Teeth angle correction

When a pinion having a uniform meshing at no load, is torqued at one end, it bends and twists according to a known algebraic combined deflection ,the load distribution is proportional to the tooth deflection. New load distributions can be calculated which can take into account alignment , bearing flexibility and thermal effects.

It can be shown that the tooth separation for double helix gears is less than that for single helix gears.

Temperature effects

Usually the pinion operates at a higher temperature than the wheel. The pinion will expand and hence the pitch will change. The change in axial pitch is most important as this wears the teeth at one end of the helix With apex trailing , the teeth bear hard on the inner ends and with apex leading the teeth bear hard on the outer ends.
Apex trailing is advantageous as apex leading teeth tend to compound the effect of heat distortion into the torque distortion