BRAKE DISC DESIGN AND THERMAL AND STRESS ANALYSIS FOR OPTIMUM HOLES

POSITIONING

A MINOR PROJECT REPORT

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF

BACHELOR OF TECHNOLOGY(Automotive Engineering)

SUBMITTED TODELHI TECHNOLOGICAL UNIVERSITY, DELHI

SUBMITTED BYAnanya Bhardwaj 2K12/AE/010Ashwani Saini 2K12/AE/018Chanchal Krishna 2K12/AE/026

SUPERVISED BY Dr. VIJAY GAUTAM

ASST. PROFESSOR, MECHANICAL ENGINEERING

May 2015

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DELHI TECHNOLOGICAL UNIVERSITY, DELHI

CERTIFICATE

This is to certify that the thesis entitled “BRAKE DISK DESIGN AND THERMAL AND STRESS ANALYSIS FOR OPTIMUM HOLES POSITIONING” submitted by Ananya Bhardwaj, Ashwani Saini, and Chanchal Krishna in partial fulfilment of the requirements for the award of Bachelor of Technology Degree in Automotive Engineering from Delhi Technological University (Formerly Delhi College of Engineering) for their Minor Project in the Sixth Semester is an authentic work carried out by them under my supervision & guidance.

Dr. VIJAY GAUTAMAsst. ProfessorDepartment of Mechanical EngineeringDelhi Technological University

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DELHI TECHNOLOGICAL UNIVERSITY

DECLARATION

We hereby declare that the work which is being presented in the B.Tech Minor Project Report entitled “BRAKE DISK DESIGN AND THERMAL AND STRESS ANALYSIS FOR OPTIMUM HOLES POSITIONING”, in partial fulfillment of the requirements for the award of the Bachelor of Technology in Automotive Engineering and submitted to the Department of Information Technology of Delhi Technological University, Delhi is an authentic record of our own work carried out during a period of 6th semester under the supervision of Dr. Vijay Gautam, Mechanical Engineering Department.

The matter presented in this Project Report has not been submitted by us for the award of any other degree elsewhere.

Signature of students

(2K12/AE/010) (2K12/AE/018) (2K12/AE/026)

This is to certify that the above statement made by the student(s) is correct to the best of my knowledge.

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Dr. VIJAY GAUTAMAsst. Professor Department of Mechanical EngineeringDelhi Technological University

DELHI TECHNOLOGICAL UNIVERSITY

ACKNOWLEDGEMENT

We would like to place on record my deep sense of gratitude to our mentor and advisor Dr. Vijay Gautam, Asst Professor, Dept. of Mechanical Engineering, Delhi Technological University, Delhi for his patience, generous guidance, help, useful suggestions, continuous encouragement throughout the course of present work and for enlightening me with his immense knowledge of the subject. His cooperation and support during the whole research process was one of the main reasons we could complete this thesis with a satisfactory conclusion. I also wish to extend our thanks of the staff of the Mechanical Dept. and other colleagues for their insightful comments and constructive suggestions to improve the quality of this project work.

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Ananya Bhardwaj 2K12/AE/010Ashwani Saini 2K12/AE/018Chanchal Krishna 2K12/AE/026

CONTENTS-

1. ABSTRACT2. INTRODUCTION3. CALCULATIONS4. INITIAL MATERIAL SELECTION5. THERMAL ANALYSIS6. COMPARISON OF DIFFERENT PLACEMENTS7. CONCLUSIONS8. REFERENCES

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1.ABSTRACT

Brake Disc Design and Analysis for ideal holes positioning is a project through which we have aimed to achieve an understanding and learning of development of a product straight from its conception to manufacturing, encompassing all the stages such as Initial Calculations, Designing, Material Selection and Manufacturing considerations.

Brake disc is an essential component of almost every Automobile, and its applications are endless. We aim to provide a refined solution or a model of working to select various brake disc parameters as well as materials depending on the use, and also Analyze the thermal loads using CAD software (solidworks) to estimate the temperatures achieved during the braking process and optimize the heat reduction and correct material selection. We aim to optimize the holes placement testing for options of radially inwards, radially outwards and comparison of few large sized holes vs many small sized ones.

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2.INTRODUCTION

In our project we’ve looked to replicate the complete professional design and manufacturing process.

So we’ve divided the project into 6 major categories

Basic Framework and Application selectionIn this part, we’ve selected a test case of a Formula Student Car, an open wheeled formula style race-car which weighs 300KG with the driver and has a strictly mechanical and hydraulic system, with no hydraulic or electric boosters

Brake CalculationsTo find the required braking torque on wheels, to calculate all the forces involved from the force applied at the pedal to the force exerted by the pads, the pressure in the brake lines for correctly selecting components such as the piston diameter of the master cylinders and the calipers, and the diameter of the discs

Study of requirements from materialThis involves basic study of the requirements of the material due to the various loads and temperature changes on the brake disc during its use

Initial Material OptionsWe consider the widely used materials generally used in this applicationCarbon CeramicsStainless Steels Aluminum MatrixCast Iron

Thermal Analysis using SolidworksWe test on solidworks for different materials using a thermal loads, and check for the

material which gives the most optimum output. (Least peak temperature)

Comparison of Different hole positionsWe compared four configurations of hole positions for thermal analysis such that we tested for the maximum temperatures and also maximum stresses for same physical and thermal loads

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3.BRAKE CALCULATIONS

BRAKE CALCULATIONS DATA

Assumed Wheelbase = 1550mm

Assumed Height of CG of car= 250mm

Static load distribution = 50:50

Weight of Vehicle = 300kg

Coefficient of Friction at tire-ground interface u = 1.6

Selected wheels and tires = 13inch rims and 260mm tire radius

At maximum deceleration of 1.6g the dynamic weight distribution is found to =210:90kg= 70:30 weight distribution

Disk Selected= 220mm on front and 200mm on rear

Assumed Brake Pad size= 3.875 cm height, 4cm width.

Most calculations done are shown in the excel file attached

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SCREENSHOT OF BRAKE CALCULATIONS MICROSOFT EXCEL FILE

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• From the calculations, Pressure on Disc part between pads found is 5.63 x 106 N/m2

• Brake Torque on the front disc was found through calculations to be 350 N.m

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4.INITIAL MATERIAL SELECTION

THE BASIC REQUIREMENTS OF THE BRAKE DISC MATERIAL ARE

1 Hardness- A brake disc is generally expected to have high hardness for slower wear rates. The hardness generally required is about 32-38 HRC (Rockwell hardness)

2 Temper Softening- As in braking applications the temperature reached are often as high as 400 degrees temper softening( a property of a metal to soften due to tempering and subsequent change in its microstructure at those temperatures) might lead to warping and deformation of disc under the physical loads.

3 Strength- The strength of the material should be high to withstand the loads and this is measured in terms of its yield/compressive strength

4 Fatigue Strength- As it is a component that is supposed to work for a long time of application depending on the use, its expected to work without failure for large number of cycles. This is measured in terms of creep strength

5 High Temperature Surface OxidationAt high surface temperatures, the reactivity of materials increases significantly, so they get oxidized and that leads to change in surface properties and deviation from ideal braking performance. So the selected material should maintain low reactivity at elevated temperatures

6 Thermally induced fracture-

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In Disk brakes heat generated by friction are very high and disks can fail due to thermally induced fracture, so the ability to dissipate heat quickly is essential to avoid fracture.

OTHER IDEAL PHYSICAL PROPERTIES

High Conductivity- As surface temperatures run high in brake disc applications , for even distribution of temperature which will lead to even wear, similar behavior and less chances of warping.

High Thermal Diffusivity

GENERAL PHYSICAL PROPERTIES OF COMMON STAINLESS STEELS

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CARBON CERAMIC ROTORS Type

C/C-SiC is a Carbon fiber phase added to a Silicon Carbide matrix. The resulting material has increased strength with a lower density and high tribological characteristics. The most predominant feature is its ability to withstand high temperatures without failure. Due to its low coefficient of thermal expansion and high thermal conductivity, this CMC can retain its strength at high temperature. This CMC was manufactured as a disc brake with 2D reinforced discontinuous fibers. The fibers are placed perpendicular to the surface of friction to maximize Thermal conductivity. The result is a disc brake that can withstand surfaces temperatures of 1000 C with minimal wear.

ProblemsCMC disc brakes are not widely used among regular cars. This is due to multiple reasons. Firstly since there is low demand for high performance brakes, these disc brakes are very expensive. The cost of raw material isn’t hugely expensive and is expected to reduce as CMCs gain popularity. In the case of regular cars that aren’t used at high speeds the amount of heat generated with low friction is small. Carbon Silicon Carbide brakes become inefficient and much weaker if used in cold conditions. The weakness is a result of thermal expansion of the composite and ceramic matrix. As the material expands at different rates under different temperatures cracking can occur on the surface

Improvements To improve this technology tests were conducted to achieve higher surface temperatures. It was found that with this ceramic composite certain areas wouldn’t dissipate heat resulting in “hot spots”. This is due to the materials ability to conduct heat in axial and transverse directions. Since the fibers are placed perpendicular to the friction surface they are unable to transfer heat in other directions. The simplest solution is to make the material with a higher ceramic content. This sacrifices the strength of the brake and while adding excess mass, since the density of ceramic is far greater than the composite fiber. Another solution is to use a more thermally conductive fiber in the ceramic matrix. This results in a higher cost of production but higher performance product.

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ALUMINIUM MATRIX COMPOSITES

ALUMINIUM MATRIXIn automotive application, brake rotors have been held up as an example where

MMCs make a difference. In terms of weight, MMC rotor designs provide up to a 60%reduction when compared to cast iron. In addition, aluminum MMC rotors outperformtheir iron counterparts in terms of their mechanical properties and practical use.

In a comparison of an aluminum MMC brake rotor vs. an iron brake rotor, the iron component will be the higher value in terms of purchase price, post-purchase processing and maintainability, but the MMC component wins out in terms of performance, marketability and maintainability.

Specifically, the wear resistance and high thermal conductivity of aluminum MMCs enable substitution in disk brake rotors, with an attendant weight savings on the order of 50 to 60%. Because the weight reduction is unsprung, it also reduces inertial forces, providing an additional benefit in fuel economy. In addition, lightweight MMC rotors provide increased acceleration and reduced braking distance. It is reported that, based on brake dynamometer testing, MMC rotors reduce brake noise and wear, and have more uniform friction over the entire testing sequence compared to cast iron rotors. The advantages of metal matrix composite over metals and other composites AMC discs show lower friction coefficients and higher wear rates than classical steel discs. By increasing the reinforcement rate upto 8 weight percentage angular SiC, the disc exhibited a low wear rate, particularly at high braking power and a relatively high friction coefficient because of high angular SiC content with same SiCcontent, the spherical SiC reinforced disc exhibited a lower friction coefficient due to spherical SiC reinforced disc exhibited a lower friction coefficient due to spherical morphology resulting in a sliding contact instead of ploughing one The thermal conductivity of AMC can be two or three times higher than cast iron.

• An MMC disc could be 60 % lighter than an equivalent cast iron component.

• The Thermal Diffusivity, which is the rate of heat dissipation compared to that of storage, is four times that of cast

It is particularly difficult to manufacture and also has very high costs so it remains to have a limited use.

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STEEL AND CAST IRON ROTORSDisc brakesW are typically made out of grey cast iron. This material is has high tensile strength and can withstand a high temperature before failing. In high performance vehicles the amount of heat generated by friction when braking can be too great so the brakes fail or must be changed often. The failure is due to thermally induced fractures. Also these brakes can be heavy and susceptible to corrosion, which cause failure. Other composites have been tested such as Metal Matrix Composite, and Carbon Carbon Composites. The challenges with these materials are the ability to dissipate heat caused by friction isn’t optimal at high enough temperatures. A typical grey cast iron disc brake can withstand a surface heat of 400 C before failure occurs

STEEL VS CAST IRON Both cast iron and stainless steels are extensively used in the automobiles as

brake rotors Cast iron has greater strength than steel and also provides a greater coefficient

of friction But cast iron has poor high temperature oxidation and corrosion resistance

properties, hence it is being replaced by stainless steels Stainless steels provide a greater coefficient of friction when used with high

performance brake pads such as sintered bronze pads or any other metallic pads.

GRADES USED IN INDUSTRYMainly martensitic steels with grades 403, 410 and 420 are used, known as 13% Cr steels.

Stability is a crucial factor as the properties of the material shouldn’t change with use

*Adding Cu to the steel increases the possibility of faster cooling rates during hardening of discs, which is restricted due to change in flatness due to faster cooling methods(0.5 to 2.5%)

Mo is added to increase corrosion resistance but it’s an expensive alternative

We’ll compare few Grades such as Austenitic vs Martensitic (Series 3XX vs 4XX)

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AUSTENITIC STEEL GRADES

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MARTENSITIC STAINLESS STEELS

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COMPARISON OF DIFFERENT MARTENSITIC GRADES

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5.THERMAL ANALYSIS OF BRAKE DISC

Thermal Analysis LogicAs on braking all the kinetic energy is converted into heat energy- which is generated at the brake pad-disc interface we will distribute this as heat power supplied at the disc pad interface.

We’ll determine kinetic energy by assuming a given speed, lets say 25m/s to be brought to a complete stop given by 0.5*mass*velocity^2.. The max coefficient of friction would provide us with the stopping time, so we’d know how the power is distributed over time.

We’d analyze the disk under this heat power provided as a flux only for the while the brakes are applied, and convection throughout the rest of the disk surface so it is simultaneously also cooling reflecting a real scenario. Initial temperature is assumed to be same as the surroundings

The solidworks simulation is prepared and will be shown separately.

Bulk Ambient temp 293K

Convection Coefficient is 90W/m^2K for all faces

From thermal loads temperature is selected to initial temp of 25celcius

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Thermal Boundary Conditions SolidWorks Simulation solves for the temperature distribution in the solid using the conduction equations and boundary conditions applied to the boundaries of the model. SolidWorks Simulation has several thermal boundary conditions that can be applied to thermal studies.

1) Temperature Allows for the definition of a temperature on a certain entity or body. 2) Convection applies a convection boundary condition to the selected faces. The

convection coefficient and ambient temperature are specified and the heat lost due to convection is calculated automatically.

3) Heat Flux Applies some amount of heat into a face per unit area.4) Heat Power Applies some amount of heat to a vertex, edge, face or component. 5) Radiation Allows surface-to-surface or surface-to-ambient radiation. In our

model, we will apply convection to all faces because all of the faces will be exposed to the air. In addition, we will apply a heat power to the faces that the brake pads touch.

Convection is the transfer of thermal energy between a surface and a fluid. The amount of heat transferred through conduction is proportional to the convection coefficient, h, the surface area, A, and the temperature difference between the surface and the surrounding fluid.

We will assume a convection coefficient of 90 W/m^2.K and an ambient temperature of 20°C, which are approximations. Actual convection coefficients and ambient temperature could be computed by running a CFD analysis in SolidWorks Flow Simulation or from experiments.

Heat PowerAs the vehicle is braked, the rotor is spinning and the brake pads are rubbing against the surface of the rotor, creating friction and heat energy. Much of the kinetic energy of the car is being transferred to thermal energy through the brake pads. The heat power will be applied to the brake rotors in the area that the pads touch. The amount of heat power can be calculated from the amount of kinetic energy carried by the car. If we assume the mass of the car is 275 kg and the car is travelling at 25 m/s, the kinetic energy of the car is as follows:

If we assume all of that kinetic energy is transferred to thermal energy during braking that lasts 3seconds, we can calculate the heat power.

Since we will analyze only one pad, and about 60% of the mass of the vehicle will be on the front, the heat power is reduced.

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The dynamic load analysis is also done by applying pressure on the disc in the part in contact with brake pad.

Simultaneously the stresses due to thermal loads are also applied such that the combined result is shown in the simulation.

http://www.wsdot.wa.gov/research/reports/fullreports/434.1.pdf

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6. COMPARISON OF DIFFERENT HOLE PLACEMENTS

TEST CONDITIONS

The test conditions which were applied for the Analysis were same for all four test cases

Thermal AnalysisTransient Analysis of a stop of 3 seconds.

Convection is applied on all faces for all tests for the whole duration of three seconds.

The ambient temperature is set at 293K

The Heat Power is set at 15000W for 1 second applied for a duration of 3 seconds.

The convection coefficient is set at 85W/(m^2 K)

Blank Disc Design

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CASE 1-

Small large number of holes

The number of holes has been kept 48, the radius of each hole is 3.53mm such that the total volume removed is same as for that of the big holes configuration with 24 holes of 5mm radius each.

The Screenshots of the Analysis are-

The Max temperature reached was 415K for the test.

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CASE 2-

Small number of large holes

The number of holes is set at 24, each hole being 5mm in radius.As this was the first iteration we did we’ve also attached the mesh detail screenshots.

Heat Power and Mesh

Convection

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The result shows a maximum temperature shows that the maximum temperature is 383K.

CONCLUSION 1

The maximum temperatures achieved for two brake discs having the same volume hence the same weight shows that the one with larger holes dissipates heat better reaching a lower temperature despite having a lesser area for convection.

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CASE 3-

Radially Outward Holes

The test cases 3 and 4 compare same sized (4mm radius) and same number of holes (24) for two configurations to see which dissipates heat better.

The holes are pushed out radially 10 mm compared to the other configuration.

The maximum temperature achieved was 496K.

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Case 4-

Radially inward holes-

This test case has 24 holes each of 4mm radius.

The maximum temperature achieved was 506K.

CONCLUSION 2-

The maximum temperatures achieved for two brake discs having the same volume hence the same weight shows that the one with radially outward holes reaches a relatively lower temperature thus it dissipates heat better.

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STRESS ANALYSIS OF DIFFERENT CONFIGURATIONS

Test Conditions-

The fixtures were applied at the disc bobbin interface as a fixed geometry.

A split line replicating the brake pad face was made for application of forces through that area.

A normal force of 5000N on each face was applied.

A total torque for both faces combined was applied at 345 N.m.

Screenshot of result on Radially Inward test case-

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Screenshot of torque applied

Comparing different results we found that due to our current design and specifications the differences in maximum stresses generated were insignificant from the point of view of failure as the weakest part was the mount of the brake disc.

However the disc with larger holes had curves of greater radius of curvature and hence lesser stress concentration. So there was a lesser value of stress developed in case 2 as compared to case 1

Case 1 offered a more favorable deflection value due to more cross section area available in the direction of the torque.

Hence the final design to be chosen is the one with larger holes in all cases except where deflection needs to be minimized.

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7.CONCLUSIONS

Producing a high performance vehicle one must consider the components, the materials used and the purpose of the vehicle. The disc brakes on a racecar emphasize different properties than a motorcycle, or airplane. A racecar would be focused on high thermal resistance since there is a great amount of friction compared to a motorcycle would focus in weight reduction since it doesn’t produce as much heat. For optimal performance and efficiency there must be a balance of mechanical properties of the material used, its performance and cost. In a racecar like the one selected for our project, the performance benefit due to using carbon ceramic discs does not justify its cost, also the heat generated is not enough for a significant advantage. Aluminium matrix provide insufficient friction coefficient, and their weight advantage is nullified due to need of a larger caliper. Hence our choice would be to use a martensitic steel which can provide good performance (coefficient of friction and heat dissipation).

Some specialized grades from researches have been studied, their large scale manufacturing possibility holds the key to their use.

From the view of positioning the holes- there has been an ideal result found- which is making holes larger and pushing them radially outwards- This allows quicker heat dissipation, less stress concentration, less rotational inertia due to removal of material from radially far positions- which leads to greater acceleration

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FUTURE TARGETS

• Other than the work already done our other targets for the current and future projects are-

• We have gone through a number of research papers on the Ideal brake disc materials, its something we aim to exploit.

• We have scouted for various active temperature sensing and data logging units, I’d attached a few pdf’s of some products along by motec and texense. We have purchased a TC Direct rubbing type thermocouple sensor which we will test on the university’s formula student racecar.

• We had stumbled upon some research on using a closer to real value of convection through a CFD simulation of air around the brake disc. I’m personally in contact with totalsim.co.uk. It would lead to a closer value of the convection coefficient that would result in a more accurate result

• We’ll aim at optimizing weight and convection rates through iterations of simulations using different designs adding ribs or holes, varying them in number and diameter to reach an optimum solution for the selected test vehicle

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8.REFERENCES

[1] Dr. Vijay Gautam, Asst. Professor Delhi Technological University.

[2] EUROPEAN PATENT APPLICATION Application number: 01308823.2

[3] Brake Safety and design Rudolph Limpert

[4]FLOQUET, A. AND DUBOURG, M.-C. Non axis symmetric effects for three dimensional Analyses of a Brake, ASME J. Tribology, vol. 116, page 401-407, (1994).

[5] TalatiaFaramarz and Jalalifar S, (2000) Analysis of heat conduction in a disk brake system. J Heat Mass Transfer 45 : 1047-1059.

[6] Yoshioka, K.; Suzuki, S.; Ishida, F.; Horiuchi, M.; Kobayashi, K. Kawasaki Steel Giho. 1983, vol. 15, no. 4, p. 266–272.

[7] Choi, B., "Thermal Performance of Disc Brake and CFD Analysis," SAE Int. J. Passeng. Cars - Mech. Syst. 7(4):2014, doi:10.4271/2014-01-2497

[8] Aero-thermal Characteristics of an Automotive CCM Vented Brake Disc, SAE 2005-39-0930

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