Design and Manufacturing Strategy of a Back-to-Back Test Rig for Investigation of Ultra High Cycle Fatigue Strength Regarding Tooth Root Strength in Aerospace Applications

Abstract

Geared turbofans increase the efficiency of aircraft engines. By decoupling the fan and low- ressure turbine, both components can be operated at their optimum speed. Due to the high number of flight hours and the high turbine speeds of n > 10,000 rpm, the number of load cycles at the gear teeth of the planetary gear unit is significantly higher than the conventional fatigue strength of gear teeth. The planetary gears under fully reversed bending load are particularly susceptible to tooth root fractures. Pulsator test rigs are not suitable for testing tooth root load capacity under fully reversed bending load. The concept of a modified 3-shaft backto-back test rig was set up to cope with this challenge. A back-to-back test rig consists of a test and reference gearbox, which are connected by shafts to form a closed power circuit. In order to perform the investigation efficiently, the test rig is operated at a speed of n > 12,000 rpm with a recirculating power of P > 5 MW. This performance class results in special requirements for the test rig design. The roller bearing arrangements normally used for back-to-back test rigs no longer offer sufficient load carrying capacity, so that a changeover to journal bearings is necessary. This leads to high power losses of the bearings of P ≈ 200 kW and oil quantities of Q ≈ 450 l/min on the cooling and bearing lubrication side. The journal bearings of the gearboxes have a decisive disadvantage. Starting under load is not possible. Therefore, a hydraulic load unit was designed which allows load application at operating speed. In the reference gearbox, this unit shifts two helical gears on a spline shaft, which are slanted in opposite directions, towards each other, so that a torsional load is generated in the test gearbox by means of a closed power circuit. The two displaceable helical gears represent the greatest challenge from a manufacturing point of view. In order to achieve an even load distribution on both gears, they must be manufactured very precisely to each other and to the spline profile. In the report, the focus is set on the double helical reference gearbox including the axial load unit. Besides the design of the components the requirements regarding manufacturing are analysed and possible manufacturing chains are discussed.