Nut and Bolt Torsion Activity

Construction Context
nutandboltYou will not have to go far from where you are right now to find a nut and a bolt that is holding parts together to make something strong. You may find nuts and bolts holding appliances such as hair dryers, electric hand tools, or a kitchen mixer together. Designer lighting may hang from the ceiling using nuts and bolts. Store displays are held by nuts and bolts as are motorcycles and bicycles. If a nut is loosened along the bolt, the store display will wobble and objects get knocked off and break. You can hear the store salesperson saying “I didn’t do it!”  A bicycle with loose nuts and bolts will wear because the parts move back and forth while riding. This can cause the bike to be unsafe, Ouch! or for it to wear to the point of needing repair. Expensive!

This activity is about the science behind how nuts and bolts work and how you can use that knowledge to repair things, make things last longer, and make things safer!  A nut and a bolt have threads cut into them. You are familiar with threads because they are how a bottle cap is tightened on a pop bottle or water bottle. If you could make yourself very small and sit on a thread, it would be like starting down a waterslide that is built like a cork screw.  As you go round and round you go down. Think of a slide wrapped around a pole. Think of how you make corkscrew curls by wrapping hair along a curling iron:

A nut is screwed onto a bolt using torque or a twisting force. When it is tightened flush and tight against the part it holding, the bolt undergoes torsion (twisting) and tension (pulling) forces. Think of how a bungee cord twisted and pulled across a load on the back of your bike. The tension on the cord holds the objects in place. The bolt is actually twisted and stretched causing tension that holds the nut against the part it is holding. The wheel is held in the fork of the bicycle. The hairdryer case is held together covering the electric elements inside. Similar to other forces we have talked about in this unit, these are not visible because there is no motion. These are static forces. There are also friction forces between the face of the nut, the face of the bolt and the object being held. There is more science to be investigated when you read the reflection questions below.

Materials
Nuts, bolts and washers for each student

Activity Description
Screw a nut onto a bolt only about half way so that it still turns very easily. The screw a second nut on to the same bolt until it comes in contact with the first nut. Then without touch the first nut, continue twisting the second nut tight against the first nut. Notice that they both stop turning and become tight. Still without touching the first nut, unscrew the second nut.  Notice that it takes effort to loosen it from the first nut. Now screw it in again until it is tight and you can feel the force against the first nut and force it takes to unscrew it again.

Part of the resistance you feel is the torsion or twisting of the bolt which pulls on the bolt. That pulling makes the first nut pull back and tighten against the threads. These forces put the bolt under tension and that holds the nuts together. If you have a washer or something else that fits on the bolt you can put that between the two nuts and then tighten the second nut against the object. It tightens up just the way it does against the head of the bolt.  The treads work as a ramp or inclined plane to push the second nut up tight against the object and friction keeps them from coming loose. There are wrenches made that measure how much force is used to tighten a nut and bolt. This becomes very important for putting the bolt in enough tension to make the connection secure and safe!

Watch the Bending and Torsion video to help better understand how torsion force affects construction materials: http://youtu.be/VuDF0Baqzog.

Science Concepts
Bending and Torsion

Next Generation Science Standards
Scientific and Engineering Practices:
– Constructing explanations and designing solutions

Crosscutting Concepts:
– Structure and function

Reflection Questions

  1. There is more science to the structure and function a nut and bolt.  Find a nut and bolt that is in place holding something together.  A good example would be on the front wheel of a bicycle holding it to the fork.  Examine the assembly closely and make a drawing of the threaded bolt, nut, and fork.  Identify as many forces as you can using arrows and labels.  Discuss this with a friend, talking about what this assembly has to do to keep you save.  Then, go to: http://home.jtan.com/~joe/KIAT/kiat_1.htm and examine the explanation of forces on the nut and bolt on a BMW motor cycle.
  1. On the same same website as above, scroll down to the question, “Who determines the strength of the bolt?”  There is a significant amount of physics involved with this question.  Use this web site, discuss with a friend, teacher, or engineer to see if you can understand how to answer this question.
  1. Go to: (http://theselsproject.org/wp-content/uploads/2013/07/TricksTVRemoteActivity.docx) to guide you through an activity with a rubber band.  See if you can present an explanation to your friend, teacher, or the class about how a nut and bolt holds parts in place.

(Download the activity in PDF, Word)