Summary
Introduction
Have you ever wanted to reach something way up high on a shelf? What about construction workers who need to reach up the side of a building or a tall utility pole? A scissor lift is a device that can extend to a great length but also fold up very compactly, making it easy to reach high places. In this project you will build your own from common household materials!
Background
Mechanical linkages are a type of machine generally made up of rigid bars connected to each other by rotating or sliding joints. When combined, links and joints can be used to create all sorts of complicated motions that are used in machines all around you (see the “More to explore” section for some cool examples and animations). They can be used to convert motion from one type to another. For example, a linkage in the engine of a car converts the back-and-forth motion of the piston to rotational motion, which is eventually transferred to the spinning wheels. Linkages can be used to amplify force, like in a pair of bolt cutters. They can also be used to amplify motion – for example if you have a lever where one side is twice as long as the other, when you push down on the short end, the long end will move twice as far (Note that in both cases, energy is conserved – you can never get “free” work out of a linkage. For example, for the bolt cutters, force is amplified, but you have to move the handles very far to get the cutting blades to move just a little bit).
In this project, you will build a mechanism called a scissor lift. Scissor lifts are used in construction equipment to lift workers up to reach high places. More generally, they are a type pantograph (see the Wikipedia link below). Scissor lifts use an accordion-like motion to contract and stretch out, allowing them to fold up compactly and extend much beyond their original length. Try this project to build your own!
Materials
- Corrugated cardboard
- Pushpins or thumbtacks
- Modeling clay or Play-Doh
- Scissors
- Optional: ruler
Instructions
- Cut the corrugated cardboard into at least 6 equal-size strips. The exact dimensions do not matter, but roughly 2-3cm wide and 10-15cm long is a good start.
- Use pushpins to poke through the center of two strips and join them to form an “X”.
- Form an “X” with a pushpin and each remaining pair of cardboard strips.
- Now, use pushpins to join the top of one X to the bottom of another X, as shown in the illustration. It is important that the pushpins be in about the same location on each strip (e.g. don’t put one right in the middle and another one close to the edge).
- Continue joining the top of one X to the bottom of the next X with pushpins. Make sure you alternate left-to-right which piece of cardboard is on top and which is on the bottom – this will keep your entire mechanism flat.
- Use small bits of modeling clay or Play-Doh to cover the points of the pushpins sticking through the back of the cardboard. Don’t prick yourself!
- Now, lay your entire scissor lift flat on a table. Gently hold the free ends of one “X” with your fingers. Try lightly pulling them away from each other. What happens?
- Now try gently pushing them towards each other. What happens?
Extra: Use a ruler to measure the total length of your scissor lift when it is contracted all the way, and when it is extended all the way. How much can it change its length?
Extra: cut more cardboard strips and extend your scissor lift. How big can you make it? You might need to use sturdier materials to make a big scissor lift (see next point).
Extra: your cardboard scissor lift is a nice example of how the mechanism works, but it is not very sturdy. Can you build a sturdier scissor lift out of different materials? Can you put a “claw” on the end so you can use it to reach and grab for things?
Observations and Results
You should find that when you pull the ends of an “X” apart, your scissor lift contracts (all the X’s get shorter and squished). When you push the ends of an “X” together, the scissor lift extends (all the X’s get tall and skinny). Depending on exactly how you cut your pieces of cardboard and where you put the pushpins, it should be able to extend to roughly twice its original length.
Since your scissor lift is made out of cardboard, there should be some flexibility in the joints, and your pin placement does not have to be perfect. However, if you were not careful enough about how you cut the cardboard or where you put the pins, your scissor lift might “jam,” or be unable to move. To work properly, the links must form parallelograms in between each pair of X’s – four-sided shapes whose opposite sides are the same length and always parallel to each other. If you cut pieces of cardboard to different lengths, or don’t put the pins in the exact same spots on each link, then the links might not form parallelograms when you connect them, and your scissor lift will not work.
The modeling clay mainly serves to protect you from pricking your fingers with the pushpins, but it does not do a very good job holding the scissor lift together. To build a sturdier scissor lift, try using something like wooden craft sticks instead of cardboard, and have an adult use pliers to bend the ends of the pushpins at 90 degree angles to prevent them from falling out of the holes.
Ask an Expert
Additional Resources
- Five Hundred and Seven Mechanical Movements, a website with animations of hundreds of different mechanisms
- Animated Mechanisms, another website with animations for many different categories of mechanisms
- Scissor Lift (by craft sticks!), another scissor lift design, from the Oakland Toy Lab
- Pantograph, from Wikipedia
- Science Activities for All Ages!, from Science Buddies
