With a reputation for supplying reliable gearboxes, Stober Drives Inc. (Maysville, KY) was understandably concerned when product returns suddenly spiked. A variety of the company’s gearboxes, which had a failure rate of just 0.5% per three years, and a mean time to failure (MTTF) of more than seven years, were being returned at an unacceptable rate.
Adding to the situation’s criticality was that the returns were coming in under the company’s three-year standard warranty. Stober quality had made the long warranties possible and they worked as a powerful purchasing incentive, but now the warranties were incurring an expense.
Suspecting that a portion of the failures were in some way related to the gearboxes’ inboard ball bearing set, Stober Drives’ Product Manager Mike Mitchell contacted bearing supplier SKF USA Inc. (Kulpsville, PA) for assistance. The purpose of the bearings is to secure the gearbox’s shaft while it is rotating under load. Mitchell knew that poor bearing quality was unlikely because the company had been sourcing bearings from SKF for 15 years without incident. Yet, something was definitely amiss.
Mitchell shipped gearboxes that had failed to SKF USA Inc. headquarters, which houses a bearing inspection and failure analysis lab. What SKF engineers found was evidence of faulty gearbox bearing installation, which could occur only during the gearbox assembly process. Equally spaced axial dents on the bearing’s outer raceway demonstrated that improper impact procedures had been employed, and blue discolorations revealed the use of excessive heat.
The solution called for expert bearing installation training. To that end, an SKF engineer developed a custom training program that would explain the need to follow proper installation procedures. “Our objective was to have SKF train our assemblers in best practices at every step,” Mitchell notes.
One essential practice concerned the process of driving a bearing onto the gearbox shaft to achieve an interference fit, a fit that is sufficiently tight to hold the bearing in place on its rotating shaft. In some cases, assembly personnel had been applying pressure or impact to the bearing’s outer race, an improper procedure that causes contact to take place on the bearing’s rolling elements and raceway, and which produced the telltale evenly spaced dents. The training program stressed the need to apply pressure or impact only to the bearing’s inner ring, a practice that moves the bearing along the gearbox’s shaft without damaging any gearbox component.
The proper use of heat, which is best achieved via induction heater or oil bath, was also stressed in the training program. When a bearing is heated, the inner ring expands for fitting onto its assigned shaft. As it cools, the inner ring grips the shaft tightly to achieve the needed interference fit. Improper heating can weaken the bearing material. SKF’s engineer instructed Stober’s assemblers to not exceed 250 F with the bearing.
The results of the training have been dramatic. Bearings returned under warranty have dropped 42%. And a bonus, scrap associated with improper assembly practices has been reduced by 48%.
“No longer is a shortcut taken by grabbing a ball peen hammer to tap an outer race to seat the bearing,” Mitchell says. “Our assemblers do the job right every time with heat and proper press tools. As a result, we are seeing improved life cycles and a more robust and reliable gearbox product for the most demanding applications.”
Since the initial training was completed, SKF has conducted additional seminars to train dozens of new personnel and refresh the skills of existing assembly workers. The ongoing program benefits Stober’s ISO quality system. “Continuous process improvement is an ISO requirement,” explains Mitchell. “The SKF bearing installation training not only improves an essential part of our gearbox assembly process, it also helps us maintain our high quality standards, which is a key Stober business priority.”
SKF USA Inc