Different types of bearing fit

When considering the bearing mechanism, a gap exists between the shaft and the bore. In order to secure the bearing firmly in place, it is considered effective to provide a structure called a ’interference fit’ to the rolling bearing. This article explains the fittings necessary for bearings.

About bearing fit

Bearings function by fitting together with the shaft and housing. The term ’fit’ refers to the degree of hardness when the shaft and inner ring and housing and outer ring are fitted together. Rolling bearings have a structure in which the inner and outer rings are fixed to the shaft and housing to receive loads. The function of a rolling bearing is to prevent relative movement in the radial, axial, or rotational direction on the fitting surfaces between the race ring and shaft or housing.

There are different types of fitting: tight fit, clearance fit, and intermediate fit, depending on whether or not the bearing has an ’interference fit)’ which is the difference when the bearing bore size is smaller than the shaft size. One of the most effective ways to securely fasten a bearing is to design a sealing ring on the fitting surface between the race and the shaft or housing. This is done by using a ’clamping clamp’. This method also has the advantage that the load carrying capacity of the bearing is not compromised.

However, it is also necessary to consider the fact that the use of a tight fit reduces the ease of mounting and dismounting the bearing. Also, when a non-separating bearing is used for the non-locating bearing, the inability to move in the axial direction must also be considered. Therefore, it is important to note that a clamping clamp is not effective in all cases.

Purpose and selection of fit

The raceway wheel rotates under load. The purpose of the fit is to firmly hold the raceway to the shaft or housing. They also prevent unwanted ’slip’' on the fitting surfaces. In order for the bearings to function as they should, it is essential that they are properly fitted to the shaft and housing.

If there is insufficient interference on the fitting surfaces, the raceway rings will slip (misalignment) in the circumferential direction with respect to the shaft and housing. This slippage is called ’creep’. Creep can cause a variety of adverse effects such as wear of the fitting surfaces, abnormal heat generation, ingress of wear debris into the bearing, and vibration. Therefore, the following points should be considered when selecting a fit.

  • Bearing type and dimensions
  • Load properties and magnitude
  • Surface finish accuracy and material or wall thickness construction of the shaft or housing
  • Clearance inside the bearing
  • Temperature distribution during operation
  • Mounting and dismounting methods
  • Whether thermal expansion of the shaft must be accommodated by the fitting surfaces

The interference fit prevents creep of the raceways of a bearing that is subjected to load and rotates. In addition, it is necessary to be aware of the relationship between the nature of the load and the fit as an essential factor in the selection of the interference fit. After fitting and installation, if the joint will not be disassembled, a ’tight fit’ is used. On the other hand, if the shaft and hole need to be removed or slid after fitting, a ’clearance fit’ is used. If an intermediate condition is needed, an ’intermediate fit’ is used.

The following table describes the properties of loads and types of fits according to the classification of rotation.

1) Rotating inner ring and stationary outer ring (load type: stationary)

A ‘ tight fit’ is suitable for the fit between the inner ring and shaft, and a ’clearance fit’ is suitable for the fit between the outer ring and housing. Major applications include electric motors and spur gear units.

2) Inner ring stationary and outer ring rotating (load type: rotation)

A ’tight fit’ is suitable for the fit between the inner ring and shaft, and a ’clearance fit’ is suitable for the fit between the outer ring and housing. Major examples of use are wheels with a disproportionate amount.

3) Stationary inner ring and rotating outer ring (load type: stationary)

The ’clearance fit’ is suitable for the fit between the inner ring and shaft, and the ’tight fit’ is suitable for the fit between the outer ring and housing. The main examples of use are traveling cars and pulleys with stationary shafts.

4) Rotating inner ring and stationary outer ring (load type: rotation)

The ’clearance fit’ is suitable for the fit between the inner ring and shaft, and the ’tight fit’ is suitable for the fit between the outer ring and housing. Major applications include vibrating sieve machines with unbalanced vibration characteristics.

5) Indefinite (when the load direction is not constant, such as with unbalanced or variable loads)

Both the inner ring and shaft and the outer ring and housing are considered to have a ‘tight fit’. Major examples of use are cranks, etc.

Why is proper fit necessary?

If the fit is not appropriate, there is a risk of ‘damage to the bearing’ or ‘shortening the life of the bearing’. Therefore, it is important to carefully consider the appropriate fit. Machine problems caused by improper fits include the following cases:

  • If the internal clearance is too small, it will lead to poor rotational accuracy and acoustics such as seizure and raceway deformation
  • Cracking or premature delamination of the raceway or shifting of the raceway ring.
  • Wear debris gets in the raceway, causing vibration and heat generation.
  • Wear and tear of raceways, shafts, and housings caused by creep and fretting corrosion (friction with oxidation)

Proper fitting is required to prevent these mechanical problems.