The Ryder Gear Test Method provides a scuffing or load capacity
rating for aviation oils. The load capacity rating is derived from
scuffing criteria. Scuffing is one of several surface deterioration
mechanisms controlling life and durability of aircraft bearing and
gear hardware. Through its use over many years as a qualification
test, the Ryder Gear Test Method has developed a large database.
The database provides a historical record for oil lubricating performance.
The U.S. Navy has supported efforts on the Wedeven Associates,
Inc. test machines, WAM1, WAM3 and WAM4 to provide Ryder-like load
capacity data of gas turbine and gearbox oils. These efforts also
expand the scope of oil characterization beyond the perspective
of a pass/fail or ranking of oils, with scuffing performance the
only criteria. To provide a continuity between Ryder Gear load capacity
data and future oil characterization methods, a “WAM Economical
Load Capacity Screening Test” was developed to rank a wide
range of engine and gearbox oils similar to the Ryder Gear Test
Method (see AIR4978, Appendix D). This test method ranks oils with
respect to a scuffing failure event. It also characterizes oils
with respect to traction (friction) behavior.
The introduction of high thermal stability (HTS) oils, and particularly
corrosion inhibited (CI) oils, has highlighted the need for greater
testing sensitivity for oils exhibiting lower than average lubricating
performance. Low lubricating performance, as evidenced in the Ryder
test, reveals itself in the form of a superficial form of scuffing
The test protocol described below was developed partially under
U.S. Navy PO No. N00421-98-M-6001, June 24, 1998. The test conditions
selected highlight the load capacity performance features of oils
that are submitted for qualification under the MIL-PRF-23699 specification.
Load capacity tests are conducted with ball and disc specimens,
which are operated under tribological contact conditions similar
to the U.S. Navy Ryder Gear Test Method. The WAM High Speed Load
Capacity Test Method has been submitted to SAE E-34C for approval
in AIR4978 “Temporary Method for Assessing the Load Carrying
Capacity of Aircraft Propulsion System Lubricating Oils”.
The purpose of this test method is to evaluate oils according to
the Ryder Gear Test Method, with enhanced sensitivity for lower
than average lubricating performance. It is important to recognize
that the Ryder Gear performance criteria are based upon the visual
observations of “scuffing” damage on Ryder gear teeth.
Since some scuffing features found on Ryder gear teeth are superficial,
a Ryder-like test method must also invoke the same type of surface
deterioration mechanism. Micro-scuffing is a superficial form of
scuffing, which is confined to the surface topographical features
of the gear teeth. Micro-scuffing is generally associated with surface
damage at low load stages where contact stresses are too low to
cause “macro” scuffing. Scuffing, or “macro-scuffing”,
is associated with the complete loss of surface integrity. Scuffing
involves gross failure of near-surface material, in addition to
surface roughness features. When traction (friction) is measured,
micro-scuffing is generally detected by a rapid decline in traction
coefficient. The decline in traction coefficient is associated with
the removal of surface roughness features. While this action actually
restores some of the EHD fluid film separation between the surfaces,
the rapid removal of surface features by plastic flow and rapid
polishing wear reflects a failure of the oil to provide adequate
surface films for boundary lubrication. In contrast, macro-scuffing
is associated with a sudden increase in traction coefficient resulting
from massive adhesion and plastic flow of near surface material.
A sudden and massive scuffing failure requires high contact stresses
in the presence of high sliding velocities.
The observation of traction coefficient during a load capacity
test is quite informative. High precision measurements of traction
coefficient clearly identify “events” like scuffing
and micro-scuffing, as discussed above. Traction behavior also reflects
the continual interactive process between oil chemistry and the
mating material pair within the contact. Subtle changes in topographical
features due to wear are reflected in traction behavior.
WARNING: This test method is a simulation of Ryder Gear Test ranking.
The test conditions are carefully selected to make the results correlate
with the Ryder Gear Test. While the Ryder Gear Test operating conditions,
in terms of rolling/sliding speeds, temperatures and contact kinematics,
are representative of helicopter gearbox hardware, slight operational
changes are likely to cause different ranking. This is based on
WAM load capacity tests conducted over a range of test conditions,
which affect EHD film generation and contact temperature. Load capacity
tests over a range of conditions are recommended. The conditions
selected here are specific to Ryder ranking using a set of five
reference oils supplied by the U.S. Navy. In addition, there is
no confirmation that scuffing load capacity performance is in any
way connected with other prominent life-limiting performance criteria,
expressed as surface distress (wear and micro-pitting). Additional
tests for surface distress, or a complete simulation of specific
hardware, are recommended to supplement scuffing load capacity results.
The load capacity test protocol is conducted with a WAM test facility
shown in Figure 1. The test machine controls specimen position,
contact load and motions of a single contact in space. A computerized
run file controls load and contact kinematics between the specimens.
Specimen temperatures are recorded with trailing thermocouples.
The high-speed test protocol uses AISI 9310 ball and disc specimens
with tight specifications for surface finish and hardness. To capture
Ryder-like oil performance features, the following test specimen
specifications and test conditions have evolved.
||2.0638 cm (13/16”) dia., AISI 9310, “hard
grind” surface roughness,
Ra = 0.25 µm (10 µin), hardness HRC 62.5-63.5.
|| 10.16 cm (4”) dia., AISI 9310, surface finish Ra =
0.15 µm (6 µin), hardness, HRC 62-64.
||Ub = 7.21 m/sec (284 in/sec).
||Ud = 7.21 m/sec (284 in/sec).
||Non-collinear velocity vectors (angle between velocity vectors
||5.72 m/sec (225 in/sec).
||8.78 m/sec (346 in/sec)
||Exponential increase from 1.8 kg (4 lbs) to 63.6 kg (140 lbs)
in 30 stages
||Until scuff, or suspension (30 stages = 30 minutes)
||Scuff defined by loss of surface integrity and sudden increase
Micro-scuff defined by rapid decline in traction coefficient.
||Oil performance is judged by load stages causing micro/macro
event(s) and traction behavior, which reflects wear of surface
||Specimen temperatures controlled by frictional heating. Surface
temperatures increase with load stage from ambient to ~200 °C.
||Computer controlled peristaltic pump, approximately 1 drop/sec.
Oil flow rate is selected for adequate lubrication without significant
The entraining velocity (Ue) and sliding velocity (Us)
are defined below:
Ue = 1/2(Ub + Ud)
Us = (Ub - Ud)
where Ub = surface velocity vector of the ball
at the contact point
Ud = surface
velocity vector of the disc at the contact point
The entraining velocity (Ue) and sliding velocity (Us) are key
parameters that control the degree of surface separation and the
rate of surface tangential shear that the oil must accommodate.
With the parameters selected, the initiation of a load capacity
test is similar to the Ryder Gear Test in that there is generally
little or no evidence of surface damage during the first load stage.
The test parameters recorded include the following:
Ball and disc temperatures
Ball and disc surface velocities
Option: video recording of running track on disc specimen
The test method utilizes the following features:
1. Slow application of load to avoid surface damage during test
2. Exponential rather than linear increase in load so that a final
scuffing event is reached, rather than a transition into a wear
mode without scuffing.
3. Prominent surface finishing features to highlight surface film
formation and wear protection through the use of traction coefficient
4. Use of frictional heating to control specimen temperature and
to cover a wide range of temperatures.
5. Continuous specimen contact rather than cyclic contact to avoid
6. Small incremental load stages to increase resolution.
7. Non-collinear velocity vectors to capture Ryder-like sliding
velocities and film thickness-to-surface roughness ratio.
The test protocol parameters focus on creating tribological conditions
which activate the same type of chemical response as the Ryder gear
test. The key parameters controlling these conditions are: (1) entraining
velocity to control EHD film thickness, (2) sliding velocity, (3)
surface topography and (4) specimen temperatures (including effects
of frictional heating). If the ranking of oils by a scuffing event
falls in line with the Ryder Gear Test, it is assumed that the key
tribological conditions invoked must be similar to the Ryder. The
progression of surface features (like abrasive scratches, polishing
of grinding ridges and surface film formation) formed prior to a
scuffing event also follows the same sequence generated in the Ryder
Prior to each test series, the ball and disc specimens are cleaned
in an ultrasonic bath with petroleum ether, followed by acetone.
The AISI 9310 “hard grind” ball specimens are processed
through the hard grind stage of a ball manufacturing process. The
“hard grind” ball specimens tend to have a consistent
surface finish (Ra = 10-13 µin) for good repeatability. The test
balls are obtained from a single manufacturing batch consisting
of approximately 8,000 balls. The disc specimens are carburized
to a hardness of HRC 62.5-64.5
Following machine calibration, checkout tests are conducted with
reference oil, Herco-A. Load capacity tests conducted with Herco-A
encounter micro-scuff events. Continued testing beyond a micro-scuff
event eventually results in a scuffing event. A scuffing event is
not always clearly defined for Herco-A when it is preceded by multiple
micro-scuffing events. Exploratory testing, conducted under Navy
PO No. N00421-98-M-6001, shows that specimen hardness influences
both micro-scuff and scuffing events. Disc specimens are heat treated
in large batches to maintain consistency.
The details of the step loading test protocol given in Table 1.
The test protocol gives an exponential rise in load with load stage.
The exponential rise is to partially offset a cube root relationship
between load and contact stress. The exponential rise in load also
balances an increase in chemical activity with temperture so a scuffing
event can be reached before the end of the test protocol with typical
jet engine oils. A mimimum of four test determinations are made
for each test oil.
Figure 2 shows a typical load capacity test plot. A test plot includes
the contact load, ball and disc temperatures and traction coefficient.
Typical traction coefficients during the first few load stages are
on the order of 0.03. The test conditions during the first few load
stages provide nearly full-film EHD lubrication. Ball and disc temperatures
increase with load stage due to frictional heating. As load and
temperature increase, the ratio of EHD film thickness to surface
roughness decreases. An increasing traction coefficient reflects
a greater degree of asperity interaction within the contact. The
rate of rise in traction coefficient reflects the ability of the
oil to form surface films at asperity sites for wear resistance.
A decreasing traction coefficient reflects polishing wear. A sudden
drop in traction is associated with a rapid loss of surface topographical
features. This is generally preceded by a micro-scuffing event caused
by adhesion and plastic flow of surface features. The surface features
following a micro-scuffing event are shown in Figure 3. Micro-scuffing
events, represented by momentary reductions in traction coefficient,
reflect marginal oil chemistry to sustain surface films for protection
against local adhesion and wear of surface features. Some oils show
multiple micro-scuffing events. Multiple micro-scuffs are characteristic
of the non-formulated 4 cSt oil, Herco-A. A macro-scuffing event
is easily detected by a sudden increase in traction coefficient.
Data Processing and Traction Behavior
The way oil chemistry reacts with the surface to control wear of
topographical features is reflected in traction coefficient. Oil
chemistry is activated by contact temperature and exposure of “clean”
metal surfaces caused by shear at asperity sites. For the selected
test conditions, oils show characteristic traction behavior. The
characteristic traction behavior of two formulated oils is shown
in Figures 4 and 5.
Since traction behavior reflects oil chemistry for wear resistance,
the traction data for each test is processed to obtain an average
traction coefficient for each load stage. The average traction coefficient
vs load stage is then plotted to compare the traction behavior of
the test oil with other oils as shown in Figure 6. The vertical
arrows on the test plots identify the average load stage at which
micro-scuffing or scuffing events occur.
Figure 6 includes the traction behavior for the reference oil Herco-A.
Figure 6 also includes the traction behavior of a test conducted
with polished surfaces. The test oil is a standard (STD) MIL-PRF-23699.
This test provides a lower bound traction, which is essentially
unaffected by surface roughness features and boundary lubrication.
The lower bound traction is attributed to the shear behavior (traction)
of the bulk oil. Interactions between surface topographical features
do not occur until late in the test protocol when the contact temperatures
are high and the EHD film is thin.
The test oils show different traction and scuffing behavior compared
to Herco-A. During the first few load stages, the high traction
of Herco-A may be associated with two factors: (1) lower base oil
viscosity (4.5 cSt @ 100°C) and EHD films thinner than the 5
cSt test oils and (2) the formation of wear protective, and perhaps
high friction, oxides. Once the oxides and organo-metallic films
are removed from the surface by wear with Herco-A, there is little
boundary lubricating chemistry available to allow continued running
without local adhesion and plastic flow of asperities. Load capacity
tests with Herco-A show multiple micro-scuff and scuffing events
between load stages 8 and 14. The traction and scuffing behavior
of Herco-A is used as a reference for low lubricating ability.
From all the load capacity traction data collected over time, there
seems to be a strong connection between traction coefficient and
wear of surface finishing features. While fluid temperature within
the contact also affects traction, the rise and fall of traction
coefficient still reflects the process associated with how the physical
and chemical properties of the oil handle the intimate collisions
of surface features within the contact during a load capacity test.
Since the WAM High Speed Load Capacity Test protocol covers a large
temperature range, we assume that the lubricating ability of the
oil, as reflected in traction, is also being tested over a large
temperature range. If this is the case, the lubricating ability
of the test oils can be differentiated with respect to preservation
of surface topographical features, at least over a limited range
of temperature or contact severity. Additional investigations are
required to determine if subtle differences in traction truly reflect
variations in chemical activity for wear resistance in service hardware.
It can be postulated that the desired lubricating attributes of
oils are good wear resistance and scuffing resistance (and surface
fatigue resistance) “across-the-board” of temperature
and stress. The WAM High Speed Load Capacity Test protocol may be
covering at least some of the desired performance features and test
conditions. For gear or other surfaces with prominent roughness
features one could argue that some mild polishing wear is desired
to topographically condition the surfaces for low asperity stress
to prevent early micro-pitting. If this were the case, good performance
would be associated with relatively low traction coefficient and
high scuffing load stages. Further tribology studies of service
hardware are needed to clarify the desired oil attributes and testing
conditions. Until this is done, we have to live with a tenuous link
between qualification testing and field performance. For now, the
traction behavior and scuffing resistance of an oil, as determined
with the present set of Ryder-like test conditions, can serve as
an initial step toward full characterization and clarification of
performance criteria. In the meantime, the collection of a database
for test oils, along with field experience, should provide greater
confidence in the test method and a near-term tool for evaluation
of oil formulations.
Preliminary Link to Service Performance
The WAM High Speed Load Capacity Test Method provides traction
and scuffing evaluation of jet engine oils with no specific connection
to service performance. The test method is designed to simulate
Ryder Gear ranking of selected reference oils supplied by the U.S.
Navy. Qualification of MIL-PRF-23699 oils with the Ryder Gear Test
Method requires oils to pass a gear scuffing load a few percentage
points above the reference oil, Herco-A. The WAM High Speed Load
Capacity Test Method provides greater differentiation between formulated
oils and Herco-A than the Ryder Gear. Although there is greater
differentiation, the alternative test method(s) for the Ryder Gear
are no more useful with respect to judging the level of performance
required for reliable performance in service. While the gear mesh
contact conditions of the Ryder Gear do come close to engineering
parameters of some aeropropulsion systems, extrapolation of results
to other thermal, kinematic and material systems is not reliable.
It is further recognized that scuffing performance is only one of
several other tribological attributes that control performance.
Other attributes include various types of wear (adhesive, abrasive,
chemical) and various types of surface initiated fatigue.
While a comprehensive picture of oil performance attributes is
unfinished business, there is a way to utilize the current database
with field experience to establish a preliminary connection between
load capacity testing and field service performance. Thanks to the
unfortunate experience in military hardware with two oils, there
is now a link between WAM High Speed Load Capacity testing and known
deficiencies in the field. The average traction data for these two
test oils are plotted in Figure 7. Oil DLA 522 resulted in high
iron content in engine oil systems with specific engine hardware
(T53). Oil TEL-0004 caused wear problems in the Canadian Air Force
Both oils in Figure 7 show a rapid rise in traction coefficient
reflecting resistance to polishing wear of surface finish protrusions.
At early load stages, these oils consistently transition into micro-scuffing
and a rapid wear mode. Micro-scuffing is a superficial form of scuffing
where loss of surface film lubrication causes local adhesion and
wear of surface features. The plastic flow and rapid wear of surface
roughness features is amply revealed by a sudden reduction in traction
The test data from these two oils are presented in Figure 8, along
with other U.S. Navy oils that are qualified products. The other
oils are currently used in the field with no apparent difficulty.
The family of oils in Figure 8 includes STD, HTS, CI and DOD oils.
The oils with known deficiencies are a CI oil (DLA 522) and a MIL-PRF-7808
Grade 4 oil (TEL0004). These oils are plotted in red. The traction
data show that both of these oils encounter micro-scuffing at low
load stages between 11 and 14. Early micro-scuffing and a rapid
wear mode are consistent with high iron content detected in service
with the engine oil DLA 522.
Figure 8 also shows two CI oils (DLA 562 and PE-5-L1859) that have
failed the Ryder Gear test. These oils are plotted in blue. While
these oils are not directly connected with field experience, the
low Ryder performance is sufficient to raise questions about their
qualification. The scuffing load stages for these oils are relatively
low (15 and 18). A unique feature of these oils is that the traction
behavior is substantially different from the previous two oils with
known deficiencies in the field. The traction coefficient rises
rapidly with load stage indicating a resistance to polishing action
of roughness features. Apparently, the surface film chemistry that
provides early resistance to polishing wear is not sufficient to
prevent early scuffing. The connection between high traction and
low scuffing performance is not understood. Anti-wear performance
(high traction) does not translate into good scuffing performance,
at least for these oils. Oil PE-5-L1874, which also has high traction,
shows good scuffing performance. The traction behavior of this oil
implies good anti-wear and anti-scuffing attributes.
The oils plotted in gray in Figure 8 are qualified products currently
being used in service by the U.S. Navy. Many of the oils, except
for the CI oils, are also used in commercial service. The family
of oils in Figure 8 shows significantly different traction behavior
due to varying degrees of polishing wear. We do not know how variations
in polishing wear translate into service performance. On the other
hand, the scuffing performance criteria do have connection with
service experience. The two oils with low scuffing (i.e. micro-scuffing)
performance have known deficiencies from service experience. The
two DOD oils, which are suspended at load stage 30 without a scuffing
event, are known to be high performing oils for power transmissions.
Based on somewhat limited field experience, one can establish a
lower bound for load capacity scuffing performance. If military
hardware has experienced difficulty with oils showing scuffing failures
at load stages around 12 and 14, then a lower bound (without margin)
could be established at a load stage 15.
OEM Proposed Criteria
The scuffing and traction performance data in Figure 8 identifies
the lower bound performance based on military hardware experience.
One OEM has proposed a qualification limit of load stage 19. The
proposed OEM scuffing limit is also identified in Figure 8. With
a proposed load stage limit of 19, some of the oils in Figure 8
would be just short of passing. It is tempting to say that the difference
between the lower bound at load stage 15 and the OEM proposed limit
at load stage 19 is a performance margin. Since performance margin
is engine and application specific, any margin above the lower bound
is generic at best.
The data in Figure 8 reflects the scuffing or micro-scuffing performance
of engine oils under specific test conditions. Extrapolation of
this data to other operating conditions is not reliable. Since wear
and fatigue attributes are not included in load capacity testing,
the oil evaluation results should be considered incomplete. Some
additional value to the scuffing results can be obtained by considering
the traction behavior throughout the test. While the linkage between
traction behavior and service performance is not clear, traction
data and its connection with polishing wear, can at least identify
which oils are in-family or out-of-family.
From preliminary examination of surface conditions of engine hardware
at overhaul shops, we believe that the tribological surface conditions
generated during the early load stages of the test protocol represent
many of the features found on bearing and gear hardware under normal
operation. If this is true, the wide variation in traction coefficient
during the early load stages indicates that the performance of these
oils may be noticeably different in service. Detailed tracking of
test engine hardware and field service hardware with specific oils
is a highly recommended activity to sort out the significance of
oil performance characteristics. This action is also an essential
step for the development of testing methods, which are truly linked
to service performance.