Need more grunt

When I came to New Zealand nearly nine years ago (it doesn’t seem that long – a sure sign of aging) I had to learn some new words. Or rather, new meanings for existing words. Things like ‘jug’, ‘dairy (as in the shop down the street), ‘five-eighth’ , ‘bush’, all have different meanings to what I’m used to. One such word is ‘grunt’. To me, ‘grunt’ is what pigs do. To my students, it’s what engines have.

Yesterday, we had a very entertaining series of talks by the third year engineering students describing a design project they’d done this year. They were set a task of building a machine that would collect squash balls from a holder and deposit them into a container a couple of metres away, as fast as possible. After the talks, there was the competition – which was great fun to watch – won by ‘The Tortoise’. As it’s name suggest, it wasn’t the quickest machine on the planet, but it had the advantage over the others of working reliably.  Sometimes (in fact most of the time in engineering) getting the job done correctly beats getting it done quickly.

Now, in the talks, a couple of the groups made a comment that the electric motors that they were supplied with didn’t have much ‘grunt’. (That, I think, was deliberate – the students had to design ways around this)  Now, grunt is not a physics word. You won’t find it in a physics dictionary. So what did the students actually mean?

 There are a few terms that can be used to describe motors. One obvious property is torque. This is the rotational analog of force – it describes the ability to impart an angular acceleration on an object. In other words, the greater the torque, the more quickly the motor can spin something up.  But ‘grunt’ doesn’t equate with ‘torque’, I think. If you have a motor with low torque, you can easily cure it with gears. Use a low gear, and you can apply more torque to a system. You know this from trying to cycle up a hill – drop down a couple of gears and it’s easier to move forward. However, this comes at a cost – it might be easier to go forward in a lower gear, but you will be doing so more slowly.

So, then, also important is rotational speed. Fast motors can spin things more quickly than slow ones. But, again, one can change the rotational speed by using gearing. If you want to turn a crank more quickly, go for a higher gear.

What I think students meant by grunt, is the combination of these two. Multiply torque by rotation rate, and you get a measure of the power. That’s the rate of supply of energy to a system. For an electric motor, torque decreases with rotation rate (the motor supplies greatest torque when it’s spinning no-where) – so it’s not immediately obvious how the product of the two will behave. Where is it maximum (i.e. at what rotation speed does an electric motor have most ‘grunt’?) In practice, with an electric motor, there’s a wide range of rotational speeds at which the motor can provide a decent power output. That means that electric motors often don’t require a gearbox, whereas with a petrol driven motor, which has a fairly limited range of rotational speeds where they can provide appreciable torque, a gear box (that is, a selection of different gears)  is essential for many applications. Another major difference between petrol and electric motors is that petrol engines don’t deliver any torque at all at low rpm (they stall), whereas an electric motor gives its maximum torque there. Hence the hybrid engine – use the electric motor at low speeds where the petrol engine is, basically, appalling; but switch to petrol at high speeds.

So, in my mind, grunt means power. At least, it does in New Zealand.








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