| INTRODUCTION As much quieter automotive transmission is required，it becomes more difficult to realize the high strength，low noise and light weight simultaneously with the traditional fine pitch design method of helical gears．A small module increases gear stress and an extremely large number of teeth cause many highfrequency resonance noise peaks. To realize Hjgh performance transmission gears for mass production vehicles，this study developed a gear optimization design system consisting of three parts． Part 1 helps gear designers to achieve sufficient load capacity，proper contact ratio and good productivity．Compared to the traditional fine pitch design method，this paper proposed a new concept to design gears with a number of teeth as small as possible．As a result，a smaller teeth number avoids many highfrequency resonance noise peaks，and the enlarged module brings higher strength and lighter weight by reducing the face width of gears． Part 2 optimizes the shape of tooth flank to have a minimum transmission error and good tooth contact pattern in wide range 0f lOad condition．The notable influence of mass production assembly errors and load deftections in actual automotive transmission can be also considered． Part 3 evaluates the mesh behavior of gears built-in actual transmission after heat treatment．A digital evaluation program of actual tooth flank shapes was developed based on the authors’ tooth flank measurement method. that can cover the whole area of tooth flank．For easy understanding．the relative tooth flank shape and the contact stress between the actual tooth flanks of gear pair are expressed by contour graphs： As practical applications，several new all-wheeld-drive type transmissions are developed by above design system．The tooth contact pattern，pitting strength and gear noise were improved remarkably． ．“ 1 NEW DESIGN CONCEPT OF HELICAL GEAR according to the experiment of measured vibration of fine pitch designed gears for actual transmission．Many resonance peaks occurred in wide mesh frequency range both for 1st and 2nd orders．It is difficult and expensive to reduce these resonance noise peaks． 1.1 Influence on strength of teeth number Gear strength can be easily improved by enlarging the module．from the result of experiment, the conclusion could be expressed that the modules of step gears for two actual manual transmissions．The new 6MT have a larger module than the conventional 5MT for each step gear while the face widths are almost same．compares the bending and contact stresses that are calculated by ISO 9336．For 5MT．since the bending stresses are too high，many strengthening schemes such as strong shot-peening processes are-applied to the 1st-3rd gears．But for 6MT．although the input torque is 20%larger than 5MT．its bendling stresses are reduced drastically．On the other hand．the contact stresses of 6MT gradually decrease compared to those Of 5MT as the gear step changes from low to high．The contact stress of the 1st gear 0f 6MT is 10% smaller than that of 5MT． 1.2 Influence on Contact Ratio of Teeth Number In the development of small teeth number gears．the largest concern is the decrease of contact ratio． However，this does not necessarily occur．Since the contact ratio depends on all items of gear dimensions，the minimally required contact ratio for small teeth number gears can be realized by properly setting other dimensions such as tooth depth，helix angle，pressure angle，face width，module，addendum coefficient． It meets the manufacturing conditions to ensure tip thickness and to prevent undercut，as well as the common designing requirements for actulal gears such as tooth depth coeficient and helix angle. As the teeth number decreases．the transverse contact ratio gradually decreases but the face contact ratio gradually increases．This is because small teeth numbers ease the limit of tip thickness and allow for deeper tooth depth and larger helix angle． Accordingly，a well-balanced design of helical gear with a small teeth number will lead higher strength as well as lower gear noise or vibration． 2 TEST OF SMALL TEETH NUMBER GEARS 2.1 Vibration test on actual transmission In order to explore the capability of gears with a small teeth number．their vibration performances were measured on an actual 4．speed automatic transmission (4AT)．the vibration test apparatus that can measure the meshing vibration of the rotating gears by accelerometers mounted on the gears．The vibration of transmission is simultaneously measured with another accelerometer fixed on transmission case． compares the actually measured vibration ofthe three test gears(Z53．Z37 and Z311)．the Z53 is designed by traditional fine pitch method．There are many vibration peal，which corresponds to the typical driving speed range from slow city cruising to fast highway driving．In particular，the vibration peak near 4300rpm is the meshing resonance peak of gear pair where the gear blank functions as mass and the teeth acts as spring．It is extremely difficult to reduce this meshing resonance peak because it inevitably occurs if the mesh frequency coincide with resonance frequency and its magnitude will be large． As the teeth number decreases，the peak positions of the vibration of transmission case shift to the right side．The vibration level decreases and the number of resonance are also steadily reduced．Therefore．the effect to narrow the mesh frequency range of small teeth number is achieved． The test results demonstrate that the structural resonance peaks of transmission and the meshing resonance peak of gear pair inevitably Occur regardless the gear teeth number when the mesh frequencies coincide with the resonance frequencies． Moreover，as the gear teeth number is decreased．the vibration of transmission is significantly reduced in the almost speed range．Therefore．it is confirmed that small teeth number is useful to decrease the vibration of actual tansmission． 2.2 Detailed vibration measurement on gearbox A simple gearbox tester is developed to measure the rotational meshing vibration of a gear pair，i．e．the exciting force of gear noise．This tester allows for the quantitative evaluation of the gear pair only，without the influence of complex transmission structure． for 5AT．4AT-gear has a teeth number 53 and center distance 108mm，while 5AT-gear has a teeth number 41 and center distance 119mm．The contact ratios of them are 3.89 and 3.46 respectively． the measurement method is that the meshing vibration of a gear pair：two accelerometers are mounted on both of drive and driven gear blanks and the meshing vibration of the rotating gears is detected by them and taken out through slip rings． According to analysis the result．5AT-gear has a same level of meshing resonance peak as 4AT-gear，but its vehicle speed at resonance is shifted from 110km／h to 150km／h since the teeth number is decreased．As a result，in common vehicle speed range up to 130km／h，the exciting force Of 5AT．gear is clearly smaller than that Of 4AT-gear．and thus small teeth number gear obviously has an advantage to decrease gear noise． 3 GEAR OPTIMIZATION DESIGN SYSTEM For simultaneously realizing low noise，high strength and light weight，this study developed a gear optimization design system based on above analysis and test results of small teeth number gears． 4 APPLICATION IN TRANSMISSION DEVELOPMENT 4.1 Strength improvement for high torque 6MT．compares the cross-section of the newly developed 6MT and its teeth number with the conventional 5MT at each gear step．The input torque increases from 350Nm to 420Nm by 20%．and thus the center distance between the gear shafts is increased from 5mm to 85mm．The new 6MT have fewer teeth numbers for all step gears compared to the conventional 5MT．In addition, the teeth number difference between 6MT and 5MT becomes larger from low step gear to to high step gear．This strategy is based on above design concept．The modules of lower step gears are expanded to improve strength by using the extended center distance，and the teeth numbers of higher step gears are reduced to decrease noise by avoiding the structural resonance in the higher mesh frequency． ， As a result，all step gears of newly developed 6MT with high torque capacity realized high strength and low noise simultaneously． 4.2 Gear noise reducfion for new 5AT the cross-sections of the standard 4AT and newly developed 5AT and the specifications of their reduction gears for the all-wheel-drive system．For avoiding the resonance peak of 4AT’s overhang driven gear，the new 5AT’s driven gear employs double support structure．The center distance of 5AT was extended to 119mm from 108mm of 4AT．but the teeth number was reduced to41 from53．compares the measured vibration of transmission case．Over the almost entire speed range，the vibration of new 5AT is significantly lower compared to the standard 4A．the small teeth number and ease-off modification achieyed the follow effect：the number of resonance peaks in high—frequency range decreased by half and the vibration level was reduced by more than 10dB．As a result．the new 5AT realized higher quietness and higher-quality driving． 5 CONCLUSIONS Based on many test results，this paper presented a new gear optimization design system in order to develop high performance transmission for actual mass production vehicles． 1 Instead of traditional fine pitch design method， a new design concept of small teeth number was proposed to improve gear strength and reduce high-frequency resonance noise peaks． 2 the tooth flank shape was optimized to have a ml。minimum transmission error and good tooth contact pattern in wide load condition range． 3 A digital evaluation program was developed to visualize the contact stress and ease-off of actual tooth flanks after heat treatment． As practical applications of above design system，several new transmissions were developed．The tooth contact pattern，pitting strength and gear noise were improved significantly． Kohei Saiki Yoshikazu Miyoshi Takeshi Watanabe (Subaru Technical Research Center,Fuji Heavy Industries Ltd，Japan) |
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