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Martempering to Improve Wear Properties of Aircraft Brake Steel Rotors by Dr

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Martempering to Improve Wear Properties of Aircraft Brake Steel Rotors by Dr Volume 23, Number 2 - April 2007 through June 2007 Martempering to Improve Wear Properties of Aircraft Brake Steel Rotors By Dr. Sudershan Jetley Peer-Refereed Article KEYWORD SEarcH Aviation Technology Materials & Processes Materials Testing Metals Quality Control Research The Official Electronic Publication of the National Association of Industrial Technology • www.nait.org © 2007 Journal of Industrial Technology • Volume 23, Number 2 • April 2007 through June 2007 • www.nait.org Martempering to Improve Wear Properties of Aircraft Brake Steel Rotors By Dr. Sudershan Jetley Abstract This article presents a study to improve Dr. Sudershan Jetley is an associate professor the wear properties of aviation brake ro- at College of Technology in Bowling Green State tors. Currently these rotors are being aus- Similarly Torque (τ), although not speci- University. He received his Ph.D. from University fied by the regulating agencies can be of Birmingham. He has taught and supervised tempered using a hot salt bath. This has graduate students in the areas of statics, materi- caused safety concerns in the plant, so derived as: als, automation, quality, research methods and manufacturing processes. His research interests an alternative martempering process was include Rapid Prototyping, Machining, Neural investigated using oil and water based Network applications and Machine vision. quenchants at lower temperature. The test samples were evaluated for hardness, distortion and wear under accelerated Where: simulated tests. The results show that al- K.E. = Kinetic Energy per wheel in though both hardness and wear resistance ft.lbs was lower compared to the austempering, W = Weight in lbs it met the design intent. Also the wear V = Speed in Knots rate of martempered samples was more R = Tire Radius in inches consistent which may provide advantages N = Number of wheels for maintenance purposes. τ = Torque per wheel in inch.lbs Introduction From these relationships, it can be The wheel has evolved from a simple seen that for a given aircraft, speed and wooden disk of antiquity to the sophis- weight are the governing factors in brake ticated composite structure of today design. This is again seen in graphical used in land and air vehicles. This form in figure1. evolution has been the result of meeting the challenge of ever increasing higher Due to the V2 term in the energy equa- speeds and loads. A vital component of tion, energy effects take on the overrid- the composite wheel is its brakes. The ing importance in brake functioning. primary requirement of modern brake Consider the scenario of a fully loaded systems is to be able to apply a braking aircraft aborting a takeoff just before the torque while resisting failure due to high actual takeoff. For a jet fighter, the speed friction temperatures developed. This at that moment may be close to 200 is reflected in national and international miles per hour. There is obviously little regulations for certification. reverse thrust to be had from the engines and for a jet fighter there is little aerody- For aircraft, Federal Aviation Regula- namic drag due to its control surfaces at tion, Code of Federal Regulation Part 23 that moment. The runway is also quickly and the United Kingdom Civil Authority running out, so the aircraft needs to be Specification No. 17 specify the required stopped quickly. Therefore, all the stop- energy capacity rating for Brakes. The ping action falls on the wheel brakes. A specifications provide the following commercial jetliner can require in the formula for deriving the Kinetic Energy order of 5,000 braking horsepower per (K.E.) absorption requirement (United brake (Garret, D.F., 1991). Kingdom Civil Aviation Authority, 2006): 2 Journal of Industrial Technology • Volume 23, Number 2 • April 2007 through June 2007 • www.nait.org Figure1. Effects of Speed and Weight of aircraft on Landing Energy Per wheel The two essential properties needed are (Johnson, 2006) heat dissipation by 1. Carrier assembly 2. Piston cup (outer) 3. Piston cup (inner) 4. Piston (outer) 5. Piston (inner) 6. Piston end (outer) 7. Piston end (inner) 8. Pressure plate For a jet fighter at peak speed, the sion of the fluid and drum leading to loss 9. Stator drive sleeve temperatures generated in the brakes due of brake action. After the World War II, 10. Auxiliary stator and lining to friction can exceed 2000o F (Hydro aircraft started to use disc brakes which assembly Aire Inc., 2005). The situation can be are used in modern aircraft. There are 11. Rotor segment further exacerbated by cross winds that two types, namely single and multiple 12. Rotor link would place additional and uneven loads disc brakes. 13. Stator plate on the different wheels due to steering 14. Backing plate compensation needed. At different alti- Pre World War II, the landing gear of 15. Torque pin tudes, K.E. generated also increases due aircraft was not retracted and stowed. 16. Adjuster pin to increased takeoff or landing speeds However, with increased speed require- 17. Adjuster clamp required to compensate for increased ments it became necessary to retract and 18. Adjuster screw density altitudes (Johnson, 2006). The stow the landing gear to reduce drag. To 19. Adjuster washer heat generated can result in rapid wear meet these requirements, wheels became 20. Adjuster return spring of the brake surfaces leading to brake smaller to reduce stow space, leading to 21. Adjuster sleeve failure, or at the least, reduction in brake the development of multiple disc brakes 22. Adjuster nut life. (Aircraft Technology Engineering and 23. Clamp hold down assembly Maintenance, 2000). This design has be- 24. Shim Early airplanes had no brakes and relied come standard and all large planes today 25. Bleeder screw upon the friction generated from their use the multiple disc brakes. An example 26. Drive sleeve bolt tails skidding on the grassy ground. diagram of these brake assemblies is 27. Dust cover (inner) When hard runways came into use, shown in figure 2 (Integrated publishing, 28. Dust cover (outer) aircraft brakes were generally adopted 2006). from automobile brakes. These early Figure 2. Segmented rotor brake–cut- brakes consisted of drum and shoes The essential part of the brake assem- away view. systems similar to the rear brakes found blies is a rotor rotating in contact against in today’s automobiles (Garret, D.F., a stationary stator during braking. The 1991). The major problem with these industry primarily uses sintered, copper types of designs is the thermal expan- based alloys as stators and steel rotators. 3 Journal of Industrial Technology • Volume 23, Number 2 • April 2007 through June 2007 • www.nait.org the stator and reduced wear rate of both ing safer quenchants. Two quenchants, This is shown in figure 3. The Hardness the stator and the rotor. Although the one oil based and other water based machine can be used with several differ- basic design has remained the same over were identified. This article reports this ent scales including Rockwell A-G, H, the years, there have been many devel- study. (Note: the identity and some of and K. In this study Rockwell scales B opments in materials and processing of the details of the process are not being and C were used. these materials (Aircraft Technology provided due to confidentiality require- Engineering and Maintenance, 2000.) ments) A significant development has been the Martempering is a commonly used use of carbon to replace steel since the heat treatment technique. The purpose 1980s. Indeed in some larger aircraft a of this process is not only to obtain the fully loaded landing and takeoff may appropriate microstructures and me- not be possible without the utilization of chanical properties but also to minimize carbon (Graphite) brakes. distortions. In martempering, the part or specimen is heated well into the aus- However, carbon brakes needs to be tenitic region. Once the entire part has thicker than steel brakes due to various reached the specified austenitic tempera- reasons such as to provide sufficient ture it undergoes an interrupted quench volume to act as a heat sink and provide at a specified temperature below the Ms structural strength. Therefore, they re- temperature of the Time- Temperature quire more space due to carbon’s lower – Transformation (TTT) diagram. The density, and are also about 1/3 more part is then slowly cooled through the costly than steel. Therefore, steel rotors, martensitic transformation. The amount also known in the industry as pucks, of time the part is kept in the martensitic remain competitive and are used in most transformation determines the properties cases (Aircraft Technology Engineering of the part. Once the temperature of the and Maintenance, 2000). part falls below the Mf line, bainite and very fine pearlite is formed. The last step The Project in the martempering process is to temper At an aircraft brake manufacturing the part at the specified temperature for Figure 3. Rockwell Hardness Tester facility, the steel rotors (pucks) are heat the specified amount of time. This re- treated using an austempering process. lieves stresses produced due to quench- Austempering is the heat treatment pro- ing and converts any retained austenite 2. Small Electric Furnace cess of heating steel into the austenitic to pearlite, thus reducing the possibility A small electric oven was used to temper phase, and then quenching to a tem- of spontaneous transformations that can the pucks after they had been quenched. perature above the martensitic transfor- lead to cracking (e-funda engineering The oven is made by Hevi Duty Electric mation temperature, which causes the fundamentals, 2006. ) Co. model number MU-56-S. The oven austenite to isothermally transform to has a voltage range of 110-200, a maxi- bainite. In the process, the quench cycle In summary, the purpose of this study is mum wattage is 3400, and a temperature is interrupted at about 700o F and held to assess the feasibility of martemper- range of 0o to1200o F.
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