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McCosker-Gear Tribology Paper-McCosker.docx
Rensselaer Polytechnic Institute
Gear Tribology
A Review of the Failure Modes, Material Selection, and
Lubrication of Gear Systems
John McCosker
12/15/2012
Table of Contents
Page
1.
Introduction ............................................................................................................................. 1
2.
Failure Modes .......................................................................................................................... 2
2.1
Macro-Pitting ................................................................................................................... 2
2.2
Micro-Pitting .................................................................................................................... 5
2.3
Scuffing ............................................................................................................................ 5
2.4
Mild Wear ........................................................................................................................ 6
3.
Material Selection and Coatings .............................................................................................. 6
4.
Lubrication............................................................................................................................... 8
5.
Conclusion ............................................................................................................................... 8
6.
References ............................................................................................................................. 10
ii
1. Introduction
In order to properly design gears so that wear is minimized and failure eliminated, failure
modes and their underlying causes must be understood. The following report documents typical
gear failure modes and the tribological phenomena that cause those failure modes. In addition,
this report will discuss how material selection and gear lubrication increase the life of a gear
system.
Gear systems are extremely susceptible to wear due to the large number of cycles in which
gear teeth are in contact with one another. In addition to the large number of loading cycles, a
tribological characterization of gear teeth contact has been recognized as extremely complex.
Even for the simplest of gears, such as the spur gears in Figures 1 and 2, contact between gear
teeth consists of a complex combination of both rolling and sliding.
Figure 1: Spur Gears [1]
In Figure 2 below, contact starts at point K2, where the tip of the driven gear initiates contact
at the pitch circle of the driving gear. As the gear moves, the contact point moves down the side
of the gear tooth to point K1. The contact consists of mostly sliding after the initial contact and
transitions to a rolling movement. When the contact point is collinear with the gear axes, the
contact consists of no sliding and only rolling.
1
Figure 2: Contact between Spur Gear Teeth [1]
Another difficulty in characterizing and predicting contact in gears is that the load sharing is
not constant and for one instant is carried by only one pair of teeth. As illustrated in Figure 2,
the load is being shared between two pairs of teeth along the line of action AB. However, as
contact point K1 moves down the tooth of the driven gear and past the line labeled “Y”, there
exists a time when the entire load is supported by the preceding pair of teeth.
2. Failure Modes
There exist at least four major types of tribological failure modes: macro-pitting or spalling,
micro pitting or gray staining, scuffing, and mild wear. The following section defines each
failure mode and describes the physical phenomenon that causes the failure.
2.1 Macro-Pitting
Contact between surfaces of gear teeth leads to shear stresses which reach a maximum
value just under the contact surface. The relative motion between gears in the form of rolling,
sliding, or both, creates a band of material that is repeatedly stressed. Eventually, cracks form at
2
subsurface stress concentrations and work upward until they reach the surface. Once they reach
the surface, a flake of material is released and a pit is formed in the gear tooth [2]. Pitting can
appear anywhere on the gear tooth that experiences contact, however most pitting in spur gears
has been shown to occur near the pitch line, as shown in Figure 3. Macro-pitting due to Hertzian
contact is shown to be greatly reduced with the use of lubrication. Lubrication reduces the
intensity of the near-surface stresses by spreading the area of contact. Many gears also utilize
case hardened materials so that the area that experiences the maximum Hertzian contact stress is
more suited to withstand the stress without cracking.
Figure 3: Pitting of a Case-Hardened Pinion [1]
Pitting has also been shown to occur in the subsurface region of a gear due to a material
inclusion, similar to the classical case of subsurface fatigue in bearings and rollers [2]. For
subsurface macro-pitting due to material inclusions, lubrication does not greatly reduce its effect.
Instead, the subsurface stress is a function of the inclusion’s size, density, and material
3
characteristics. Investigations of gear pitting have revealed that spherical particles of about 1 μm
are discharged from the gear cracks prior to the formation of pitting [2]. Figure 4 shows a
detailed Finite Element Model (FEM) consisting of two gear teeth in contact with linear elastic
material properties. As mentioned, the maximum stress due to contact pressure occurs just under
the contacting surface.
Figure 4: Elastic FEM of Gear Tooth Contact
4
2.2 Micro-Pitting
While a gear system is lubricated, asperities on the surface of contacting gear teeth do not
contact one another. However, after the lubrication deteriorates, contact between the gear teeth
is mainly through asperities. When the asperities contact one another, groups of very small
cracks propagate quickly through the material and create grey patches on the gear teeth. The
patches of grey, also called “grey staining” and “frosting”, are comprised of individual pits on
the scale of a micron. Micro-pitting is shown to degenerate the material very slowly, and unless
it leads to macro-cracking, is considered to be nondestructive.
2.3 Scuffing
The tip and root of the gear tooth, where the slide-to-roll ratio is the highest, temperatures
can elevate and break down the lubrication. Once the lubrication is broken down to a point
where it is no longer able to keep asperities on the surface of the gears from contacting, the
asperities plastically deform. Failure can occur due to the rapid wear and transfer of material
between gear teeth and in severe cases scuffing can cause seizing of the gears. Figure 5 shows a
spur gear experiencing substantial scuffing at the tip and the root of the gear teeth.
Figure 5: Typical Scuffing on a Spur Gear [1]
5
2.4 Mild Wear
Operating conditions that do not cause pitting and scuffing may still slowly remove material
from the gears through mild wear. Mild wear exists due to the adhesion, abrasion, and corrosion
that slowly remove surface asperities while sliding across one another. The rate at which the
asperities are adhered, sheared, and removed depends on the asperity contact temperatures. If
the unlubricated area reaches a critical temperature, scuffing will begin to occur.
Another type of mild wear is called corrosive wear in which lubricant additives slowly
corrode rough asperities. The result of the corrosion is a smooth polished surface. If the
corrosive wear is controlled so that material is not abraded away too quickly, it is desirable to
have a polished surface between gears contacting. Doing so increases the lubrication film
thickness and reduces the risk of pitting and scuffing.
3. Material Selection and Coatings
The ideal material selection for two bodies bearing and sliding against one another would
be selecting two hard, smooth surfaces, perfectly aligned with no edge contacts. Due to the cost
and fabrication limitations with such precise gears, there are very few applications where this is
cost effective. Another approach is to make one of the two materials softer than the other so that
material plasticity can distribute discrete Hertzian pressures. Since the softer alloys that are
better for bearing have limited structural strength and fatigue resistance, they are generally used
as thin overlays on steel, bronze, or aluminum bodies. These alloy overlays are applied with the
following methods: casting, sintering, or electroplating of individual layers.
As previously discussed, wear of gears is a possible failure mode. Abrasive wear is caused
by material penetrating and cutting the surface. Two-body abrasion is the main cause of abrasion
which is caused by asperities on one surface removing material on the other surface. Three-body
6
abrasion is wear caused by foreign matter being trapped between two moving surfaces, which
happens when particles are trapped in bearing clearances. It has been indicated that abrasive
particles or asperities must have an angle of attack of about 80 to 120 degrees to cut the surface,
which means that two-body abrasion will generally cause more wear than three-body abrasion
[3]. Figure 6 illustrates that at large angles of impact, hard materials show more erosive wear
than soft materials. Also illustrated is that elastomeric coatings are very capable of resisting
erosion at high angles of impact.
Figure 6: Rate of Material Erosion for a Range of Impact Angles [3]
For annealed steels and nonmetallic hard materials such as ceramics, wear resistance is
directly proportional to penetration hardness. For this reason, many steels used in gear design
are hardened through a number of different processes. Case hardening, which was previously
discussed, must penetrate deep enough into the material to provide sufficient hardness without
allowing penetration of foreign particles.
7
4. Lubrication
Predicting lubrication performance in gears is done by using the Hertzian theory of two
contacting cylinders or ellipsoids and the kinematics of determining relative surface velocities.
The two main concerns in determining which lubricant is best for the gear system are: locating
the area with the thinnest film thickness that can lead to contact fatigue; and locating the areas of
high contact temperature which can initiate scuffing and the levels of contact temperatures. The
following are a list of boundary lubricant properties that can be used to characterize which
lubricant should be used for a given application: entrainment, cushioning, thermal insulation,
and coherence and friability [4].
For gear systems, entrainment is important because not only does the film need to cover the
macroscopic surfaces in contact, but it must also be carried by microscopic surfaces in between
material asperities. The entrainment, or ability of a lubricant to conform to a surface, for a liquid
is based on the liquid’s viscosity while the entrainment for a solid lubricant based on its modulus
of elasticity. Another important characteristic of a gear system lubricant is its thermal insulating
properties. The reduction of bearing surface temperatures reduces the risk of scuffing and
increases the entrainment tendencies of the boundary film. Increasing the film entrainment,
increases surface separation, and reduces stress on asperities. Another benefit of increasing film
entrainment is reducing the corrosive wear created when boundary film containing substrate
material is forced along the surface.
5. Conclusion
Considering tribological failure modes is a requirement for the design of any gear system.
Most gear teeth are loaded with a very large number of cycles, and each cycle consists of a
combination of sliding and rolling. The relative motion of the gear teeth give way to several
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failure modes that are each caused by different phenomena. Hertzian contact pressure causes
pitting under the surface, contact between asperities only causes micro-pitting, high frictioninduced temperatures create scuffing, and the removal of gear tooth asperities causes general
wear.
All of these failure modes can be controlled with the correct material selection, material
treatment, and lubrication. Material treatments and material selection ensure that if gear teeth are
not precision machined, that they will be able to either distribute the load over more area or be
able to withstand more load. The use of lubricants also helps to better distribute the load
between surfaces by creating a film between surface asperities. Lubricants have also been shown
to reduce the temperature of gear systems which reduces the risk of scuffing and creates more
separation between surfaces.
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6. References
[1]
H. Cheng. “Modern Tribology Handbook, Two Volume Set”, CRC Press LLC. Copyright
2001
[2]
F. Harwell. “Handbook of Lubrication (Theory and Practice of Tribology), Volume II”,
CRC Press LLC. Copyright 1983.
[3]
S. Murray. “Wear Resistant Coatings and Surface Treatments”, CRC Press LLC.
Copyright 1983.
[4]
R. Fein. “Characteristics of Boundary Lubrication”, CRC Press LLC. Copyright 1983.
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