# timing pulleys setup for children's pedal car

I want to convert my son's pedal car to electric. The car is meant for 3-6 year old children.

So far I got the following:

• 50W, 3300 RPM electric motor with 8mm round shaft (no keyway or shaft groove)
• 10mm threaded rod made of some fairly soft metal to use as a shaft

I was thinking to connect the motor to the shaft with XL timing pulleys. With a 10 teeth pulley connected to the motor and a 100T one on the shaft, I would get a 10X RPM reduction and with 10-inch wheels, I calculate that the max speed of the car will be ~15KM/h. Probably still a bit to high but I could do further electronic limiting.

Now I have a few questions:

1. Would there be issues if I combine the smallest pully in the range with the largest one? It seems XL-10T is the smallest and XL-100T is the largest.
2. Considering these easily available pulleys are almost always mounted with set screws would the grip be sufficient for this application?
3. What is the required belt length if the distance between shafts is ~90mm?
4. What is a practical way to tension the belt for something like this?
5. Is 10mm thick enough for this belt?
• See my comment at the RedHelment answer: you should be able to find a tensioner (pulley mounted on adjustable mount) commercially available for the belt you've chosen. – Carl Witthoft Mar 13 '18 at 15:18

The belt length can be calculated using the formula in the end below. It comes out from geometry.

Just try plugging in the values for R1, R2 and d.

Now, the thickness of the belt required depends on the amount of tension it will be handling which depends upon the torque it has to transfer. Now, we know the torque to be transferred. So, we know the maximum tension it has to handle. What remains then is to identify the designed elastic tensile limit for belts of differing thicknesses. Now, I suppose that isn’t mentioned. So we can try searching online for this material’s modulus of elasticity E. Would be best if there is a stress-strain curve as well.

Find the elastic limit stress and multiply it by the cross-sectional area of the belt. That should give us then it’s max. Limit.

• As is done in production automobiles, I'd recommend adding a tensioner somewhere. The OP should be able to purchase a stock tensioner to match the belt size (width) he's using, and mount it somewhere sensible. – Carl Witthoft Mar 13 '18 at 15:16
• Or, if you don’t have a scientific calculator to hand, once the pulleys are mounted, wrap a piece of string around both pulleys and measure.... – Solar Mike Mar 13 '18 at 18:43
• @SolarMike wow ....nice – RedHelmet Mar 14 '18 at 1:32

Comments: You want to use solid bar stock instead of a threaded rod. The websites McMaster and Misumi have shafting, pulleys, and belts. Also, a flat for a setscrew can be easily added to a shaft to increase torque transmitted. A belt length calculator is at: https://www.bbman.com/belt-length-calculator/

I suggest computing the torque the motor can produce or use an online timing belt calculator to see if a larger pitch belt such as L is needed. If the belt is tensioned by using a tensioner on the belt return side, or mounting the motor on slots, the ratio is okay.

Not being an engineer, I'd run whatever setup fits the confines of the car and wire in a pulse width modulator. You can tune the output of the electric motor to fit your application. Not very pretty, but it will work and PWMs are about $$10-$$15 on ebay.