Abstract
An attempt is made to predict the pitting strength of cast iron and copper alloy materials from their compressive yield or compressive proof strength for a reliability of 99% at 107 load cycles. The compressive yield or compressive proof strength is related to the tensile strength of ductile cast iron and copper alloy materials by a proportionality factor. Two proportionality factors are used for brittle cast iron materials. The pitting strength formulation incorporates a nominal design factor at 99% reliability which is estimated from a probabilistic model based on the lognormal probability density function. Pitting strength estimates from the predictions are compared with those of American Gear Manufacturers Association (AGMA) estimates and data from other sources. The predicted values for gray cast irons had variances in the range of -11.28% to 25%. Ductile cast iron pitting strength estimates deviated from those of AGMA by -30.28% to 1.73% and 16.76% to 36.34% for Austempered ductile irons. The variances obtained for cast bronze were from 11.17% and 14.73%, but the sample size was small. These variances appear to be reasonable due to the many factors that can influence pitting resistance. Since pitting strength data for many grades of cast iron and copper alloys are not available (especially in the public domain), they may be estimated by the expressions developed in this study for initial design sizing. Also, the pitting strength of new cast iron and copper alloy materials could likewise be estimated for initial design sizing. This will eliminate long and costly contact fatigue testing at the initial design phases, which of course is necessary for design validation.
Highlights
An attempt is made to predict the pitting strength of cast iron and copper alloy materials from their compressive yield or compressive proof strength for a reliability of 99% at 107 load cycles
By Hertz stresses, load cycles, hardness, surface rough- Contact fatigue is extremely important for all engineering ness, temperature, and degree of lubrication [1]
Eq (15) indicates that the compressive yield or compressive proof strength can be related to the tensile strength of linearly-asymmetric materials by two proportionality factors αyt and αuc
Summary
Hertz contact stress bears the name of the German physicist, Henry Hertz who first developed expressions for the stresses and deformations created when curved frictionless surfaces are statically loaded normally in 1881 [12]. A) Two cylinders in contact b) Contact pressure distribution Fig. 2: Cylindrical contact. In frictionless Hertzian contact under a static normal load, a localized complex stress state that is concentrated in a small volume of material is produced. Relative rolling motion between contacting bodies cre– ates the same type of stress field as in static normal load but the contact patch and the stress field is in continuous motion. Speci– fically, the presence of sliding introduces a tensile stress component in the contact zone and leads to increases in contact stress component values as well as cause the location of the maximum shear stress below the contact surface to migrate upward [2, 13]
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