Roller bearing design – the most common errors in practice

Choosing the wrong lubricant is the number one error in error ranking. The roller bearing specialist considers this fact “not very surprising.” “It’s almost impossible to properly assess the countless products of all the different manufacturers.” Uncharacteristic noises, no smooth running or irregular temperature behavior are common signs that the wrong lubricant or too little or too much is being used. For operating conditions that aren’t according to plan, for example unexpected high temperatures, the lubricant may evaporate, the coagulant settles in the bearing, and the following dry run leads to a defect in the bearing.
 
The devil is in the details
Factors such as operating temperatures, speed, pressure conditions, environmental impacts and bearing loads should be taken into account in your selection. Special greases with properties such as low noise levels, smooth running or longevity are recommended by various roller bearing manufacturers. But quantity counts too, not just quality. As a rule, roller bearings are offered that are 30 percent filled by empty space. However, there are variants with reduced greasing of, for example, 20 percent. The greasing has to be customized in particular for aspects such as starting torque, energy efficiency and smooth running.
Findling’s tip: the construction phase should already ensure that no moisture or dirt can reach the bearings. Focusing on quality is also recommended for applications that theoretically don’t have to meet any special conditions. “There’s a rule of thumb here. If there’s too much grease, the adhesive sliding effect causes greater wear. If there’s not enough grease, the grease service life will decrease,” says Klaus Findling.

A matter of tolerance
According to the specialists at Findling Wälzlager, the second most common error is choosing the wrong tolerances in the interaction between the shaft and housing fit as well as the chosen bearing clearance. Here is a practical example: a 624-ZZ miniature deep groove ball bearing is installed with a lower M2 bearing clearance and a standard P0 tolerance class. Due to its tight fit, the shaft has an interference fit of n6. Theoretically, an excess of about 24 µm is possible in this combination. Calculating the 3 µm minimum bearing clearance with the 24 µm excess results in a deflection of about 10 µm per rolling element. In a miniature bearing, such an excess can result in uncharacteristic noises and irregular temperature behavior. To prevent this, Findling Wälzlager recommends a miniature bearing with M3 or M4 bearing clearance and a corresponding shaft tolerance of j6 or k6 according to the application.
Sluggishness, noise development or high wear are often signs of too low or too high radial pre-loading in the bearing. When choosing bearings and tolerances, the maximum tolerances should always be taken into consideration. Experienced design engineers choose bearings with a high bearing clearance for applications that require a firm bearing fit and where slightly uneven running is acceptable. In critical cases, the supplier should provide information about what average and variation within a tolerance class and the bearing clearance is used as standard for production. Premium manufacturers often manufacture products in tighter, manufacturer-specific tolerances that may unexpectedly cause problems when switching manufacturers.

Savings in the wrong place
The savings achieved with a cheaper seal are not worth mentioning in comparison to the higher risk, but it is still common practice. The third most common design error can cause contamination of the lubricant and reduce a bearing’s service life. Even slight contamination can result in a service life reduction of up to 50 percent. The better the seal, the more dirt-free the grease and the longer the service life. A distinction is made in general between non-contact and contact seals. The former are suitable for applications with a focus on energy efficiency. ZZ gap-type seals with sheet steel washers are the most cost-effective variant and also the simplest for sealing a bearing. Many users choose this design for economic reasons without taking into consideration the fact that it has a gap of several tenths of a millimeter between the inner ring and the seal. Dirt particles more than 0.1 mm in size can get into the bearing interior through this gap. The better but more costly solution for a non-contact seal is the labyrinth seal. Because of its design, it is highly resistant to external influences and contributes to lower power dissipation. Contact seals are suitable for applications with a medium to high risk of contamination. One of the various RS variants is usually used – a sheet metal core coated with NBR that contacts the inner ring with at least one lip. This design is also available with multiple sealing lips guided in a groove (DDU, LLU) They offer significantly better protection against contamination but also against moisture and grease leakage. The advantage of the groove is that the lubricant settles in it and also contributes to the seal. Findling recommends that security-conscious users who prioritize the service life over power dissipation always use contact seals. For special applications with particular external influences, the multi-lip seal guided in a groove is the method of choice. Standard seal forms and alternatives should be discussed with the supplier and bearings should be additionally protected under very rough operating conditions, for example using a shaft seal.

Corrosion doesn’t stand a chance
Roller bearings are predominantly manufactured from 100Cr6 steel, which corrodes when it comes into contact with water and aggressive media. Upon the first signs of rust, it’s only a matter of time until the bearing runs unevenly and ultimately locks up. For many bearing types, there are designs in stainless steel, for example for deep groove ball bearings and housing bearings, but also for cam followers and other needle roller bearings. They are corrosion-resistant but have a load rating that is about 30 percent lower. In the case of housing bearings, coated (e.g. burnished) bearing inserts are also available and have a surface treatment that prevents corrosion. However, in cases of environmental conditions involving water and aggressive media, the cage, lubrication and seal of the bearing are also important in addition to the bearing material. Lubricants and seals can also be defect-causing factors in corrosion-resistant bearing technology. Design engineers should make an effort to avoid environments like this by planning accordingly ahead of time.