Distortion of gearing during heat treatment is inevitable and varies
with the hardening process. The part design and manufacturing process
must consider movement during heat treatment. Tolerancing must consider
these changes. Section size modification may be required along with
added stock for grinding or machining after heat treatment.
Dimensional changes of gearing resulting from heat treatment occur
principally when steel is quenched. These changes occur in both quenched
and tempered and surface hardened gears. Distortion is due to mechanical
and thermal stresses
and phase transformation. Process variables and design considerations
have a significant effect upon the amount of distortion. High induced
stress can result in quench cracking. Thermal processes such as
annealing, normalizing, and diffusion controlled surface hardening
processes such as nitriding, which do not require liquid quenching,
result in less distortions than processes that require liquid quenching.
- Quenching and Tempering.
Quenched and tempered gearing changes size and distorts due to
mechanical and thermal stresses and microstructural
transformations. Quenching the structure to martensite prior to tempering
results in steel growing in size. Tempering of the hardened structure
reduces the volume, but the combined effects of quenching and tempering
still result in a volume and size increase.
Distortion of quenched and tempered gearing occurs generally as follows:
- Outside and bore diameters grow larger and go out of round.
- Side faces become warped, and exhibit runout.
- Pinions. Pinions become bowed, with the amount of bowing increasing
with higher length/diameter ratios and smaller journal diameters; amount
of bowing or radial runout is often confined to journal diameters and
shaft extensions for integral shaft pinions.
Normally, rough gear blanks (forging, bars tock, or casting) have
sufficient stock provided so distortion can be accommodated by
machining. High L/D ratio pinions may require straightening and a
thermal stress relief prior to finish machining. In some exceptional
instances, straightening, thermal stress relief, rough machining, and a
second stress relief
prior to finish machining may all be necessary to keep the pinion
dimensionally stable during finish machining.
Sequence of manufacture is dependent upon design considerations and the
temperature used for stress relief. Stress relief temperature is
dependent upon specified hardness and temper resistance of the steel.
Modified methods of quench hardening, such as austempering of ductile
iron, reduces distortion and forms a modified hardened structure at
higher quenchant temperatures than those conventionally used.
- Surface Hardened Gearing.
Distortion must be minimized, controlled and made predictable to minimize
costly stock removal (lapping, skiving, or
grinding), when tooth accuracy requirements dictate. Selective surface
hardening of gear teeth by flame and induction hardening results
essentially in only distortion of the teeth because only the teeth are
heated and quenched. Amount of distortion increases with case pattern
depth and increases as more of the tooth cross section is hardened,
compared to profile hardened tooth patterns. Distortion is not limited
to gear teeth, however, when the entire gear is heated and quenched as
- Carburized Gearing.
Distortion of carburized gearing makes it one of the least repeatable of
surface hardened processes. Lack of repeatability
is due to the greater number of variables which affect distortion. Close
control is, therefore, required. Distortion results from microstructural
transformation, and residual stress (from thermal shock, uneven cooling,
etc.) considerations. Transformation in the case results in growth which
sets up residual surface compressive stress. This stress is balanced by
corresponding residual tensile stress beneath the case. Principal
variables affecting the amount of growth, distortion, and residual
NOTE: Direct quenching generally results in less distortion than slow
cooled, reheated and quenched gears, providing gears are properly cooled
from the carburizing temperature to the quench temperature before
- Hardenability (carbon and alloy content) of the base material.
Higher hardenability increases growth and distortion.
- Fixturing techniques in the furnace and during quenching.
- Carbon potential of the carburizing atmosphere.
- Carburizing temperature and temperature prior to quenching.
- Time between quench and temper for richer alloys.
- Quenchant type, temperature and amount of agitation.
- Resultant metallurgical characteristics of the case, such as carbon
content, case depth, amount of retained austenite, carbides, etc.
Once a component is designed to minimize distortion, processing
techniques should be optimized to make distortion consistent. At times,
redesign of components may be required to reduce distortion.
Stock removal by grinding after carburize hardening should be limited to
approximately 0.007 inch (0.18mm) per tooth surface or 20 percent of the
case depth, which ever is less. Exception may be made for coarser pitch
gearing with cases 0.080 inches (2 mm) or greater. Surfaces other than
the tooth flanks and roots may tolerate greater stock removal.
General design considerations of
gearing related to
distortion include the following:
Distortion of carburized gearing also exhibits the following typical
- Larger teeth (lower DP) distort more.
- Rim thickness should be the same at both end faces.
- Radial web support section under the rim should be centrally
located. Web support section thickness under the rim is recommended to
be not less than 40-50 percent of the face width for precision
Near solid “pancake” gear blanks, designed with moderate recess on both
sides of the web section, distort less. The recess is provided to enable
clean-up grinding of the rim and hub end faces after hardening.
- Holes in the web section close to the rim, to reduce the weight or
provide holes for lifting, may cause collapsing of the rim section over
- High length/diameter ratio pinions distort more. Journals may be
required to be masked in order to prevent carburizing and then be finish
machined after hardening with sufficient stock for clean-up. Masking can
also be used for ease of
- Cantilever pinions, with teeth on the end of the shaft, and “blind
ended” teeth on pinions, where the adjacent diameter is larger than the
root diameter, present problems from both distortion and
- Reduction in tooth helix angle (“helix unwind”), which often
requires an increased helix angle to be machined into the element prior
to carburizing (more prevalent in pinions). Teeth on larger diameter,
smaller face width
gears may exhibit “helix wind-up” after hardening.
- End growth on gear teeth at both ends of the face due to increased
case depth (carburizing from two directions, 90 degrees apart, followed
by improved quench action for the same reason) may appear as reverse
tooth crowning on narrow face gearing.
Teeth are often crown cut prior to hardening to compensate for reverse
crown or are chamfered at the ends of teeth. Teeth may also be both
crown cut and chamfered.
- Eccentricity (radial run-out) of gears and their bores is dependent
upon how they are fixtured in the furnace.
- Taper across the face (tapered teeth), bore taper and
“hour-glassing” of the gear bore can occur due to non-uniform growth of
teeth across the face and non-uniform shrinking of the bores.
- Bowing of the integral shaft pinions. Integral shaft pinions should,
whenever possible, be hung or fixtured in the vertical position (axes
vertical) to minimize bowing.
Gears may be fixtured vertically through the bores or web holes on a
support rod (axes horizontal), or fixtured horizontally (individually or
stacked) to minimize distortion, depending on size and face width.
Larger ring gears are positioned horizontally with sufficient stock for
clean-up of the teeth. Bores and web sections can be masked to prevent
and enable subsequent machining. Thin section gears, such as bevel ring
gears, may be press quenched to minimize distortion.
- Flame and Induction Hardened Gearing.
Flame and induction hardened gearing generally distort less than
carburized gearing because only the teeth are heated and subsequently
quenched. Contour induction hardening of tooth profiles produce less
distortion and growth than spin hardening methods.
During both spin flame and spin induction hardening, the entire tooth
cross section is often hardened to the specified depth below the roots
For high bending strength applications, it is not desirable to have the
hardening pattern terminate in the roots of the teeth because of
residual tensile stress considerations. Distortion increases as a
greater cross-section of a tooth is hardened. Spin flame and spin
induction hardening generally produce the following distortion
- Helical unwinding of the gear teeth, as with carburized
- Increased growth of the teeth (greater than for carburized gearing)
because the entire tooth cross section may be hardened in finer pitch
- Crowning or reverse crowning of the teeth across the face dependent
upon the heat pattern.
Crowning is more desirable from a tooth loading standpoint.
- Taper of teeth due to varied heat pattern and case depth across the
Distortion of the teeth from spin induction hardening is often
considered more repeatable than with spin flame hardening, because of
fewer human error factors involved during machine and inductor set-ups
with induction hardening. Spin flame hardening involves more manual
set-up factors, which include positioning of the flame, gas flows, etc.
However, spin flame hardening can be engineered with special flame heads
and fixtures for required control.
CAUTION: Deep spin hardening of gear teeth may cause excessive tooth
growth and may affect bore size.
- Nitrided Gearing.
Nitriding of gearing results in less distortion, compared to
flame, and induction hardening. Prior quench and
temper heat treatment, which results in distortion, is done before
machining and nitriding. Parts are also not heated above the
transformation temperature or previous tempering temperature of the
steel during nitriding, and are not quenched, as occurs during
carburizing, flame or induction hardening. Therefore, nitrided
teeth are not generally required to be
ground or lapped after hardening to meet dimensional tolerance
requirements. Bearing diameters of shaft extensions are often ground
nitriding with only minimum stock provided. Surfaces can also be
masked for subsequent machining.
When close tolerances are required, gearing can be rough machined and
stress relieved at 50_F(28_C) below the prior tempering temperature to
relieve rough machining residual stress prior to finish machining and
nitriding, outer surfaces grow approximately 0.0005-0.001 inch
(0.013-0.025 mm). Bores size may shrink up to 0.0015 inch (0.04 mm)
depending upon size.
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