Simple Machines: Spinning with Screws

Everyday Einstein concludes his 5-part series on simple machines with an examination of screws and salad spinners.

Lee Falin, PhD,
May 31, 2013
Episode #056

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Simple Machines: Spinning with Screws

Would you believe that before I got married, I had never heard of a salad spinner? It’s true; I was that sheltered. For any of my listeners who live equally sheltered lives, a salad spinner is made of a basket that sits inside of a bowl with drain holes on the bottom. The idea is that you put your salad in the basket, spray it with water to clean it off, and then spin the basket around to make the water fly off. The water flies out of the basket, hits the sides of the bowl, and drains out through the holes, giving you a clean and dry salad..

There are two types of salad spinners that I’ve seen on the market, both of which use simple machines to make the basket spin. The most common type uses a wheel and axle, which we talked about last time. The fancier kind uses a screw, a simple machine that we’ll discuss today.

Taking Things in a New Direction

One of the neat things about screws is that they allow you to change back and forth between rotational force and linear force. For example, when you apply rotational force to a screw with a screwdriver, the threads of the screw turn that rotational force into a linear force that pulls the screw into whatever you’re trying to screw it into.

A salad spinner uses that same principle in reverse. The shaft of the screw is sticking out of the top of the basket, and is threaded like a screw. You push down on the shaft and the threads of the screw get pushed into the threaded hole in the lid of the salad spinner. As the threads of the shaft pass through that hole, they cause the shaft to spin, converting the linear force you apply to the top of the shaft to a rotational force that spins the basket around.


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