Page 77 - Machinery Component Maintenance

62 Machinery Component Maintenance and Repair

check bearing clearance with this type of bearing is by using a mandrel
the size of the journal and a flat plate. The clearance in a sleeve-type
journal bearing can be checked with Plasti-Gage@. Be sure to torque the
bearing cap bolts correctly or you may get a false reading.
When assembling bearings be sure the anti-rotation dowels are in place
and look to be sure.the oil dam (if used in that particular bearing) is in the
correct direction of rotation. Some of these steps will sound obvious, but
each one results from a problem experienced in the field. It is also useful
to check the alignment of oil supply holes in the housing with oil feed
grooves in the bearing. For want of a %-in. groove in the housing of a
replacement bearing, one user lost a high speed shaft and impeller assem-
bly in a plant air compressor package.
Before the upper half of the casing of a horizontally split machine is
bolted in place, a final rotor mid-span bow check is recommended. This
is particularly useful if you, as the responsible engineer on an overhaul,
have not been able to personally witness all rotor movements during the
course of the job.
Coupling hub fit is another area requiring consideration. The assump-
tion that the taper is correct provides a false sense of security. By lightly
bluing the shaft and transferring the bluing to the coupling bore, the fit
can be properly checked. It is prudent to require at least 85 percent con-
tact. If the contact pattern is not acceptable, the question of whether to
lap or not to lap needs to be addressed. We will not lap using the coupling
half for obvious reasons, but will lap using a ring and plug gauge set. The
advance of the coupling on the taper must be correct and should be wit-
nessed and recorded by a knowledgeable individual. Coupling bolts must
be torqued to the coupling manufacturer’s specifications as the clamping
force, not the bolt body, is generally the means of transmitting the
torque.
As the machine goes back together, fill in the information in the critical
dimension diagram. Labyrinth and bearing clearances, total rotor float,
thrust clearance, coupling advance, bolt torque, etc., should all be mea-
sured and logged. Shaft alignment and cold baseline data for comparison
with hot growth data taken after startup should also be logged on the ap-
propriate sheet. Remember to check the shaft end gap, as not all rotors
are created equal and the wrong dimension could damage your coupling.
When leaning into an open machine, it is well to remember to remove all
loose objects from shirt pockets!
There are some other checks which may or may not have been incorpo-
rated in the critical dimension diagram, most notable of them being
whether the rotor is free to turn and whether oil is flowing to and from
the proper places. This latter item can be viewed just prior to bolting
bearing caps or covers in place, assuming the oil lines have been recon-