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70 Gas Turbine Handbook: Principles and Practices
Squealer Cut—a change in blade length from the convex surface to
the concave surface, thus allowing a smaller portion of the blade
tip to rub into the tip shroud. This design also serves to seal
against tip leakage.
Lacing Wire—used to dampen blade vibration is usually installed at
or near the mid-point of the airfoil (Figure 3-7).
Turbine blades and vanes tend to change pitch with continued
use, straightening toward low pitch. Blades also undergo distortion
and thinning and lengthening of the airfoil. This thinning and lengthening
of the blade is known as “ creep.” This condition is cumulative.
The rate of creep is determined by the load imposed on the turbine
and the strength of the blade, which changes as a function of the
temperature within the turbine. Creep also affects the stator vane.
However, since the vane is held in place by the inner and outer platforms,
the distortion takes the form of bowing. This is almost always
accompanied by cracking of the airfoil.
References
1. “Gas Turbine Analysis And Practice,” Jennings and Rogers, McGraw-
Hill, 1953.
2. “Gas Turbine Engine Parameter Interrelationships,” Louis Urban, Hamilton
Standard, 1969.