Four-wheel and all-wheel drive popularity has never been higher and motorists that buy the extra traction that four driven wheels affords report some of the highest owner satisfaction ratings anywhere. More driven wheels, however, mean more gears, which means additional lubrication. With a typical four-wheel drive SUV or light truck having four or five individual gear cases (including the engine and transmission) there is a lot of lube to service, especially as mileage increases. Compared to engine lubrication, however, the gear oils lubricating differentials and transfer cases operate under very different conditions. Without combustion processes or the diluting effect of gasoline or diesel, the environment may seem less severe, especially with the use of ball and roller bearings instead of the plain sleeve bearings in most engines. In fact, automotive gear lubes operate under severe conditions. Different, but definitely severe.
What makes a good lube?
Many properties of a good gear lube are similar to those of engine oils. Automotive gears run in a hot environment, so thermal stability is a must, as is resistance to oxidation and foaming. Good shear stability and film strength is essential for minimum friction and resistance to corrosion and deposits are necessary to protect gear trains. The lube must flow easily when cold, lubricate when hot, and be compatible with automotive seals and gasketing materials. Limited slip mechanisms are another factor. And it has to be cheap. It’s a tall order, but the ability of modern lubes to last so long that they’re rarely considered by motorists as a service item places the responsibility for thinking about them with the technician.
What happens if the differential or transmission doesn’t get adequate lube? Gears can fail in a number of ways. Surface fatigue occurs when the lubricant film on the gear teeth is insufficient to protect the surfaces from the stress, resulting in pitting in the contact region. Scuffing can also occur when the lubricant film fails, raising temperature which in turn accelerates both the breakdown of the lubricant and wear. Mechanical failure occurs when bending stresses at the root of the tooth exceed the strength of the gear metal. This bending phenomenon is the reason why failures shear the entire tooth at the base, even though this is the thickest part of the tooth, especially in hypoid sets found in differentials. Shock loads also cause failure, which is another reason for addressing excessive lash. While it’s important to set up ring a pinion depth to create the proper contact pattern between the gears, don’t forget the consequences of shock loads, especially in hard-working applications such as snow plowing, racing and off-road driving. Obviously, manual transmissions transmit shock loads more severely that automatics, but don’t forget the influence of the clutch. Heavy-duty, racing or improperly adjusted clutches can hammer drivelines. If U-joints are breaking or wearing out at an accelerated rate, the axle is also feeling the strain.
Changing Gear Lube
Changing gear lubes is such a no-brainer that it’s a task often left to apprentices or semi-skilled installers. It is important, however, so remember to keep a few things in mind. One is to, wherever possible, avoid using suction pumps to remove old oil if a drain plug or rear cover is available. Many drain plugs have magnets which can tell a lot about internal wear. Some replacement plugs are available with magnets, and can be a good idea for heavy-duty applications like four-by-fours or tow vehicles. If you’re servicing an axle, wipe out the bottom of the housing and inspect the crown gear. When reinstalling the cover, check it for flatness and extrusion around the bolt holes. Hammer a steel cover’s flange flat if necessary. RTV sealants are commonly used and tolerate uneven flanges well, but a badly extruded or distorted series of holes (usually caused by excessive tightening torque) can leave gaps between the bolt holes, causing leaks. And if you’re servicing an abused solid axle with a chronically low oil level, think about the seals at the end of the axle tube. If they’re worn, the newly topped-up lube might migrate out and at the rear, into the brake drums. It’s a comeback that’s hard to explain to customers, and if you have the rear cover off anyway, why not change the seals? On a high-mileage vehicle, it makes sense for you and your customer.
What the Codes Mean
There are several standards for automotive gear lubes, but the most common one in use in North America is the standard set up by the American Petroleum Institute. The most common in use today is API GL-5, which describes lubricants intended for gears, particularly hypoid gears, in axles operating under various combinations of high-speed shock loads and low-speed, high-torque conditions.
API MT-1 describes lubricants intended for nonsynchronized manual transmissions used in buses and heavy-duty trucks. Lubricants meeting the requirements of API MT-1 provide protection against the combination of thermal degradation, component wear and oil seal deterioration. Can you use MT-1 lubes in light duty applications? No. API MT-1 is strictly a non-synchromesh lube and can’t be used in synchro boxes, whether automotive or heavy-duty.
Will the next standard be GL-6? Surprisingly, no. GL-6 was developed for lubes intended for gears designed with very high pinion offsets, which need extra scoring protection not offered by API GL-5 gear oils. Current designs use modest pinion offsets, which combined with the obsolescence of original API GL-6 test equipment and procedures, have left API GL-5 at the top of the game in light duty lubes.
Engine oils as gear lubes?
An increasing number of automotive gearboxes use automatic transmission lubes as gear lubes. The reasons are simple: they’re good at controlling oxidation and wear and can maintain a lubricant film better than earlier generation products. And these higher-performance fluids allow power train engineers to design gear sets to take advantage of the energy saving properties of these low-viscosity lubes. Can you use modern engine oils as transmission lubes? Officially, no, but racers (and original Mini owners) have used engine oils as gearbox lubes for years when looking for a slight edge, but at a cost in durability. Better automatic transmission fluids have allowed them to migrate to the gearbox, and motor oils are improving, too. The API (American Petroleum Institute) Engine Oil Licensing and Certification System (EOLCS) has just introduced a new passenger car gasoline engine oil service category, API Service SM. Engine oils meeting the new API SM service category designation are designed to provide improved oxidation resistance and deposit protection, better wear protection, and better low-temperature performance over the life of the oil. Some SM oils may also meet the latest International Lubricant Standardization and Approval Committee (ILSAC) GF-4 specification, its latest minimum performance standard for gasoline-fueled passenger car engine oils. The adventurous might want to try a synthetic SM engine oil as a manual transmission lube for racing, but remember that you can’t replace an extreme pressure gear lube with an engine or automatic transmission lube if the gearbox wasn’t designed for it. But do consider synthetic versions of the approved oils as a way to offer consumers extra protection.