This invention relates generally to rotating machinery and more particularly to apparatus for balancing rotors.
Gas turbine engines typically include several rotor stages, each having a rotor disk carrying an array of airfoils, i.e., compressor or turbine blades. Turbine rotors must be balanced to prevent damage and excessive loads on bearings and supporting structures, as well as efficiency losses caused by loss of clearance between the airfoils and the surrounding structure (caused by, e.g., shroud rubs).
Despite efforts to first balance their constituent components, turbine rotors still require dynamic balancing following assembly. For this purpose, it is desirable to use balance weights that can be re-positioned to redistribute the mass of the rotor as needed and allow the system unbalance to be fine-tuned to meet precise requirements. Separable balance weights are a common practice in larger gas turbine engines. These include bolts, washers, nuts and other fasteners of varying sizes.
In some gas turbine rotors, notably those in smaller engines, CURVIC couplings and friction joints are assembled using a single bolt or a group of bolts (referred to as a “tie rod” or “tie bolts”) spanning the length of the assembly. A tie bolt configuration weighs less than a conventional bolted joint, but the absence of bolt holes eliminates convenient features on the rotor disk which could otherwise be used to attach separable balance weights. Accordingly, the current state of the art for smaller turbine engines is to balance the assembly by selectively machining a sacrificial surface on the rotating part. Material is removed at the location of peak unbalance to redistribute the mass of the rotor about the axis of rotation. This process is irreversible and risks damaging a component such as an integrally-bladed rotor or “blisk”, which is both safety-critical and expensive.