Simulation and Vibration Analysis of Shaft Cracks
Fei Xie, Lin
Liu,
Suri
Ganeriwala
SpectraQuest Inc., 8201 Hermitage Road, Richmond, VA 23228
Published: April 2007
Abstract
A Shaft
crack is one of the most common defects in a rotor system and
detection of such shaft crack is a very serious matter. In this
study, shaft cracks were simulated and analyzed using SpectraQuest’s
rotor Machinery Fault SimulatorTM (MFS). A series of experiments
were conducted to observe the behavioral changes of the cracked
shaft in critical speed, 1X and 2X frequency responses. The
experimental results were found to be consistent with the
theoretical prediction of the shaft crack.
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Introduction
A shaft crack is a slowly growing fracture of the
rotor. If undetected in an operating machine, as a crack grows, the
reduced cross section of the rotor will not able to withstand the
dynamic loads applied to it. When this happens, the rotor will fail
in a fast brittle fracture mode. The sudden failure releases a large
amount of energy that is stored in the rotating system, and the
rotor will fly apart. This kind of failure may cause serious injury
or even death to anyone unfortunately standing near the machine at
that moment. Obviously, shaft crack detection is a very serious
matter, and machines that are suspected of having a crack must be
treated with the utmost caution.
Cracks are initiated in the shaft in regions of
high local stress. Shafts are subjected to large-scale stresses due
to bending, torsion, static radial loads, constrained thermal bows,
thermal shock, and residual stresses from heat treatment, welding
and machine operations. All of these stresses combine to produce a
local stress field that changes periodically. In a small, local
region where stresses exceed the maximum that the material can
withstand, a crack will form in the material.
If the cyclic stresses are sufficiently high, the
leading edge of the crack will slowly propagate so that the plane of
the crack is perpendicular to the orientation of the tensile stress
field. The orientation of this stress field is determined by the
type of stress (bending or torsional) and by geometric factors. If
the rotor is subjected only to simple bending stresses, then the
stress field will be oriented along the long axis of the rotor, and
the crack will propagate directly into and across the rotor section,
forming a transverse crack. The pure torsional stress will produce a
tensile stress field that is oriented at 45° relative to the shaft
axis. A crack in this stress field will propagate into the rotor and
tend to form a spiral on the shaft surface. Bending stress, however,
is usually the dominant component, thus the crack will usually
propagated into the rotor more or less as a transverse crack.
Fig 1: Changes of
critical speeds as the crack conditions change
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