1 HO CHI MINH UNIVERSITY OF TECHNOLOGY AND EDUCAITON DESGIN AUTOMATIC TRANMISSION FOR FORTUNER CAR INSTRUCTOR PhD NGUYỄN MẠNH CƯỜNGPREFACE1 CHAPTER 1: Overview of automatic transmission2 1.1. Tasks, requirements, structure automatic transmission.2 1.1.1. Tasks2 1.1.2. Requirements2 1.1.3.General structure of automatic transmission (EAT)2 1.2. Operational characteristics and advantages and disadvantages of automatic transmission3 1.2.1. Operational characteristics:3 1.2.2. Comparison between normal transmission and automatic transmission.3 1.2.3. Advantages and disadvantages of automatic transmission compared to normal transmission.5 1.3. Automatic transmission classification6 1.3.1.According to the layout6 1.3.2 According to the transmitter after the torque converter.7 1.3.3. By control7 1.4. General requirements when designing gearboxes for 8-seater vehicles.9 CHAPTER 2: Torque converters and basic planetary transmissionsTorque converters and basic planetary transmissions10 2.1 Hydraulic torque converter10 2.2 Model transformation tasks10 2.3 Structure of the model variable:11 2.4. Basic planetary transmitter20 2.4.1. Wilson planetary transmission20 2.4.2. Simpson’s planet transmission21 2.4.3. Ravigneaux planetary transmission22 CHAPTER 3: CALCULATION OF STUDY DYNAMICS25 3.1. Selected and given vehicle parameters25 3.2 Basic parameter of hydraulic torque converter26 CHAPTER 4: Calculation of the planetary gear box34 4.1. Calculation desgin of planetary transmission34 4.1.1. Structure diagram and calculation load.34 4.1.2. Determine the gear ratio of the main transmission i034 4.1.3. Determine the gear ratio of each gear in transmission35 4.2. Kinematics equation of the base planetary transmission37 4.2.1 Calculate the gear ratio for each gear individually39 4.2.2. Calculation of teeth ratio between gear pairs in base transmissions.45 4.3. Calculating the design of the cones, the brakes46 4.4 Durability of details48 4.5. Calculating the dimensions of the transmission gears.49 4.5.1. Select material49 4.5.2. Determination of the contact stress and allowable bending stress.49 4.5.3 Calculate stress when overloaded51 4.5.4. Determine the parameters of the Wilson transmitter52 4.6. Desgin each part of planetary gear54 CONCLUSION59 REFERENCE60
1 HO CHI MINH UNIVERSITY OF TECHNOLOGY AND EDUCAITON … … DESGIN AUTOMATIC TRANMISSION FOR FORTUNER CAR INSTRUCTOR : PhD NGUYỄN MẠNH CƯỜNG BY : NGUYỄN ANH QUỐC 20145431 PHẠM VĂN HUY 20145452 TƠ HỒI NAM 20145420 PHAN THANH PHONG 20145425 18th December 2022 TABLE OF CONTENT PREFACE CHAPTER 1: Overview of automatic transmission 1.1 Tasks, requirements, structure automatic transmission 1.1.1 Tasks 1.1.2 Requirements 1.1.3.General structure of automatic transmission (EAT) 1.2 Operational characteristics and advantages and disadvantages of automatic transmission 1.2.1 Operational characteristics: 1.2.2 Comparison between normal transmission and automatic transmission 1.2.3 Advantages and disadvantages of automatic transmission compared to normal transmission 1.3 Automatic transmission classification 1.3.1.According to the layout 1.3.2 According to the transmitter after the torque converter 1.3.3 By control 1.4 General requirements when designing gearboxes for 8-seater vehicles CHAPTER 2: Torque converters and basic planetary transmissionsTorque converters and basic planetary transmissions 10 2.1 Hydraulic torque converter 10 2.2 Model transformation tasks 10 2.3 Structure of the model variable: 11 2.4 Basic planetary transmitter 20 2.4.1 Wilson planetary transmission 20 2.4.2 Simpson’s planet transmission 21 2.4.3 Ravigneaux planetary transmission 22 CHAPTER 3: CALCULATION OF STUDY DYNAMICS 25 3.1 Selected and given vehicle parameters 25 3.2 Basic parameter of hydraulic torque converter 26 CHAPTER 4: Calculation of the planetary gear box 34 4.1 Calculation desgin of planetary transmission 34 4.1.1 Structure diagram and calculation load 34 4.1.2 Determine the gear ratio of the main transmission 𝒊𝟎 34 4.1.3 Determine the gear ratio of each gear in transmission 35 4.2 Kinematics equation of the base planetary transmission 37 4.2.1 Calculate the gear ratio for each gear individually 39 4.2.2 Calculation of teeth ratio between gear pairs in base transmissions 45 4.3 Calculating the design of the cones, the brakes 46 4.4 Durability of details 48 4.5 Calculating the dimensions of the transmission gears 49 4.5.1 Select material 49 4.5.2 Determination of the contact stress and allowable bending stress 49 4.5.3 Calculate stress when overloaded 51 4.5.4 Determine the parameters of the Wilson transmitter 52 4.6 Desgin each part of planetary gear 54 CONCLUSION 59 REFERENCE 60 PREFACE With the strong development of information technology, informatics in the path of automation is constantly developing, going deep into production technology and into products In particular, the automotive industry is always leading the application of new technologies Automation does not make users feel more comfortable with their car, but it improves safety in the application In the development of the world auto industry, the systems on cars are constantly being perfected The powertrain is no exception to that rule The purpose of the complete transformation is to: Reduce fuel consumption, increase power, reduce noise, increase maximum engine speed, make the best use of engine power generated and create convenience, simple for the driver Part of the powertrain in cars today is the transmission, which is used to change the gear ratio of the powertrain to create traction at the active wheels in accordance with the moving conditions Transmission today includes basic types: normal gearbox, automatic gearbox, and stepless gearbox The current trend of the automotive industry is to create gearboxes capable of automatic transmission or Infinitely variable transmission ratio However, stepless gearbox has low durability and efficiency, so it is still less popular Therefore, the topic selected Automatic transmission to design for the required vehicle CHAPTER 1: Overview of automatic transmission 1.1 Tasks, requirements, structure automatic transmission 1.1.1 Tasks Gear used to: - Transmit, change the speed and transmission moment (or traction) to the wheels in accordance with the engine load and vehicle speed, - Change the direction of motion (forward or reverse) for the car, - Permanent disconnection of the engine from the transmission 1.1.2 Requirements - There is a reasonable range of transmission ratios, distribution of optimal transmission ratio change intervals, in accordance with the required dynamics and transport economy - Must have high power transmission efficiency - When working without making noise without generating dynamic loads, shifting gears gently - Compact structure, easy to maintain and repair - It is possible to arrange a power extractor cluster to drive other auxiliary equipment 1.1.3.General structure of automatic transmission (EAT) The automatic transmission cluster in cars today includes a moment converter and a planetary gearbox, which is a cluster that shares a shell that is mounted behind the engine In the powertrain, the function of the automatic transmission assembly has a complex electro-hydraulic control system that works together with a small electronic computer to automatically switch gears inside the box main number Moment converters used on conventional cars are capable of varying moment between 1.6 and 2.5 times the torque of the engine Therefore, moment converter cannot meet the driving conditions of the car, so tmoment converter is often used together with a stepless or stepless mechanical transmission 1.2 Operational characteristics and advantages and disadvantages of automatic transmission 1.2.1 Operational characteristics: For cars with a normal transmission, the gearshift lever is used to change gears to change the traction at the wheel to suit the moving conditions When driving uphill or when the engine does not have enough power to climb the hill in the running gear, the transmission is shifted to low gear Therefore, the driver must constantly be aware of the load and engine speed in order to shift gears appropriately That will cause an unnecessary loss of engine power, in addition, it will cause difficulties when controlling and excessive concentration for the driver In an automatic transmission, such awareness of the driver is not necessary, the driver only needs to select a number of gears, and then shifting up or down to the most appropriate gear is done automatically at the time of automatic transmission Most suitable according to engine load and vehicle speed The automatic transmission has increased the comfort of the car In addition, during operation, it is possible to stop the vehicle without having to disconnect the clutch and switch to N The hydraulic automatic transmission has a straight transmission speed as well as an increased transmission 1.2.2 Comparison between normal transmission and automatic transmission Compared with normal transmission, automatic transmission has the following differences: Normal transmission Automatic transmission With clutch (taper) up with taper shank In the automatic transmission, the role (clutch pedal) of the clutch is to change the The moment flow from the engine to the hydraulic moment, and the moment transmission passes through the clutch and transmitted from the engine to the it is only capable of transmitting all the transmission when passing through the moment converter is increased K moment generated by the engine times where K is the coefficient of variation The gearbox arm have usually sprinkled The gearbox arm of the automatic shape transmission is in a straight direction P R N D R The gears when shifting gears are engaged The gears are engaged and when together direct insertion or locking shifting gears only control the synchromesh clutches the taper are controlled automatically through valve number In the process of shifting gears, the traction For automatic transmission, the graph is partially delayed due to the delay gearshift is automatic by controlling caused by the downshifting process shown the hydraulic flow to close the various in the black part of the graph valves so the time lag is almost zero For cars with a normal transmission, the gearshift lever is used to change gears to change the traction at the wheel to suit the moving conditions When driving uphill or when the engine does not have enough power to climb the hill in the running gear, the transmission is shifted to low gear For these reasons, it is essential for drivers to constantly be aware of engine load and speed in order to shift gears appropriately That will cause an unnecessary loss of engine power, in addition, it will cause difficulties when controlling and excessive concentration for the driver In an automatic transmission, such awareness of the driver is not necessary, the driver does not need to shift gears, but shifting up or down to the most appropriate gear is done automatically at the most appropriate time according to engine load and vehicle speed 1.2.3 Advantages and disadvantages of automatic transmission compared to normal transmission • Advantages: - Thanks to the structure of the planetary transmission, the planetary gearbox when automating the gearshift process has many advantages - The process of shifting gears is done automatically, so it reduces clutch and transmission control, reducing labor intensity for the driver, enabling the driver to handle other situations on the road This makes the comfort in use of the car significantly increased - The moment is transmitted to the active wheels smoothly and continuously, corresponding to the engine load and vehicle speed, reducing the dynamic load on the powertrain and finishing kinetic ability - When using hydraulic torque converter, or belt transmission, it is possible to limit dynamic loads, improve the life and durability of the engine and transmission system - Shift gears continuously without cutting the flow of force from the engine - Longer service life, simultaneous transmission of force through several pairs of mating gears, small tooth stress Inner meshing, so the diameter of the engagement circle is large Capable of self-suppressing axial force - Reduce noise when working - High working efficiency because the energy flows can be parallelized For suitable gear ratio but not large size • Disadvantages: - Besides, automatic transmission also inevitably has disadvantages: - The speed change is also accompanied by slippage of the transmission elements, resulting in a small loss of engine power - The ability of the car to move does not completely depend on the driver's actions but also on the condition of the road surface, sometimes there may be situations where it is difficult to control the movement of the car on the road - Manufacturing technology requires high precision because the most used axes are cage shafts, many gears are matched with one gear, and the control mechanisms require high precision - Complex structure, many cage assemblies, cage shafts, brakes, locking clutches The centrifugal force on the planetary gears is large due to the large angular speed - Using more clutches and brakes can increase power loss when shifting gears, and performance will decrease - However, with current machine-building technology, the disadvantages of planetary gearboxes will gradually be overcome when choosing the optimal operating scheme 1.3 Automatic transmission classification 1.3.1.According to the layout • Front engine, active front axle (FF): Wheels Engine Gearbox Sell shaft Match posts Modulation Figure 1.1: Front-mounted engine layout, active front-wheel drive (FF) • Engine placed in front, active rear axle (FR): Engine 2, 5.Wheels 3.Variables 4.Planetary gearbox 6.Sell shaft 7.Active bridge Figure 1.2: Engine layout in front, active rear axle (FR) The gearboxes used on FF cars are designed to be more compact than those used on FR cars because they are installed in the same compartment with the engine Transmissions used in FR cars have a final gear drive with an external axle housing The gearboxes used on FF cars have the last gear transmission with the axle housing installed inside, so the automatic transmission used on FF cars is also called a axle housing transmission 1.3.2 According to the transmitter after the torque converter Continuously Variable Transmission (belt transmission) Automatic transmission with speed (normal fixed shaft and planetary transmission movable shaft) Continuously Variable Transmission Figure 1.4: Automatic transmission with speed (planetary) 1.3.3 By control Table 4.3: Table of tooth ratio between gear pairs Number of tooth ratio ZR1/ZS1 100/40 ZR2/ZS2 142/100 ZR2/ZS3 178/100 ZS2/ZS3 126/100 From the coaxial condition for the transmission ZR1 = ZS1 + 2.ZP1 have: ZP1 = (ZR1 – ZS1)/2 On the other hand, ZR1/ZS1 = 100/40 Substitute into the above equation: ZP1 = 60/200.ZR1 Likewise we have ZP2 = 21/100.ZR2 With the basic dual planetary gear transmission, we cannot accurately calculate the tooth ratio of gear P3 to sun gear S3, so choose the number of teeth of this gear P3 later 4.3 Calculating the design of the cones, the brakes • Strip brake The brake band is wound around the outer diameter of the brake drum One end of the brake band is secured to the gearbox housing by a pin, and the other end is in contact with the brake piston through a hydraulically driven piston rod The brake piston can be moved on the piston rod by compressing the springs The piston rods of two different lengths are arranged so that the clearance between the brake strip and the brake drum can be adjusted When replacing the brake strip with a new one during an overhaul of an automatic transmission, the strip must be soaked for 15 minutes or so in the automatic transmission (ATF) fluid before mounting 46 Figure 4.10 Strip brake diagram • Wet multi-disc brake Brake B2 works through the 1st one-way joint to prevent the front and rear sun gears from turning counter-clockwise The friction discs are bolted to the outer ring of the d.c joint and the steel discs are fixed to the gearbox housing The inner ring of d.c joint (front and rear sun gears) is designed so that when turning counterclockwise it locks, but when turning clockwise it can rotate free The purpose of brake B3 is to prevent the rear guide lever from rotating The friction discs mesh with hub B3 of the rear guide rod The hub B3 and the rear guide rod are arranged in a cluster and rotated together Steel discs are fixed to the gearbox housing Figure 4.11 Diagram of wet friction disc brake 47 • Hand Con C1 and C2 are power connection and disconnection clutches Clutch C1 operates to transmit power from the torque converter to the front ring gear via the primary shaft The friction discs and steel discs are arranged alternately The friction discs are keyed to the front ring gear and the steel discs are keyed to the forward clutch drum The front ring gear is keyed with the ring gear flange, and the forward clutch drum is keyed to the front hub of the straight gear clutch Clutch C2 transfers power from the primary shaft to the drum of the directshift clutch (sun gear) The friction discs are keyed to the hub of the straightthrough clutch, and the steel discs are keyed to the straight-through clutch drum The direct transmission clutch drum is engaged with the sun gear input drum, which is in turn engaged with the front and rear sun gears The structure is designed so that the three assemblies of friction discs, steel discs and drums rotate together Figure 4.12 Structure diagram of the cones in automatic transmission 4.4 Durability of details In principle, the calculation of the gear transmission design of the planetary gear transmission is not different from that of the conventional gear transmission design Calculations are performed for each gear pair, including design and testing steps When designing calculations, there are a few things to keep in mind: 48 - Because the applied force and the modulus of engagement of the gear pair when engaged are the same, while the inner gear pair has higher durability, when using the same material, we only need to calculate the gear pair strength external fit When using different materials, we calculate the strength of internal gears for the purpose of material selection or testing - To reduce working noise, reduce the modulus and increase the ring width Select a unified module according to the standard to ensure repair, replacement and armoring technology - With the planetary gearbox, the gear pairs are always engaged and can carry the load or just rotate the cage, thus making the gearbox have a lot of noise Therefore, in the gearbox we use inclined teeth because it has many outstanding advantages over straight teeth, quieter working, reduced dynamic load, and high load capacity - However, with inclined gears, axial force is generated, if the axial force is large, the bearings will be large and the axial structure is large To reduce the axial force, we should arrange - symmetrical so that the generated axial forces will cancel themselves Choose a small angle of inclination β The angle of inclination β is chosen as follows: - For passenger cars β = 30 ÷ 45 degrees - For trucks, β = 20 ÷ 30 degrees - For the design vehicle, we choose the angle of inclination β = 30 degrees 4.5 Calculating the dimensions of the transmission gears 4.5.1 Select material Unified in the view that the car can be used after many overhauls, repairs, replacements and is convenient for mass production, so we choose the same gear material However, the gears have the same module, so when the gears are engaged, the large gears will bear less load, so when the small gear has to be overhauled, the big gear is still usable We choose 25CrMnTi alloy steel, surface quenching reaches hardness HRC = 56 ÷ 63, tooth core hardness HRC = 28 ÷ 35 Strength limit δ= 1600 Mpa The yield strength δ ch = 1400 Mpa 4.5.2 Determination of the contact stress and allowable bending stress [ σH ] = σHolim.KHL/ SH 49 [ σF ] = σFolim.KFL.KFC/ SF In there : σHolim : allowable contact stress ΣFolim: allowable bending stress SH, SF : Factor of safety when calculating contact and bending Look up table 6.2 (Mechanical drive system design calculation book, volume Author: Trinh Chat – Le Van Uyen) We choose SH = 1.1 SF = 1.6 KFC is a factor that takes into account the effect of loading, with the gearbox being calculated, since The sun gear of the first base transmission is locked, so this transmission can only transmit in one specified direction, taking KFC = KFL and KHL are life factors, taking into account the effect of service life and load mode of the transmission, determined by the following formulas: mH,mF is the degree of the fatigue curve when testing for contact and bending: mH =10, mF = NHO is the number of cycles of change of the base stress in the contact test: NHO = 100.106 NFO is the number of cycles of base stress change in bending test: NFO = 4.106 NHE and NFE are equivalent stress change periods Treat it like a car operating at an average speed of 60 (km/h) for most of the time of use use of automatic transmission Therefore, it can be calculated according to the following formula: NHE =NFE =60.c.n.t In there 50 c, n, t∑ are the number of times of engagement of a revolution, the number of revolutions in a minute, and the working time of the gear in question, respectively Total working time of the gear under consideration can be considered as the use time of the automobile transmission between overhauls 𝑠 300000 = = 5000 (ℎ) vtb 60 From the formula to calculate the average speed of the car we have: t = vtb = 0,377 𝑟𝑏 𝑛𝑡𝑏 𝑣𝑡𝑏 𝑖ℎ 𝑖𝑜 => 𝑛𝑡𝑏 = 𝑖ℎ 𝑖0 𝑟𝑏 0,377 ih : Gear ratio of the gearbox, we take the highest gear ratio in increasing gears ih=0.85 𝑛𝑡𝑏 = 60.0,85.4,12 0,32.0,377 = 1741 (vg/ph) Hence we have: NHE= NFE= 60.1.1741.5000 = 522,3.106 Thus, NHE and NFE have larger values than NHO and NFO, so the value KHL = KFL = is used Substituting all the above values into the equation for the stress allows us to have: H =1180 =1072,73(MPa) 1,1 1,1 F1=730 =456,25(MPa) (Thus, the neighborhood requirement for planetary 1,6 gears is satisfied.) 4.5.3 Calculate stress when overloaded For nitrided, carbonized gears, the contact stress under overload is calculated according to the formula Hmax=40.HRCm In there HRCm is the tooth surface hardness according to Rockwell With a surface hardness of 750 HV, converting to HRC has a hardness of 62 HRC So: Hmax=40.62=2480 (Mpa) 51 Allowable bending stress at overload: Fmax=0,8. ch =0,8.1400=1120 (Mpa) 4.5.4 Determine the parameters of the Wilson transmitter The input gear of the gearbox is the outer gear of the base transmission Wilson We have the formula to calculate the rolling ring of the sun gear In there: Kd : is a factor that depends on the tooth material and type Look up Table 6.5 book for calculating mechanical drive system design, volume Author Trinh Chat - Le Van Uyen we have Kd = 43 (MPa) MS1 : Torque on drive gear MS1 calculated according to the following formula MS1 = Mt2./(Z1+1) Với Mt2 : Torque on the output shaft of the hydraulic torque converter.Mt2 = 536 (N.m) MS1 = 536./(1+2,44) = 155,81 (N.m) = 155,81.103 (N.mm) i : Gear ratio of ring gear R1 and planetary gear P1 when instantaneous fixed drive lever i= 𝑧𝑟1 200 = = 3,33 𝑧𝑃1 60 KH∑ is the coefficient used to replace the coefficient KHβ in the calculation of planetary gears KH∑ calculated according to the following formula K H =K C +K0 H −1 In there: KC is the load uneven distribution factor for the planetary gears, K C = 1,1 for planetary gears 52 K H is the coefficient of uneven load distribution over the ring width, with the transmission internally engaged and having a planetary gear ratio of strokes K H = 1,21 K H = K C + K0 H −1=1,1+1,21−1=1,31 c : is the number of planetary gears used in the transmission, take c = [σH] is the allowable contact stress, [σH] = 1072,73 (MPa) bd is the coefficient that determines the ratio of the gear width to the tooth diameter bd = 0,35 Substituting numbers into the formula we have: 𝑑𝜔𝑠1 = 67,5 √ 155,81 103 1,31 (3,33 + 1) = 40,72 (𝑚𝑚) [1072,73]3 3,33.0,35.3 Sun gear width: bωS1 = dωS1.ψbd = 40,72.0,35 = 14,2 (mm) Lấy bωS1 = 14 (mm) Preliminary calculation of the rolling diameter of the ring gear: dωR1 = dωS1.Z1 = 40,72.2,44 = 99,35 (mm) Preliminary calculation of the rolling diameter of the planetary gear: dωP1 = dωR1.60/200 = 30,82 (mm) Select the normal module for the basic Wilson transmitter as m = Above we have pre-selected the tooth inclination angle β = 30’ Number of sun gear teeth: 𝑑ωS1 𝑐𝑜𝑠𝛽 40,72 𝑐𝑜𝑠30 = = 17,63 (𝑡𝑒𝑒𝑡ℎ) 𝑚 = 17( 𝑡𝑒𝑒𝑡ℎ) 𝑍𝑆1 = =>chose 𝑍𝑆1 Number of teeth of ring gear ZR1 = ZS1.100/38 = 17.100/40 = 42,5 (𝑡𝑒𝑒𝑡ℎ) choose ZR1 = 43 (𝑡𝑒𝑒𝑡ℎ) According to the coaxial condition to ensure the gears are centered properly: 53 ZR1 = ZS1 + 2ZP1 ZP1 = (ZR1 - ZS1)/2 = (43 - 17)/2 = 13 (𝑡𝑒𝑒𝑡ℎ) According to the assembly condition, the number of planetary gears is ZR1 + ZS1 = 43 + 17 = 60 Notice: 60/3 = 20, is an integer, ensuring the assembly condition: the planetary gears are arranged with equal distances Recalculate the radius of the ring gear of the planetary gear: Since the gear does not shift, the diameter of the ring dωP1 = dP1 𝑍𝑃1 𝑚 2.13 = = 30.02(𝑚𝑚) 𝑐𝑜𝑠30 𝑐𝑜𝑠30 Check the adjacency of the planetary gears in the Wilson transmission: 𝑑𝑃1 = 𝑑ωP1 = ZR1.sin(π/4) – ZP1(1-sin(π/4) > → 43 sin(π/4) – 13.(1-sin(π/4) = 26,59 > Thus, the neighborhood requirement for planetary gears is satisfied 4.6 Desgin each part of planetary gear • Carrier 54 • Helical gear (planetary gear) • Sun gear 55 • Ring gear • Helical gear pin & Washer 56 • Liner ring 57 • Final assembly drawing design 58 CONCLUSION In this project, my team calculated the basic geometrical dimensions of the planetary gearbox, ensuring the reasonable size of the gearbox when installed in the vehicle's powertrain the vehicle's ability to pass through various terrains Because of the lack of knowledge, some parts are still lacking I hope you and your friends can give suggestions to make the project better Although my team tried a lot and received emotional guidance from the instructor, there were some limitations in terms of time and knowledge, so my project could not be avoided Mistakes I look forward to receiving comments from teachers in the subject to improve my project 59 REFERENCE [1] Tập giảng tính tốn thiết kế tơ – PGS.TS Nguyễn Trọng Hoan [2] Cấu tạo hệ thống truyền lực ô tô – PGS.TS Nguyễn Khắc Trai – NXB KHKT, 2005 [3] Nguyên lý máy, Tập – Đinh Gia Tường, Tạ Khánh Lâm – NXBGD, 2005 [4] Tính tốn thiết kế hệ dẫn động khí, Tập – Trịnh Chất, Lê Văn Uyển – NXBGD [5] Tính tốn thiết kế hộp số tơ – Nguyễn Văn Tài [6 File hướng dẫn CHƯƠNG : TÍNH TỐN THIẾT KẾ HỘP SỐ TỰ ĐỘNG thầy Nguyễn Mạnh Cường 60