The following tables are a guide for establishing shaft and housing fit for miniature and instrument bearings when the expansion coefficients of the shaft and housing are similar or when the operating temperature differential between them is nominal. In other conditions, modification in fits and internal clearance may be required.
** For closer running accuracy or reduced axial play K13 internal clearance may be used provided that the fit of the outer ring is line to line or looser
** For closer running accuracy or reduced axial play K25 internal clearance may be used provided that the fit of the outer ring in the housing is line to line or looser.
The ideal fit is where the shaft and housing fit 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: e.g. a bearing in a vacuum cleaner motor belt 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. This application requires an interference shaft fit and a clearance housing fit.
Another possibility is 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. This applications require an interference shaft fit and a clearance housing fit.
Outer ring rotating load: e.g. a bearing in a pulley. The shaft and inner ring are fixed while the outer ring and housing (the pulley) do rotate. 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 application requires a clearance shaft fit and an interference housing fit.
This example involves a static outer ring and rotating inner ring, the load rotating 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. Both of these applications require a clearance shaft fit and an interference housing fit.
This means that usually only one ring is subjected to an interference fit. There may be instances where a fluctuating load direction 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.
The material of the shaft and housing fit should be taken into consideration. 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.
Care should also be taken where shaft and housing fit materials have a different expansion coefficient to the bearing steel. This may lead to an increase or reduction in radial play. This is a danger when using ceramic bearings on a steel shaft. Silicon nitride has a very low coefficient but will withstand very high temperatures so if a silicon nitride bearing is used on a stainless steel shaft at 500 °C, there is a risk of the inner ring breaking or cracking particularly as ceramics are more brittle than steel. Much looser fits should be considered to accommodate these differences. There is less of a risk with zirconia as the expansion coefficient is much higher but the differences in expansion should always be considered.
For commonly used bearing materials the coefficients are:
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) – 10.3 x 10-6 (0.0000103) per °C
Si3N4 (silicon nitride) – 3.3 x 10-6 (0.0000033) per °C
To calculate the expansion, first work out the difference in initial temperature and final temperature. Next multiply this figure by the expansion coefficient and multiply that new figure by the relevant bearing dimension. For example, a 440 stainless steel bearing bore is 30mm at ambient temperature 20°C. What is the bore size at 250°C?
Final temperature 250°C minus initial temperature 20°C = 230°C increase in temperature
Expansion coefficient of 440 grade steel is 0.0000105 per °C so…
230 (temperature increase) x 0.0000105 (expansion coefficient) x 30mm (bearing bore) = 0.072mm
Therefore, at 250°C the bearing bore will be 30mm + 0.072mm = 30.072mm
Interference fits can affect rotational accuracy by distorting bearing rings. The standards of roundness and surface finish which apply to the bearing should also apply to shaft and housing fit. This is very important for electric motor and other quiet-running 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 and housing fit tolerances should be used to obtain the correct fit with the minimum interference.