Methods and procedures for trim balancing the LM2500 marine gas turbine in the test cell and aboard ship

Abstract

Extensive fleet experience with LM2500 marine gas turbines shows that engines with higher than normal vibration are more likely to show early wear. Gas generator rotor unbalance has been identified as the main cause of high vibration. Rotor rebalancing reduces vibration to acceptable levels, at the same time reducing or eliminating many wearout modes. Initially, the only rebalance option was to remove the gas generator from the ship and send it to the depot. The high cost of this option led to a search for alternatives, and the successful development of a procedure for rebalancing the gas generator rotor aboard ship. The method adopted was the well known influence coefficient procedure, developed by the National Aeronautics and Space Administration (NASA) in the late 1960's. This method is well suited for implementation on portable computers, and fits readily into a practical procedure for use by trained technicians. The NASA program originally included a procedure to minimize peak residual vibration. Navy engineers added an improved optimizing procedure and a method to account for engine nonlinearities. Rebalancing involves mounting four external accelerometers on the engine, along with a tachometer to give a one-per-rev signal for phase angle measurement. Baseline vibration measurements, together with stored influence coefficients for the LM2500 engine series, permit first shot multi-plane, multi-speed trim correction weights to be calculated. The compressor case is readily opened and the weights installed without disturbing the engine. Application of this procedure has been highly successful: vibration levels of less than 0.001 inch peak--to-peak over the entire speed range have been achieved. The avoided cost of removal, replacement and repair of an LM2500 is estimated to be about $500,000.