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Torque and Spark


When it comes to gear oils, heavy-duty and light-duty vehicle manufacturers have no lack of tasks on their to-do lists: More towing capacity. Longer service intervals. Higher torque outputs. And not least, every ounce of fuel economy possible.

One of the biggest challenges for our vehicles is towing, said Harold Chambers of Ford Motor Co., Livonia, Mich. We also need to run quietly for the life of the vehicle. We want to optimize the power density – that is, get the maximum torque capability with the least fatigue life in the smallest space possible.

Other needs listed by the lubrication technology specialist, with Ford Transmission & Driveline Engineering, include higher performance in the axles of sportscars such as the Mustang, and greater heat transfer capabilities. Also, our customers are begging for the best fuel economy possible. Were trying through geometry to maximize sliding speeds, but if we increase the offset of the gears greatly, we risk losing strength.

To minimize heat generation, one of the gearboxs greatest enemies, Ford has been looking at mechanical factors such as the gears spiral angle and hypoid offset. It also is scrutinizing ways to reduce surface roughness and coating variables. Gear coatings are a great expense, Chambers explained, and it can be difficult to control the process of applying a protective manganese or phosphate layer. The gear lubricants base stock and additive package need to be compatible with such coatings, of course.

Overall, because of higher durability and towing needs, were seeing more wear and distress issues with gear sets.

Heavyweight Needs

We have a lot of issues similar to Harolds, commented Diann Hua, senior engineering specialist in the Advanced Materials Technology Group at Caterpillar Inc., in Peoria, Ill. Her company sees the same needs for meeting customer expectations, gear design challenges and lubrication design – and may also face a grueling off-highway environment.

For off-highway trucks, wheel loaders, excavators and so on, the stakes are very high. Were operating in a different environment as far as most of the automotive industry is concerned. But like theirs, our customers always want optimum performance.

Chambers and Hua spoke in May to the Society of Tribologists and Lubrication Engineers, at the groups annual meeting in Philadelphia. Both described their toughest hurdles, and what they expect gear oils to do to help them.

Hua started by pointing to Caterpillars off-road haul trucks. Our biggest off-highway truck is bigger than a three-story house, she said. Its huge. Especially favored for mining outfits, these leviathans can be found in a wide range of operating and ambient temperatures – from ice-bitten oil sands projects in Canada to the torrid, dusty deserts of Arizona. They also see a wide range of speeds and loads. The filtration and forced-lubrication systems on board are constantly challenged.

For the majority of final drives and axles, we use a very thick lubricant, Hua said, but fuel economy, smaller gearbox sizes and temperature needs have forced Caterpillar in some cases to use lower viscosity lubes. Worse yet is the contamination you cannot avoid. Grit passes through the seals into the drivetrain system, she noted.

Our customers always want two things. One is a higher payload, so you need higher power density. Two, they want lower operating costs. Often, users want to go at least 20,000 operating hours before needing to bring the unit in for regular overhaul, and not to be derailed with costly breakdowns. For a mine-haul truck, Hua said, each breakdowns average repair cost is $70,000 – not including lost productivity during the downtime.

Users also expect gears to live to the second overhaul – thats 40,000 hours. They also want to see longer oil life as well as the minimum number of lubricants. Were seeing customers want the same lubricant in the gearbox as in the transmission.

Teething Pains

She went on to provide a closer look at gear design in these behemoths: each gear can be greater than two feet. The teeth are large, high modules (up to 14 of them for each spur gear) and generally are not finished beyond heat treatment. All grinding of the shape takes place before heat treatment, none after. These gear sets move at very low speeds, with highly loaded contact, and pressures greater than 2 gigaPascals.

In terms of lubrication, these are the worst case you can deal with, Hua declared. No hydrodynamic lubrication is possible, where a fluid film can build up to separate the rubbing surfaces. Instead, its all metal-to-metal contact, and it really relies on the functionality of the gear oils additives.

Initially, the gears begin with a relatively high surface roughness. (We can make them smoother but it would cost too much, said Hua pragmatically.) That means the designer is relying on the equipments break-in period to resolve any roughness issues. However, theres also a need to prevent other surface failure modes, such as pitting and micropitting.

On the lubrication side, theres a challenge between the transmission side and the axle side of the system. In the transmission, Caterpillar must balance gear life and bearing life with friction material performance. It also finds that lower viscosity fluids are best here to optimize clutch control and minimize churning.

On the axle side, by contrast, we need a high viscosity, an SAE 60 or greater, Hua said. This is a growing challenge, and to maximize lubrication, Cat has been testing a wide variety of lubricants in a ball-on-disk apparatus, to pinpoint the differences among fluid types, viscosities, surface coatings, and so on. Were looking at traction coefficients, wear, and so on. What were finding is that lubricant and surface engineering have a significant impact on performance, Hua stated.

Torque to Tow

Fords passenger cars and pickups have a more human scale, but the challenges are still daunting. The needs vary by the vehicle type and how much room it has on board for components, Chambers indicated. Smaller pickups, for example, get smaller fluid reservoirs but still have to deliver plenty of power for their size. With our F-150 pickups, we see more problems because of power density needs. Thats a very big challenge. At least with the Super Duty, we have more space to put everything we need.

Driveline challenges often come down to the gear lubricant. Todays Fords demand lower viscosity gear oils, made with a higher viscosity index base oil, and improved viscosity modifiers. The fluids also need to have improved thermal transfer properties, and to effectively balance antiwear systems.

Currently, Fords standard gear oil product is an SAE 75W-140 viscosity grade. This product, however, seems to be limited to handling 418 foot-pounds of torque, and we need it to do 600, Chambers emphasized – an improvement of nearly 30 percent. We also have one of the most severe requirements for towing duty cycles, he continued. Even gear oils surpassing current industry standards may fail to provide sufficient robustness for this trailer-towing durability test. Its even more of a challenge for fuel-efficient lubricants, which tend to be lighter vis grades and may not provide the needed film strength.

Ford also wants to see reduced axle temperatures during break in, and better heat transfer out of the gear box. It would prefer also to avoid using hypoid gears with phosphate coatings. Can lubes help? asked Chambers. These high performance needs are all an uphill climb. Theyre all challenging for the oil, even for synthetics.

The optimal gear oil also will need to be compatible with a wide range of seal elastomers and clutch materials (including HNBR, ACM, NBR, silicone and others), and to survive the severe Ford split Mu vehicle test without chatter.

A particular challenge, Chambers said, is foaming. Weve used a liquid silicone RTV gasket since 1975, and that tends to add to the foaming tendencies. So far, nobody in the oil, additive or RTV industry has been able to get rid of this challenge for us.

Currently, Ford requires a 75W-140 gear oil for factory fill, but its hungry for better performance. While the idea of using a lower viscosity grade such as 75W-90 is tempting for its fuel economy potential, the automaker is not yet ready to take that route. Were still testing it, and were open to looking at any combination that would pass our tests – but we cant go too low in viscosity because of durability concerns, Chambers concluded.

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