How does a lubricant work?
There are hundreds of moving parts whirring away in your car’s engine and gearbox. Sometimes it can feel like you’re driving a clock! The pistons pump up and down in the cylinders, the crankshaft spins, the gears race round at top speed. Every one of these things rubs against something else as it moves—making noise, losing energy to friction, and gradually wearing out. The way to reduce friction between two moving parts is to lubricate them (coat them in oil)—but how does lubrication actually work?
Solids are materials that have a built-in resistance to changing shape, whereas liquids can flow. Think of the difference between ice (which just sits there in a lump) and water (which flows easily as you pour it). If you put a liquid like oil between two solid gears, it will shift about and change its shape as much as it needs to, cushioning the microscopic bumps between the gears as they mesh together and reducing the friction between them.
Imagine two unlubricated surfaces (maybe they’re the teeth of two meshing gears) moving roughly past each other, slowed down by friction. They might look smooth to your eyes, from a distance, but they’re rough at an atomic level and one surface drags horribly against the other, wasting energy and wearing out the materials. A lubricant helps in two different ways: it smooths and cushions the bumps between the two surfaces. Also, because it’s a liquid, it can easily change shape and flow. Ideally, it will flow in perfect layers, as shown here, in what’s known as laminar flow. The layer of lubricant near the top surface will move toward the left while the layer near the bottom surface will move toward the right. The layers will slide freely past one another, so helping to reduce friction.
What makes a good lubricant?
Although some lubricants are used at reasonably cool and constant temperatures, many have to work in engines and machines at very high temperatures. Car engines use thick, syrupy oils for lubrication because these stay liquid at over 300°C (570°F), which is hot enough to survive the kind of temperatures that engine parts heat up to. Water would quickly evaporate and turn to steam in those conditions but it also makes metal parts turn rusty, so it’s not really a good choice for a lubricant.
Often, lubricants have to work well at a range of temperatures. For example, in a chilly country like Iceland or Switzerland, car axle lubricants need to do their job both when the car is just starting from cold and when it’s been running for a while. In practice, that could mean a wide range of operating temperatures from −10°C to 100°C (−14 to 212°F). Lubricants are much like any other substance: the colder they get, the harder, more solid, and less fluid (more viscous or “treacly”) they become. Sometimes, that means they work less effectively: the poorer performance of lubricants at lower temperatures is one of the reasons why engines and transmissions are less efficient before they’ve properly warmed up.
On the other hand, the hotter you make a lubricant, the thinner and less viscous (more runny) it becomes. Although that might sound like a good thing, it isn’t always. On heavy-duty machines working slowly at high power, you need a thicker, more durable layer of lubrication, and a low-viscosity lubricant isn’t going to be helpful. The thinner a lubricant becomes, the more likely a machine is to seize up. Usually, there’s a critical temperature above which lubricants no longer form a strong and effective coating that sticks to the parts they’re touching, performance falls off dramatically, and seizure becomes an alarming possibility.