The Design of Rolling Bearing Mountings

9–18 Work spindles of machine toolsThe heart of every machine tool is its main or workspindle and its work spindle bearings. The main qualitycharacteristics of the spindle-bearing system arecutting volume and machining precision. Machinetools are exclusively fitted with rolling bearings of increasedprecision; mainly angular contact ball bearingsand spindle bearings (radial angular contact ball bearingswith contact angles of 15° and 25°, respectively),double-direction angular contact thrust ball bearings,radial and thrust cylindrical roller bearings and, occasionally,tapered roller bearings.Depending on the performance data required for amachine tool, the spindle bearing arrangement is designedwith ball or roller bearings based on the followingcriteria: rigidity, friction behaviour, precision,speed suitability, lubrication and sealing.This yields a viscosity ratio = / 1 ≈ 4; i. e. the rollingcontact areas are fully separated by a lubricant film.With = 4, a basic a 23II factor of 3.8 is obtained fromthe a 23 diagram. Since the bearings, as a rule, are relativelylightly loaded (f s* > 8), a very good cleanlinessfactor (s = infinite) is obtained with increased (V = 0.5)and utmost (V = 0.3) cleanliness. Consequently, thefactor a 23 (a 23 = a 23II · s), and thus the attainable life(L hna = a 1 · a 23 · L h ) becomes infinite; the bearing isfailsafe.So, as long as f s* ≥ 8 and the main spindle bearings arelubricated well ( ≥ 4), only the cleanliness in the lubricatinggap determines whether the bearing is failsafeor not.Out of a multitude of possible spindle bearing arrangementsfor machine tools a few typical arrangementshave proved to be particularly suitable for applicationin machine tools (figs. a, b, c).DimensioningUsually, a fatigue life calculation is not required for thework spindles since, as a rule, to achieve the requiredspindle and bearing rigidity, bearings with such a largebore diameter have to be selected that, with increasedor utmost cleanliness in the lubricating gap, the bearingsare failsafe. For example, the index of dynamicstressing f L of lathe spindles should be 3...4.5; this correspondsto a nominal life of L h = 15,000...50,000 h.Example: The main spindle bearing arrangement of aCNC lathe (fig. a) is supported at the work end inthree spindle bearings B7020E.T.P4S.UL in tandem-O-arrangement (contact angle 0 = 25°, C = 76.5 kN,C 0 = 76.5 kN). At the drive end, the belt pull is accommodatedby a double-row cylindrical roller bearingNN3018ASK.M.SP. The cutting forces cause 50 %each of the axial reaction forces for the two tandemarrangedspindle bearings. The front bearing at thework end accommodates 60 % of the radial forces. It isloaded with F r = 5 kN, F a = 4 kN at n = 3,000 min –1 .If the bearings are lubricated with the lithium soapbase grease FAG Arcanol L74V (base oil viscosity23 mm 2 /s at 40 °C), an operating viscosity of = 26 mm 2 /s will be obtained at an operating temperatureof 35 °C. With the mean bearing diameterd m = 125 mm and the speed n = 3,000 min –1 a ratedviscosity of 1 = 7 mm 2 /s is obtained.a: Spindle bearing arrangement with universal-design spindle bearings(spindle bearing set), subjected to combined load, at the workend and a single-row or double-row cylindrical roller bearing at thedrive end which accommodates only radial loads.b: Spindle bearing arrangement with two tapered roller bearings inO arrangement. The bearings accommodate both radial and axialloads.c: Spindle bearing arrangement with two double-row cylindricalroller bearings and a double-direction angular contact thrust ballbearing. Radial and axial loads are accommodated separately.FAG 20