Page 114 - An Introduction To Predictive Maintenance

104 An Introduction to Predictive Maintenance

For example, most of the logic trees state that when the dominant energy contained
in a vibration signature is at the fundamental running speed, then a state of unbalance
exists. Although some forms of unbalance will create this profile, the rules of machine
dynamics clearly indicate that all failure modes on a rotating machine will increase
the amplitude of the fundamental or actual running speed.

Without a thorough understanding of machine dynamics, it is virtually impossible to
accurately diagnose the operating condition of critical plant production systems.
For example, gear manufacturers do not finish the backside (i.e., nondrive side) of
gear teeth. Therefore, any vibration acquired from a gear set when it is braking will
be an order of magnitude higher than when it is operating on the power side of
the gear.

Another example is even more common. Most analysts ignore the effect of load on a
rotating machine. If you were to acquire a vibration reading from a centrifugal com-
pressor when it is operating at full load, it may generate an overall level of 0.1ips-
peak. The same measurement point will generate a reading in excess of 0.4ips-peak
when the compressor is operating at 50 percent load. The difference is the spring con-
stant that is being applied to the rotating element. The spring constant or stiffness at
100 percent load is twice that of that when operating at 50 percent; however, spring
constant is a quadratic function. A reduction of 50 percent in the spring constant will
increase the vibration level by a factor of four.

To achieve maximum benefits from vibration monitoring, the analyst must understand
the limitations of the instrumentation and the basic operating dynamics of machinery.
Without this knowledge, the benefits will be dramatically reduced.

Application Limitations
The greatest mistake made by traditional application of vibration monitoring is in its
application. Most programs limit the use of this predictive maintenance technology to
simple rotating machinery and not to the critical production systems that produce the
plant’s capacity. As a result, the auxiliary equipment is kept in good operating condi-
tion, but the plant’s throughput is unaffected.

Vibration monitoring is not limited to simple rotating equipment. The microproces-
sor-based systems used for vibration analysis can be used effectively on all electro-
mechanical equipment—no matter how complex or what form the mechanical motion
may take. For example, it can be used to analyze hydraulic and pneumatic cylinders
that are purely linear motion. To accomplish this type of analysis, the analyst must
use the time-domain function that is built into these instruments. Proper operation of
cylinders is determined by the time it takes for the cylinder to finish one complete
motion. The time required for the cylinder to extend is shorter than its return stroke.
This is a function of the piston area and inlet pressure. By timing the transient from
fully retracted or extended to the opposite position, the analyst can detect packing
leakage, scored cylinder walls, and other failure modes.