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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 is
the 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 300°C (300
to 570°F). 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.
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