REPAIRING CRANKSHAFTS AND JOURNAL BEARINGS
REPAIRING CRANKSHAFTS AND JOURNAL BEARINGS
The repair of crankshafts and bearings varies depending on the extent of damage. There is no doubt about the necessity for replacing such items as broken or bent crankshafts. Out-of-round journals may be reground and undersize bearing shells may be installed, but this requires personnel skilled in the use of precision tools. If a new shaft is available, it should be installed and the damaged shaft should be sent to a salvage reclamation center. Under certain conditions, scored crankshaft journals or damaged journal bearings may be kept in service if proper repair is performed. Repair of SCORED JOURNALS depends on the extent of scoring. If a crankshaft has been overheated, the effect of the original heat treatment will have been destroyed. In this case, the crankshaft should be replaced.
If journal scoring is only slight, you can use an oilstone for dressing purposes if you take precautionary measures with respect to abrasives during the procedure. During the dressing operation, plug all oil passages within the journal and those connecting the mainbearing journal and the adjacent connecting rod journal. In the dressing procedure, use a fine oilstone, followed with crocus cloth, to polish the surface. After dressing journals, always wash them with diesel oil. This procedure must include washing the internal oil passages as well as the outside journal surfaces. Some passages are large enough to accommodate a cleaning brush; smaller passages can be cleaned by blowing them out with compressed air. Always dry the passages by blowing compressed air through them. NEVER STOW A CRANKSHAFT OR BEARING PART ON ANY METEL SURFACE. When you remove a shaft from an engine, place it on a wooden plank with
—Using a strain or deflection gauge between crank webs.
all journal surfaces protected. If the shaft is to be exposed for some time, protect each journal surface with a coating of heavy grease. Always place bearings on wooden boards or clean cloths.
CRANKSHAFT overhaul consists of an inspection, servicing for scoring and wear, and a determination of each crank web deflection. Take crank web deflection readings according to the Planned Maintenance System (PMS).
A strain gauge, often called a crank web deflection indicator, is used to take deflection readings. The gauge is merely a dial-reading inside micrometer used to measure the variation in the distance between adjacent crank webs as the engine shaft is barred over. Figure 3-29 shows a strain gauge between crank webs. When you install the gauge, or indicator, between the webs of a crank throw, be sure to place the gauge as far as possible from the axis of the connecting rod journal. Rest the ends of the indicator in prick-punch marks in the crank webs. If these marks are not present, make them so that the indicator can be placed in its correct position. Consult the manufacturer’s technical manual for the proper location of new marks.
Readings are generally taken at the four crank positions: top dead center, inboard, near or at bottom dead center, and outboard. In some engines, it is possible to take readings at bottom dead center. In others, the connecting rod may interfere, making it necessary to take the reading as near as possible to bottom dead center without having the gauge come in contact with the connecting rod. When the gauge is in its lowest position, the dial will be upside down, making it necessary to use a mirror and flashlight to obtain a reading.
NOTE: Once you have placed the indicator in position for the first deflection reading, do not touch the gauge until you have taken and recorded all four readings.
Deflection readings are also used to determine correct alignment between the engine and the generator or between the engine and the coupling. However, when determining alignment, you should take a set of deflection readings at the crank nearest the generator or the coupling. In aligning an engine and generator, you may need to install new chocks between the generator and its base to bring the deflection within the allowable value. You may also need to shift the generator horizontally to obtain proper alignment. To align an engine and a coupling, first, correctly align the coupling with the drive shaft; then, properly align the engine to the coupling, rather than aligning the coupling to the engine.
