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Obtaining an interference fit

During mounting, the degree of interference is normally determined by one of the following methods:
  • measuring the clearance reduction
  • measuring the lock nut tightening angle
  • measuring the axial drive-up
  • measuring the inner ring expansion
For self-aligning ball bearings, feeling the clearance reduction by swivelling the outer ring is an additional method (→ Mounting bearings with a tapered bore).
Measuring the clearance reduction
A feeler gauge is most often used to measure the radial internal clearance in medium- and large-size spherical and CARB toroidal roller bearings. Recommended values for the reduction of radial internal clearance to obtain the correct interference fit are listed in the relevant product section.

Before mounting, the clearance should be measured between the outer ring and uppermost roller (fig. 1). After mounting, the clearance should be measured between the inner or outer ring and lowest roller, depending on the bearing internal design (fig. 2).

Before measuring, the inner or outer ring should be rotated a few times. Both bearing rings and the roller complement must be centrically arranged relative to each other.

For larger bearings, especially those with a thin-walled outer ring, the measurements are affected by the elastic deformation of the rings, caused by the weight of the bearing or the force to draw the feeler gauge blade through the gap between the raceway and an unloaded roller. To establish the “true” clearance before and after mounting, use the following procedure (fig. 3):
  1. Measure the clearance “c” at the 12 o’clock position for a standing bearing or at the 6 o’clock position for an unmounted bearing hanging from the shaft.
  2. Measure the clearance “a” at the 9 o’clock position and “b” at the 3 o’clock position without moving the bearing.
  3. Obtain the “true” radial internal clearance with relatively good accuracy from 0,5 (a + b + c).
Measuring the lock nut tightening angle
This method can be used for mounting small to medium-size bearings with a tapered bore (d ≤ 120 mm). Recommended values for the tightening angle α are listed in the relevant product section.

Before starting the final tightening procedure, the bearing should be pushed up on the tapered seat until it is firmly in position. By tightening the nut through the recommended angle α (fig. 4), the bearing is driven up over the proper distance on the tapered seat. The bearing inner ring then has the requisite interference fit. The residual clearance should be checked whenever possible.
Measuring the axial drive-up
Mounting bearings with a tapered bore can be done by measuring the axial drive-up of the inner ring on its seat. Recommended values for the required axial drive-up are listed in the relevant product section.

However, the SKF Drive-up Method is recommended for medium- and large-size bearings. This method provides a reliable and easy way to determine the degree of interference. The correct fit is achieved by controlling the axial displacement of the bearing from a predetermined position. This method incorporates the use of an SKF hydraulic nut (1) fitted with a dial indicator (2), and a hydraulic pump (3) fitted with a pressure gauge (4), appropriate to the mounting conditions (fig. 5).

The SKF Drive-up Method is based on a two-stage mounting procedure (fig. 6):
  • Stage one
    By applying a predetermined pressure in the hydraulic nut, the bearing is pushed from the “zero” position to a reliable start position.
  • Stage two
    By increasing the pressure in the hydraulic nut, the bearing inner ring is pushed further on its tapered seat to the final position. The displacement ss is measured by the dial indicator.
Recommended values for the requisite oil pressure to reach the start position and the axial displacement to reach the final position for individual bearings are available at or SKF Drive-up Method Program.
Measuring the inner ring expansion
Measuring the inner ring expansion is a quick and accurate method to determine the correct position of large spherical and CARB toroidal roller bearings on their seats (d ≥ 340 mm, depending on the series). To apply this method, use common hydraulic mounting tools and SensorMount, which consists of a bearing with a sensor embedded in the inner ring and a dedicated hand-held indicator (fig. 7). Aspects like bearing size, shaft material and design (solid or hollow), and surface finish do not need any special consideration.

For additional information about SensorMount, contact the SKF application engineering service.
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