Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
This master thesis comprises analysis and dimensioning of a keyed joint between a generator axis and a hub with aid from the FEM-software ANSYS Workbench 13.0.
The generator axis is a part of a hydroelectric power station that generates electricity with a reactions turbine, a Kaplan turbine. The existing keyway consists of one parallel key with the width 90 mm and height 45 mm. Due to a planned increase of the power of the generated power with 32 % a design study was needed to see if the keyed joint would be suitable for the extra force. Because of the fact that the axis diameter is bigger than the standards covers/recommends a the keyway had to be re-dimensioned. In an earlier assignment from Skellefteå Kraft AB to Sweco Energuide the keyway was planned to be properly dimensioned, but no final solution was approved.
During this master thesis a number of different numerical tests were made. The analyses in ANSYS Workbench showed that the old keyed joint only had a safety factor of one. The tests showed that it was the hub that was the weak link and the decider of dimensions and design when the materials yield strength and tensile strength was too low and therefore essential. A number of attempts were made to increase the safety factor by adding both two and three keys to the keyed joint, but without success.
With the existing material in the hub and axis it was not possible to reach the requested safety factor of three in the joint, no matter what dimensions the keyways had. The solution was to change the material in the axis and the hub to a more high quality steel with a larger yield and tensile strength. By using a typical steel for turbine axis, with a yield strength of 700 MPa for the axis and 455 MPa for the hub, a safety factor of 2,8 was achieved. The new keyed joint consists of two keyways spread 180 degrees apart with a standardized size of 100 mm width and 50 mm height together with a shrink-fitting between the axis and the hub. The main reason why two keyways were used instead of three was to get a more robust solution.
As a conclusion the maximum effective stress was lowered with 55 % in the axis and 82 % in the hub.