Page 217 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating Gas Engines and Compressors 199

As each door is removed, its back should be inspected for foreign ma-
terial thrown there by centrifugal force of the connecting rod. Bronze
cuttings from a faulty wrist-pin bushing will adhere to the door. The same
is true for babbitt from bearings and cast iron from liners, pistons, etc.
The walls of the crankcase as well as the bottom should also be scruti-
nized for particles of those metals. The condition of the oil can be
checked by looking for lacquer formations on machined surfaces or de-
posits of sludge that could come from trouble with valves, rings, or pis-
tons. All nuts and bolts should be tapped with a hammer for the familiar
ring common to tightness. Each piston should be moved to top center and
the liner checked for scuffing.
Generally, two mating parts that have had a tendency to seize while in
operation will generate enough local heat to discolor the casting support-
ing them. This is particularly true of main and connecting rod bearing
caps or the wrist-pin end of the connecting rod. Consequently, the entire
crankcase should be observed for any blue discoloring, and if any is
found it should be thoroughly investigated. Any parts that have been hot
enough to become discolored will normally be warped, cracked, or both.
Therefore, they should be Magnafluxed or dye-checked for cracks. Con-
necting rods and main bearing saddles should be measured for warpage.
The inspection outlined so far has not consumed any more time than it
takes to look at every square inch of the crankcase. The operator should
make these observations every time a door is removed and certainly at
intervals of not more than every three months. At all intervening crank-
case inspections, the main and connecting rod bearings should be
checked for clearances. As the following explanation will show, these
bearings do not have to be dismantled for this check, but it will take at
least two hours to complete, depending on the size of the engine.

Determining Bearing Clearances

The crankshaft web deflection test and crankcase inspection are good
indicators of main bearing condition, but they must be supplemented by a
clearance check.
Excessive clearance in all main bearings, which could be caused by
abrasives in the oil, may not show up in web deflection tests. Excessive
clearance should not be ignored in any engine, but it is less dangerous in
a two-cycle engine than a four-cycle engine.
The method of determining the clearance is very controversial. Al-
though many people use the lead wire method, this method is not recom-
mended due to two main factors: lead wire expands after removal from
the cap, or the wire can become embedded in the babbitt, especially in
soft high-lead-base babbitt bearings.
~