The length of action divided by base pitch, giving the average number of tooth pairs sharing load. Spur gears typically run 1.4 to 1.8, meaning one pair or two pairs alternately, and high contact ratio designs push above 2 so two pairs are always engaged. Helical gears add face contact ratio from the helix overlap, and a total above about 2 is a common target for quiet running.
Spur set's only at 1.3 contact ratio, go to a finer pitch and longer teeth if you want it quieter.
Also heard as
- profile contact ratio
- transverse contact ratio
Related terms
Line of action
The straight path along which the point of tooth contact travels, tangent to both base circles.
Spur gear
A cylindrical gear with straight teeth parallel to its axis, the simplest gear for parallel shafts.
Helical gear
A cylindrical gear whose teeth wrap at an angle around the blank, engaging gradually for quiet, strong running.
Transmission error
The tiny difference between where the output gear actually is and where a perfect gear pair would put it.