Background

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 (“micro-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”.

Scope

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.

Approach

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.

The entraining velocity (U_e) and sliding velocity (U_s) are defined below:

U_e = 1/2(U_b + U_d)

U_s = (U_b - U_d)

where U_b = surface velocity vector of the ball at the contact point
         U_d = surface velocity vector of the disc at the contact point

The entraining velocity (U_e) and sliding velocity (U_s) 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
Traction coefficient
Ball and disc surface velocities
Contact load
Time
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 startup
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 behavior.
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 load/unload damage.
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 test.

Test Procedure

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.

Test Description

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.

Performance Criteria

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 (AMAD hardware).

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 coefficient.

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.