Vibration and Force Signatures of Overhung Rotor
Rotating Machine with Unknown Initial Conditions
Eric Li, Suri Ganeriwala,
Zhuang Li
SpectraQuest Inc., 8201 Hermitage Road, Richmond, VA 23228
Published: July 2008
Abstract
Rotating
machinery with overhung rotors is very common in industry.
Unbalanced rotor and misaligned shafts usually causes
excessive machine vibration, generates large forces on bearings and
thus reduces the machine life span and may lead to property loss
and
even human life loss. Two-plane balancing of overhung rotors is one
of the most challenge problems maintenance engineers may encounter.
As a prerequisite, Successful diagnosis of unbalanced overhung rotor
system must be performed. The vibration and force signatures of
unbalanced overhung rotors with unknown initial conditions are
different from those of the systems with center hung rotors and have
been studied in this tech note. Experiments were carried out on a
Machinery Fault Simulator(MFS) for both balanced
and unbalanced rotor systems. The data were analyzed using the
VibraQuestTM software and efforts were
focused on identifying the system characteristic
signatures.
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Introduction
Unbalance is one of the most common reasons of
excessive machine vibration. It has been said that 80% of the
vibration problems of the rotating machinery can be solved by
balancing and alignments. A small amount of unbalance weight in
rotating system may have devastating effects as the system is
operating with high speed or running near the critical speed.
Therefore, extreme care must be taken in balancing high speed
rotating system to avoid any potential damages. Overhung rotors are
commonly used in fluid turbo-machinery, such as pumps, propellers
and fans. The vibration and force signatures of the overhung rotors
are different from that of the center hung rotors, which have been
well studied.
The vibration and force signatures of unbalanced
overhung rotor can be simply explained as below: the unbalance
forces will cause the bearing to move in a circle in its own plane.
when the force is upward on the outboard bearing, the tilting of the
bearing will be away from the rotor, so the axial phase at the top
of the bearing will be out of phase with the vertical. At the
inboard bearing (the one away from the overhung rotor), the
situation is reversed relative to the outboard bearing at any given
time. When the rotor is forcing the inboard bearing upward, it will
force the outboard bearing downward, so the vertical phases will be
180 degrees apart between the two bearings. Also, the bending of the
shaft between the bearings will causes the inboard bearing to tilt
toward the rotor, opposite to the tilt of the outboard bearing.
Therefore, the axial phase at the top of the inboard bearing will be
out of phase with the vertical.
Fig 1: Configuration of Single Plane Overhung Rotor
Fig 2:
Dimensions of the
Test Rig (a) one rotor overhung (b) one centered and one overhung
(Blue: Triaxial accelerometer; Red: Force transducer)
Fig 3: 1X spectrum magnitude of the
bearing housing vertical force
as function of shaft speed and
unbalance weight: outboard
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