AQA GCSE Engineering 8852: systems (mechanisms)

spec ref: 3.3.1 textbook ref: ch 3.2 pages 75-79

What is a mechanism?

A system of parts that transmits or changes motion and forces

What is a linkage?

A system of rods or bars used to change motion, direction or force

What can linkages change?

The size of force, direction of motion and type of motion

What types of motion are there?

Rotary, linear, reciprocating and oscillating

What is rotary motion?

Movement in a circle

What is linear motion?

Movement in a straight line

What is reciprocating motion?

Back-and-forth motion in a straight line

What is oscillating motion?

Back-and-forth motion in an arc

What is a gear train?

Two or more gears that mesh to transmit rotary motion and torque

What is torque?

A turning force

What happens when a large gear drives a small gear?

Speed increases and torque decreases

What happens when a small gear drives a large gear?

Speed decreases and torque increases

What happens to direction in a gear train?

The driven gear rotates in the opposite direction to the driver

How do you calculate gear ratio?

Number of teeth on driven gear ÷ number of teeth on driver gear

What is a chain and sprocket system?

A mechanism using a chain and toothed wheels to transmit rotary motion

Advantages of chain and sprocket

Simple, efficient, can transmit motion over distance

Disadvantages of chain and sprocket

Needs maintenance, chain can slip or break

How is ratio calculated in chain and sprocket?

Teeth on driven sprocket ÷ teeth on driver sprocket

What is a cam and follower?

A mechanism that converts rotary motion into reciprocating motion

What is dwell?

When the follower remains stationary

What is rise?

Upward movement of the follower

What is fall?

Downward movement of the follower

What affects the motion of a cam and follower?

The shape of the cam and position of the shaft

What is an eccentric cam?

A circular cam with an off-centre shaft producing symmetrical rise and fall

What is a pear (egg-shaped) cam?

A cam that gives long dwell and quick rise/fall

How are cams used in engines?

To open and close valves in time with pistons

What is a camshaft?

A rotating shaft with cams that control engine valves

What is a pulley system?

A system of wheels and a belt/rope used to transmit motion or lift loads

What happens when a large pulley drives a small pulley?

Speed increases and torque decreases

What happens when a small pulley drives a large pulley?

Speed decreases and torque increases

How do you calculate pulley ratio?

Diameter of driven pulley ÷ diameter of driver pulley

What direction do pulleys rotate?

The same direction (if not crossed)

Advantages of pulleys

Cheap, simple, fewer parts

Disadvantages of pulleys

Belt can slip or break

What is mechanical advantage (MA)?

The ratio of load force to effort force

How is mechanical advantage calculated?

Load ÷ effort

What is the MA of a single fixed pulley?

1 (no force reduction)

What is the MA of a movable pulley?

2 (effort is halved)

How can mechanical advantage be increased?

Add more pulleys

What is a load?

The weight being lifted

What is a bearing?

A component that reduces friction and controls motion

What is a plain bearing?

A simple bearing with sliding contact

Example of a plain bearing

A shaft rotating in a hole

What is a rolling element bearing?

A bearing with rolling elements between races to reduce friction

What are the two types of rolling element bearings?

Ball bearings and roller bearings

What is a ball bearing?

A bearing using spherical balls

What is a roller bearing?

A bearing using cylindrical rollers

Why are bearings used?

To reduce friction and allow smooth movement

What is mechanical advantage used for?

To reduce the effort needed to lift a load

Do crossed pulley belts change direction?

Yes, they reverse the direction of rotation

What is reciprocating vs oscillating (difference)?

Reciprocating moves in a straight line; oscillating moves in an arc

What motion conversion does a cam and follower provide?

Rotary motion to reciprocating motion

What motion conversion do gears provide?

Rotary motion to rotary motion (change speed/torque)

What motion conversion do pulleys provide?

Rotary motion to rotary motion (transfer motion)

What motion conversion can linkages provide?

Can convert between linear, rotary, reciprocating and oscillating motion

Give an example of a linkage mechanism

A scissor lift or robotic arm

How do linkages change force?

They can increase or decrease force depending on design

What does mechanical advantage mean?

It shows how much a mechanism reduces the effort needed to lift a load