Page 268 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 253

It is vital to install bearings correctly. Improper installation is the most
common cause of bearing failure. Tightening bearings for correct crash is
of prime importance; otherwise, bearings may come loose and fail from
constant pounding action or poor heat transfer. Check with the bearing
supplier for more specific information on bearing crush.
We've already commented on the importance of determining the cause
and extent of damage when a bearing fails. Damaged or failed main bear-
ings or rod bearings should prompt checks of the crankshaft, main bear-
ing caps, bedplate saddle bores, and connecting rod bearing bores.

CHECK TOP CLEARANCE FOR MAIN BEARING ALIGNMENT

The clearance between the upper main bearing shells and main bearing
journals gives preliminary indications of bedplate alignment. All top
clearances should be within approximately .001 inch of one another (see
Figure 4-13). A tight top clearance may indicate a low saddle. If so, jack
down on the crankshaft and check the top clearance for settling.

CHECK CRANKSHAFT FOR DEFLECTION AND RUNOUT

Deflection readings give some indication of whether the crankshaft is
straight, properly supported and running in correct alignment. Deflection
is normally tested with a deflection gauge. Deflection gauges measure
changes in the spread between adjacent crank webs while the crank is
rotated (see Figure 4-4 and 4-6).
Whenever a main bearing fails or any time bearings are replaced, it is
vitally important to check both runout and deflection. Reading deflection
alone does not give the full picture because zero deflection is no guaran-
tee that the crankshaft is straight. Actually, some nominal deflection on a

s / \^ -*=: »•- / \.^\.
/ / / ^ — --^A N >v FIGURE 4-13. Compressor main
bearing.