Where does vibration come from? What causes it? Any force changing in direction, changing in magnitude or any active component interaction generating a rub even as minimal as friction within rolling element bearings or moving fluids within pipes will cause vibration movement of some small to intolerable amplitude. Any combination of the previous would generate more complex vibration patterns. In practical terms, this means a long list of potential sources whenever an analyst tries to quantify and/or decompose machinery and structure behavior.
The (presumably, unless something escapes this writer) full list breaks down into:
Rotor related sources (rotor behavior and forces at play)
- unbalance
- static or force, couple, dynamic or combined
- misalignment, residual, growth-related or dynamic (reactionary)
- angular, parallel and combined
- bent shaft
- rubs
- macro rub
- rotor-casing
- rotor-product or moved fluid (when dense or solid)
- shaft-seal
- shaft-sleeve bearing
- gear coupling-jaws
- improperly lubricated gears
- micro rub
- belt-pulley
- sliding (instead of rolling) elements
- rolling elements themselves onto raceways (through lubricant film)
- pumping action or film-creation action within the bearing
- rolling elements and retaining cage
- fluid and casing or piping
- eccentricity
- whirl (of shaft or of the shaft within a fluid-film bearing) and whip
- gyroscopic effects (fairly common in inertia wheels)
- torsional vibration
Process related sources (what the machine does)
- hydraulic or aerodynamic forces
- blade / vane pass (BPF) or blade rate (BRF) vibration
- eccentricity relative to casing
- rotor mesh (lobes or screws)
- cavitation, recirculation, turbulence, stonewalling, surging, stalling
- compressed fluid wave or pulse (pipes, etc), turbulent flow
- reciprocating forces (can come close to shock)
- shock (presses, etc)
Drive related sources
Electromagnetic torque pulses from a motor and the interaction between armature and stator components a.k.a. rotor-bar and slot pass frequency
Pulses used to recreate Variable Frequency altered electrical line frequency
Pole or coil pass frequency
Magnetic centering
Coupling problems (lock-up, wear, etc)
Belt or chain drive w. eccentricity, belt problems, belt slip, cogs
Gear mesh and gear problem related vibration
Couple transmission discontinuities or variability from load transmission from one tooth to the next (simplified to picture the matter)
Hydraulic or aerodynamic forces from gas, steam or water turbines
BPF (Blade Pass Frequency) from turbines
BRF (Blade Rate) interaction between blades and stationary elements
Reciprocating forces from engines, unbalanced cam action
Other factors may cause the appearance or perception of an excessive vibration problem, but they are REACTIONS to one or more of the above sources as opposed to being themselves a source of vibration. The most easily recognized of these are looseness and resonance. Critical speeds of a rotor could be assigned to resonance for purposes of this listing. Oil whirl on sleeve bearings also fit into this category: it is an instability reaction due to lack of static load or dynamic loads overcoming static load. In this latter case, increased tolerances facilitate overcoming the static load. From the point of view of reactions, we can also add:
Reactive Problems
- dynamic misalignment (occurring from loads moving component positions)
- resonance
- looseness (really a change in response due to lessened stiffness)
- instability (rotor, hydraulic, oil film, other)
- base and retaining / supporting mechanism or structural problems
- distortion from pipe strain
- transmitted vibration (structure borne transmission from other nearby machines)
- acoustic excitation (airborne or structure borne noise exciting a plate, casing or other component; structure borne usually or closely falls under transmitted vibration)
© 2005, 2006, 2007, 2008 by François Gagnon