Parallel shaft application of helical gears result in smoother action than spur gears because, the load is transferred uniformly and gradually as successive teeth come into play. They also reduce noise and vibration.
To substitute normal pitch helical gears for spur gears while
maintaining center distances and ratio, proceed as follows: Select
a diametral pitch finer than the spur gearing employed (called
the new pitch) and divide the spur gear diametral pitch by the
new pitch. This results in the cosine of the helix angle required.
Therefore the helix angle = 27 degrees 16 minutes and the diametral pitch in the plane of rotation remains 64. One would therefore employ a helical gear cut with a standard spur gear 72 pitch hob set to cut a helix angle of 27 degrees 16 minutes.