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Introduction

Rotary motion is the most common type of motion to be found in machines. In many machines rotary motion must be transmitted from one shaft to another. An example of this is the pillar drill. The exercise below requires you to combine several calculations explored previously in earlier examples. These involve the calculation of velocity ratio and the output speed:

Velocity ratio = No. of teeth on driven / No. of teeth on driver
Output speed = Input speed / Velocity ratio


You will use these calculations in the following exercise.

Task 1: Velocity ratio of a chain drive


Chain drive and motor

  1. Open Yenka file Model 1.
  2. The model shows a chain drive being driven by a motor. In what direction are the motor and the chain drive turning? Describe the type of motion illustrated by the motor.
    Answer
    The motor and chain drive are turning clockwise using rotary motion.
  3. Identify the driven and driver gear wheels. Explain your answer.
    Answer
    Gear wheel A is the driver as this is attached to the motor. B is the driven.
  4. Does the above arrangement result in a speed reduction or increase? Explain your answer.
    Answer
    The driven gear wheel is larger than the driver. Therefore the ratio is greater than 1, resulting in a speed reduction.

Summary

If the gear ratio is greater than 1, the result is a speed reduction.

Task 2 : Calculating speed reduction & increase


Chain drive and motor

  1. Load Yenka Model 2.
  2. Given that gear wheel A has 8 teeth and gear wheel B has 40 teeth, calculate the velocity ratio and the output speed. Check your answer by dragging the target of the number object to the gear, and choosing 'ang. vel. (gear 1)'.
    Answer
    Velocity ratio = No. of teeth on driven / No. of teeth on driver = 8 / 40 = 0.2
    Output speed = Input speed / Velocity Ratio = 5 / 0.2 = 25 rpm.
  3. State whether this is a speed reduction or increase.
    Answer
    Increase.

Summary

The velocity ratio must be calculated before calculating the output speed. If the velocity ratio is less than 1, the result is a speed increase.

Teacher Summary