Lubricants

by Chris Woodford. Last updated: May 23, 2011.

When a car screeches round a bend at high speed, the only thing that stops it skidding is the friction between its tires and the road—a force that can be several times greater than its weight. Here friction is working like a kind of glue that sticks the car to the road, but it's not always so helpful. Under the hood, friction is busily wearing out the engine parts—it's one of the main ways that a car wastes energy. That's why we need lubricants: substances that fight friction and allow moving machine parts to slide past one another more smoothly.

Photo: Lubricants crop up in some unusual places. Safety razors like this usually have some lubricant on them so they move easily over your face. The lubricant is the green strip at the top. When the razor gets wet, it turns into a greasy cream that makes it glide more smoothly over the hairy skin that's heading toward the blades.

What is friction?

Sparks from a circular saw

Friction is caused by forces between two things when they touch, like when you drag your foot along the sidewalk and feel and hear the surfaces scuffing together. It doesn't matter how smooth things look or feel—peer at them under a microscope and you'll see they're always rougher in close up. When you drag your foot on the ground, the jagged bits on the two surfaces snag against each other, slowing you down and stealing your energy. The rougher the touching surfaces, the more friction there is, the harder you have to work, and the more energy you waste.

Photo: Right: Friction makes the sparks when the teeth on this power saw start to bite. Photo by Eduardo Zaragoza courtesy of US Navy and Defense Imagery.

Friction also happens because of a microscopic "gluing" effect when two things touch. When a gecko climbs a wall, millions of tiny hairs on the base of its feet are touching the wall at the same time. Molecules on the tips of each hair attract to molecules on the outer surface of the wall with a microscopic amount of what's called van der Waals force—a really weak kind of electromagnetism. Although each hair provides only a smidgen of magnetic force, there are so many hairs on the gecko's foot that, together, they can provide enough force to support its weight. Read more about this in our article on adhesives.

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?

How unlubricated surfaces rub through friction while lubricated surfaces glide past one another.

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.

This little illustration shows you what's happening. On top, you can see 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 (1). At the bottom, we can see how a lubricant helps in two different ways: it smooths and cushions the bumps between the two surfaces (2). 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 (3).

What makes a good lubricant?

Car engines use thick, syrupy oils for lubrication because these stay liquid at over 300°C (570°F)—the kind of temperatures 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. Engine parts like a car's gearbox are coated with oil or grease to reduce friction. Some of these parts are not machined smooth, as you might expect, but deliberately left a bit rough so that lubricants will cling to them and keep on doing their job.

In case you're wondering, the science of friction, lubrication, and surface wear is called tribology. So next time you meet a tribologist on your travels, you'll know exactly what to talk about!

Greasing machine gears. A ball bearing taken to pieces

Photo: Two different approaches to lubrication. Left: Applying thick grease will keep these gears turning for a few more weeks or months. Photo by Angel Roman-Otero courtesy of US Navy. Right: Ball bearings like these help to reduce friction in moving machine parts. There's an outer metal ring fastened to one part of the machine and an inner metal ring fastened to another part. The two rings are separated by a circular collar (here colored reddish-brown) with holes inside it. The collar can rotate freely on metal balls that rest in the holes. Bearings are usually lubricated to keep them running smoothly. Photo courtesy of NASA Glenn Research Center (NASA-GRC).