What is thrust load?
THRUST load is load parallel to the shaft of the gear. It is produced by helical gears because the helix angle, not the pressure angle. It is not produced by spur gears, which have straight teeth that are parallel to the shaft axis. RADIAL load is the load that tends to separate the gears. It acts perpendicular to the shaft. This is what is produced by the pressure angle. Both spur gears and helical gears produce this kind of load. Spur gears do not have any substantial thrust load, but they are noisy. Helical gears, by comparison, can be much quieter and potentially longer-lived, but they are more expensive to manufacture and you must provide thrust bearings to handle the substantial thrust load. For extremely demanding applications, double-helical gears can be used, with the teeth forming a herringbone pattern. However, these have a high penalty in cost and limited performance advantage over helical gear sets, aside from the cancellation of thrust load.
