Superfinishing Rear Axle Gears Reducing Greenhouse Gas Paper Image

Superfinishing Rear Axle Gears – A Significant Step Toward Reducing Automotive Greenhouse Gas Emissions


Government regulators have established lower Greenhouse Gas (GHG) emissions for automotive emissions. For example, the European Union is imposing strict financial penalties of about ati€95/(g-CO2/km) for emissions above the standards in Europe. Since the automotive market is global, US car manufacturers must also meet equivalent standards. Currently, CAFE non-compliance is only fineable by the EPA. On the other hand, emission levels are mandatory. NHTSA can refuse to certify a car for sale if emission regulations are missed. Mandatory recalls can also be implemented if it is discovered that vehicles are not meeting emission requirements due to discovery of original certification, false positives, or observed field degradation of emission compliance equipment. Now that the EPA has included CO2 as a GHG over which it has statutory authority; CO2 per mile goes right along with MPG. Therefore, the financial consequence of missing CAFE MPG can possibly result in loss of ability to sell, exposure to recalls and worse. The rear axle is a major contributor to energy loss in today’s passenger vehicles. See Figure 1.


Figure 1iii. Shows that 5.6% of a passenger car’s fuel is wasted on friction and slippage in the drive train.

Hypoid gears are used in passenger cars since they are stronger, quieter and can be used for higher reduction ratios. Since they experience sliding action along the teeth, energy is expended to overcome friction which is shed in the form of heat. It has been well over a decade since the motorsport industry took advantage of isotropic superfinishing to increase fuel efficiency and durability. For this reason it should also be noted that a number of aerospace companies use or plan to use isotropic superfinishing to increase performance, especially on gears that are highly loaded such as those in rotary aircraft.

In the OEM automotive industry, Fordiv found that isotropic superfinished hypoid gears “resulted in about a 0.5% improvement in fuel economy in chassis roll dynamometer tests under metro/highway cycles”. Others have seen similar fuel efficiency increases for isotropic superfinished rear axle gears.

A recent study on the energy consumption by passenger cars due to friction states: The topography of a surface in sliding contact has a remarkable influence on friction in both dry and lubricated sliding. This friction has long been considered only as a surface roughness property of the component. Recent research has shown that the influence of the micro-and nanoscale topography is more complex but, at the same time, offers interesting possibilities for friction reduction. Changing the surface topography of gears to be smoother by superfinishing has been shown to reduce friction by typically 30%.

Another recent study on hypoid efficiency makes an interesting pointvii: On the other hand, the efficiency of the drive-train is highly dependent on temperature. A further complication is that most legislation is based upon a small number of, typically low-power, drive cycles. Testing for compliance with the rules on fuel economy and emissions involves rig testing of the vehicle under conditions that do not necessarily reflect their real-world usage. For example the European fuel economy test (which follows the ‘‘New European Drive Cycle, NEDC’’) requires a starting temperature of 20–25 0C and lasts for only 20 minutes. During this period, the engine warms up to its thermostatically controlled temperature, but the axle, which is dip-lubricated and physically separate, remains relatively cool, as will be shown.

In contrast, the same vehicle may be subject to severe usage involving hot weather, high towing loads, steep gradients and high speeds leading to very much higher temperatures and appreciably different tribology. Therefore, isotropic superfinished hypoid gears will impart even more significant fuel savings than that determined by legislative testing.

When production gears are evaluated in head-to-head comparison testing to isotropic superfinished gears, the production gears usually go through a “proper run-in” whereas superfinished gears require no run-in. Since the majority of passenger vehicles do not experience a “proper run-in”, the performance of vehicles with superfinished gears will have even greater performance benefits. The data shows this under carefully controlled testing.

In addition, isotropic superfinished gears do not demand the higher viscosity and/or additive packages like production gears. Therefore, the fuel efficiency can be further increased by lowering the rear axle lubricant viscosity to minimize losses while still maintaining their durability.

A number of major automotive manufacturers have evaluated the isotropic superfinishing of rear axle gears, and have obtained promising fuel efficiency savings. Why then has this technology been so slow to gain acceptance in the automotive industry?

The main reason is that the process is not user friendly for high volume production of rear axle gears. In the past, conventional isotropic superfinishing was carried out in vibratory machines with finishing times ranging from approximately half an hour to an hour, and required manual labor. Up until this time, there has been no pressure put on the industry to lower GHG emissions or suffer severe financial penalties.

The following sections of the paper discuss the nature of hypoid gears, Rapid ISF technology and data on the benefits that it provides.


• Roughness Average
• Supermagnets
• Accelerated Chemistry
• Media
• Drag Finisher
• TOE (narrow part of the gear tooth)
• HEEL (wide part of the gear tooth)
• DRIVE SIDE (convex side of the gear tooth) • COAST SIDE (concave side of the gear tooth)


Description of the Hypoid Gears

Hypoid gears are used in heavy trucks, four-wheel- drive and rear wheel drive passenger cars, sport utility vehicles, and light trucks. They and are the type of gears used in this study. The convex side of the ring gear and the concave side of the pinion gear are the drive sides of this hypoid set. Since the vast majority of operating time is spent on the drive side, these are the most important surfaces to finish to obtain increased fuel economy (i.e., or lower greenhouse gas emissions). The ten hypoid gears isotropic superfinished in this paper have a gear ratio of approximately 2.8 with the diameter of the ring gear being approximately 140mm. The gears were carburized and then ground. The average surface roughness of the convex side of the ring gear was 0.56 micron Ra; the average surface roughness of the concave side of the pinion gears was 0.57 Ra. The ring gear may or may not contain bolt holes depending on whether the ring gear is bolted or welded to the differential case assembly. See Figure 2 showing a close-up of the ground ring gear and pinion gear. page2image3543703808 page2image3543704096


Initial ground surface of drive side (convex) of ring gear.