DRIVE SYSTEMS
A small vehicle or buggy powered by an electric motor may use a drive or transmission system connecting the motor to the driving wheels. This transmission system allows the vehicle to run efficiently ( as quickly as possible). All cars have a transmission system which normally includes a gearbox.
PULLEY DRIVE SYSTEM
Pulleys are wheels with a 'V' groove around the edge to hold a drive belt. A pulley drive system is very easy to make and it will work even if the various parts are not quite lined up. The pulleys supplied in the Mechathlete pack are approximately 6mm diameter (to fit onto motor spindle) and 36mm diameter (to fit onto an axle).
When these pulleys are connected by a belt, the motor has to turn 6 times to make the larger pulley turn just once. This is because the larger pulley is 6 times the diameter of the smaller one. We say that such a drive system has a speed reduction ratio of 6 : 1. This ratio allows the motor to turn quite rapidly but also gives high torque or "turning power" to the axle. To get even higher torque, we can increase the size of the larger pulley or decrease the size of the motor spindle pulley. However, this will also reduce the speed of the axle.
To calculate the pulley ratio, divide the diameter of the smaller pulley into the larger one. E.g. 36/6 = 6:1.
What is the ratio if the larger pulley is changed to 48mm in diameter?
To calculate the speed of the output shaft, divide the motor speed by the larger number in the ratio.
E.g. 3000rpm/6 = 500 rpm.
This is not quite the right answer! Because of friction and other energy losses we have to derate the speed of the motor by about 50percent and assume it actually runs at only 1,500 rpm.
What then is the correct answer in the above example?
To calculate the speed of a buggy, multiply the output shaft speed by the circumference of the drive wheels. The circumference is found by multiplying the diameter by Pi, e.g. 75mm x 3.142 = 235mm. (Pi is a special figure in maths which makes many engineering calculations possible. If you measure the circumference of a circle, it will always be 3.142 times longer than the circle's diameter.)
The answer is:
1) 1500rpm/6 = 250rpm
2) 250rpm x 235 = 58,750mm/minute
or 58.75 M/minute
These same calculations also apply when using gears instead of pulleys.