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Constant Mesh Transmission are extensively used in agricultural tractors. Such transmission generally give rise crash noise during upshift and downshift due to lack of synchronisation between constant mesh and shift sleeve. This paper identifies factors that lead to such a situation. It presents some case studies and based on experience gained offers guidelines for design of constant mesh transmissions.

An approach is proposed for optimum design of gears for achieving required bending life,contact life as well as noise less transmission. Various gear design parameters to be optimised at design stage to produce first time right transmissions have been discussed. Factors considered are a) The method to fix best total addendum correction, to get maximum possible contact ratio. b) Distribution of addendum correction factors to gear and pinion, based on criteria of equal bending strength; and minimum top land required for addressing heat treatment and tip strength limitations. c) Fine tuning of vehicle ground speeds. d) Higher modules for better bending strength e) Tooth profile and lead modifications based on deflection of teeth and shafts in loaded condition. Above factors if considered upfront would lead to satisfactory design of transmission.