Angular Velocity

• Gearbox manufacturers sometimes provide tables of gearbox efficiency versus angular velocity (the rate of change of angular position of a rotating body). In practice, these are not helpful because of power losses due to surface resistance, or friction.
  
  

Types of Friction

• Friction in the bearings of a gearbox creates a loss of torque dependent on velocity. Friction between the teeth of the gear creates a loss of torque dependent on the torque itself.
  
  

Causes of Friction

• Friction ultimately determines gearbox efficiency. Friction depends on the quality of the bearings, gearing, the number of times one wheel drives another -- the number of tooth engagements -- and the load torque.