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sensors that,.as the name implies, provide an electrical output proportional to the acceleration of
the vibration of the machine being checked. Although accelerometers are available that can
measure low frequency vibration, (less than 5 Hertz [Hz]) they are primarily used for higher
frequency vibrations such as 1,800-revolutions-per-minute (rpm) electric motors. Accelerometers
are generally ineffective for use on slow speed equipment such as hydroelectric units as the
primary frequencies are low and the critical measurement is displacement.
the vibration of the machine being checked. Although accelerometers are available that can
measure low frequency vibration, (less than 5 Hertz [Hz]) they are primarily used for higher
frequency vibrations such as 1,800-revolutions-per-minute (rpm) electric motors. Accelerometers
are generally ineffective for use on slow speed equipment such as hydroelectric units as the
primary frequencies are low and the critical measurement is displacement.
Depending on the system, accelerometers may be permanently mounted, handheld, or attached
with a magnetic base. A typical accelerometer system requires periodic readings to be taken at
different points on each machine. The data from these readings may be stored in a portable
recording instrument, plotted directly on what is known as a signature card, or displayed on a
digital readout to be manually recorded. This data must then be analyzed and compared to
previous readings to determine if there is a significant increase in the vibration levels indicating an
impending failure.
with a magnetic base. A typical accelerometer system requires periodic readings to be taken at
different points on each machine. The data from these readings may be stored in a portable
recording instrument, plotted directly on what is known as a signature card, or displayed on a
digital readout to be manually recorded. This data must then be analyzed and compared to
previous readings to determine if there is a significant increase in the vibration levels indicating an
impending failure.
Signature Analysis.
A common means of analyzing vibration data is through the use of a
vibration frequency, usually in cycles per minute or cycles per second (Hertz), and the Y axis
represents vibration amplitude in acceleration, velocity, or displacement units. A spectrum plot
features amplitude spikes or peaks corresponding to operating frequencies of components of the
equipment being tested. The initial plot provides a "signature" of the vibration for that particular
piece of equipment. An increase in the amplitude of vibration at any of the various frequencies or
the appearance of a new spike in subsequent plots may indicate an operational problem or
impending failure.
represents vibration amplitude in acceleration, velocity, or displacement units. A spectrum plot
features amplitude spikes or peaks corresponding to operating frequencies of components of the
equipment being tested. The initial plot provides a "signature" of the vibration for that particular
piece of equipment. An increase in the amplitude of vibration at any of the various frequencies or
the appearance of a new spike in subsequent plots may indicate an operational problem or
impending failure.
A signature analysis program can help in scheduling outages for bearing replacement on small
motors and pumps. The amplitude of vibration at the bearing pass frequency will increase as a
antifriction bearing starts to fail. Signature analysis is also a good tool for hydroelectric units.
Spectrum plots from proximity probes at each of the guide bearings can be used to diagnose
problems such as misalignment, unbalance, or draft tube surging. To be effective with
hydroelectric units, spectrum plots should be taken frequently, as vibration levels will vary with
the water level of the forebay and tailrace. Subsequent readings can then be compared to readings
under the same operating conditions.
motors and pumps. The amplitude of vibration at the bearing pass frequency will increase as a
antifriction bearing starts to fail. Signature analysis is also a good tool for hydroelectric units.
Spectrum plots from proximity probes at each of the guide bearings can be used to diagnose
problems such as misalignment, unbalance, or draft tube surging. To be effective with
hydroelectric units, spectrum plots should be taken frequently, as vibration levels will vary with
the water level of the forebay and tailrace. Subsequent readings can then be compared to readings
under the same operating conditions.
In order to perform vibration analysis, a basic understanding of the characteristics of machine
vibration and some knowledge of use of the test equipment is required. Training is available from
many of the manufacturers of vibration monitoring systems.
vibration and some knowledge of use of the test equipment is required. Training is available from
many of the manufacturers of vibration monitoring systems.
2.10 Oil and Lubricants
Lubrication.
The primary purpose of a lubricant is to reduce friction and wear between two
moving surfaces, but a lubricant also acts as a coolant, prevents corrosion, and seals out dirt and
other contaminants. In order for a lubricant to perform as intended, careful attention must be
other contaminants. In order for a lubricant to perform as intended, careful attention must be