Loc Duong

Evaluation of Ring Gear Tooth Stress

To optimize the operating speeds of the low spool of a gas turbine engine together with the overall transmitted horsepower to weight ratio, an epicyclic gear train is used to transmit power from the turbine section to the propeller shaft (as in PWC PT6 engine) or to the fan shaft (as in Honeywell TFE-731 engine). In order to achieve an optimum design in view of structural integrity, the stress characteristics of each component of the epicyclic gear train needs to be optimized. In general, the external gear mesh (as in the sun and planet mesh), needs a back up rim to tooth thickness ratio of not less than 1.2. However, this is not always the case for internal gear mesh such as the ring gear. The objective of this paper is to present analytical results on the stress behavior of the ring gear under different loading conditions. Three dimensional finite element method is employed to study the internal tooth fillet stress under the effects of fillet radius, gear rim thickness, pressure angle and helix angle. This study is the first part of a work aiming to determine the failure mode of the ring gear and leading to design optimization of epicyclic systems.Copyright

Radial Inflow Turbine: Synchronous Vibration Failure

Radial inflow turbine is frequently used in small gas turbine application where ruggedness and simplicity are prime requisites. For compactness, the radial turbine is mounted back to back with a radial compressor resulting in an overhung rotor in both compressor and turbine wheel. In such configuration the structural integrity of the radial turbine is crucial since due to its large inertia its failure could result in an uncontained egress of the engine. This paper presents one class of radial inflow turbine wheel failure due to high cycle fatigue of synchronous nature caused by two different excitation sources, namely mechanics and aerodynamics. While the first source of excitation is associated with the dynamics of the rotor, the second one is related to the dynamic of fluid flow and the thermal characteristics of the combustor, expressed in terms of hot streak. These sources of excitation can act individually or in combination. Two distinct types of failures were illustrated — blade mode — and disc-blade coupling mode. The failure phenomenon is characterized by fatigue crack originated at location corresponding to maximum dynamic stress for each type of failures, followed by the released of one portion of the blade.Analytical methods including finite element method, rotor dynamics analysis, and computational fluid dynamics are used to illustrate the root cause of such failure and also to its underlying solutions. Laser vibrometry and optical method were used to obtain the blade dynamic characteristics and to validate the solutions.Copyright