Tuesday, September 27, 2016

Bearing Design Guide: Chapter Twelve: Recommended Shafting and Journal Material

           The shaft or journal is the mating part of any bearing application and, therefore, requires ample consideration when mated with one of the many bronze alloys which have varying mechanical properties compatibility and hardnesses.

          The designer must first choose a shaft or journal material that will satisfy the applications requirements of torque, shear stress, fatigue strength, fracture toughness, rigidity, wear resistance, corrosion resistance and have the ability to provide a good surface finish and sufficient hardness.

          Various type of shaft or journal materials such as cast gray iron, modular iron, forged steel, induction hardened steel, case hardened, chrome-plated and polished steels are used.
The cast iron, gray and modular iron shafts offer low cost but since they do not pose all of the desirable shaft properties of steel, they have somewhat limited usage. Further, they require specific grinding and polishing instructions.
          The more popular shafting steels are mild SAE 1020 and low-carbon steels of SAE 1040. The highercarbon steels- AISI 1045, 1060,4140,4340, 52100 and M 50 tool steels and stress-proof steels are used after hardening, grinding and polishing finish.

                      The general range of such shafts are classified by hardness as follows:

                                                      Soft 165 BHN to 290 BHN
                                                      Medium 300 BHN to 390 BHN
                                                      Hard 400 BHN to 1000 BHN

          There are commercially available standard shafts which have soft cores but a sub-surface which is case hardened to various depths, chrome-plated and polished.

          Tests have proven that various hard bronze alloys, when mated with a soft shaft, will tend to seize or weld to the shaft at substantially lesser loads than when mated with a hard shaft.

          The harder shaft will permit an appreciably higher unit load to be sustained with a lower wear rateresulting in extending the life of the assembly.

          With the high leaded bronze aUoys, which range in hardness of 50 to 70 BHN, a soft or mild steel, or cold-rolled steel shaft ranging in hardnesses of 165 BHN to 290 BHN are a suitable combination.

          Shafts mated with the high copper-tin alloys which contain little or no lead having a range of hardnesses of 70 BHN to 80 BHN, would be adequately served by the medium shaft hardness range of 300 minimum.

           The aluminum bronze alloys, not heat-treated, with average hardness of 140 BHN to 170 BHN are best combined with the hard shaft category of 400 BHN minimum.

          Heat-treated aluminum bronze alloys, including the manganese bronzes with hardness ranges from 180 BHN to 240 BHN, require the shaft hardness range to be above 500 BHN or recommended to be used with shafts RC60 minimum being preferred.

          In bearing tests conducted by various sources, the results indicated that the harder shafts with the highest polished finishes offer the best combination for improved load carrying ability over the speed ranges tested with the lowest wear rate.
          Initially, most bronze alloys will show a high wear rate while "bedding" or "running in" but will level off to a more constant lower value.

          The surfuce finishes of the shaft have a profound effect in any bearing application on the lubricating mode.

          In full hydrodynamic conditions, the bronze bearing alloy should have a RMS finish in the range of 25 to 32 RMS and the shaft or journal held to polished to 6 RMS to 12 RMS.

          In mixed film conditions, the bronze bearing alloy can range between 32 RMS to 43 RMS and the journal polished to 8 RMS to 16 RMS. Since the bearing goes in and out of full hydrodynamic mode, the better the finish the less the initial wear.

          For boundary conditions, where the surface speed is much lower, the relative roughness of the bearing .and journal are not as critical. However, it is best to maintain at least a 43 RMS to 63 RMS finish on the bronze bearing and the journal to 32 maximum.

          A hardened and super finished shaft has the ability to double the load before seizure when compared to a soft shaft finished to 10 RMS under the same conditions. In loads over 3000 PSI, a 10 RMS shaft finish is recommended.

          In the case of sintered powdered metal self-lubricating bearings, a cold-rolled steel or mild steel shaft would be acceptable. However, a hardened carbon steel shaft such as C1137 with chrome-plating will double the PV factor while reducing shaft and bearing wear.

          Whenever a stainless steel shaft must be considered because of corrosion conditions, it is not
recommended that the 303 austenitic stainless series be used in combination with sintered bronze bearings unless it is chrome-plated or the sintered bearing is re-impregnated with a special lubricant containing oxidation and corrosion inhibiting additives.

          If a stainless steel shaft is necessary, it is recommended the 400 SS Series with the 440 C being the preferred shaft since it does not require the special lubes.

           Remember, in all cases, that the harder the shaft, the greater the load-capability and the better the surface finishes, the less the wear rate with longer bearing life. Shaft roundness and shaft size control without nicks, gouges or sharp edges will offer the most satisfactory performance.

 Next Up: Chapter Thirteen: Lubrication & Lubricants
Chapter Fourteen: Lubricant Selection


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