NKE Hauptkatalog / General Catalogue 2011

Selection ofBearing Type and SizeAccuracy of gearhigh precision gearspitch and form errorsless than .02 mmstandard accuracypitch and form errorsbetween .02 and .1 mmTable 6.6Gear factorf Z1.05 ÷ 1.11.1 ÷ 1.3Additional Forces ofChain and Belt DrivesChain and belt drives create additional forces thatmust be considered for bearing dimensioning.Belt drives always run under preload to enablethe transmission of forces. This invariably causesvibrations.In case of chain drives vibrations and shockloads occur frequently.Some empirical values for consideration of theseadditional forces are listed in table 6.7 by applyingthe factor f Z .FactorType of driveChain drives 1.1 ÷ 1.5Belt drivesV-belt 1.5 ÷ 2.5Toothed belt 1.1 ÷ 1.5Flat belt 3 ÷ 4Flat belt with pulley 2.5 ÷ 3Table 6.7f ZCalculation of Bearing Load andSpeed under Variable Operating ConditionsIt is the exception that machines operate atuniform load and constant speed all the time.Normally the magnitude of load, forces, and therotational speed vary during operation.However, more often the parameters follow acertain pattern, such as during a CNC machineproduction cycle, when this cycle loading andspeed change is repetitive. In some cases loadpatterns are defined by customer requirementsand as such included within the bearing designarrangement.To determine a realistic magnitude for theestimation of bearing life the variable loads andspeeds have to be transferred into an imaginary(fictitious) constantly acting mean load F m andrespectively a uniform mean speed n m .Depending upon the individual conditions or theload or speed pattern the mean load F m and themean speed n m may be calculated according tothe formula shown on page 288 Ep. 6.9 and Ep.6.10, respectively.Rectangular Course (fig. 6.1):A typical load and speed pattern for powertransmissions, e.g. in a mechanical gear box isrepresented by a staircase input of load and/orspeed.Fig. 6.1264www.nke.at