The right front ball joint of a 2001 Volvo V70T5 (VIN YV1SW53D111016071) failed while entering the driveway of a restaurant in Bartlesville, Oklahoma shortly after noon on May 21, 2010. Minor damage was done to the bottom rear of the fender of the car and the adjoining area of the fender liner and plastic bottom trim under the door. The ball joint that failed was ScanTech brand and had been installed new less than 2 years ago.

Findings:
Visual and macroscopic examination of the fracture surfaces revealed that the failure mode was corrosion fatigue initiating at multiple points on the OD of the straight portion of the pin where it joins the taper. The fracture surface was typical of high-cycle corrosion fatigue with eccentric loading and the fatigue had progressed to the point that the final failure ocurred over an area about 1/8" in diameter.
Examination of the failed ball joint and another OEM ball joint revealed that the OEM ball joint had a radius at the point where the straight portion of the pin joins the taper, while the failed ScanTech ball joint appeared to have no radius at that point. Additionally the OEM ball joint had a much smoother machined surface on the pin than the ScanTech ball joint. There was a coppery residue and considerable corrosion on the surface of the steel pin and the threads were tight enough that it took at least 100 ft-lb of torque to remove the nut from the threads on the pin, although the threads did not appear rusty. The threads on the OEM ball joint were finger tight. It is suspected that a copper-based antiseize agent was used on the pin of the failed ScanTech ball joint upon installation (but not on the threads). A thread-locking compound may have been used on the threads of the failed ScanTech ball joint, based on the difficulty removing the nut.

Discussion:
Several factors working together contributed to this relatively early failure in a component whose predecessor lasted at least 8 years with no failure (and was replaced as a matter of preventive maintenance because the lower control arm bushings were also being replaced). It is fairly certain that multiple initiation points provided by the lack of a radius where the taper joins the straight portion of the pin and the rougher machined surface were major contributors.
Additionally a corrosive environment (moisture and the possible presence of copper, which would bring on galvanic acceleration of the corrosion) also was a major contributor. Other contributions MIGHT be such things as overtorquing the nut, road conditions (potholes, uneven paving, etc) and bumping the curb occasionally, although there was no apparent wheel or tire damage.

Conclusions/Recommendations:
Based on the observed differences in quality between the ScanTech ball joint and the OEM ball joint (and the assumption that they are typical), it seems worth the extra cost to buy the higher quality OEM ball joint. Additionally if an antiseize agent is to be used, it should be something that is NOT electropositive to the surrounding structural parts (steel, aluminum). Chassis grease or marine grease would be good options. Finally, since overtorquing increases the tension loading on the pin, the manufacturer’s torque recommendation should be observed on installation.

I'll post pictures if/when I am able to get good ones.


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George Downs, Bartlesville, Heart of the USA!