Build a Crank and Slider Mechanism
A crank and slider is a common, simple mechanism used to convert rotational motion into reciprocating linear motion, or motion that goes back and forth along a straight line. Familiar examples are the piston and crankshaft on a locomotive or in a car engine. A simple, working model can be built out of household materials, wood, or metal.
Steps
- Choose materials. This example will use cardboard and wire, since just about anybody can readily obtain and cut them, but a sturdier or more attractive model could use balsa wood, Popsicle sticks, wood, stiff craft foam, metal, or found materials such as a container lid. Choose materials according to what you can find and whether you can work with them easily.
- Create a base. The base simply supports the other mechanism parts and provides pivot points for the rotating members. In this cardboard model, a large sheet of cardboard cut from the side of a cereal box will serve as the base.
- Cut out the crank. The crank needs two pivot points. They may be on a circular cutout as shown here, or they may be on either end of a short rectangular piece. If you'd like a circle, use a compass or trace around a circular object of a suitable size, such as a can or glass. Make a hole in the center of the crank and make another near the edge of the circle. For a linear crank, make holes near each end. Don't put any holes too close to the edge.
- Cut out the shaft. Make it about twice as long as the distance between the two holes. Make holes near either end of the shaft.
- Cut out the slider. This can be a simple rectangle of whatever material you are using. If you will place the slider into a tube, such as a pipe, you may prefer a cylindrical object, such as a cork or a piece of dowel or rod. Place a hole in one end of the slider.
- Fashion a means of retaining the slider along a linear path. For a two-dimensional cardboard model, you can use rectangles folded and glued to your base. Make these retainers two times as long as the crank, plus the length of the slider.
- Prepare pivots. This model uses craft wire and buttons, but any assemblage that will fit freely through the holes and hold the pieces together while allowing them to rotate against each other will do.
- Put a hole in your base material and create a pivot between the center (or one end) of your crank and the base.
- Create a pivot between the crank and the shaft. If you're making a two-dimensional model on a base of wood or cardboard, place the shaft on top of the crank.
- Create another pivot between the shaft and the slider. In a two-dimensional model, place the slider under the shaft.
- Rotate the crank so that the slider is in its fully retracted position. Arrange the retainers on either side of it so that it has a path to move and fasten them down. Glue or even staples will do for paper.
Tips
- Use a ruler to cut on straight lines, especially for the slider.
- Make the pieces fairly broad, especially if you are working in a delicate material such as cardboard or balsa wood. It will help keep them from bending around too much or folding instead of turning.
- Because the crank end follows a circular path, the motion of the slider will follow a sinusoidal pattern. Put another way, the location of the slider follows the sine of the angle of the crank. This also means that for a constant rotational speed of the crank, the velocity of the slider will be greatest in the middle of its travel and least at the ends of its travel.
- In this model the rotary motion of the crank drives the slider, but it is also possible for the slider to drive the crank. For this to be effective, the crank should have a bit angular momentum to keep it spinning at the ends of travel of the slider. If you push the slider back and forth in this model, you may have to give the crank a push to get it past the center of its travel.
Warnings
- Wear safety glasses if you will be operating a drill or using a craft knife.
- Metals can have sharp edges, especially when cut. Use appropriate caution.
- If your materials are more heavier rigid than cardboard, be careful of pinching your fingers between moving parts.
Things You'll Need
- Materials of your choice: wood, metal, stiff cardboard, craft foam, or found objects.
- A means of cutting holes in these materials.
- A means of fastening the materials as a pivot: wire, brads, screws, etc.
- Retainers for the slider: blocks or folded, fixed walls.
