Modelling friction

The Nullarbor Plain, the longest stretch of straight railway track in the world, 428 kilometres. It’s part of the 4,300 kilometre journey across Australia from Perth to Sydney. Trains can be a couple of kilometres long and need two or three locomotives to pull them, most sometimes. Here’s a freight train transiting Sydney. We want minimum rolling friction from the waggons, but we want maximum friction between the locomotive driving wheels and the rail. Each axle of the waggon supports 20 tonnes. It can be much higher on iron ore trains. They use roller bearings, heavy duty ones. The shape of the rail matters, and the shape of the wheel tread matters, too. It took a lot of research to get that right.

You need to understand friction to do it well. And you need to understand friction if you’re going to get maximum traction before the locomotive wheels slip. This week we’ll look at how engineers model friction. They need something complete enough to be useful and simple enough to give insight. We’ll explore the most basic model, dry friction, which is where engineers often start. And we’ll look at two applications, tipping and sliding, and rope around the bollard. We could present many more, friction clutches, drum brakes, friction vibration dampers, bearings, screw threads. But you will be well on your way to gaining engineers’ eyes with these two. Your design task this week is a belt drive for a miniature car.

To specify the pulley diameters and the belt tensions, you will need to understand dry friction. In the analysis videos, you will learn about the work of two Frenchmen, Coulomb and Amontons. But first, some experiments.