Machine Motion: Motion in Engineering System – JSS3 Home Economics Lesson Note (Second Term)

Motion is the change in position of an object relative to its surroundings over time.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define motion in engineering;
2. identify and describe linear motion and rotary motion, with examples;
3. name reciprocating and oscillating motion and give one example of each;
4. describe the conversion of rotary motion to linear motion, and of linear motion to rotary motion;
5. state applications of motion in engineering.

What is motion in engineering?

Motion in engineering is the movement of the parts of a machine or system so that it can do useful work.

• Fields of use: motion is a basic idea in mechanical, civil, aerospace and robotics engineering.
• Axis: the fixed line or point about which an object rotates.
• Mechanism: an arrangement of parts that passes on or changes motion.
• Power source: the part that starts the motion, such as an electric motor or an engine.

What are the types of motion?

There are four common types of motion in machines: linear, rotary, reciprocating and oscillating.

Linear motion

• Meaning: motion of an object in a straight line, where all points on the object move the same distance in the same direction.
• Examples: a car along a straight road, an elevator moving up and down, a train on a straight track.

Rotary motion

• Meaning: rotation of an object around an axis or a point, so that all parts move in circular paths around a centre.
• Examples: a spinning wheel, a rotating fan blade, a key turning in a lock.

Reciprocating motion

• Meaning: repeated straight-line movement forwards and backwards, or up and down, along the same path.
• Examples: a piston in an engine, a hand saw, the needle of a sewing machine.

Oscillating motion

• Meaning: swinging to and fro along a curved path about a fixed point.
• Examples: a pendulum, a windscreen wiper, a child’s swing.

How do linear and rotary motion compare?

Why is motion converted in engineering?

Motion is converted because the power source gives one kind of motion and the work to be done often needs another.

• Rotary source: electric motors and engines usually give rotary motion.
• Linear need: many jobs, such as moving materials or pumping, need straight-line motion.
• Mechanism: a mechanism placed between the source and the load changes the type of motion.

How is rotary motion converted to linear motion?

Rotary motion is converted to linear motion by mechanisms that turn the circular movement of a shaft into a push, pull or straight-line travel.

Where it is used

• Translation of power: power from a motor or engine comes as rotary motion and is translated into linear motion to do useful work.
• Conveyors and material handling: a motor turns rollers, and the belt carries materials in a straight line.
• Pumps and compressors: in reciprocating pumps and compressors, rotary motion drives a piston back and forth to move fluid or compress air.

Common mechanisms

• Crank and slider: a turning crank pushes and pulls a slider or piston in a straight line.
• Cam and follower: a turning cam pushes a follower up and down.
• Rack and pinion: a turning gear (pinion) moves a straight toothed bar (rack) along a line.
• Screw: a turning screw moves a nut along its length, as in a vice.

How is linear motion converted to rotary motion?

Linear motion is converted to rotary motion when a push or pull in a straight line is made to turn a shaft.

Examples

• Piston engine: in a car engine, the linear motion of the pistons is converted into the rotary motion of the crankshaft.
• Treadle sewing machine: the up and down push of the foot pedal turns the wheel through a crank and connecting rod.

How does a piston engine produce rotary motion?

1. Fuel burns in the cylinder and produces hot gas under pressure.
2. The gas pushes the piston down in a straight line.
3. The connecting rod carries the push from the piston to the crankshaft.
4. The crankshaft turns, giving rotary motion.
5. The rotary motion passes through the transmission to the wheels.

What are the applications of motion in engineering?

Motion principles are used in many engineering fields, as listed below.

• Automobiles: engines, transmissions and suspension systems are designed for efficient and safe movement on roads.
• Conveyor belts: they transport materials along production lines.
• Machine tools: lathes and milling machines shape and cut materials.
• Power generation: generators and turbines convert mechanical energy into electrical energy.
• Machine parts: gears, cams and linkages are designed using motion principles.
• Steering and handling: understanding motion helps in designing suspension and steering and improves vehicle handling.
• Aircraft and spacecraft: motion analysis helps design for stability, control and performance.

Evaluation

1. Define motion in engineering.
2. Name four types of motion and give one example of each.
3. Describe the conversion of rotary motion to linear motion.
4. State five applications of motion.

Assignment

Describe the conversion of linear motion to rotary motion, giving two examples from the lesson.