Recently, our STEM club did a four session electricity unit. We challenged the kids to design and build a battery powered toy that either moved or had lights and sound. Without exception, each team chose to build a toy that used a motor for movement. We provided them with a DC motor, 3v battery pack, wheels and axles, and various miniature pulleys and gears.
Always budget minded, we’d purchased the motors from Amazon in a kit that offered six motors and dozens of gears and pulleys for less than $15! The kids were excited when they connected the motors to a power source and watched them spin little propellers. But when the teams tried to use the motors to move a vehicle, they hit a major obstacle. Though the little vehicles were small – usually less than 300 grams, the little motors couldn’t move them at all.
Gizmo Rig Exploration
To troubleshoot the problem we had the students experiment with the motors on their Gizmo Rig test stands. When mounted, it became obvious that the motors turned too fast to be a good power source for the cars. In checking the fine print of the motor description, I learned these little motors spin over 4000 rpm! The kids measured the circumference of their wheels and calculated that, if the wheel turned once for every turn of the motor shaft, the toy would travel nearly ¼ mile in a minute! It was obviously a good thing that the
motors couldn’t move the little contraptions since 15mph was too fast for a toy! By experimenting with various pulley configurations, the teams found they could reduce the rpm by using a small pulley to drive a large pulley. In addition, they discovered that the slower wheels had more power. Eureka – mechanical advantage in action! When a small pulley drives a large pulley the resultant motion trades speed (in the form of rpm) for power.
In this picture of a Gizmo Rig test, both sets of wheels are spinning. Each stage reduces rpm by using a small pulley to drive a much larger pulley. In the picture below, the lower wheels are rotating much slower than the motor shaft, but they’re still too fast to see the wheel spokes. The wheels in the second stage are turning substantially slower than the first set. By repeating the same configuration (small pulley driving large), the second stage wheels are turning slow enough to see the wheel spokes.
Once the teams learned how to reduce the rpm of the wheels, they were able to build toys that their little motors could move. The chassis in the picture below shows a two-stage drive system in what eventually came to be a bus vehicle. The 1st stage turns the axle in the middle of the car. The red pulley on the motor shaft has a diameter of 8mm. The yellow pulley diameter is 100mm. This reduces the rpm on the 1st stage axle by a factor of 12.5 – slowing 4000 rpm to just over 300 rpm. The orange pulley on the 1st stage axle is 30mm in diameter and drives a second 100mm pulley on the front axle. This reduces the rpm of the front wheels to under 100. The car was still pretty fast, but at least the motor could move it!
Another team built a robot truck. This vehicle used a two-stage power train too but added the innovation that the motor was located above the rest of the powertrain. As with the previous example, the first stage is an 8mm pulley driving a 100mm pulley. The second stage has a 50mm pulley driving a 100mm pulley. The robot truck was a lot faster than the bus because the rpm reduction from 50mm to 100mm was only 50%.
Merlin’s Robot Truck
This unit was extremely successful. The kids loved their little contraptions, and we kept the cost down by using cheap motors. The teams also learned about mechanical energy transmission and mechanical advantage.
Kits used in this unit:
- Motors and pulley set: https://amzn.to/2T6lWwP
- Assorted gears and pulleys: https://amzn.to/2tuX1YQ
- 10mm x 2mm small plastic pulleys: https://www.amazon.com/dp/B00EPQMMRE