Page 195 - Machinery Component Maintenance

Machinery Alignment 177

Figure 5-2 can be applied to determine allowable misalignment for ma-
chinery equipped with nonlubricated metal disc and diaphragm cou-
plings, up to perhaps 10,000 rpm. If the machinery is furnished with
gear-type couplings, Figure 5-2 should be used up to 3,600 rpm only. At
speeds higher than 3,600 rpm, gear couplings will tolerate with impunity
only those shaft misalignments which limit the sliding velocity of engag-
ing gear teeth to less than perhaps 120 in. per minute. For gear couplings,
this velocity can be approximated by V = (TDN) tancr, where

D = gear pitch diameter, in.
N = revolutions per minute
2 tan cr = total indicator reading obtained at hub outside diameter, di-
vided by distance between indicator planes on driver and
driven equipment couplings.
Say, for example, we were dealing with a 3560 rpm pump coupled to a
motor driven via a 6-in. pitch diameter gear coupling. We observe a total
indicator reading of 26 mils in the vertical plane and a total indicator
reading of 12 mils in the horizontal plane. The distance between the flex-
ing member of the coupling, i.e., flexing member on driver and flexing
member on driven machine, is 10 in. The total net indicator reading is
[(26)2 + (12)?]''* = 28.6 mils. Tan CY = (1/2)(28.6)/10) = 1.43 milslin., or
0.00143 in./in. The sliding velocity is therefore [(~)(6)(3560)
(0.00143)1 = 96 in. per minute. Since this is below the maximum allowa-
ble sliding velocity of 120 in. per minute, the installation would be within
allowable misalignment.

Choosing an Alignment Measurement Setup
Having taken care of the preliminaries, we are now ready to choose an
alignment setup, or arrangement of measuring instruments. Many such
setups are possible, generally falling into three broad categories: face-
and-rim, reverse-indicator, and face-face-distance. The following
sketches show several of the more common setups, numbered arbitrarily
for ease of future reference. Note that if measurements are taken with
calipers or ID micrometers, it may be necessary to reverse the sign from
that which would apply if dial indicators are used.
Figures 5-3 through 5-8 show several common arrangements of indica-
tors, jigs, etc. Other arrangements are also possible. For example, Fig-
ures 5-3 and 5-4 can be done with jigs, either with or without breaking
the coupling. They can also sometimes be done when no spacer is pre-
sent,by using right-angle indicator extension tips. Figures 5-6 and 5-7
can be set up with both extension arms and indicators on the same side,
(Text wnrinurd on pap 180)