The service life of the tool spindle is largely dependent on the service life of the spindle bearings, the components most affected by wear. The nominal service life calculation for roller bearings is based on fatigue theory. During operating times the roller surfaces of the bearing are subjected to cyclical compressive stresses, which result in the failure of the bearing after a foreseeable period due to fatigue. Moisture, inadequate lubrication or contamination, which result in wear and rust formation, are largely disregarded by this theory. For this reason, the calculated service life does not generally apply reliably to practical situations.
Alongside the pure fatigue period, the calculation of the period of use, which takes account of the external factors, is more relevant. The period of use therefore yields the time for which the bearing can remain in practical use. The modified service life calculation therefore incorporates additional adaptation factors such as operating temperature, material characteristics or lubrication, which are determined through operating conditions that are already known. However, it should be noted that all conventional calculation processes can only provide a guide value. External stresses due to the cutting conditions or minor collisions can result in a bearing reaching the end of its service life much earlier. In contrast to this, good care and careful handling of the spindle can also result in a far longer service life.
The conventional service life calculation and the modified service life calculation are briefly presented below. There are numerous other calculation processes, which are not listed here however.
The factor a2 stands for the characteristics of the bearing material based on its heat treatment and alloy composition.
In the case of conventional, hardened roller bearing steel a2 = 1.
Improvements in steel manufacturing, refined production methods and special manufacturing qualities have the effect of extending the service life,
i.e. a2 > 1. For example, a factor of a2 = 0.5 is used for calculations for bearings made from stainless steel.
The factor a3 primarily considers the lubrication, the cleanliness and the operating temperature. For example, excessively high operating temperatures reduce the workpiece hardness. [DAHLKE94], [WECK06]