Buying Time

| December 12, 2008

Checking the transmission fluid requires getting underneath and pulling a plug. But, unless seals are defective, leakage is negligible, and all you’ll need to do is pull the plug for a second to see that the level is satisfactory.

Today’s axles and transmissions are beasts of burden: Smaller than their predecessors, these components have higher power densities but with less lubrication capacity than earlier models. At the same time, engine torque through the driveline is higher
and higher, says Jack Zakarian, consulting engineer with Chevron Corp. In other words, “The stress on oil has gone up,” he says.

That’s why these workhorse components need lubes tough enough to handle the rigors of over-the-road operation. There are two basic types of lubricants available for axles and transmissions: mineral lubes, which are distilled, or separated, out of crude oil with heat and solvents, and synthetic lubes, made through a process that enhances the most basic hydrocarbon molecules.

That process produces a more pure lubricant that offers improved performance and longer component life and that remains stable for longer periods, says AmSoil’s Ed Newman. For these reasons, component makers have authorized transmission and axle fluid change intervals up to five times longer than with mineral lubes. Longer change intervals mean lower overall cost because you’ll use less lube over the life of the component and have less downtime. Yet, despite these advantages, only about half of owner-operators use synthetic lubricants in their transmissions and axles, according to an Overdrive survey.

That’s too bad, experts say, because synthetics do a superior job at tackling the two biggest challenges transmission and axle lubes face: heat and friction. Today’s transmissions carry more and more torque and power with the same size gears in the same case, Zakarian says. “Many transmission coolers today dump their heat into the engine cooling system, which in many vehicles raises transmission temperatures even more,” he says. The problem is compounded by aerodynamics, which minimizes airflow around the transmission, meaning almost all the heat has to be carried away by the cooler. The result is a high concentration of heat there.

Truck drive axles, on the other hand, use gears that have many advantages, including strength, but which create a lot of sliding friction. Although the total amount of heat created is less than in the transmission, the fluid still must work very hard – much harder than in the past. The loads on the teeth are more concentrated than in the transmission. There is also less cooling air passing around the axles than in the past.

For these reasons, axle lubes need EP (extreme pressure) additives, which create a surface coating on the gearteeth that’s “almost a solid under load,” says Zakarian, and which prevents welding and seizure. The coating enables those hypoid gears to slide over one another without scoring, while helping resist “much higher loads as well as the shock loading,” says Alex Bulkhovsky, a technical adviser with ExxonMobil.

When it comes to dealing with heat, synthetics can run at high temperatures without getting too thin because their viscosity is more stable than mineral lubes. “My shop people tell me that heat destroys components,” says Harold Mast, owner of BRT Transport in Parkesburg, Pa., which has used synthetics in transmissions and axles since 1996. With synthetics, “we believe the units run cooler and will last longer,” he says.

In fact, synthetics can endure thousands of hours at high temperatures without forming deposits that interfere with cooling. A synthetic will degrade less in 500,000 miles than a mineral lube will in 50,000 miles, Bulkhovsky says. Deposits result when heat attacks the lube and converts it from oil into sludge and varnish. “There is a lot of heat generated in the transmission and axle,” says Bulkhovsky. “That heat needs to dissipate or the lubricant degrades.”

Deposits and sludge slow heat transfer. Heat is trapped, and there can even be a spiraling cycle in which the extra heat then speeds up the rate at which the deposits are formed.

In the end, partly because of heat-related expansion and partly because of the space they occupy, the deposits “change the clearances in the geartrain,” says Bulkhovsky. “The clearances get tighter, and the equipment works harder,” because the moving surfaces are clamped closer together and there is more friction. “This happens because the lubricant can’t get between the working parts,” he says.

The ability of synthetics to run at high temperatures hour after hour without either getting too thin or forming deposits enables them to solve these wear-causing problems.

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