Over the years, SKF has built its reputation by meeting or exceeding customer expectations. And that can be difficult. Especially in those cases where quality is in the eye of the beholder. To illustrate this, let’s take a look at two very different cases where a customer’s perception of quality became a quality issue. In one case, the customer was a consumer group that purchased small, single row deep groove ball bearings for use in skate-boards and in-line skates. In the other case, the customer was a manufacturer of air conditioning units for use in cars and light trucks.
   

When SKF engineers sat down and listened to the concerns that in-line skaters and skateboarders had about bearing quality, what they found was that this group of consumers wanted a bearing that could pass a “spin test.” What’s a spin test? It’s where you hold the bearing’s inner ring between your thumb and forefinger and spin the outer ring race of the bearing. If the outer ring race spins easily, the bearing is good. If the bearing doesn’t spin easily, the bearing is bad. In this case, the customer didn’t understand that “spinning” is not a function of bearing quality. It’s the interaction between the lubricant, rollers, cage and seals or shields. If they knew that, would they change their perception of what a quality bearing should be? No. They want a bearing that will pass the spin test because that is their measure of a good, fast bearing.
The solution

When SKF engineers heard that the spin test was truly the measure of quality, they designed the new series of bearings for in-line skates that will be available later this year. The new series will include a stunt bearing and a speed bearing. Both were designed with the user and the operating conditions in mind. And yes, both will pass the spin test.
Though that may sound like a simplistic example of where a customer’s perception of quality became a quality issue, let’s look at a case where an industrial customer had a similar perception of quality.
   

Some years ago, when automotive air-conditioner manufacturers were looking for viable alternatives to Freon¬, SKF started working with an OEM to develop a bearing that would work well in a hot, contaminated environment. The bearing grease would have to lubricate the bearing, inhibit corrosion, and be compatible with the bearing seal. The grease and seal would also have to be tolerant of the refrigerant and oil used in the compressor’s system.
   

Before work actually began, SKF sat down with the air conditioner manufacturer and identified 16 performance tests that the bearing would have to pass before it would be accepted for production. Tests would include a durability test, a high-speed durability test, corrosion tests, seal tests, heat tests as well as a series of lab and field tests which would incorporate real life “four-season” testing on closed test tracks and on fleet vehicles.
   

To meet the needs of the customer, NATC engineers studied the application thoroughly and developed tests that would simulate real-life under-the-hood conditions.
In one case, engineers designed a test rig and chamber that would run the bearings at a constant speed with an ambient temperature of over 107degrees F (43degrees C) to simulate the engine compartment. Imagine running the bearings in a chamber that’s so hot, water would come to a boil! They also ran a high-speed durability test where as a part of the test, the bearings had to operate at 275 degrees F at 12,000 rpm. That translates into running a car’s engine at 6,000 rpm, because the air conditioner bearing is turning twice as fast as the engine.
   

Throughout all the tests for this application, where bearings were run to failure, our analyses showed that the bearing failures were a function of the bearing/ seal/lubricant system and that it was not a question of fatigue. In other words, the design of the bearing, cage and seal and the materials used to make them didn’t fail, it was an issue of the lubricant breaking down.
   

Lubrication can be a difficult issue, especially in an application like this one where there are so many compatibility issues combined with outer ring rotation. For months, SKF engineers in the U.S. and at the SKF Engineering Research Center in the Netherlands worked together to find the right formula for a grease that could be used in this application. The grease had to have very specific operating characteristics. It couldn’t be too soft or too thick. If it were too soft, it would be flung from the center of the bearing out to the outer race and interfere with the rolling elements. The result, called “churning”, would create excessive heat. If the grease were too thick, the oil would not “bleed” out of the grease bearing cavity and the bearing would fail due to lubricant starvation. The key was to develop a specially formulated grease for this application – and in time, the SKF engineers found an exceptional solution.
The testing phase went smoothly and the bearings were approved for production. Shortly after the bearings were delivered to the customer’s manufacturing facility, SKF received a phone call. The bearings were too “rough” and wouldn’t pass a “spin test.”
   

What’s a spin test? It is a test that installers perform once the bearing is mounted on the shaft of the compressor. Installers spin the bearing and shaft to be sure that all of the compressor’s components are moving freely and that there is no roughness between mating parts. The problem, according to the customer, was that the roughness (caused by the lubricant) in the bearing was masking the results of the spin test.
   

The roughness problem was fixed by altering the composition of the lubricant, but to this day, it’s hard to believe that the spin test is the gauge by which bearing quality is measured.
Now, whenever I embark on a new project with a customer I ask them whether or not they use the “spin test” as their measure of quality. Those customers who have been around bearings for a long time know that a free-spinning bearing is not necessarily a good sign. When they look at me in a way that says, “Do you know much about bearings?” I just look at them and say, “I just had to ask.”
Dave Pine,  
the testing resources coordinator for SKF’s Automotive Division.