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Introduction

Gears are used to transfer rotary motion. When two or more gears are used together they are known as a gear train. Gears can be used to change the speed and direction in which something moves. The output speed can be calculated if the input speed and velocity ratio are known.

Task 1: Changing gear ratio


Gears and motor

  1. Open Yenka file Model 1.
  2. The model shows a simple gear train being driven by the motor. Identify which gear wheel is the driven and which is the driver. In what direction is the motor and gear wheel A turning?
    Answer
    Gear wheel A is the driver and gear wheel B is the driven. The motor and gear A are turning anticlockwise.
  3. At what speed are the motor and gear wheel A turning?
    Answer
    5 rpm
  4. Is gear wheel B turning at the same speed? Explain your answer.
    Answer
    No; gear wheel B is turning at 10 rpm. This is because B has fewer teeth than A and is therefore turning faster.

Summary

If two gears are meshed together, the gear being driven will turn in the opposite direction to the driver. If the driver gear has a greater number of teeth than the driven then the driven will turn faster.

Task 2 : Calculating output speed


Large and small gears with motor

  1. Load the Yenka Model 2
  2. Move the mouse pointer over the gear wheels and state the number teeth on gear wheel A and gear wheel B.
    Answer
    A = 40; B = 16
  3. Look at the number objects recording the speeds of the gears. Which gear wheel is moving faster? Explain your answer.
    Answer
    A = 5 rpm; B = 12.5 rpm; B has fewer teeth and therefore make 2.5 revolutions for every 1 revolution of A.
  4. The ratio between two gear wheels is known as the velocity ratio. This is the number of times the driver gear must turn to make the driven gear make a complete revolution. The output speed can be calculated if the input speed and velocity ratio are known.

    Output speed = Input speed / Velocity ratio


    Double click on the gear wheel and change the number of teeth to complete the table below. Use the formula to calculate the output speed. Please note: input speed remains constant at 5 rpm.
    Remember to check your answers using the simulator. You may need to reattach the motor each time.
    No. of teeth on driver No. of teeth on driven Velocity ratio Output speed (rpm)
    40 16 0.4 12.5
    8 8
    Answer
    1
    Answer
    5
    16 40
    Answer
    2.5
    Answer
    2
    24 16
    Answer
    0.667
    Answer
    7.5

Summary

The ratio between gear wheels is known as the velocity ratio. This can be used to calculate the output speed using the formula:

Output Speed = Input Speed / Velocity Ratio

NB. If the driver gear is larger than the driven, the velocity ratio will be less than 1 and the output speed will be greater than the input speed.

Teacher Summary

  • You may need to revise calculations of velocity ratio and ensure students understand the meaning of 'driven and driver.'