Handbook of Turbomachinery Second Edition Revised - Ventech!

earing supports are reduced. The third critical speed is no longer a pure
bending mode since the forward bearing possesses a significant percentage of
the total strain energy.
A single overhung configuration, as discussed here, has a packaging
advantage in that it allows both bearings to reside in a single sump;
however, this design configuration presents several rotordynamic challenges.
For comparable support stiffness, external damping, and mass eccentricity,
overhung disks will have larger response at the ‘‘cylindrical’’ mode than a
straddle-mounted configuration. Larger response at the disks increases the
probability of encountering nonlinear behavior, possibly resulting in rotor
instability. The unbalance response at the ‘‘conical’’ mode will be very
sensitive to an out-of-phase unbalance distribution of the disks.
Bearing Mount Considerations
Several factors should be considered if a bearing outer ring is ‘‘hardmounted’’
to the bearing support structure:
1. The bearing internal clearance must be properly accounted for in
any unbalance response calculations by using the secant stiffness
of the bearing’s nonlinear load-deflection relationship [6].
2. Between rest speed and approximately 50% of the critical speed,
the dynamic magnification is small and the bearing load is almost
equal to the mass unbalance force ðmeo 2 Þ.
3. The motion of the bearing support structure will essentially be the
same as the rotor whirl at that location.
A soft connection between the rotor and the bearing support structure
allows the rotor to whirl about its mass center without imparting that
motion to the support structure and case. Reducing the transmissibility
via a soft rotor–structure interface, e.g., a squeeze film damper, can thus
effectively isolate the structure from the rotor motion.
Blade-Out
Most turbomachinery must be designed to shut down safely in the event of
failure/loss of an entire blade. For a propulsion engine, a safe shutdown
implies that an engine blade-out unbalance response does not adversely
affect the aircraft other than the loss of the engine’s thrust. The engine
mounts must not fail and damage to the rotating and static components
must not result in a fire. To maintain rotordynamic integrity, sufficient axial
clamp-load must be maintained on the rotating components to overcome
Copyright © 2003 Marcel Dekker, Inc.