Gas turbine engines have a compressor for pressurizing an air stream, a combustor for burning fuel in the pressurized air to generate high energy output in the form of a hot gas stream, and a turbine, which uses a portion of the energy in the hot gas stream to drive the compressor. The remainder of the energy of the hot gas stream may be converted to a useful output by being discharged from a propulsive nozzle; by driving a turbine powering an output shaft, or by combination of both in a turbo fan engine. The powered output shaft may be used for several different purposes, for example, driving engine components such as a fuel pump and other components which are generally referred to as accessories.
A gearbox is generally used to convert the rotating power from an output shaft, which may rotating at speeds as high as 100,000 revolutions per minute in smaller engines or 8,000-10,000 revolutions per minute in larger engines, to more useful speeds.
A gearbox is particularly useful to convert the high rotational speed of an output shaft to usable speed for accessories. Gearboxes are typically mounted to an engine by means of a band encircling the engine and attaching to the gearbox.