Finding the Perfect Fit

 


Bearings are precise mechanisms produced with great care. Yet their performance is dependent on selecting the perfect shaft and housing fits. If the fits are too loose or too tight, performance and design can suffer.

Improper fitting can lead to noisy operation, excessive heat and poor running accuracy. If the fits are too loose there can be bearing seat damage and excessive wear, along with vibration, reduced radial accuracy and unusual noise. On the other hand, if the fits are too tight, there is an excessive reduction in radial clearance, overheating and unintended preload.

Selecting the correct fit will minimise failures. Correcting the fit can be difficult, depending on the application. This requires an entire teardown to access the shaft and housing.

The different types of Fits
  • Clearance Fit - A clearance fit enables clearance between the inner and outer diameter of a bearing. It makes installation easier. If the fit is loose, it can cause problems, where a bearing can spin on the shaft. This can generate heat and cause premature bearing failure.
  • Interference Fit - An interference fit is the opposite of a clearance fit. In this fit, there is interference between the bearing ring and the matting part. If the bearing bore is smaller than the shaft diameter or the outer diameter is larger than the housing bore. This interference will make the parts challenging to assemble and so will require pressing into place or the use of heat fit for easier assembly.
  • Transition Fit - Transition fit comes between the two described above. Either a clearance or interference fit may be required depending on the actual size of the bore, outer ring and housing.
When selecting fits, you should consider the following:
  • Condition of rotation - Three rotation conditions exist for bearings: rotating load, stationary load and indeterminate load direction. Rotating loads require an interference fit to prevent creep and wear. Stationary loads need no interference fit. Indeterminate loads, like variable or heavy ones, also benefit from an interference fit to prevent fretting corrosion or wear, especially when axial movement is required in the outer ring's housing.
  • Magnitude of load - When a bearing carries a load, it can change shape. In cases where the inner ring rotates and carries the load, this change in shape can make the fit between the inner ring and the shaft less tight, causing the inner ring to move on the shaft seat. If the load is heavier, you need a tighter fit to prevent this.
  • Temperature differences - During operation, bearing rings can get hotter than the components they connect to, potentially loosening the shaft fit and hindering axial movement in the housing. Rapid heat-up from seals can worsen this issue. Temperature differences and heat direction can impact fits, requiring consideration of steady-state and transient conditions.
  • Precision requirements - To minimise vibration in high-speed applications, interference or transitions fits are required.

Design and material of shaft and housing - Avoid any distortion in bearing rings caused by poor shaft or housing design, like uneven surfaces or irregular wall thickness. If you're using thin-walled housings or hollow shafts, go for tighter interference fits. Keep in mind that different materials may require different fits depending on their strength and thermal properties.

Ease of mounting and dismounting - Loose fits are beneficial for easy mounting and dismounting. In applications where interference fits are required for the shaft, bearings with tapered bore should be chosen.

Premier Bearings, a leading FAG & INA bearing supplier in India, has the right professionals with the expertise to help you find the perfect fit for bearings. For more information, visit our website https://www.premierbearing.com/


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