Gearbox manufacturer Stober Drives Inc. (Maysville, KY) prides itself on the quality of its products. The company was therefore very concerned when it experienced a sudden increase in the number of returns for one of its products-a gearbox that had historically experienced a failure rate of just 0.5 percent and which boasted a mean time to failure rate of more than 7 years. Adding to the severity of the situation was an attendant increase in warranty costs. As part of its commitment to quality, Stober offers a 3-year standard warranty, as well as a special 5-year warranty on selected products.

Suspecting that at least some of the failures were related to the gearboxes’ inboard ball bearing set, Stober Drives product manager Mike Mitchell contacted bearing supplier SKF USA Inc. (Kulpsville, PA) for assistance. After 15 years of working with SKF, Mitchell was fairly sure the bearings themselves were not the problem. Nonetheless, something was definitely amiss.

As part of the subsequent investigation, Mitchell shipped some of the failed gearboxes to SKF headquarters in Kulpsville where they could be analyzed by engineers in the company’s bearing inspection and failure analysis lab. What SKF engineers found was evidence of faulty gearbox bearing installation. Dents on the bearings’ outer raceway, for example, indicated that operators had been hitting them with something-probably a hammer-during installation. Similarly, blue discolorations revealed the use of excessive heat.

To solve the problem, SKF created a new custom training program to ensure that operators would follow the correct installation procedures. Central to this program was an emphasis on the importance of using only approved methods when driving bearings onto the gearbox shaft to achieve an interference fit.

The program also outlined the correct method for applying heat to the assembly, via an induction heater or an oil bath. 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. Because too much heat can weaken the bearing material, Stober’s assemblers were instructed to not exceed a temperature of 250 F.

The results of the training were dramatic and immediate. The number of bearings returned under warranty dropped 42 percent. The amount of scrap resulting from incorrect assembly practices dropped 48 percent.

“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, SKF has conducted a number of additional seminars to bring new workers up to speed and reinforce correct practices among existing employees. A natural byproduct of these efforts has been a further strengthening of Stober’s already robust ISO quality system.

“Continuous process improvement is an ISO requirement,” Mitchell says. “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.”

For more on bearings and bearing installation, call 800-440-4SKF or