Browse Publications Technical Papers 2012-01-2005

Selection of Gear Ratio for Smooth Gear Shifting 2012-01-2005

Manual transmissions are characterized by gear ratios that are selectable by locking selected gear pairs to the output shaft inside the transmission. Top gear is selected to get a maximum speed and is limited by the engine power, speed and the fuel economy. Lower gears are selected to get maximum speed at maximum gradient. Lower gears are also expected to give creeping speed to avoid usage of clutch and brake in city traffic. Selection of intermediate gears is such that it provides a smoother gear shift. Gear spacing is done in geometric progression. Spacing between the higher gears is usually closer than in the lower gears because drivers shift more often between the lower gears. This is opposed to the conventional idea of progressive spacing where higher gears had more space between them.
An objective method is provided for selecting gear ratios for use in vehicle transmission having multiple selectable gears. The method includes selecting gear ratios for a specific application followed by calculating a low gear ratio and a high gear ratio based upon vehicle parameters and performance requirements. The total ratio spread is determined by dividing the low gear ratio by the high gear ratio. Using the total ratio spread a geometric sequence is created with a plurality of terms, such that each of the terms respectively represents the ratio steps between the gears. Lastly, each gear ratio is divided by its respective ratio step plus one to find the gear ratio for the next gear. This method provides an objective method for selecting gear ratios, such that the steps between each of the ratios are uniformly progressive. Not only the procedure is generalized and validated but also a performance prediction tool is developed in house for quick validation and results.
The theoretical gear ratio thus calculated was formulated and made using ROMAX as indicated in fig. 1. Thus, for a range of torques different gear boxes like 5 MT- 320Nm transmission, 6MT transmission, 6MT transaxle, 5MT-100Nm were tested. Performance prediction for vehicles with the designed gear ratios was rigorously done and required parameters were recorded and compared. An experience with the wide range of gear boxes with different number of gears and varying engine torques has proved that the above adopted method of gear selection is optimum and can be made a standard for gear selection. With this standard method of gear ratio selection the optimum gear ratio can be selected with ease and best results can be obtained.


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