Mechanical Devices
Overview
Pick up a pair of nail clippers. Your thumb pushes down with a force you could apply all day, and at the other end two blades cut through a material that would blunt a kitchen knife. Nothing in the clippers generates any force at all. Every newton at the blades came from your thumb, and the clippers simply rearranged it, trading a long movement for a short one and a small force for a large one.
That trade is what mechanical devices do, and it is the whole of this lesson. You will learn the functions of mechanical devices that produce linear, rotary, reciprocating and oscillating movements, and how they change the magnitude and direction of force. You will meet levers of the first, second and third classes, linkages including bell cranks and push and pull arrangements, and rotary systems: cams and followers, simple gear trains, and pulleys and belts. You will calculate mechanical advantage, gear ratios and output speeds, and you will choose between three ways of doing the same job on a real product. This is core technical principles content, examined in Section A of Paper 1, where it supplied question 8 of the specimen paper.
Objectives
- The functions of mechanical devices to produce linear, rotary, reciprocating and oscillating movements.
- Changing magnitude and direction of force
- Levers:
- levers, first, second and third classes.
- Linkages:
- bell cranks
- push/pull.
- Rotary systems:
- cams and followers
- simple gear trains
- pulleys and belts.
Mind map
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Lesson Note
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Lesson Evaluation
Congratulations on completing the lesson on Mechanical Devices. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- A windscreen wiper blade moves backwards and forwards along an arc. This motion is described as A. linear. B. oscillating. C. reciprocating. D. rotary. Answer: B
- In which class of lever is the load always between the fulcrum and the effort? A. First class B. Second class C. Third class D. All classes Answer: B
- A driver gear of 12 teeth meshes with a driven gear of 36 teeth. The driver turns at 600 revolutions per minute. What is the output speed? A. 100 revolutions per minute B. 200 revolutions per minute C. 1,800 revolutions per minute D. 3,600 revolutions per minute Answer: B
- Which cam gives the follower a period of dwell in every revolution? A. An eccentric cam B. A pear cam C. A snail cam D. No cam can Answer: B
- An idler gear is added between a driver and a driven gear. What is its effect? A. It doubles the output speed B. It halves the output torque C. It reverses the direction of the driven gear without changing the ratio D. It has no effect at all Answer: C
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