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Performance Crankshafts, Duane Boes
Repair or Replace Following Engine Failure?
By Duane Boes
Across the United States it’s "Racing Season 2003." Along with the festivities and excitement will be the usual situations of despair that arise from occasional engine failures. Aside from determining the root cause of the failure your biggest challenge will be in deciding what parts are worthy of going back into service.
Placing existing components back into service will significantly reduce both cost and downtime. Crankshafts involved in a failure will typically exhibit one or more familiar signs of distress. With the following illustrations and photographs we’ll take a look at what can be considered acceptable and what should be thrown away or stepped down in severity of application.
For the sake of discussion all cranks referenced in this article will be assumed to be of 5140, 4340 or EN30 high alloy steels that have been either nitrided or induction hardened.
Main bearing runout (also known as total indicated runout or "TIR") is often the result of localized heating that quickly develops during a rod or main bearing failure. While temperatures are elevated the shaft will move toward the heated surface. Upon cooling, the opposite scenario takes place, with the shaft moving away from the heat-affected surface (Figure 1).
The initial growth and subsequent shrinkage of heat-affected material in the journal overlap areas can create extreme TIR, over .020˝ in some cases. Your choices will be to straighten the shaft or scrap it. If the shaft is found “without stress cracks” it can be straightened, reground and polished with minimal consequence, providing several details are addressed.
The project of straightening a nitrided or induction hardened high alloy shaft should not involve a press. The shaft bent as the result of residual stresses developed during the localized heating and cooling. These stresses are difficult to eliminate entirely but can be easily and permanently offset by strategically placed offsetting compressive stresses. This practice has been around for years with both OEM and aftermarket manufacturers, the reliability of mechanically induced compressive stresses have been thoroughly documented.
Two practices are common: OEM cranks are fillet rolled while in-service repair cranks are peened. A critical factor with peening nitrided shafts is the minimization of impact marks or fractures. Impact marks will show up during a magnetic particle inspection examination. They can be minimized by carefully shaping the peening punch to match the contour of the journal fillet radius. Once created, impact marks can be removed by sanding the immediate area involved.
Evaluating crankshaft main bearing runout is a straightforward process. With the shaft resting upon the end main bearings, a dial indicator, preferably a “.0001˝ reader,” registers the amount each journal deviates from the crank’s actual centerline during one complete revolution. Your comfort level for the amount of allowable total individual bearing runout is often determined by previous experience or application.
An important consideration when evaluating TIR should always be the clock position of adjacent main bearing runout. We worry about runout because of its effect on the actual operating oil clearance of a journal and its surrounding bearing insert. A runout measurement indicates to what degree a journal is orbiting around the shaft’s centerline. As a journal orbits within its surrounding bearing inserts, the load-carrying oil film barrier is unnaturally squeezed down to a reduced thickness. Depending on the extent of this runout, the resulting reduction in oil film barrier can be devastating.
Taken at face value, the TIR values shown in red in Figure 2 would appear to be acceptable by most standards. When clock position of this runout is considered another picture quickly develops, i.e., adjacent runout jumps to .0012˝. With adjacent main bearing runout readings of opposite sign, the orbit of each journal is significantly increased. In effect the journals are prying on each other, and as they do so the load carrying oil film is squeezed down to an abnormally thin level.
In Figure 2, the readings in green are of the same overall magnitude but different clock position. With the green readings, adjacent runout drops to .0006.˝ Development of 180 degree runout within certain crankshaft types is a common phenomenon. Ideally, the difference between adjacent main bearing runout readings should be no more than half of your acceptable standard tolerance if clocked at 180 degree separation.
In situations having a 90 degree separation between TIR high points, the allowable difference between adjacent main bearing runout readings can be increased to three-quarters of your accepted standard tolerance. Typical 90 degree V-8 shafts are prime candidates for 90 degree adjacent runout.
Crankshaft failures often involve severe impact to the rod journal bearing surface. In these situations size and location will be your main consideration in evaluating impact zones. In most cases impact damage occurs on the leading half side of a rod journal’s circumference. Rod journals exhibit their greatest wear from loads generated at the bottom of each power stroke and at the top of each exhaust stroke. These loads are great enough to deplete the oil film; over a period of time they can create out-of-roundness.
Ideally there should be no surface deviations in this heavily loaded area typically found between the crank webs. In situations where there is no alternative, the location of the impact crater in relation to the bearing insert should be considered. Every automotive rod journal has an oil hole that is centrally located to the insert.
Loss of the load carrying surface area represented by the oil hole poses no threat to the rotating system. The same holds true for impact craters providing there aren’t so many as to significantly reduce the load carrying surface area of the journal.
Consider the crater’s location in relation to bearing insert contact area. Ideally the entire crater should fall within the footprint of the bearing insert creating a cavity that is totally captive. In this situation the crater holds oil with the only negative being the loss of surface area. As seen in Figure 3 (page 49) if the crater extends beyond the insert’s footprint it will create a leak path.
Oil will take the path of least resistance to squirt from under the insert as the heavily loaded film barrier comes in contact with a crater that is not captive under the insert. This localized loss of oil will result in an area of diminished load carrying ability. Depending on the crater’s location in relation to the journal’s circumference, and its heavily loaded areas, this loss of film barrier may be an issue.
Rapid heating and cooling cycles occurring during a bearing failure can result in what are commonly called heat cracks. Heat cracks come in all sizes, but are primarily isolated to the load carrying area of the journal. While not desirable, they usually are not fatal to the crank.
The journal shown in Figure 4 is showing evidence of severe heat cracking. This particular main had previously suffered an oiling failure requiring it to be ground -.010˝. The shaft was then put back into service in a blown alcohol engine. Even with this quantity of apparent cracks, the surface held up well causing no related problems during extended severe use.
Heat cracks originate in the bearing surface directly under the footprint of the insert. Since load carrying areas are not subject to typical crankshaft bending stresses heat cracks rarely propagate into failure causing fractures.
Heat cracks around oil holes can be an exception. Areas surrounding oil holes (rod journals in particular) are subject to torsional stresses that can drive surface cracks to wick toward the hole, as seen in Figure 5. Rod journal oil holes are subjected to wicked torsionals that are most severe in the thin section of the journal directly over the oil transfer hole between rods and mains. A crack extending into an oil hole should always be polished or filed out leaving a stress riser free surface.
While heat cracks are surface in nature, bending fatigue cracks are an outward sign that significant stresses have been applied to the component. A look at the shaft in Figure 6 revealed these serious cracks. Stress cracks in the fillet radius of any journal are worthy of serious scrutiny. These types of cracks are often deep. For your reference, the journal in Figure 6 had been ground -.050˝ prior to this photograph. A crankshaft having stress cracks this severe should never be repaired for use in a high output engine.
Duane Boes wears a variety of hats with Callies Performance Products, Fostoria, OH, including performance marketing, technical support, product development and heat treat.