# Design considerations

## Adjusting for clearance or preload

When mounting a cylindrical roller bearing with a tapered bore, radial internal clearance or preload is determined by how far the bearing inner ring is driven up on its tapered seat. The further up the seat the ring is driven, the more it expands and the less radial internal clearance there will be, until eventually, there is a radial preload in the bearing. To quickly and accurately obtain the specified clearance or preload when mounting a bearing, SKF recommends using gauges. Gauges are particularly useful when mounting two or three bearings as it is not necessary to measure and calculate the axial drive-up distance for each bearing (→ Mounting).
If obtaining an exact radial internal clearance or preload is not critical or SKF gauges are not available, it is possible to determine the required axial drive-up distance. To do this, locate the assembled bearing at a reference point on the shaft and measure the radial internal clearance with a dial indicator positioned on the outside surface of the outer ring (→ Mounting bearings with a tapered bore by measuring radial clearance prior to mounting).
With the radial internal clearance measured using either of the above methods, the axial drive-up distance can be obtained using

If the bearing is to be mounted against a distance ring (fig. 1), the width of the distance ring must be adjusted to obtain the value Ba.
If there is no fixed abutment and a threaded nut is used to drive the inner ring assembly up on its tapered seat, the angle through which the nut should be turned can be calculated using

where

 Ba = axial drive-up [mm] α = requisite nut tightening angle [°] c = measured radial internal clearance at the reference point plus the required preload [μm] for preload minus the required clearance [μm] for clearance minus the adjustment [μm] for an interference fit in the housing bore when not using SKF gauges (→ Mounting bearings with a tapered bore by measuring radial clearance prior to mounting) e = a factor depending on the diameter ratio of the hollow shaft and the bearing series  (fig.2 and table 1) p = thread lead of the nut [mm]
For mounting procedures for super-precision cylindrical roller bearings, refer to Mounting.
Calculation example
Determine the axial drive-up for a double row cylindrical roller bearing mounted on a hollow shaft. Input data:
• bearing NN 3040 K/SPW33
• measured residual radial internal clearance = 10 μm
• requisite preload = 2 μm
• mean bearing seat diameter dom = 203 mm
• internal diameter of the hollow shaft di = 140 mm
From table 1 e = 18 for di/dom = 140/203 = 0,69
With c = 10 + 2 = 12 μm

## Free space on both sides of the bearing

To be sure that N 10 and NN 30 series bearings, with a polymer cage (designation suffix TN, TN9, TNHA or PHA), can accommodate axial displacement of the shaft relative to the housing, free space must be provided on both sides of the bearing (fig. 3). This prevents damage that might otherwise occur if the cage makes contact with an adjacent component. The minimum width of this free space should be

Ca = 1,3 s

where
 Ca = minimum width of free space [mm] s = permissible axial displacement from the normal position of one bearing ring relative to the other [mm] (→ product tables: single row bearings and double row bearings).