**4.3. Optimised heat treatment and grinding mode**

The heat treatment method (Table 6) followed by grinding as described in 4.2 is not sufficient to achieve the required ovality of 0.1 mm needed by the customer for the special bearings. It was necessary to develop, technologically master and verify a different, additional method of heat treatment and subsequent grinding of the rings, which would guarantee lower internal tensions, deformations and ovality values, whereas the required hardness of the rings should remain unchanged. Tempering is known to be used to eliminate internal tensions. This had been already done after hardening (see Section 4.2, Table 6), but resultant values of ovality were inadequate. It is also known that hardness of tempered parts decreases when tempering is done as described in Table 6, so any further tempering using this process was not possible.

Considering these facts and drawing from our experience, a procedure was suggested to ensure lower levels of ovality from 0.2 to 0.1 mm:



**Table 7.** Technological procedure of additional tempering

Table 5 shows that two parameters have been modified for additional tempering. The first one is the tempering temperature, which is now lower, i.e. 155+5 °C. This is a substantial change that will ensure lower internal tensions and ovality reduced by 50% against the standard design. The second one is the heating time, which is 65+5 minutes. It is a sufficient time. However, if a longer time was preserved, the effect would be the same without any risk of lower hardness values. Tempering, referred to as "artificial ageing" is followed by hardness tests on selected pieces. The required hardness after tempering remains constant of 60 to 63 HRc.

Ovality on finished rings was measured using the Talyrond 73 device, see Figure 9. The measuring device ensures very high accuracy, better than 0.1 μm.
