Page 134 - An Introduction To Predictive Maintenance

124 An Introduction to Predictive Maintenance

monic (2X), and third harmonic (3X) of the shaft’s true running speed. In addition,
this movement increases the axial amplitude of the fundamental (1X) frequency.

Forces resulting from air or liquid movement through a machine also generate unique
frequency components within the machine’s signature. In relatively stable or laminar-
flow applications, the movement of product through the machine slightly increases the
amplitude at the vane- or blade-pass frequency. In more severe, turbulent-flow appli-
cations, the flow of product generates a broadband, white-noise profile that can be
directly attributed to the movement of product through the machine.

Other forces, such as the sideload created by V-belt drives, also generate unique fre-
quencies or modify existing component frequencies. For example, excessive belt
tension increases the sideload on the machine-train’s shafts. This increase in sideload
changes the load zone in the machine’s bearings. The result of this change is a marked
increase in the amplitude at the outer-race rotational frequency of the bearings.

Applied force or induced loads can also displace the shafts in a machine-train. As a
result the machine’s shaft will rotate off-center, which dramatically increases the
amplitude at the fundamental (1X) frequency of the machine.

7.3.2 Reciprocating and/or Linear-Motion Machinery
This section describes machinery that exhibits reciprocating and/or linear motion(s)
and discusses typical vibration behavior for these types of machines.

Machine Descriptions
Reciprocating linear-motion machines incorporate components that move linearly in
a reciprocating fashion to perform work. Such reciprocating machines are bidirec-
tional in that the linear movement reverses, returning to the initial position with each
completed cycle of operation. Nonreciprocating linear-motion machines incorporate
components that also generate work in a straight line but do not reverse direction
within one complete cycle of operation.

Few machines involve linear reciprocating motion exclusively. Most incorporate a
combination of rotating and reciprocating linear motions to produce work. One
example of such a machine is a reciprocating compressor. This unit contains a rotat-
ing crankshaft that transmits power to one or more reciprocating pistons, which move
linearly in performing the work required to compress the media.

Sources of Vibration
Like rotating machinery, the vibration profile generated by reciprocating and/or linear-
motion machines is the result of mechanical movement and forces generated by the
components that are part of the machine. Vibration profiles generated by most recip-
rocating and/or linear-motion machines reflect a combination of rotating and/or linear-