What is clearance sequencing in engine assembly and provide an example?

• A. Randomly set clearances based on guess
• B. Only verify exhaust casing clearance
• C. Increase all clearances equally
• D. Stepwise setting of rotor/stator clearances to prevent rubbing

Answer: (D)

Clearance sequencing means adjusting rotor-to-stator gaps in a staged, stepwise manner during assembly so that rubbing is avoided as parts warm up and settle. In a turbine engine, clearances change with temperature and bearing loading, and a gap that looks acceptable cold can become too tight or too loose once the engine runs. By setting clearances one stage at a time and using small, measured adjustments, you ensure each gap reaches the correct operating value without risking contact.

An example helps make this concrete: you assemble a turbine with several stages and begin with a cold clearance slightly larger than the final operating target. You install the first stage and set its gap to the specified cold value. After a warm-up run, you check the gap again. If the rotor has expanded more than the stator and the clearance has decreased toward the final spec, you adjust by shaving or adding a shim in small amounts to bring that stage into its correct operating clearance. You then repeat the process for the next stage, using the same stepwise verification, until all stages meet their final clearances. This approach prevents rubbing during startup and throughout operation and preserves efficiency and blade life.

Randomly guessing clearances, focusing only on one area, or widening all clearances equally would not reliably prevent rubbing or maintain performance, which is why the stepwise, staged method is the correct concept.