Key concepts
Physics
Mechanics
Geometry

Introduction
Have you ever wanted to reach something way up high on a shelf? Or have you ever watched construction workers who need to reach up a tall utility pole? A scissor lift is a device that can extend to a great length but also fold up very compactly. In this project you will build your own scissor lift 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 many complicated motions that are used in machines all around you. They can be used to convert motion from one type to another. For example, a linkage in a car engine converts the back-and-forth motion of the piston to rotational motion, which is eventually transferred to turn the car’s wheels. Linkages can be used to amplify both force, such as in a pair of pliers, and motion. An example of the latter is if you have a lever in which 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.)

In this project you will build a mechanism called a scissor lift. Such lifts are used in construction equipment to elevate workers to high places. More generally, they are a type of pantograph. A scissor lift uses an accordionlike motion to contract and stretch out, allowing it to both fold compactly and extend much beyond its original length. Try this project to build your own—and see what heights you can reach!

Materials

• Corrugated cardboard
• Pushpins or thumbtacks
• Modeling clay or Play-Doh
• Scissors (and an adult to help cut and use pushpins, if necessary)
• Ruler (optional)

Preparation

• Cut the corrugated cardboard into at least six equal-size strips. The exact dimensions do not matter but roughly two to three centimeters wide and 10 to 15 centimeters long is a good start.

Procedure

• Carefully 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. (For example, 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 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.
• 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 toward 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 long can you make it?
• 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.

Because 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. If you were not careful enough about how you cut the cardboard or where you put the pins, however, 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 do not 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 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.

More to explore
Mechanical Movements, from Five Hundred and Seven
Scissor Lift (by craft sticks!), from Instructables