6.3 Planet Carrier Cum One-way Clutch This device as shown in Figure 10 is a modification of planet carrier. It fulfills the purpose of both planet carrier and ratchet simultaneously. The design of planet carrier is modified in such a way that the spring loaded fingers are on its periphery and the saw tooth, i.e. the complementary part of the ratchet, on the inner face of the casing. Thus, the assembly of planet carrier and casing function as a ratchet. The device is mounted on bearing which holds the drive shaft. This set up enables rotation of device only during braking but during idling and recovery, it remains stationary. Figure 9. The spring housing safety factor is Planetary Gear System This system enables energy transfer to and from the hub by acting as clutch, eliminating the use of an additional clutch mechanism. Added advantages are reliability, efficiency, simplicity in design. Figure 10. CAD model of the unique planet carrier cum ratcheting device. 6.4 Analysis As explained above, the planet carrier cum one-way clutch will transfer the load from the planet gears to the Indian Journal of Science and Technology 5

6 Novel Design of Flat Spiral Spring based Regenerative Braking System spiral spring and it will wind the spring but as soon as the load is removed the action of one-way clutch will come into the picture and it won t allow the spring to unwind. The geometry was imported to ANSYS workbench and constraints and loads were applied for FEA. The material used was 2 mm thick sheet of AL 6061 for planet carrier and the casing material is of structural steel of 2 mm thickness. 7. Working of The Mechanism The complete assembly and its component have clearly been explained in previous sections. In this section, working of the proposed mechanism has been discussed. The energy flow to and from the mechanism is shown by a block diagram (Figure 11). Figure 11. Configuration of the Regenerative Braking System and the flow of energy through various components of the system. The wheel hub rotates on the axle. While braking, the vehicle braking system stops the ring gear. As the ring gear stops, the sun gear which is keyed to the hub causes the planet gear to rotate around it. The planet gear, in turn, rotates the planet carrier which ultimately twists the spring. Energy thereby gets accumulated as elastic energy in spiral spring. The modified planetary carrier cum oneway clutch assembly allows planet carrier to turn freely during braking and energy storage mode of operation, but engages and restrain the reverse motion of planet carrier and spring (i.e. restrains the involuntary unwinding of spring). As explained in the previous section, one end of the spring is fixed to the barrel, the barrel being held by a lever. When the driver sets the barrel free by releasing the lever, the spring releases energy and provides instant boost or torque to the wheel hub. As the barrel is mounted on the hub through the one-way clutch, the energy is delivered till the hub overrun the barrel. 8. Concept Modes The developed mechanism is intended to store brake energy, discharge the stored energy, and remain idle during usual operation. In storage mode, the spring deforms but spring returns to its original shape during energy recovery mode. In idle mode, the spring will retain energy, if previously stored and the system will remain inactive allowing the vehicle to function as normal. a) Idle- During idling the sun gear will rotate with the wheel hub and the ring gear is free. As the planet gears are loaded, since they are attached to the spring s one end via planet carrier, it will cause the ring gear to rotate in a direction opposite to the direction of the sun gear. b) Storage- During energy storage, the vehicle braking system locks the ring gear and this will cause the planet gears to revolve around the sun gear which in turn will rotate the planetary carrier. The planetary carrier will thus compress the spring. The modified planetary carrier will allow the planet carrier to twist the spring but will prevent the spring to release the stored energy involuntarily. c) Discharge- When energy is required to be released from the system the barrel is disengaged and set free and thus output will be made available from the barrel. All operations are summarized in Table 1. Table 1. Modes of operation Components Modes of Operation Idle Mode Regenerative Braking Energy Recovery Sun Engaged Engaged Engaged Spring Free Engaged Free Barrel Locked Locked Free 9. Conclusion The developed spring based regenerative braking system is definitely practical and is much simpler as compared 6 Indian Journal of Science and Technology

7 Ankit Raj to other systems. Few concepts already exist but there is no detailed design available to implement that concept. In this paper, a complete mechanism is developed and described as to how the system will store and release energy. The problem before us was to tackle the sense in which a spiral spring stores and releases energy. The developed mechanism can successfully tackle this problem. The mechanism can easily be installed on a wheel hub and are cheaper than electrical KERS and light weighted than other mechanical KERS. Its simplicity in design makes it feasible to mount on a human powered vehicle to enhance its performance. The biggest advantage of this system is its ability to install on a wheel hub with not much modification in an existing vehicle transmission system. In coming days, spring based regenerative system will gain more attention with the advancement in spring design and spring material. The vehicle with start-stop cycle of driving will be affected the most with the introduction of this technology. 10. References 1. Husain I. Electric and hybrid vehicles: design fundamentals. Second Edition. CRC Press: Taylor and Francis Group, USA; p Ludlum K. Optimizing flywheel design for use as a Kinetic Energy Recovery System for a bicycle [Senior Theses]. Claremont, California: Pomona College; Nieman JE. A novel, elastically-based, regenerative brake and launch assist mechanism [MS thesis]. Dayton, Ohio: University of Dayton; 2014 May. 4. Mu-oz-Guijosa JM, Caballero DF, de la Cruz VR. Generalized spiral torsion spring model. Mechanism and Machine Theory May; 51: Gravino MC. Regenerative braking system. U.S. Patent A1; 2011 Aug Ahmed A, Zhou H. Synthesis of nonlinear spiral torsion springs. International Journal of Engineering Research and Technology Jun; 3(6): Duan W, Feng H, Liu M, Wang Z. Dynamic analysis and simulation of flat spiral spring in elastic energy storage device. Proceedings of Asia-Pacific Power and Energy Engineering Conference (APPEEC); p Tang J, Wang Z, Mi Z, Yu Y. Finite element analysis of flat spiral spring on mechanical elastic energy storage technology. Research Journal of Applied Sciences, Engineering and Technology. 2014; 7: Budynas J, Nisbett R, Shigley Mechanical Spring. In: Shigley s mechanical engineering design. 9th Edition, McGraw-Hill Publishing Company: New York; p Khurmi RS. Design of spring. In: Machine design and elements. 1st edition, S. Chand Publications: India; p Indian Journal of Science and Technology 7