Page 276 - Cam Design Handbook
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THB9  9/19/03  7:26 PM  Page 264

          264                      CAM DESIGN HANDBOOK

             Rolling-element fatigue can be initiated by hard inclusions in the material; corrosion;
          surface stress raisers such as dents, grinding imperfections, or geometric stress concen-
          trations; microspalling; and surface interactions dependent on roughness and lubrication
          film thickness. Sometimes thin layers of case hardening, surface checks from heat treat-
          ment and high sliding velocities, excessive temperature, type of lubricant, or contamina-
          tion can accelerate the action of fatigue.
             If the surface is stressed in a corrosive environment, a phenomenon called corrosion
          fatigue or stress corrosion occurs. Corrosion on either the cam or roller follower will ini-
          tially roughen the smooth rolling surface and form minute pits from which a fatigue crack
          will start due to the local stress concentration. The use of a lubricant has another function
          besides the reduction of friction of the surfaces, which is to prevent corrosion and ensure
          longer fatigue life.

          9.4.4.2 Surface Fatigue Design.  As mentioned previously cams, rolling-element bear-
          ings, and gearing are machine elements that are similar in their performance, and they fail
          most frequently by surface fatigue. Unfortunately statistical wear data have been devel-
          oped  for  rolling-element  bearings  and  gearing  but  not  for  cam-follower  systems.  The
          diversity and complexity of cam-follower systems has prevented a complete wear design
          methodology to evolve.
             Nevertheless,  this  section  presents  a  simplified  design  procedure  for  the  cam
          engineer. The data presented will be of initial value as a guide in selecting compatible
          material  combinations  for  the  cam  and  follower.  Ultimately,  the  proper  materials,
          wear performance, and lubricant must be confirmed in the field under actual operating
          conditions.
             Test data are presented for the comparison of cam and follower fatigue under pure
          rolling and rolling with some sliding conditions. Experience has shown that some sliding
          may occur, even at low speeds, affecting the wear life of the contacting surfaces.
             Life  surface  fatigue  tests  were  conducted  by  Talbourdet  (1950),  Morrison  (1968),
          and  Cram  (1956)  utilizing  radially  loaded  contacting  cylinders  with  (a)  pure  rolling
          and (b) rolling with 9 percent sliding. Three-inch diameter mating rollers were used; one
          roller  had  a  hardened  steel  surface  (60-62  RC)  and  the  other,  softer  roller  was  cast
          iron, steel (of different hardnesses), bronze, and nonmetallic materials. The weaker roller
          material wore out under the cyclical loading which established the life of the materials in
          combination.
             The stress test data algorithm relates s max to the hertzian compressive stress Eq. (9.11)
          for dissimilar metals in surface contact.
             For a Poisson’s ratio m = 0.3 and the normal force per cylinder length
                                          K
                                    P ¢ =       lb in                   (9.13)
                                       Ê 1  1  ˆ
                                       Á  +   ˜
                                       Ë r  r  ¯
                                         1   2
          where
                                       s  2  Ê 1  1  ˆ
                                    K =  max  Á  +  ˜                   (9.14)
                                        . 035  Ë  E 1  E ¯
                                                 2
          which is called the load-stress factor for cylinders in contact. Table 9.3 lists values of K
          for 100 million stress repetitions for pure rolling and rolling plus 9 percent sliding action.
          One roll is of hardened steel 60-62RC while the other is of various selected materials.
          Values of K have been determined as a function of the number of stress cycles necessary
          for surface failure. The test lubricant was mineral oil 280-320 SSU and 100°F at a surface
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