Change the speed. Explain the motion.

01 / UNDERSTAND

Your design brief

A demonstration mechanism needs to turn an output more slowly than its input. Design a 24-tooth driver and a 48-tooth driven gear, and explain the change in speed and direction.

What you will learn

• Use matching gear parameters.
• Calculate a nominal centre distance and speed ratio.
• Distinguish a geometric prediction from evidence gathered from a working model.

You will need: Forma, calculator, ruler and a teacher-prepared hand-turned gear model if available. Forma draws the profiles; it does not simulate their movement.

02 / EXPLORE

Make it in Forma

Menu names refer to the current browser edition. On a touchscreen, use Properties to show the inspector. A keyboard is helpful for exact dimensions.

1. 01

   Make a prediction

   Sketch two touching gears with 24 and 48 teeth. Mark the driver. Predict how far the driven gear turns during one full driver revolution and which way it rotates.

   Check: Write the prediction before seeing a working model.

2. 02

   Create the driver

   Start a new millimetre drawing at scale 1:1. Open Draw → Design wizards → Spur gear. Set Number of teeth 24, Module 2, Pressure angle 20°, Bore diameter 5, Tooth thinning 0 and Cut width / kerf 0. Add it.

   Check: The wizard reports a 48 mm pitch diameter and 52 mm outside diameter.

3. 03

   Create the driven gear

   Reopen the wizard and use 48 teeth with the same module and pressure angle. Keep bore 5, thinning 0 and kerf 0. Under Position on sheet, choose a clear area so the drawings do not overlap. Add it.

   Check: The second pitch diameter is 96 mm and the outside diameter is 100 mm.

4. 04

   Calculate the centre distance

   Add the two pitch diameters and divide by two. Sketch or draw a reference line of that length. Do not use outside diameters for this calculation.

   Check: The nominal centre distance is 72 mm.

5. 05

   Explore placement

   Select the complete first gear group and use Arrange → Place centre to place it at one endpoint of the reference line. Place the second at the other endpoint with Snap on. Adjust the orientation to explore a tooth facing a gap.

   Check: The wizard profiles are editable drawings. Visual contact does not prove a manufactured mechanism will run freely.

6. 06

   Test the ratio

   Use the teacher’s hand-turned model, if available. Mark one tooth on each gear and count turns. If there is no model, present the result as a mathematical prediction rather than a measured test.

   Check: One driver turn should produce half a driven turn in the opposite direction for this external gear pair.

7. 07

   Explain and extend

   Save the .forma file. Explain the driver, driven gear, tooth counts and predicted ratio. Swap their roles on paper and predict the new result.

   Check: Separate any practical observations about friction or fit from the ideal ratio.

03 / REFLECT

Check the design. Explain the decisions.

These ticks are temporary and are not submitted or saved. Keep your project file and written reflection as evidence.