Truck Parts and Service

February 2012

Truck Parts and Service | Heavy Duty Trucking, Aftermarket, Service Info

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Tech Updates INPUT SHAFT BREAKAGE Th ere are two types of input shaſt failures, 45 degree angle fracture and star fracture. According to Roadranger, a 45 degree fracture is the result of an extreme torque spike. Th e torque spike causes a surface crack that grows over time. As the crack grows, it weakens the shaſt until it cannot transfer torque, then it fractures. Th e root cause of a 45 degree input shaſt fracture is be- lieved to be aggressive lever shiſt s or splitter shiſt s — a shiſt completed out of the rpm synchronous range for a normal shiſt . Th is could include a shiſt where the lever is forced into gear or a splitter shiſt that is not selected properly. (See Fig- ures 1 and 2.) Another possible cause of a 45 degree fracture is spinning tandem wheels coming into contact with pave- ment causing an extreme torque spike. A star fracture results from a shaft being subjected to continuous operation at or near the design limit of the shaft strength, Roadranger says. These cracks often start below the surface. A star fracture is the result of operating the vehicle at the engine's maximum torque for extended periods of time, ac- cording to Roadranger. (See Figures 3, 4, 5 and 6.) Figure 1: In a 45 degree fracture typically there will be secondary damage to the shaft diameter just forward of the input bearing and the main drive gear drive spline ends are rounded. This type of fracture (surface initiated torsional fatigue) with the 45 degree helical fracture, often results in secondary damage to either (or both) the engine or transmission. The damage is the result of separate thrust forces — the fracture surfaces act as screw threads forcing the rearward section of the shaft into the transmission case and the for- ward section of the failed shaft into the engine flywheel area. Figure 2: In this close-up of the rearward end of the fracture, you can see the fracture began at the bottom of a spline root and progressed slowly across the diameter of the shaft at a 45 degree angle. The crack grew until the cross section was reduced to a threshold where the shaft could no longer transfer torque and final fracture occurred. Figure 3: This photo shows typical secondary damage caused by the input shaft being driven by the vehicle and flailing around due to lack of restraint. The shaft rubs against the bearing cover resulting in severe scoring on the shaft diameter forward of the input bearing. Figure 4: The area rearward of the clutch engagement area and forward of the end of spline full depth is the area of highest stress on the shaft. There are multiple torsional shear cracks that began internally and spread to the surface where they were connected by torsional tensile fatigue cracks that began on the surface. 10 Figure 5: Torsional shear cracks and surface initiated torsional tensile fatigue cracks are visible. Torsional tensile fatigue cracks from the surface connect with internal torsional shear cracks leaving the star type fracture. Figure 6: A cross section of the forward end of the fracture shown in Figure 4 and 5. Torsional cracks often initiate below the surface of each spline, extending to the sur- face and center of the shaft. TRUCK PARTS & SERVICE | February 2012

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