Producing gear teeth with high form accuracy and fine surface
finish using water-lubricated chemical reactions
K. Oobayashia, K. Iriea, F. Hondab,*
aAISIN AW CO., LTD, 10 Takane, Fujii-cho, Anjo, Aichi 444-1192, JapanbToyota Technological Institute, 2 Hisakata, Tempaku, Nagoya 468-8511, Japan
Abstract
A new method was investigated for high-accuracy fine finishing of gear teeth surfaces using a water-lubricated tribo-chemical technique. A
pair of shaved gears with rather low surface roughness was rotated in water lubricant for 30 min so that the gear tooth surface contacting the
mating tooth was ‘worn’ to a mirror surface and ideal tooth profile, due to the mechano-chemical mild erosion of the contact area. The wear
rate was 2.0 mm per 20,000 meshings, corresponding to a wear of one atomic radius thickness per meshing. Oxidation of the steel surface by
water molecules is proposed as the dominant wear process. Operation noise from the gear pair rotation was drastically reduced to lower than
about 10–15 dB compared to conventionally machined gear surfaces (30 dB in average), as a result of the wear of the tooth surface to form a
best-fit profile. The noise increased with further processing of the gear pair. Thus, there is an appropriate number of rotations for suitable
surface wear treatment. This new and simple procedure for surface treatment assures saving in energy, and does not require expensive honing
techniques or high-accuracy grinding tools.
The wear mechanisms used in this process are discussed along with the application of the technique to other processes for precision
finishing.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Water-lubrication; Tribo-chemistry; Mild wear; Fine finishing; Gear tooth; Low noise; Mirror surface
1. Introduction
For the machining of accurate and fine gear teeth, the
numerically best-fit profile of the tooth has been developed
using sophisticated calculations [1,2]. Manufacturing of the
tooth shape near a best-fit involute curve profile can be
carried out through the rough cutting, shaving, honing, and
fine finishing procedures using rather high-accuracy cutting
tools. Additional and final finishing involves a running-in
process to obtain the best-fit pair contact alignment and of
the lowest noise during gear revolution. Each of these
processes and the manufacturing time accounts for a great
percentage of the total energy of gear production. For
example, after the honing process, a variety of GRz 2.5 mm
still remains between the produced surface profile and the
numerically ideal one.
Only a small fraction of a tooth surface requires a high-
accuracy profile to contact the mating gear surface, so
0301-679X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.triboint.2004.08.022
* Corresponding author.
the high-accuracy finishing can be limited to a small contact
area. The remaining area (outside of the contacted area) on
the tooth can remain relatively rough.
The important parameter to monitor the fitness of the
surface profile is the noise level of the pair of gears in
operation. Many experiments and simulation studies [3]
have been carried out to reduce the noise level but are not
completed due to the non-linear combined effects of mutual
relationships among complicated shapes, vibration and
friction. An additional objective of the surface finishing of
the gear would be improvement of the mechanical strength
of the finished surface layers. A high-energy fluctuating load
applied to the gear tooth in operation induces surface fatigue
or undesirable exfoliation of the stressed dedendum surface
layers on the gear [4,5]. Micro-cracks on the machined
surface must usually be minimized to maintain the longer
service life of the gear.
In this report, we present the results of a wear process
by which a pair of gear teeth are worked in water, thereby
using the tribo-chemical oxidation for the fine machining.
Tribology International 38 (2005) 243–248
www.elsevier.com/locate/triboint
Fig. 1. Systematic diagram of the test bench for water-lubricated surface
finishing.
K. Oobayashi et al. / Tribology International 38 (2005) 243–248244
We monitored the wear rate as a function of the number of
teeth meshings and examined the wear process for obtaining
the best conditions for the oxidation reaction with water [6].
2. Experimental procedure
A pair of gears (120 mm in diameter) was mounted on a
bench tester designed especially for the tribological and
noise monitoring tests during rotation, using water as the
lubricant. The apparatus is designed to apply a variable load
between the drive and driven teeth (190–1500 N). Fig. 1
shows the schematic diagram of the tester. The geometric
alignment of the contacted position of each gear and the
distance between the two gears are adjustable. Distilled
water (1000 ml/min) was supplied continuously between the
mating gear teeth under contact. The gears were not
immersed in water, to avoid additional rotation resistance.
The applied load, rotation rate, and noise level were
measured continuously by sensors. The noise sensor signal
was analyzed in the range from 0 to 10 KHz at the point
closest to the gears.
The gears were made of alloy steel (JIS SCM420H
standard steel which almost corresponds SAE 5120, and
DIN 20CR4), forged, and shaved to a surface roughness of
Rz 2.5, and then carburized. The hardness of the curburized
surface was 750 Hv. After testing at a given rotation number
and load, the tooth surfaces were analyzed by a high-
accuracy profilometer, measuring surface roughness, wear
Fig. 2. Surface roughness of the water-finished gear tooth, along and across the p
grinding is presented for reference.
position and tooth profile at the contact area. The wear
position and profile of the contacted surface clearly showed
great changes as a function of the number of gear meshings
(i.e. calculated from the number of revolutions). The
numerically ideal involute curve of the gear was referred
to, in evaluating the wear surface.
3. Results and discussion
3.1. Surface roughness of worn teeth
The surface roughness of the water-lubricated gear tooth
(referred to as ‘tribo-assisted oxidation for fine finishing’:
TOFF) is shown in Fig. 2, compared with surfaces after the
procedures previous to TOFF. The first row in Fig. 2 indicates
the roughness along the profile direction (across the pitch-
line), and the second row indicates the lead direction (along
the pitch-line) of the tooth, together with the quantitative
expression of roughness Rz. These are results for a reference
condition of 500 rpm, 415 N of load, and 20,000 revolutions.
The contacted regions on the surface had a mirror surface (Rz
0.5 mm), much better than that before finishing (Rz 2.5 mm).
The mirror-smooth area expanded to a larger area by
increasing the number of gear revolution.
In Fig. 3, the profile of (a) the shaved surface is shown,
plus the results of (b) worn surface after 20,000 revolutions,
and (c) after 50,000 revolutions. The straight lines in Fig. 3
indicate the ideal involute curve. The involute curve is
exactly three-dimensional curve but mathematically
expressed by a straight line to simplify the deviation of
the measured surface from the ideal involute profile. Both
ends of the gear profile are manufactured to deviate largely
from the involute curve for smooth release of the load
during meshings. Therefore, a profile closest to the straight
line (excluding the both gear ends) in Fig. 3 is a best-fit
profile. A smooth profile closest to the ideal involute profile
was obtained between 15,000 and 20,000 revolutions. Over
this range, the entire tooth surface was smooth but the
profiles became worn beyond the calculated ideal involute
curve by 50,000 revolutions, as is shown in Fig. 3(c).
itch-line. Average surface roughness before finishing and after honing and
Fig. 3. Two-dimensional profiles of the water-lubricated gears as a function of the number of mesh contacts. Profiles across and along the pitch-line are
presented.
K. Oobayashi et al. / Tribology International 38 (2005) 243–248 245
Fig. 4 presents three-dimensional maps of the surfaces
(a) before water-lubrication and (b) after 20,000 evolutions
showing close to the best-fit profile, while (c) reveals
the over-worn surface after 50,000 revolutions. Fig. 4(b)
indicates that the suitably treated tooth surface is very
Fig. 4. Three-dimensional surface profile of drive and driven surfaces. Excess
smooth in local area with the mesoscopical roughness and
matches well macroscopic involute curve. In Fig. 4(c), the
roughness in local regions is still fairly smooth, but
measurements show high waviness especially in dedendum.
In addition to drive and driven gear surface profiles,
wear is observed after 30,000 mesh contacts with the mating gear tooth.
K. Oobayashi et al. / Tribology International 38 (2005) 243–248246
the third row in Fig. 4 indicates the mathematical
summation of the two surface profiles. Therefore, the third
row do not show any real gear profile but realizes that the
highest points in these maps are the closest points between
the gears which come into contact at first. The relatively
smooth surface suggests mild wear.
From the area of the mirror surface and profiles before
and after the wear-test operation, the wear rate was
determined within an accuracy of G0.1 mm. The load was
fixed here at 415 N. The observed result of this wear rate
was 2.0 mm per 20,000 revolutions, corresponding to a wear
of 0.10 nm thickness in the surface layer per revolution. An
average thickness of a single atomic radius of iron surface is
thus removed only from the meshing area of the tooth.
Fig. 5 shows the tooth profile after running under high
load of 1132 N. The worn surface area was larger under
increasing load in comparison with the results of the same
number of meshings and load of 415 N, as is shown in
Fig. 5(a) and (b). The wear rate, in term of thickness,
however, was almost the same, equivalent as determined by
the profile. Based on these results, the mating tooth surfaces
apparently were deformed under high load and that the area
of gear contact between the mating surfaces increased, yet
the wear ‘thickness’ on the surface did not change.
In the contact mode examined here, a tooth surface mates
once per gear revolution. The tooth pair in contact is not
fixed because each gear has different number of teeth.
The best-fit contact condition, therefore, is averaged on all
Fig. 5. Tooth profiles as a result of high load contact. The mirror-finish area increas
the tooth pair of the gear. It is reasonable to consider that the
highest spot on the tooth surface comes into contact
preferentially and undergoes mechano-chemical wear,
depleting layer-by-layer [7,8] beginning from the top-most
atomic layer on the tooth surfaces. By scanning electron
microscope (SEM) observations, at least, no etch pit or
micro-cracks were observed on the smooth contacted
surfaces. The time interval between each mating until the
next mating contact is 0.12 s at 500 rpm, and 0.01 s at
6000 rpm. This is long enough for the clean (intrinsic) iron
surface to react with the surrounding water molecules,
because the rate of water adsorption to a clean surface is
known to be of the order of 1 ms (1 Langmuir). The surface
top-most iron atoms are allowed to react with water
molecules during one turn of the gear mating contact, and
the reaction product layer is removed from the surface. We
assumed that the surface structure is composed of Fe–O or
Fe–OH bonds on the adsorbed surface of two-dimensional
layer, because two-dimensional adsorbed structure cannot
be defined as compounds such as FeO, Fe2O3, and Fe3O4
have been identified on the slid surfaces in water [6]. If
reaction products were accumulated as oxide layers and
formed crystalline oxides, multiplayer of oxides can be
removed from the contacted area, due to weak shear strength
in the oxide layers. In this contact condition, wear do not
proceed layer-by-layer and do not make smooth metallic
surface. In the relevant wear of gear surface, meshing
number determined the wear rate as half-monolayer per
ed with the load, but the wear rate was almost equal to the result at low load.
Fig. 6. The typical noise level observed on the test bench. First- and second-order noise decreased abruptly at minimum.
K. Oobayashi et al. / Tribology International 38 (2005) 243–248 247
contact of the iron surface, on an average. The wear
mechanism is still unclear by our experimental results but
the surface observations suggested the wear proceed layer-
by-layer, which is different from the sliding tests in water.
3.2. Noise level of gear revolution
The ideal smooth profile produced a minimum noise
level under repeated meshing and unmeshing of the gear
teeth surfaces. The noise level could, therefore, be an
important diagnostic parameter in determining the most
desirable surface profile. Fig. 6(a) shows a typical result of
the first- and second-order vibration observed on the TOFF
tester. Starting from 25 dB of the first-order vibration, the
noise decreased to 12 dB at minimum, and the second-order
noise showed the same decrease to 20,000 revolutions. The
noise reduction of K13 dB on the test bench correspond to a
noise reduction from 96 to 85 dB in an automatic
transmission (AT) assembly (87 dB is the standard noise
limit for compliance). The minimum noise appeared after
about 20,000 revolutions from the test start for both
vibration modes in the case of Fig. 6(a), but in some
cases, the primary noise minimum was reached at 18,000
revolutions as shown in Fig. 6(b). For some specimens (c),
even two minimum stages appeared in the secondary noise.
If one presumes this to be due to some size-fluctuation
particular to various original specimens, at an average of
between 15,000 and 27,000 meshings, a minimum noise
would appear, and minor hump of secondary noise would
Fig. 7. Noise level observed on an automatic transmission assembly, after
finishing treatment on the test bench.
depend on the original surface profile. The noise drop by the
TOFF treatment was rather drastic compared to other
treatment such as honing and shaving, so the profile closest
to the ideal one was very effective for noise suppression.
When TOFF tested gears were placed into an AT
assembly, the noise level was found to be lower than with
conventional gears. This is shown in Fig. 7, which is a graph
of gear noise as a function of gear revolution rate. Within
the observed range of revolution rate, the noise levels were
suppressed satisfactorily, especially the sympathetic
vibrations at 700 and 1200 rpm. A small percentage of the
tested specimens, however, showed almost the same noise
level. This is probably due to misalignment of the gear pair
and/or inhomogeneous deformation during transfer from
the TOFF device to the AT assembly. The exact same
geometric configuration on a TOFF device is very difficult
to reproduce on the AT assembly; results showed some
fluctuation on the best-fit contact reproduction. As has been
indicated above, the best-fit profile of a tooth appears after
almost the same number of revolution: 15,000–22,000.
Thus, it is reasonable to consider that the three-dimensional
best-fit profile of the tooth produces the lowest noise
condition. Monitoring of the vibration in the TOFF
operation is therefore recommended.
Operation beyond the minimum noise stage resulted in
smooth surfaces but the worn profile emitted higher noise as
before the TOFF treatment.
In this report, we supplied plain distilled water to the gear
surface mating points so that the reactions with water were
obviously accelerated tribologically. In fact, an area on the
gear surface not contacted is not oxidized or worn visually
at least. No rust on the surface is observed after a few hours
of running water. The wear rate of plain distilled water was
suitable for controlling for the relevant purpose. For general
application of this finishing technique to other complex
shapes of surface and/or inner surface of work to be
polished, one could use combinations of planetary gears, or
smaller size rotators could be inserted inside the work.
4. Conclusions
When water-lubricated boundary condition was used
between gear teeth in finishing, a very smooth metallic
surface was obtained as follows.
K. Oobayashi et al. / Tribology International 38 (2005) 243–248248
(1)
The tooth surface was finished with a profile extremelyclose to the numerically designed best-fit profile, or to a
profile of best-fit curve for a pair of gears. The mesh
contact had a mirror finish.
(2)
The wear of the tooth surface proceeded layer-by-layeras a linear function of the number of mesh contacts and
preferentially at the contact point or area.
(3)
The noise level abruptly decreased from 25 to 12 dB onthe test bench at minimum, on the best-fit tooth profile
of the gear.
The new procedure introduced in this study reduced the
total energy for fine, high-accuracy finishing of gear teeth
and other surfaces, with applying minimum shear stress on
the surfaces.
Acknowledgements
The authors express their gratitude to AISIN AW Co, Ltd
and Toyota-Shokki Co Ltd, for their generous financial
support for our study.
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