12.25 Number of contacts per revolution

For a single pinion in mesh with a single gear, each member has one contact per revolution. Some gears have more than one cycle of load contact per revolution. An epicyclic gearset (planetary or star gear) is shown below:

• The gear has Q contacts/rev, where Q = number of planets. For the example shown, the sun gear has 3 contacts/rev.
• The planet gear has 1 contact/rev because the loads from the sun gear and ring gear occur on opposite sides of the planet gear teeth. The reverse bending that occurs on the planet gear teeth is accounted for with the ”Loading-type Code” (See chapter 12.24).
• (planetary gear train) The internal gear has Q contacts per revolution, where Q = number of planets. Although the internal gear in a planetary gearset is fixed, it is analyzed as if it were rotating at the planet carrier speed.
• (star gear train) – the internal gear has Q contacts per revolution of the internal gear where Q = number of planets. An example of a split-power-train (branched) gearset is shown below:

In this example, if the pinion is the driver or is driven, it has 2 contacts/rev. If the pinion is an idler, it has 1 contact per revolution and reversed bending. The mating gears each have 1 contact/rev.