Hypoid gears in a vehicles differential are inherently inefficient due to high sliding velocities where the gear teeth meet, which is where power is transferred. The efficiency of a hypoid differential, including all its bearings and seals, is typically 85 to 95 percent. The U.S. Army alone has some 130,000 Humvees, and many of them are expected to be in service for decades. Even a small improvement in drivetrain efficiency will have significant transportation and operational benefits.

Tribology research and testing company Wedeven Associates Inc. proposed oil formulation attributes for run-in polishing to improve surface finish at the hypoid gear mesh. Project success required an aftermarket oil for in-service vehicles as a one-time oil change, no drain-and-fill break-in oil.

Run-in Polishing

Run-in polishing is an oil quality allowing the chemical formulation to polish away asperities, or roughness features, on a metal surface. Reducing roughness decreases traction to improve efficiency.

Wedeven Associates developed a test method (SAE ARP6156), originally meant for aviation applications, that characterizes oil for scuffing limits while measuring the traction coefficient through increasing load stages. Traction is used here instead of friction to acknowledge combined rolling and sliding motion at the interface.

Traction is like the heartbeat of the contact. Precision measurements of its behavior are essential for revealing the effects of additive chemistry on gear mesh efficiency and surface durability.

Exploratory tests using the SAE ARP6156 method with a wide variety of additive chemistries, including sulfur, phosphorus, zinc and boron, revealed an enormous range of traction behavior. Examining the traction coefficient versus load stage for 5 percent additive concentrations in a 5 centistoke polyol ester base stock revealed both extreme pressure and antiwear behavior. The test used AISI 9310 gear steel specimens with a combined surface roughness of 0.35 microns (using Ra calculation, or an average of the measurements of surface peaks and valleys).

A baseline test with polished surfaces and low traction coefficient (0.02) clearly differentiates the effects of asperities. This lower-bound traction represents the internal shear resistance, or traction coefficient, of an elastohydrodynamic oil film. The low traction of the EHD film, relative to the asperities, is due to the high sliding velocity (8.8 meters per second) and heat generation within the oil film.

Asperities, under severe thermal and shear conditions like those found in high-speed gears, activate oil chemistry and create a characteristic response. Good extreme pressure behavior is reflected in operation up to high load stages without a catastrophic scuffing event-a required attribute for hypoid gears. Strong antiwear behavior is reflected in high traction coefficient (0.08) because the roughness features are protected by boundary lubricating films.

Certain additive chemistries were found to lower the traction coefficient to a level (0.03-0.04) well below antiwear behavior (0.08) and above the lower bound traction (0.02) of an EHD oil film. This was attributed to polishing wear of the asperities, reflecting a new oil attribute identified as run-in polishing.

RIP behavior is not the antithesis of antiwear behavior. The high tribological intensity (stress, shear and temperature) at prominent asperities initiates chemical polishing action, which then reduces the tribological intensity. Subsequent action from a good RIP formulation creates an antiwear film with low friction for stable, long-term operation. The RIP behavior needs to be carefully orchestrated with the elastohydrodynamic oil film thickness, which is viscosity and speed dependent.

It should also be noted that one approach to increasing differential efficiency is to lower oil viscosity in order to reduce viscous churning losses. This is opposite to what is needed for gear mesh efficiency, where the EHD film is necessary to avoid asperity features. RIP behavior is helpful for both causes.

Gear Mesh Analysis and Simulation

Wedeven analyzed the Humvee differential hypoid gear mesh for contact conditions in service. These conditions were then simulated with a company-designed tribology testing machine in a single-contact configuration.

The gear mesh analysis and simulation addressed the motions, stresses, temperatures and tribology in service. The materials and operating conditions were consistent with the U.S. Army Fuel Efficient Gear Oil program and its full-scale axle testing conducted at Southwest Research Institute. The test used AISI 4320 gear steel with a surface roughness of 0.5 microns, which represented the fully run-in finish of used ring and pinion gear teeth. The researchers designed a test protocol representing Humvee operation from a high speed of 72 miles per hour to an ultra-low speed of 0.4 mph.

The SAE 80W-90 viscosity RIP formulations targeted the SAE J2360 standard for gear oils. Based on results from traction testing with the Wedeven Associates Machine, a low-traction base stock blend was prepared using two polyalphaolefins with an ester for additive solubility. Additive suppliers provided proprietary chemistry to complete the RIP formulation. After testing to narrow down the selection, Wedeven prepared a final RIP concept oil that was consistent with a fully-formulated axle oil.

To evaluate the RIP concept oil, the Humvee test protocol was conducted with superfinished surfaces as well as surface finishes representing used hypoid gears. With almost all asperities removed, the level of traction was the shear resistance of an EHD film. The reduction in traction from 0.1 to 0.02 as speed increased was the result of shear heating within the film, and represented the lower-bound traction for polished surfaces.

Test cycles conducted with typical surface roughness clearly showed RIP behavior. The reduction in traction at a given speed stage was attributed to polishing action, which was most pronounced during moderate speeds. This was consistent with the tribological intensity at asperities activating the RIP chemistry. Roughness was reduced by 62 percent for the test machine ball specimen and 42 percent for the disc specimen. RIP oil testing is generally conducted with six test cycles over five hours to provide time for antiwear surface films to form subsequent to RIP action. The completion of the RIP action coincides with lower tribological intensity, creating long-term, steady-state operation.

Surprisingly, the benefits of RIP action were significant over the entire speed range. This is evident from the traction difference between the superfinished test and the first and second test cycles. The benefits at high speed (and high EHD film thickness) were attributed to less localized pressure generation and shear resistance at asperity sites.

The RIP benefits at low speed extended the EHD film operating range with lower traction before transitioning to boundary film lubrication. An RIP oil formulation with a good low-traction boundary film can be similar to EHD film traction at low speeds. Additionally, the lower-bound full-film EHD traction indicated ample scope for additional RIP action and efficiency.

The hypoid gear mesh simulation test revealed a remarkable amount of information about the sources of traction and loss of efficiency. The test methodology showed the impact of the controlling parameters during operation, which involved contact motion, thermal effects and EHD film thickness. The chemical polishing action from the RIP oil formulation was easily revealed through precision traction measurements and post-test surface topography measurements.

Hypoid Axle Validation Test

Next, Wedeven Associates tested the run-in polishing concept in an operating differential, which contained other mechanical components contributing to efficiency as well as viscous churning losses in the lubricant.

The researchers acquired a used differential from field service to evaluate the RIP concept oil. They made surface roughness replicas of several gear teeth on the pinion and ring gear before and after testing. The replicas were prepared without disassembly of the differential components in order to avoid disturbing the seal and bearing interfaces or the gear mesh alignment. Testing was conducted at Southwest Research Institute in a new U.S. Army Tank Automotive Research, Development and Engineering Center axle test facility developed for the Fuel Efficiency Gear Oil program.

FEGO uses axle hardware that has been completely run in before the test. This required a special RIP drive cycle protocol to highlight the improved efficiency of a used differential from the field. The RIP drive cycle for a light-duty Humvee axle consisted of typical torque and speed cycles over two 16-hour test stages. Test stages were run at different temperatures and torque loading, and total drive cycle was 2,000 kilometers (1,220 miles).

Thirty-two hours of testing with the RIP concept oil demonstrated an efficiency improvement of 0.5 percent to 1.0 percent with a used and fully run-in differential. The hypoid gear mesh roughness was reduced by 20 percent. This efficiency improvement was attributed to the hypoid gear mesh portion of the total losses within the system. While there may have been some RIP efficiency improvement associated with bearings and seals, Wedeven believes that the high relative sliding (0.7 sliding/rolling) and power transfer at the hypoid gear mesh was the dominant component that benefited from the RIP concept oil.

It is clear from the roughness measurements in the axle validation test that there is ample room for additional RIP action for efficiency. It is also clear that there is a large amount of RIP action that is needed to reach the lower-bound EHD traction for optimum efficiency. The original surface finish, including its phosphate conversion treatment, is too coarse for the desired corrective action from an RIP oil formulation. Yet the efficiency improvement attributed to a first-generation RIP concept oil is significant. Additionally, the RIP improvement in efficiency at the gear mesh is compatible with enhanced protection against scuffing and micropitting. RIP action may also help reduce noise, vibration and harshness.

It is significant that the test methodology, particularly the gear mesh simulation, provides a measure of the traction, or efficiency, of a gear mesh relevant to service operation. This methodology reveals all the controlling mechanisms for efficiency and surface durability.

While the research has focused on an aftermarket oil formulation for existing vehicles, perhaps the most important outcome of this project is the outlook for a factory fill oil. The formulation can then be optimized with more compatible surface topography manufactured during the final lapping stage of a hypoid gear-and-pinion matching pair.

Lavern Wedeven, Ph.D., is president of Wedeven Associates Inc. He is a fellow of STLE and a graduate of Calvin College, University of Michigan and Imperial College. He previously worked at NASA and SKF. Wedeven can be reached at vwedeven@wedeven.com.