Page 98 - An Introduction To Predictive Maintenance

88 An Introduction to Predictive Maintenance

Helical Gear

Figure 5–7 Internal bullgear drive’s pinion gears at each stage.

Crankshaft frequencies. All reciprocating compressors have one or more crank-
shaft(s) that provide the motive power to a series of pistons, which are attached
by piston arms. These crankshafts rotate in the same manner as the shaft in a cen-
trifugal machine; however, their dynamics are somewhat different. The crankshafts
generate all of the normal frequencies of a rotating shaft (i.e., running speed,
harmonics of running speed, and bearing frequencies), but the amplitudes are much
higher.

In addition, the relationship of the fundamental (1X) frequency and its harmonics
changes. In a normal rotating machine, the 1X frequency normally contains between
60 and 70 percent of the overall, or broadband, energy generated by the machine-train.
In reciprocating machines, however, this profile changes. Two-cycle reciprocating
machines, such as single-action compressors, generate a high second harmonic (2X)
and multiples of the second harmonic. While the fundamental (1X) is clearly present,
it is at a much lower level.

Frequency shift caused by pistons. The shift in vibration profile is the result of
the linear motion of the pistons used to provide compression of the air or gas. As
each piston moves through a complete cycle, it must change direction two times.
This reversal of direction generates the higher second harmonic (2X) frequency
component.