University of Houston uses Patran for computational-mechanics modeling of Gas Turbine Shaft
In an aero-derivative gas turbine, rapid airflow-induced loading acts on the valve disk, causing dynamic motion of the system. In various operational conditions, the shaft (with a keyway) transfers the aerodynamic torque acting on valve to pinion and rack gears. Stress concentrations in the keyway under the aerodynamic load are found to depend on: (1) keyway size and geometry, (2) shaft and key materials, and (3) contact conditions between the keyway and the key. The keyway must be designed such that stress concentrations are minimum under the maximum designed (operational) aerodynamic loading to ensure the life of the shaft. In this study, aerodynamic load and the geometry of the shaft with a selected keyway are modeled with the MSC.Patran. Stress concentrations in the shaft with different keyway geometric are determined through computational dynamic analyses of the system.
MODELING OF CONTACT BETWEEN SQUARE KEYWAY AND KEY
A geometric model of a shaft with a selected keyway in contact with the key has been constructed by using MSC.Patran as shown in Fig.1. The aerodynamic torque is applied at one end of the shaft. The surfaces between the keyway and the key are assumed in normal contact with a friction coefficient of 0.2. The upper part of the key surface is connected to pining gears and is hence assumed with a prescribed displacement condition in the normal direction.
STRESS CONCENTRATIONS IN THE KEYWAY UNDER CONTACT
Von Mises stresses in the shaft with a square keyway (with a fillet radius 0.01 inch) during contact are shown in Figs.2 and 3. Also, the von Mises stress distribution around the square key is shown in Fig.4. It is recognized that the keyway shape, contact conditions and component material properties all affect the stress concentrations in the shaft. From the results obtained, for the shaft with a square keyway with no contact, the maximum stress concentration occurs at the root of the fillet in the keyway section, due to the geometrical effect of the sharp fillet curvature. In the dynamic contact case, the maximum contact stress occurs at the corner of the keyway. It is found that the maximum stress during contact is much higher than that in the case without the contact. To reduce the contact stress, the shaft may be better designed with a stiffer material (compared with the key material) and more suitable keyway geometry.
Fig.1 Modeling Contact between Keyway and Key.
Fig.2. Von Mises stress around the square keyway in contact
Fig.3. Detailed von Mises stress distribution at keyway section (with contact)
Fig.4. Von Mises distribution in key section.
Dr. Tung-Pei Yu
Composites Engineering and Applications Center
University of Houston