When considering specifications for a bearing, many engineers take into account a number of important aspects, but one that is sometimes overlooked is radial clearance. Radial Clearance refers to the distance the inner ring is displaced in relation to the outer ring of a bearing in the radial direction under a gauge load. Radial clearance allows for free rotation of rolling elements, compensation for thermal expansion and optimum load distribution. The influence that radial clearance has on a bearing is far reaching and directly affects many aspects such as endplay, contact angle and bearing stiffness. Understanding radial clearance and can lead to increased bearing life and potential cost savings.

Radial clearance is classified on a C1, C2, CN, C3, C4 and C5 scale, with clearance increasing toward the highest number, which is C5. When noise or vibration is an issue, internal clearance is reduced, typically to the C1 or C2 range depending on the application. Larger clearance classes of C4 or C5 are used in applications requiring a heavy interference fit or experience high operating temperatures.

Applications experiencing very heavy loading, shock loads or significant shaft deflection require a large interference fit to offset fretting between the bearing bore and shaft. Additionally, a bearing inner ring can reach higher temperatures than the surrounding components causing an easing of the fit on the shaft. The heavier interference offsets these issues. The problem is that an insufficient starting radial clearance can lead to radial preloading and a significant reduction in bearing life or load capacity in the case of angular contact ball bearings. Thermal differences between the inner ring and outer ring can further reduce the operational radial clearance.

The bearing design engineer must therefore determine the proper starting radial clearance to ensure the correct mounted clearance taking into consideration fitting practice and thermal conditions. Historically, bearing manufacturers have provided very good guidelines for fitting practice and adequate available options for radial clearance. However that is not the case today. Most manufacturers do not stock anything different than a C3 radial clearance. This puts a strain on the bearing supply chain to meet the needs of individual application conditions.

Fortunately, internal clearance can be modified to provide the best performance in the application. Napoleon Engineering Services (NES) routinely performs radial clearance changes for a wide range of customers in both ball and roller bearings. As an ISO 9001:2008 certified custom bearing manufacturer, NES is tooled up to support the needs of specialty manufacturing and modification. We have dedicated grinding and honing equipment to meet the most stringent raceway manufacturing requirements. Typically, radial clearance changes can be completed in 7-10 days depending on quantity. For more information contact us at sales@nesbearings.com.