Typically, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a rotor assembly for producing power. The rotor assembly is formed from a plurality of turbine blades extending radially outward from a rotor. Each turbine blade is formed from a root portion having a platform at one end and an elongated portion forming a blade that extends outwardly from the platform. The blade is ordinarily composed of a tip opposite the root section, a leading edge, and a trailing edge. The root typically is attached to a disk of the rotor assembly with a plurality of serrations extending from the disk that mesh with a plurality of serrations extending from the disk to fixedly attach the blade to the rotor. While the turbine blade is fixedly attached to the rotor, there typically exists sufficient play in the attachment mechanism that the turbine blade may move.
During use, the rotor, in one particular turbine engine, rotates at about 3600 revolutions per minute (RPM). In this operating mode, the centrifugal forces created cause the turbine blades to extend radially outward without any movement relative to the rotor assembly to which the turbine blades are attached. However, in another mode in which the rotor is rotated very slowly by a turning gear, such as about 2 RPM, the turbine blades rock back and forth causing wear as the rotor is turned slowly. In particular, many turbine engines are kept in a ready state through use of turning gears that enable a turbine engine to be quickly brought to steady state operating conditions. In some situations, turbine engines are run with a turning gear for long periods, such as for several continuous months. At such a slow RPM, gravitational forces are stronger than centrifugal forces, thereby causing the turbine blades to rock back and forth, causing turbine blade root serration wear on both the rotor serrations and the blade root serrations as well. The rocking motion also causes hard face coating damage on shrouded turbine blades and, in some instances, has caused rotor cracking.
The wear caused by the rocking action of the turbine blades also frustrates efforts to take blade tip readings as well. In particular, when blade tip readings are performed in the field, the operator must physically lift or wedge each blade into the running position while taking the blade tip reading. Lifting each turbine blade by hand takes time and often results in inaccurate blade tip readings.
Previous attempts to curb root serration wear have been attempted but have not been successful. For instance, seal pin slots on the turbine blades have been enlarged and larger pins have been used. However, the turbine blade serrations have continued to wear resulting in rotor repair and scrapping of the turbine blades. Thus, a need exists for reducing wear on turbine blade root serrations.