Page 327 - Intro Predictive Maintenance

318 An Introduction to Predictive Maintenance

Unless there is a damaged bearing, all of the profiles contain low-level roll frequen-
cies (1¥) and bearing rotational frequencies.

The top roll set is also properly aligned, but the strip tracks to the bottom of the roll
face. In this case, the vibration profile from all of the bottom bearing caps contain much
lower-level broadband energy, and the top bearing caps have clear indications of
mechanical looseness (i.e., multiple harmonics of rotating speed). The key to this type
of analysis is the comparison of multiple rolls in the order that the strip connects them.
This requires comparison of both top and bottom rolls in the order of strip pass. With
proper tracking, all bearing caps should be nearly identical. If the strip tracks to one
side of the roll face, all bearing caps on that side of the line will have similar profiles,
but they will have radically different profiles compared to those on the opposite side.

Roll misalignment. Roll misalignment can be detected and isolated using this same
method. A misaligned roll in the series being evaluated causes a change in the strip
track at the offending roll. The vibration profiles of rolls upstream of the misaligned
roll will be identical on both the operator and drive sides of the rolls; however, the
profiles from the bearings of the misaligned roll will show a change. In most cases,
they will show traditional misalignment (i.e., 1¥ and 2¥ components) but will also
indicate a change in the uniform loading of the roll face. In other words, the overall
or broadband vibration levels will be greater on one side than the other. The lower
readings will be on the side with the higher strip tension, and the higher readings will
be on the side with less tension.

The rolls following the misalignment also show a change in vibration pattern. Because
the misaligned roll acts as a steering roll, the loading patterns on the subsequent rolls
show different vibration levels when the operator and drive sides are compared. If the
strip track was normal before the misaligned roll, the subsequent rolls will indicate
off-center tracking. In those cases where the strip was already tracking off-center, a
misaligned roll either improves or amplifies the tracking problem. If the misaligned
roll forces the strip toward the centerline, tracking improves and the vibration profiles
are more uniform on both sides. If the misaligned roll forces the strip farther off-center,
the nonuniform vibration profiles will become even less uniform.

14.2.7 Shaft
A bent shaft creates an imbalance or a misaligned condition within a machine-train.
Normally, this condition excites the fundamental (1¥) and secondary (2¥) running-
speed components in the signature; however, it is difficult to determine the difference
between a bent shaft, misalignment, and imbalance without a visual inspection.
Figures 14–32 and 14–33 illustrate the normal types of bent shafts and the force pro-
files that result.

14.2.8 V-Belts
V-belt drives generate a series of dynamic forces, and vibrations result from these
forces. Frequency components of such a drive can be attributed to belts and sheaves.