Page 250 - Machinery Component Maintenance

232 Machinery Component Maintenance and Repair

during the measurement, leading to inconsistencies. Measurement of
torque and hydraulic effects will also be absent by this method. Some
training courses advocate this technique, but we do not. If used, how-
ever, three sets of data should be taken, at close time intervals-not two
sets as some texts recommend. The cooling, hence shrinkage, occurs at a
variable rate, and three points are required to establish a curve for back-
ward graphing.

Make Mechanical Measurements of Machine Housing Growth During Opera-
tion, Referenced to Baseplate or Foundation, or Between Machine Ele-
ments. This method can be used for machines with any type of coupling,
including continuous-lube. Essingefl describes one variation, using base-
plate or foundation reference points, and measurement between these and
bearing housing via a long stroke indicator having Invar 36 extensions
subject to minimum expansion-contraction error. Hot and cold data are
taken, and a simple graphic triangulation method gives vertical and hori-
zontal growth at each plane of measurement. This method is easy to use,
where physical obstructions do not prevent its use. Bear in mind that base
plate thermal distortion may affect results. It is reasonably accurate, ex-
cept for some machines on long, elevated foundations, where errors can
occur due to unequal growth along the foundation length. In such cases,
it may be possible to apply Essinger’s method between machine cases,
without using foundation reference points. A further variation is to fabri-
cate brackets between machine housings and use a reverse-indicator
setup, except that dial calipers may be better than regular dial indicators
which would be bothered by vibration and bumping.

Same as Previous Paragraph Except Use Eddy Current Shaft Proximity
Probes as the Measuring Elements, with Electronic Indicating andlor Re-
cording. Of all the various methods, this one lends itself the best to keep-
ing a continuous record of machine growth from startup to stabilized op-
eration. Due to the complexity and cost of the instrumentation and its
application, this technique is usually reserved for the larger, more com-
plex machinery trains. Judging by published data, the method gives
good results, but it is not the sort of thing that the average mechanic
could be fully responsible for, nor would it normally be justified for an
average, two-element machinery train. In some cases, high machine tem-
peratures can prevent the use of this method. The Dodd bars offer the
advantage over the Jackson method that cooled posts are not needed and
thermal distortion of base plate does not affect results. The Indikon sys-
tem also has these advantages, and in addition can be used on unlimited
axial spans. It is, however, more difficult to retrofit to an existing ma-
chine.