For years, transmission and engine rebuilders have focused their sights on determining the cause of crankshaft thrust bearing failure. While diagnosis can be difficult at times, many have observed that failure is more likely to occur with an automatic transmission setup. It is worth noting that this may not be the only reason, and failure can be traced to a single problem or a combination of problems. To better understand why crankshaft thrust bearings fail, we will cover their history, causes of failure, how to pinpoint the problem, and more.
Though thrust bearings run on a thin film of oil, they cannot support nearly as much load as their radial journal bearing counterparts. For instance, radial bearings can support loads measured in thousands of pounds per square inch, while thrust bearings can only carry a few hundred pounds per square inch. Thrust bearings generally have two flat mating surfaces with no natural wedge shape in the clearance space, promoting the formation of an oil film to support the load. For this reason, many heavy-duty diesel engines utilize separate thrust washers with contoured faces in order to support higher thrust loads.
There are four primary factors that lead to failure, those of which include a poor crankshaft surface finish, misalignment, overloading, and overall surface finish. Generally, crankshaft thrust faces are difficult to grind, but it is not impossible. As they are made using a grinding wheel, grinding marks may appear on the face, those of which should be removed by polishing the surface. In addition, shot peening is also helpful. Shot peening is a cold working process that produces a compressive residual stress layer and modifies the mechanical properties of metals and composites.
A grinding wheel that does not make clean cuts can create hot spots on the workpieces, leading to a bumpy surface. For this reason, the side of the wheel should be aligned 90 degrees to its outside diameter. This configuration will produce a thrust face that is perpendicular to the axis of the main bearing journal. The crankshaft grinding wheel should be fed into the thrust face slowly to avoid excess waste. It is also paramount to provide ample coolant between the grinding wheel and thrust surface to prevent stone loading and “burn” spots on the thrust surface. Keep in mind that grinding should end with complete “spark out” before the grinding wheel is moved away from the workpiece.
In terms of assembly, one must begin by tapping the main cap toward the rear of the engine with a soft-faced hammer. Then, tighten the main cap bolts until they are finger tight. With the help of a bar, push the crankshaft as far forward into the block as possible to align the bearing rear thrust faces. Hold the shaft in a forward position when tightening the main cap bolts Furthermore, complete torquing the main cap bolts to the required specifications, in 2 to 3 equal steps. This series of steps should align the bearing thrust faces with the crankshaft, maximizing the amount of bearing area in contact to support varying loads.
Overloading is quite common for bearings; thus, you must familiarize yourself with some key contributing factors. To begin, an uneven crankshaft surface finish and geometry can result in overloading. Moreover, excessive torque converter pressure, improper throw out bearing adjustment, and riding the clutch pedal are all likely causes as well. Other associated issues include torque converter ballooning, installing the incorrect flex plate bolts and/or torque converter, as well as installing the pump gear backward. All of the aforementioned problems can impose undue force on the crankshaft thrust surface.
Typically, diagnosing the issue that leads to crankshaft thrust bearing failure begins with making observations. Damage is evident, so one must look at the physical appearance of the bearing to narrow down the problem. For instance, look for evidence of distress in other areas of the engine such as insufficient lubrication or foreign particle contamination. In some cases, failure is transmission-related. As there are many areas that can be cause for concern, you cannot complete repairs until you have found the source of the issue. Once you have figured out the issue, simple modifications and adjustments can be carried out to ensure your crankshaft thrust bearing is working optimally. However, if you require a bearing replacement, look to the experts at Aviation Distribution.
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