Page 195 of catalogue Super Precision Bearings of Schaeffler Technologies GmbH & Co. KG

Bearing arrangement design and application examples
Preload · Rigidity

Bearing arrangement design and application examples
In practice, a large number of various Spindle bearings is used. The selection of the spindle bearing type and arrangement is generally determined by the specific end use they will see; turning, Milling, grinding etc. However, both operation conditions and economic considerations also play a significant role in the final selection and specification. In operation, bearings typically must run preloaded or at least clearance free, in order to attain high precision requirements (P4 and better). The lowest possible operating temperatures are required for highest speeds (with grease lubrication up to n · d m = 2 · 10 6 mm/min. and with oil lubrication up to 3,1 · 10 6 mm/min.). This requires the use of Super precision bearings and correspondingly precise mating parts. The following information should help in the search for a selection of optimal uses for bearings and bearing arrangements. The following aspects are covered: • Preload • Rigidity • Contact angle • Ball size and material • Distance between the bearings • Sealing • Steps for bearing design • Bearing arrangement comparison • Bearing examples.

Preload Rigidly preloaded bearings react sensitively to temperature changes between the shaft and housing, especially if they have a short distance between the bearings. Preloading within the bearing set could significantly increase or even cause the bearings to loose all internal clearance if the axial sliding fit fails. Radial stress can occur, especially in spindle bearings with a 15° contact angle. This applies to cylindrical Roller bearings and spindle bearing pairs with a sliding fit as well. In comparison, bearings with elastic preload, bearing sets with a wider distance between bearings and bearings with 25° contact angle are not as susceptible. Normally, bearings with ceramic rolling elements have lower operating temperatures. In a rigid system preloading here does not increase with ΔT as it does with steel balls. Speed reducing factors (see Table 2) need to be used with rigidly preloaded bearings. In elastically adjusted (by Springs or hydraulically) bearings speeds are attained in accordance to the bearing Tables due to reduced thermal sensitivity. The chosen preloading force is at least equivalent to medium bearing preload M (see Bearing tables).

Rigidity The bearing system rigidity is affected by the shaft diameter, number of bearings, bearing size, preloading and contact angle. Bearings with a 15° contact angle have only 45% of the axial rigidity of bearings with a 25° contact angle, whereas the advantage in radial rigidity is only around 10%. If the whole spindle bearing/ cantilever system is considered, then the radial rigidity of a bearing arrangement with 25° bearings, due to wider support basis, is normally better than one with 15° bearings. The rigidity of a rigidly preloaded bearing arrangement increases, in comparison to catalog data, during the assembly because of the fitting influence. During operation, it normally increases through ring expansion from centrifugal force at high speeds and the thermal expansion of shaft and inner ring.

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