The crankshaft is forged of chrome-alloy steel, heat-treated to ensure strength and durability. The main and connecting rod bearing journal surfaces and fillets are induction hardened.
The crankshaft assembly also includes counterweights attached to the main bearing lobes. Crankshaft end play is controlled by thrust washer bearing set installed on the No. 4 main bearing shell. See Figure
1.Pulse Wheel (DDEC III)
2.Crankshaft Timing Gear
3.No. 4 Thrust Bearing Shell
Full pressure lubrication to all connecting rod and main bearings is provided by drilled passages inside the crankshaft and cylinder block. See Figure
Crankshaft Lubricating Oil Holes
Ten tapped holes, equally spaced, are provided on the rear butt for attaching the flywheel. Eight tapped holes, equally spaced, are also provided at the opposite end for attaching the crankshaft pulley and vibration damper.
Repair or Replacement of Crankshaft
To determine if repair is possible or replacement is necessary, perform the following procedure. See Figure
If the crankshaft is worn beyond acceptable limits, the crankshaft must be replaced.
Inspection of Crankshaft for Cracks
Carefully check the crankshaft for cracks that start at an oil hole and follow the journal surface at an angle of 45 to the axis. Any crankshaft with such cracks must be replaced. Several methods for locating minute cracks not visible to the eye are outlined below.
Magnetic particle method
Magnetize the crankshaft.
Cover it with a fine magnetic powder or solution. Flaws, such as cracks, form a small local magnet that draws the magnetic particles in the powder or solution to itself, effectively marking the flaw.
Demagnetize the crankshaft after completing the test.
Fluorescent magnetic particle method
Cover the crankshaft with fluorescent magnetic particles
Examine the crankshaft under a black light. Fine cracks that may have been missed using non-fluorescent magnetic particles will be disclosed under a black light.
Fluorescent penetrant method
Apply fluorescent liquid penetrant to the crankshaft.
Remove excess penetrant from the surface and dry the part.
Apply a developing powder to help draw the penetrant out of the flaws by capillary action.
Inspect the crankshaft under a black light.
The majority of flaws revealed by the preceding inspection methods are normal and typically do not impair crankshaft reliability. Nonetheless, the flaws revealed must be carefully evaluated to determine whether they are serious.
Crankshaft failures are rare. When one cracks or breaks completely, it is important to isolate contributory factors. Unless these factors are discovered and eliminated, crankshaft failure is likely to reoccur.
Normal service imposes two types of loads on a crankshaft: bending force and twisting force. The crankshaft is designed to ensure that these forces produce practically no stress over most of the surface. Certain areas, designated as critical areas, sustain most of the load. See Figure
Critical Crankshaft Loading Zones
Inspection of Crankshaft for Bending Fatigue
Crankshaft bending can result from a load pulling on the crankshaft.
The crankshaft is supported between each cylinder by a main bearing. The load imposed by gas pressure on top of the piston is divided between the adjacent bearings. Abnormal bending stress in the crankshaft, particularly in the crank fillet, may result from misalignment of the main bearing bores, improperly fitted bearings, bearing failures, a loose or broken bearing cap, or unbalanced pulleys. Overtightened drive belts will impose a bending load on the crankshaft.
Failures that result from bending start at the pin fillet and progress throughout the crank cheek, at times extending into the main journal fillet. If main bearings require replacement because of one or more badly damaged bearings, make a careful inspection to determine if the crankshaft itself has started to crack. Cracks, if they occur, are most likely on each side of the damaged bearing.
Inspection of Crankshaft for Torsional Fatigue
High frequency torsional vibration may cause crankshaft failures. A combination of abnormal speed and load conditions may create twisting forces which, in turn, create torsional vibrations that impose stress at critical locations.
Torsional stresses may produce fractures in either the connecting rod journal or the back of the crank cheek. Connecting rod journal failures are usually at the fillet or oil hole at 45 to the shaft axis.
A loose or defective vibration damper, a loose flywheel, or the introduction of improper or additional pulleys or couplings are typical causes of torsional failures. Overspeeding the engine, or overriding the engine electronic controls to permit overspeeding, may also contribute.
Inspection of Crankshaft for Crankshaft Cracks
Two critical faults include circumferential fillet cracks and cracks 45 to the shaft axis, starting from either the fillet locations or the connecting rod journal holes. See Figure
. If this kind of crack is discovered, replace the crankshaft.
Crankshaft Fatigue Cracks
Installation of Crankshaft
Before installing a new crankshaft, steam clean it to remove rust preventive coating.
To avoid injury from flying debris when using compressed air, wear adequate eye protection (face shield or safety goggles) and do not exceed 40 psi (276 kPa) air pressure.
Blow out oil passages with compressed air. Install the crankshaft as follows:
Install the upper main bearing shells in the block. If the old bearing shells are to be reused, install them in the location from which they were removed. See Figure
When a new crankshaft is installed, all new main and connecting rod (upper and lower) bearing shells must also be installed.