The fits between the shaft and bearing inner ring and the housing and bearing outer ring can have a significant effect on bearing performance.

The ideal fit is where the shaft/housing is the same size as the bore/O.D. of the bearing. This is known as a line-to-line fit and gives optimum bearing performance. Looser fits are commonly used and often preferred for ease of assembly or where spring preloading is used (see "Preload" in the Radial Play section). Where heavy radial loads or excessive vibration are present, bearing rings under a rotating load may need to be firmly located by an interference fit or other means such as a nut or adhesive. This prevents them from creeping in a circumferential direction which gives rise to increased wear. A bearing ring is subjected to a rotating load when the load is applied to all points of that ring during operation. For example:

Inner ring rotating load/outer ring static load

(interference fit for inner ring and clearance fit for outer ring)

Bearing inner ring rotating loadE.g. a bearing in a vacuum cleaner motor driving the roller brush. The shaft and bearing inner ring are rotating. The load is in a constant direction in relation to the bearing so as the inner ring turns, all parts of it are subjected to the load. The outer ring does not rotate so the load acts on only one point of the outer ring.

Bearing inner ring rotating load 2Another example has a static inner ring and rotating outer ring but this time, the load rotates with the outer ring. As above, the load acts on only one point of the outer ring while all parts of the inner ring are subjected to the load.


Outer ring rotating load/inner ring static load

(clearance fit for inner ring and interference fit for outer ring)

Bearing outer ring rotating loadE.g. a bearing in a pulley wheel. The inner ring is static while the outer ring rotates. The load is in a constant direction in relation to the bearing so as the outer ring turns, all parts of it are subjected to the load. The inner ring does not rotate so the load acts on only one point of the inner ring.

This example involves a static outer ring and rotating inner ring. The load rotates with the inner ring. As above, the load acts on only one point of the inner ring while all parts of the outer ring are subjected to the load.

Fluctuating load/unbalanced load

(interference fit for inner ring and interference fit for outer ring)

Bearing outer ring rotating load 2

Usually only one ring is subjected to an interference fit but there may be instances where a fluctuating or unbalanced load will require interference fits for both shaft and housing. This may also be true where there is excessive vibration associated with the application.

Make sure that interference fits do not reduce the radial play of the bearing to an unacceptable level or early failure will occur. These fits will stretch the bearing inner ring or compress the outer ring, reducing the bearing's internal space. Excessive interference fits can also cause high stress which may fracture rings. It should be noted that an interference fit can reduce radial play by up to 80% of the size of the interference fit. Let's use a shaft with a 10mm diameter and a bearing with a 10mm bore as an example. Imagine the shaft diameter is actually 10.007mm and the actual bearing bore is 9.993mm. This gives an interference fit of 0.014mm (i.e. the shaft is 0.014mm or 14 microns larger than the bearing bore). The radial play of the bearing may be reduced by as much as 80 percent of this figure or approx 0.011mm. If the bearing radial play (before fitting) is less than 0.011mm, the bearing may become tight and fail quickly.

Shaft and housing fits should always accommodate differences in expansion coefficients. These may cause an increase or reduction in shaft/housing fit and a resultant change in radial play. An aluminium housing will expand more than a steel housing so requires a greater interference fit than a steel housing. Greater interference fits are required in thin walled or plastic housings and also on hollow shafts. Silicon nitride has a much lower expansion coefficient than  steel so if a silicon nitride bearing is used on a steel shaft at high temperature, there is a risk of the inner ring cracking.

The standards of roundness and surface finish which apply to the bearing should also apply to shaft and housing. An out-of-round shaft or housing can affect rotational accuracy by distorting bearing rings.  This is very important for low noise applications. Miniature and thin-section bearings are particularly susceptible to distortion which leads to higher noise and vibration levels. If rotational accuracy is important, a combination of close bearing tolerances and close shaft/housing tolerances should be used to obtain the correct fit with the minimum interference.

Expansion coefficients for commonly used bearing materials
52100 chrome steel:  12.5 x 10-6 (0.0000125) per °C
440 stainless steel:  10.5 x 10-6 (0.0000105) per °C
316 stainless steel:  16 x 10-6 (0.000016) per °C
ZrO2 (Zirconia):10.5 x 10-6 (0.0000105) per °C
Si3N4 (silicon nitride):  3.3 x 10-6 (0.0000033) per °C

Sample calculation for the expansion of a 30mm stainless steel bearing inner ring at 250°C
250°C minus 20°C (ambient temperature) = 230°C increase in temperature
230 (increase) x 0.0000105 (coefficient of 440 grade steel) x 30mm (bearing bore) = 0.072mm
At 250°C the bearing bore will be 30mm + 0.072mm = 30.072mm

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