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State the ‘Lewis equation’ for spur gear design. State SI unit of each term in the equation.

Question: 

State the ‘Lewis equation’ for spur gear design. State SI unit of each term in the equation.

Answer: 

Lewis equation: WT = σw.b.π.m.y, WT= Tangential load acting at the tooth in N σw= bending stress in N/mm2 b= width of the gear face in mm m= module in mm y= lewis form factor.