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Operating Deflection Shapes It is well known that
in the case of many types of variable speed machines, there usually are
certain speeds at which the machine performs poorly, either evidenced by
excessive vibration levels or poor
quality of the product. A good example of this is found in the paper
industry, where production capacity is reduced because the machine may
not safely be operated at certain speeds. The primary reason
for the erratic behaviour of complex machines at different speeds is
that mechanical resonances in the structure are excited when
forcing frequencies approach a
natural frequency of the structure. A
large machine will have a great many modes of
vibration, each one at a particular
natural frequency, and it is usually very difficult to determine
by inspection how and where the structure is moving at any resonant
condition.
In the past, bump testing of machinery has been used to ascertain crude
measures of a machine’s natural frequencies. However, bump tests have
limited capability to precisely determine a machine's resonant
frequencies. Success is governed largely by the skill of the user and
the right combination of structural parameters. Furthermore, bump tests
very often require the machine under test to be off-line, shutting down
production. Operating Deflection
Shape analysis (ODS), is a technique where
vibration measurements are made at many locations on a machine
and transfer functions are calculated between a reference location and
all the other sensor locations. These TRFs contain
phase and
amplitude information about the motion of the machine when it is
running. The operation of the machine itself provides the
excitation forces for the measurement
-- unlike other techniques, no external
excitation is used for ODS. After the
measurements are made, a computer program examines all the data and
produces a series of animated 3-D pictures on the screen that shows the
motion of the machine parts at selected frequencies. The ODS analysis
provides information to the designer about how to modify the structure
to solve the vibration problem by
pointing out the locations and directions in which the excessive motion
is occurring.
These Days
Portable Multichannel Instruments And Associated Windows Based Software,
Makes It Easy To Visualise Machine Movement
In
the past, analysts required extensive knowledge and expertise in
applying complex mathematical Pruftechnik, now
provides an intuitive, easy to use approach to performing Operating
Deflection Shape Analysis. By designing an interface between the
portable data collector / analyser and the advanced, easy to use
operating deflection shape/modal analysis software, which gives every
user the opportunity to perform expert analysis. A user begins the
easy and convenient process simply by selecting the structure to be
analyzed from the software’s extensive template library. Once selected,
the active test points are downloaded into the instrument. After
collecting the data, it is uploaded to the software to animate the
structure for review and analysis. Panning, rotating, and zooming allow you to move the structure for more detailed observations. Colored contour plots, displayed during deflection animation, let you view critical node lines and identify logical options for “What If” structural modification.
Observe, analyse and document the dynamic behavior of a machine, under
varying load conditions
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