Handbook of Turbomachinery Second Edition Revised - Ventech!

DESIGN STRATEGIES AND PROCEDURES
Rotordynamic system response presents the following concerns: generated
noise, rotor deflection at close clearance components (e.g., seals, rotor-tostructure
rubs and rotor-to-rotor rubs), lateral bearing loads, and support
structure loads. All of these are concerns associated with the overall goal of
minimizing the response of a rotor-bearing system for both synchronous
and nonsynchronous whirl. The largest responses usually occur when a
forcing function acting on the system, e.g., mass unbalance, excites a natural
frequency with its inherent dynamic magnification. Strategies for designing
rotor-bearing systems with acceptable rotordynamic response characteristics
thus focus on the critical speeds and associated mode shapes.
For most rotor systems the preferred design sequence to minimize
dynamic response is
1. Size the system, whenever possible, so the natural frequencies are
outside the machine’s operating range.
2. Introduce damping if design constraints place natural frequencies
within the operating range.
3. Reduce the excitation levels acting on the system.
Designing the system so the natural frequencies are outside the operating
range is an obvious benefit to improving rotor dynamic response
characteristics. This strategy, however, can place severe constraints on the
key design parameters perhaps requiring unrealistically stiff bearings and/or
support structure, large shaft diameters, short bearing spans, etc. Larger
shaft diameters naturally lead to larger diameter bearings, seals, compressors,
and turbines. These larger components then limit the maximum
allowable speed for overall system durability. The introduction of damping
to minimize response near a system natural frequency is typically
accomplished using an oil film between a bearing outer ring and the
support structure. These ‘‘squeeze film dampers’’ can be quite effective in
adding the beneficial external damping to the rotor provided that the
excitation does not drive the oil film beyond its useful linear range. The
excitations acting on the system should obviously be minimized (e.g.,
balancing the individual components) whenever possible even if the natural
frequencies are outside the operating range. This method of controlling
response, however, typically adds significantly to the manufacturing cost.
Straddle-Mounted Rotor: Disks Inside the Bearings
Consider, as an illustrative example, a straddle-mounted rotor-bearing
system: 20.0-in. (50.8-cm) steel shaft with a 0.70-in. (1.78-cm) inner radius
Copyright © 2003 Marcel Dekker, Inc.