Planetary Train
WARNING: This page does not describe the selected car, but rather 12 other vehicles, including the 2014 Volvo XC90, 2013 Volvo XC90, 2012 Volvo XC90, 2011 Volvo XC90, and 2010 Volvo XC90. However, it is still accessible from the selected car via links, so may be relevant.
When the starter motor rotates, the sun wheel rotates clockwise. The planetary gears, which are constantly engaged with the sun wheel and ring wheel, rotate counter-clockwise. As the ring wheel is secured, the planetary gears are forced to circle around the sun wheel clockwise. The planetary gears rotate more slowly than the sun wheel. It is the clockwise rotation of the planetary gears which drives the pinion. The speed reduction that occurs between the sun gear and the planetary gear, results in greater torque. This means that a smaller and lighter starter motor can be used.