Tolerance Relief on Transmissions Castings for Producibility

Helicopter gearbox transmission castings must be designed for adequate strength, adequate stiffness for gear meshes and flight loads, clean-up of all machined surfaces, adequate edge distance for all clamped connections and minimum weight. Castings manufacturers generally request relatively large tolerances due to the nature of the casting processes in the order of +/- 0.100 inch while designers will require casting tolerances in the order of +/- 0.030 inch for cast surfaces. Excess casting material can lead to assembly interferences and excess weight while insufficient material can lead to thin wall conditions, lack of damage repair capability and potential performance issues. This paper describes how skewed (unilateral or unequally disposed) casting tolerances can eliminate casting rejections, eliminate Material Review Board (MRB) delays, eliminate reworks, ensure proper performance, and have negligible impact on weight.