Differentials are often one of the most forgotten components in a vehicle, particularly in rear wheel drive and four wheel drive vehicles, but why is that? It’s probably because they’re hidden away below your vehicle and their service intervals are normally quite long when compared to say your engine service intervals which can make your diff easy to forget about. Sometimes the only time diffs get any attention at all is when they have failed or on their way to failure, whirring and whining away as you travel down the highway.
Just like your engine & gearbox, your diff requires regular maintenance too. After all the differential gear set transmits the most torque in the drive train and other diff components are subject to some extreme forces, both potentially causing severe component wear if not managed well through proper maintenance. The crazy thing is that differentials are usually very simple and quick to service providing that you don’t mind getting under the vehicle and getting your hands dirty!
Just imagine the extreme forces acting on the differential components in the following scenario. A 2.5 tonne four wheel drive vehicle is towing a 3 tonne caravan fully loaded with equipment. The vehicle is also carrying 4 passengers travelling at varying speeds and up inclines along a winding country road through the mountains. In this instance there would be some incredibly high forces applied to the diff such as the massive amount of torque being transmitted by the gear set as the vehicle hauls the load up hill, the thrust load being applied to the bearings as the vehicle corners around the bends and the shock loads as you drop the gearbox back a gear or two and release the clutch pedal when you hit those extra steep incline sections.
Yes you’re correct in thinking that this is probably the ‘nth degree’ of heavy duty passenger car diff abuse but there are many other applications where significant diff wear is also a probability, such as a modified street car that gets driven hard and occasionally taken to the race track or drag strip, sound familiar? Often diffs in these vehicles desperately require maintenance and usually doing regular oil changes with the correct grade and type of gear oil will suffice.
Just like an engine oil does, a gear oil protects components from wear in 2 main ways. Firstly by separating the components under load with an oil film and secondly, when the oil film can no longer support the load, with EP (Extreme Pressure) anti-wear additives within the oil. Ideally we want to rely on the oil film to support the load so in extreme cases, such as the scenario described above, increasing the gear oil viscosity grade can help to achieve this, for example moving up from an SAE 80W-90 up to an SAE 80W-140. Higher viscosity grade means that the oil is more resistant to temporarily shear therefore able to support a higher load and separate the components in question. However in differentials it is not always possible to separate these components particularly in a shock load situation so it is inevitable that we need to rely on the EP additives in the gear oil at some point. The EP additives themselves are normally in the form of Sulphur and Phosphorus and the levels of their concentration are determined by the API GL categorisation system (GL-4, GL-5 etc). These chemical compounds react with component surfaces when under pressure forming layers of sacrificial, wear reducing, slippery goodness between the mating components to protect them from wear.
The problem is that over time even the most heavy-duty gear oil will degrade somewhat when subject to substantial load and temperature. Mechanically some of the other gear oil additives will permanently shear under extreme load, reducing the gear oils viscosity and chemically the oil may oxidize in high temperatures changing some of its base properties. Regular oil changes in differentials are just as essential as engine oil or gearbox oil changes, so don’t ignore your differential. Stay tuned for the next diff BLOG where we will talk about diff types…