# Pulleys

## 1 Introduction

• The drag racer has a pulley system which drives the rear wheels from the electric motor.

### Pulley Ratios

In order to calculate the speed the wheels will turn at, we need to calculate the pulley ratios and speeds.

• The input (drive) pulley diameter is 5mm and the motor (drive) speed is 5000rpm.
• You will need to choose the driven pulley diameter from the list below, we are going to use 50mm in this example.
• Velocity ratio (VR) = diameter of driven pulley / diameter of the drive pulley.
• VR = 50 / 5
• = 10
• The output speed = input speed / VR
• = 5000 / 10
• = 500rpm

This driven pulley is connected directly to the drive wheel, therefore the wheel will be travelling at the same speed as the driven pulley. This can now be used in the speed calculations.

### Calculating Speed

We can now look at the circumference of a wheel you could choose to calculate the speed at which your racer could travel.

• The diameter of the wheel X Pi (3.14)= The circumference of the wheel
• The circumference of the wheel X the rpm of the shaft (this would be 500rpm in the example) = the distance travelled per minute
• The distance travelled per minute X 60 = the distance travelled per hour.

Remember, you can change the pulley diameters to change the rpm of the shaft. Changing the diameters of the pulleys and wheels will have a big impact on the speed at which your racer will travel. If you have large wheels, it will have a high top speed, but it will start very slowly. This is because it will have less torque (power). The same applies to the pulley systems. You can adjust your pulley wheel system to suit you.

### Wheel Choices…

• The wheel diameter choices are: 70, 60, 50, 40 and 30mm
• The driven pulley diameter choices are: 56, 50, 44, 40 and 34mm
• The driver pulley cannot be changed, it is 5mm

Calculate the speed at which your racer will travel per hour using the following pulley wheel system. Make sure you show all your working out.

• Driver pulley: 5mm
• Driven Pulley: 40mm
• Wheel Diameter: 50mm
• Motor rpm: 5000rpm

## 2 Design

### Techsoft 2D Designs

• Now go to the open drive at G:\Design Engineering\Year 9\5 - Drag Racer
• Select the Techsoft file 'Body template 200X300' open this document. This template has all the parts you need to complete the racer except for the wheels and the final driven pulley.
• You need to open the 'choose wheels' file and choose your front and rear wheels. You will need to copy one of each across and then make 3 copies of them each (you need 4 of each wheel).
• Now open the 'choose inner pulley' file and choose the diameter pulley you want and copy this across to your template.
• You should now have a complete racer. Sheets of MDF measuring 200mm x 300mm have been cut out for you. To ensure we make efficient use of material, you now need to team up with two peers and create a single 200mm X 300mm cutting sheet before you cut out your work with all your wheels and pulleys on them.