Critical Scuffing Temperature for Gearss

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Critical Scuffing Temperature for Gears - Research has shown that for a given mineral oil without anti-scuffing or extreme pressure additives, there is a critical scuffing temperature that is constant regardless of operating conditions. Evidence indicates that beyond the critical temperature, scuffing will occur. Therefore, the critical temperature concept provides a useful method for predicting the onset of scuffing. The critical scuffing temperature is a function of the gear bulk temperature and the flash temperature and is expressed as:

where the bulk temperature Tb isthe equilibrium temperature of the gears before meshing and the flash temperature Tf is the instantaneous temperature rise caused by the local frictional heat at the gear teeth meshing point. The critical scuffing temperature for mineral oils without anti-scuffing or extreme pressure additives increases directly with viscosity and varies from 150 to 300C (300 to 570F). However, this increased scuffing resistance appears to be directly attributed to differences in chemical composition and only indirectly to the beneficial effects of increased film thickness associated with higher viscosity. Examination of the critical temperature equation indicates that scuffing can be controlled by lowering either of the two contributing factors. The bulk temperature can be controlled by selecting gear geometry and design for the intended application. The flash temperature can be controlled indirectly by gear tooth smoothness and through lubricant viscosity. Smooth gear tooth surfaces produce less friction and heat while increased viscosity provides greater film thickness, which also reduces frictional heat and results in a lower flash temperature. Furthermore, judicious application of lubricant can cool the gears by removing heat.

For synthetics and lubricants containing anti-scuff additives, the critical temperature depends on the operating conditions and must be determined experimentally for each case. Anti-scuff additives commonly used are iron sulfide and iron phosphate. These additives react chemically with the protected metal gear surface to form very strong solid films that prevent metal contact under extreme pressure and temperature conditions.

The following guidelines should be observed to prevent scuffing in gear units:

  • Specify smooth tooth surfaces produced by careful grinding or honing.

  • Protect gear teeth during the running-in period by coating them with iron-manganese phosphate or plating them with copper or silver. During the first ten hours of run-in, new gears should be operated at one-half load.

  • Use high-viscosity lubricants with antiscuff additives such as sulfur, phosphorus, or borate.

  • Make sure the gear teeth are cooled by supplying adequate amount of cool lubricant. For circulating-oil systems, use a heat exchanger to cool the lubricant.

  • Optimize the gear tooth geometry. Use small teeth, addendum modification, and profile modification.

  • Use accurate gear teeth, rigid gear mountings, and good helix alignment.

  • Use nitrided steels for maximum scuffing resistance. Do not use stainless steel or aluminum for gears if there is a risk of scuffing.