Simple Machines: Spinning with Screws
Ask Science concludes his 5-part series on simple machines with an examination of screws and salad spinners.
Lee Falin, PhD
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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.
Give Me Some Space
Aside from letting you change the nature of your force, as simple machines, screws can also give you mechanical advantage. As we mentioned in previous episodes on levers, pulleys, and ramps, mechanical advantage is a fancy name for how much a machine multiplies your force. So if you apply a force of 10 newtons to a machine with a mechanical advantage of 3, the force coming out of that machine will be 30 newtons.
To calculate the mechanical advantage of a screw, we first need to figure out how far the screw travels forward when it makes a full turn, which is called the lead. For most screws, the lead is equal to the pitch, which is the distance between two threads.
The mechanical advantage of a screw is the circumference of the shaft divided by the lead.
So imagine we have a screw with a lead of 3mm whose shaft has a radius of 10mm. In our episode on wheels and axles, we mentioned that circumference is equal to the radius multiplied by 2 times pi. So the circumference of the shaft would be 10mmx2xpi, or about 62.8mm. If we divide 62.8mm by 3mm, we get a mechanical advantage of about 21.
That means that for every newton of rotational force you use to turn the screw, it moves forward with 21 newtons of force. However, just as with all simple machines, that increased force comes at the price of increased distance. In order to move the screw 3mm forward, you have to spin it 62.8mm.
See also: Newton’s 3 Laws of Motion
Some Non-Mechanical Advantages
Another important feature that most screws have is the ability to resist overhauling. Overhauling is the term used to describe what happens when the force being applied to the shaft of a screw causes it to unscrew. Obviously screws that don’t have this ability wouldn’t make for very good fasteners. How likely a screw is to overhaul depends on the force applied to the shaft, the angle of the threads, and the amount of friction between the threads and whatever material the screw has been inserted into.
In some cases, we want the screw to overhaul, such as with our salad spinner example. When we push down on the spinner, what we’re really doing is applying a force to the shaft of a large screw, which starts to unscrew, making the basket spin.
Conclusion
So now you know how to calculate the mechanical advantage of screws, how they change rotational force to linear force, and why most screws make good fasteners. This article just scratches the surface, as the mathematics behind screws gets extremely complicated.
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Salad Spinner and Screw images from Shutterstock
