Field of the Invention
The present invention relates generally to gears. More particularly, it relates
to gears, for example transmission gears, having a reduced surface roughness
resulting in increased contact fatigue life, improved wear resistance and improved
performance.
Background
Many methods of finishing gear teeth are known. For example, in gear
hobbing and shaving, a gear is rotated in mesh with a gear-like cutter tool. The gear-
like tool has cutting edges that extend up and down the sides of the teeth parallel to
the plane of rotation. This is accompanied by a relative traverse between the gear and
the cutter in a plane parallel to the axis of the gear and the cutter. The resulting
finished surface roughness is in the range of 40 to 80 micro-inches Ra, where Ra is
the arithmetic mean roughness.
Another method of finishing the teeth of a gear is known as gear grinding. In
gear grinding, the resulting finished surface roughness is typically 15 to 35 micro-
inches Ra.
In another method of gear finishing, a honing operation is performed. Here,
the gear is rotated in mesh with a gear-shaped hone. Portions of the hone at the said
of the gear teeth are fabricated from a plastic material that is relatively hard yet highly
resilient. The honing operation occurs by rotating the hone in mesh with the gear
while providing a traverse stroke parallel to the axis of the gear. This distributes the
finishing action evenly throughout each gear tooth. The resulting roughness is
typically 15 to 35 micro-inches Ra. Fine grit honing may yield surface roughness as
low as 12 to 13 micro-inches Ra.
However, none of these finishing methods can improve the surface finish
below approximately 10-12 micro-inches without significantly increasing the cost
and process time.
Polishing compounds used for preparing metal parts for electroplating have
been available, using liquid polishing compounds containing fine abrasive particulate.
For example, U.S. Patent No..4,491,500 teaches a physicochemical process for
refining metal surfaces. The disclosed two-step process first utilizes a liquid chemical
followed by a burnishing liquid. It further involves the development of a relatively
soft coating on the surface being treated, followed by the physical removal of the soft
coating and continuous repair. Rougher areas greater than 70 urn are first leveled
through some form of mechanical action. U.S. Patent No. 4,818,333 describes a
similar process, focusing on the composition of a high density burnishing media used
in the process. These processes can result in finishes less than 3 micro-inches Ra.
Thus, processes and chemicals are described for reducing surface roughness.
Chemical finishing techniques such as etching and bright dipping are also
widely known in the art of electroplating preparation for the purpose of achieving an
ultra-smooth and clean surface.
The main failure modes for gears are pitting or micropitting, wear and
scuffing.. When a gear and pinion interact, the gear teeth necessarily contact each
other. Without lubrication, the teeth scratch against each other, scuff each other, wear
down, pit and crack. Lubrication postpones the onset of these effects. Thus, the better
the lubrication, the longer the life of the gear. Gears with surfaces that are too rough
have surface peaks that will damage the gear teeth as they interact. Gears with
surfaces that are too smooth, for example below 3 micro-inches Ra, cannot retain
sufficient lubrication between adjacent teeth, resulting in an increased tooth wear rate.
Consequently, there is a need for gears having properly shaped teeth with
improved surface finishes below approximately 10 micro-inches Ra in order to
maximize the life and overall performance of gears.
Summary of the Invention
The present invention is directed to a gear having a surface finish between
approximately 5 micro-inches to approximately 10 micro-inches Ra for improved
contact fatigue life, improved wear resistance, reduced friction and improved gear
performance.
Most gears are manufactured using various gear cutting and shaping
techniques, including hobbing and shaving, which result in surface roughness greater
than 15 micro-inches Ra. Numerous methods exist for polishing metal surfaces in
order to get a reduced surface roughness, including chemically accelerated vibratory
polishing, electrochemical polishing, and mechanical polishing. When surface
roughness is reduced to between approximately 5 micro-inches Ra to 10 micro-inches
Ra, the maximum contact stress can be reduced more than fifty percent (50%).
Similarly, the subsurface shear stress can be possibly reduced by approximately thirty
percent (30%) to fifty percent (50%). The reduced contact stress and reduced shear
stress results from improved lubrication conditions between the gear teeth. That is,
the smoother the gear surface, and therefore the lower the roughness, the higher the
film thickness ratio^ X, and the greater the overall lubrication. As the film thickness
ratio is increased, the lubrication is better, therefore the friction is lower, and the
surfaces are better separated with a layer of lubricant. As a result of both surfaces not
having at most limited direct peak contact, the contact pressure and the subsurface
stresses are reduced. Also, the heat generation and the temperature rise in the gear
tooth contact will be reduced due to the fact that direct rough surface peak contact and
rubbing
robbing are the main cause of high friction and heat generation. Reduce operating
temperature and better controlled thermal equilibrium condition would greatly
improve castomer satisfaction, increase life and reduce warranty cost for geared
products such as transmissions. It will also reduce energy consumption and improve
efficiency.
It is also possible, however, to have a gear that is too smooth. For example, a
gear with a surface roughness of approximately I micro-inch Ra to approximately 3
micro-inch Ra is too smooth, resulting in reduced oil retention for lubrication. This is
because the lubricant requires some degree of surface roughness from which to
adhere.
Accompanying
Brief Description of the/Drawings
The preferred embodiments of this invention will be described in detail, with
reference to the following figures, wherein:
Fig. 1 is a perspective view of a spur gear pair;
Fig. 2 is a partial side view of a spur gear depicting the different parts of the
gear and teeth;
Fig. 3 shows a perspective of a surface with traditional hobbed and shaved
surface finish.
Fig. 4 shows a perspective of a surface following physicochemical polishing.
Detailed Description of the Preferred Embodiment
Fig. 1 is a perspective view of a spur gear combination 10. Spur gear
combinations 10 is made up of a gear 12 and a pinion 14. The pinion 14, by
convention, is the smaller of the two gears. Spur gears 10 are used to transmit motion
and power between parallel shafts 16, 18. The teem 20 are generally straight and run
generally parallel to the shaft axis.
Fig. 2 shows a partial side view of a spur gear depicting the different parts of
the gear 12 and teeth 20. The pitch circle diameter 22 is the basis of measurement of
gears 12, and represents the size of the gear. The circular pitch 24 is the distance from
the center of one tooth 20 to the center of the next tooth measured around the
circumference of the pitch circle 26. The diametrical pitch of the gear 12 is the
number of teeth to one inch of the pitch diameter. For example, if a gear 12 has
sixteen (16) teeth and the pitch diameter is four (4) inches, the gear has a four (4)
teeth to each inch of its pitch diameter and is called 4 diametrical pitch, or 2 D.P.
Spur gears 10 are generally fabricated from cast iron, steel, bronze and brass, or other
strong metals. However, they may also be fabricated using nylon or other plastics for
silent running.
It will be understood by one of skill in the art that this invention relates to
gears of different types, including, but not limited to, rack and pinion gear systems for
converting rotary motion to linear morion; internal gear systems, helical gear systems,
herringbone gear systems, bevel gear systems, worm gear systems and planetary gear
systems.
Table 1 shows analysis results of different surface finishes and corresponding
data, including roughness and film thickness. The composite roughness, o, is given in
microns. The film thickness ratio, X., is the ratio of the average film thickness ha
divided by the composite roughness. The contact load ratio, Wc, is the ratio of
asperity contact load to total load. The maximum dimensionless pressure is
calculated as P/Ph, where Ph is the maximum Hertzian pressure. It can be readily seen
that the lower the overall composite roughness, or arithmetic mean roughness, the
higher the film thickness ratio, X. Accordingly, a lower maximum pressure ratio and
a lower maximum substrate shear stress is observed, as well as reduced contact load
ratio an coefficients of friction. The smooth surface represents the ideal case and is for
comparison. When the surface is ideally smooth, it has a calculated coefficient of
friction of 0.02679, and there is little of any lubrication present because the surface is
too smooth. Retention of sufficient lubrication on the gear surface is necessary for
optimal gear wear. In contrast, when it is too rough, not enough lubrication is present,
and increased surface peak interaction results. The result is greater lubrication of the
gears, which corresponds to numerous benefits known to those skilled in the art
including reduced stress on the gear and extended time to failure caused by fatigue.
For example, gear fatigue life can be significantly improved from 11-12 hours before
polishing to greater then 100 hours after polishing.
Accordingly, the optimum gear surface finish is between approximately 5
micro-inches and 10 micro-inches Ra.. When surface roughness is reduced to
between approximately 5 micro-inches Ra to 10 micro-inches Ra, the maximum
contact stress can be reduced more than fifty percent (50%). Similarly, the subsurface
shear stress can be reduced by approximately thirty percent (30%) to fifty percent
(50%).
Figure 3 shows a perspective of a surface with traditional hobbed and shaved
surface finish. In the figure, Ra is the arithmetic mean roughness, Rq is the root mean
square of the roughness, or a, and Rz is the maximum peak to valley measurement.
The Ra is approximately 0.6988 um, or approximately 25.5 micro-inches, and Rq is
approximately 0.8864 um, or approximately 34.9 micro-inches.
Figure 4 shows a perspective of a surface following physicochemical
polishing. Here, following treatment to polish the surface of the gear teeth, the Ra is
approximately 0.1987 um, or approximately 7.82 micro-inches, and Rq is
approximately 0.304 um, or approximately 11.97 micro-inches.
The result of polishing the surface of the gear is an enhanced surface with
reduced surface irregularities.
In the preferred embodiment, a gear surface is finished, or polished, to
between approximately 5 micro-inches Ra and 10 micro-inches Ra. Any metal gear
can be used, and the surface to be finished or polished is preferably the gear
functional surface. The gear functional surface is also known as the surface of the
gear teeth that contact gear teeth of another gear. Any gear size is suitable, including
transmission gears having approximately 3 to approximately 8 teeth per inch diameter
to gears greater than 8-10 teeth per inch diameter. Additionally, the finished or
polished surface topgraphy can be isotropic or non-isotropic.
While advantageous embodiments have been chosen to illustrate the invention,
it will be understood by those skilled in the art that various changes and modifications
can be made therein without departing from the scope of the invention, as defined in
the appended claimij For example, the gear can be finished or polished using any
known method, for example, electrochemical polishing, mechanical super finishing,
mechanical abrasive polishing, and chemically accelerated vibratory polishing. The
actual polishing or finishing used does not matter so long as the finish is relatively
uniform and between approximately 5 to approximately 10 micro-inches.
Additionally, it is preferable that the finishing or polishing process does not
negatively effect the geometric shape and function of the gear teeth.
We Claim
1. A device in particular transmission gear device having reduced surface
roughness, comprising:
- a first gear (12) having a first set of teeth (20) with a first surface
finish thereon represented by a first composite RMS roughness (a)
and resulting into a first lubrication film thickness ratio (A), which is
a first average film thickness divided by said first composite
roughness; and
- a pinion gear (14) engaging said first gear (12) and having a
second set of teeth with a second surface finish thereon
represented by a second composite RMS roughness (a) different
from said first surface finish, said pinion gear (14) resulting into a
second lubrication film thickness ratio (A1) , which is a second
average film thickness divided by said second composite roughness
(ct), such that said second ratio (A1) is greater than said first ratio
(A);
- wherein said first surface finish has an arithmetic mean roughness
(RA) of 0.0762 ^m (3 micro-inches) to 0.3048 |^m (12 micro-
inches).
2. The device as claimed in claim 1, wherein said first surface finish has an
arithmetic mean roughness of 0.127 i^m (5 micro-inches) to 0.254 ^m (10
micro-inches).
3. The device as claimed in claim 1, wherein said second surface finish has
an arithmetic mean roughness of 0.127 µm (5 micro-inches) to 0.254 µm
(10 micro-inches).
4. The device as claimed in claim 1, wherein said first gear is one of a spur
gear, an internal gear, a helical gear, a herringbone gear, a bevel gear, a
and a planetary gear.
5. The device as claimed in claim 1, wherein said first gear (12) has a set of
teeth (20) comprising to 12 teeth.

This invention relates to a gear system in particular transmission gear system
having reduced surface roughness, comprising, a first gear (12) having a first set
of teeth (20) with a first surface finish thereon represented by a first composite
RMS roughness (σ) and having a first lubrication film thickness ratio (Λ), which is
a first average film thickness divided by said first composite roughness; and a
second gear (14) engaging said first gear (12) and having a second set of teeth
with a second surface finish thereon represented by a second composite RMS
roughness (σ) different from said surface finish. The second gear is having a
second lubrication film thickness ratio (Λ), which is a second average film
thickness divided by said second composite roughness, such that when said
second composite roughness (σ), said second ratio (λ) is greater than said first
ratio (λ). The first surface finish has an arithmetic mean roughness (RA) of
0.0762 μm (3 micro-inches) to 0.3048 μm (12 micro-inches).