T53 – Tachometer Drive Gearbox [N2]

Identification & Location

Mounted on the inlet housing at the 10 o’clock position and has
a mounting pad for the Torquemeter Boost Pump.

System Design

The N2 System consists of two power turbine nozzles, two power turbine wheels, power shaft and reduction gearing. The function of the two turbine nozzles and wheels are the same as the gas producer nozzles and wheels. The N2 system is mechanically independent of the N1 system which is called “Free Power Turbine.” Free power turbine allows constant power turbine speed while varying gas producer speed to maintain power. Thermal energy extracted by the power turbine is delivered through a coaxial power shaft to the reduction gear. The reduction gear is a two step helical gear, with co-axial input and output shafts which functions to deliver energy to a transmission at usable speeds and torque. The forward end of the power turbine shaft is supported by No. 3 Roller Bearing (Radial Support) and the aft end is supported by No. 4 Main Ball Bearing (Radial and Axial Support). The aft end of the Power Shaft is supported by being internally splined to the power turbine shaft and the forward end is supported by No. 21 Roller Bearing (Radial Support).

The N2-driven overspeed governor and tachometer drive gearbox receives its drive from a spur gear pressed on the power shaft aft of the sun gear. This gear engages the N2 drive and driven gear package located within, and at the top of the accessory gear carrier. This package, a series of 90 degree conversion gears provides an internally splined drive for the overspeed governor and tachometer gearbox driveshaft which passes up through the 10-o’clock inlet housing strut and into the gearbox.

The drive shaft then engages the internal splines of the 90 degree upper drive gear which provides drive to the tachometer drive gear and power turbine overspeed magnetic pickup housing. This gear meshes directly with an idler gear, which in turn, transmits the drive to the combination torquemeter boost pump and overspeed governor drive gear.