Nature, causes, and prevention of labyrinth air seal failures

This paper is devoted to the problems of labyrinth air seals where radially extending teeth are carried by rotating components. In view of the possibility of resonance, determination of the natural nodal frequencies of seal components and supporting structure by analysis, tests, or both is of vital importance in the design and development procedure. Construction of Campbell frequency diagrams is a basic requirement to assure adequate frequency mistuning margin against vibration and fatigue failures caused by rubs. Aerodynamic disturbing forces have also excited flexural vibration and fatigue failure of seals. A w/R stability criterion has proven effective in evaluating the hazard of aeroelastic vibration due to pressure drop across multi-tooth seals. Acoustic oscillations in the annular chamber into which the seal discharges have coupled with the corresponding natural nodal flexural vibration of seal components, which increases nodal flexural vibration of seal components, and increases the hazard of vibration and fatigue failure. Transient thermal instability may occur if the heat generated during rub causes the inner component to grow at a faster rate than outer. Seals must have compatible thermal expansions to maintain close clearances even when engine operating transients are rapid. The design and application of severed damper sleeves and ring to rotor components are effective in protecting air seals and the cylindrical walls of annular chambers aft of seals against flexural vibration and fatigue failures excited by aerodynamic disturbing forces. Since acoustically coupled oscillations persist over relatively long distances and thus involve additional components, severed damper sleeves have proven effective in protecting these thin-walled components excited by aerodynamic disturbing forces.