Section A- Mechanical devices (3.1.5)

Tension

Pulling force exerted by each end of an object such as a string or rope.

Torsion

A twisting force

Compression

A pushing force/ being squashed

Bending

A bending force is applied at an angle causing a material to be in compression and tension at the same time.

Shear

A force across a material, eg scissors cut by applying a shear force.

Mechanical advantage

Load ÷ effort

It allows change of direction force, making lifting easier but weight will feel the same.

Fixed pulley

Uses two or more pulleys, one fixed, one moveable. Reduces efforted needed to lift. Provides mechanical advantage.

Block and tackle pulley

Linear

One direction

Reciprocating

A back and forth movement.

Oscillating

swinging back and forth

Rotary

Mechanical advantage

Making something easier to move/ lift.

First order lever

Have fulcrum between force and load e.g. Pliers

second order lever

Have fulcrum one end, effort at the opposite end, load in the middle e.g. Wheelbarrow.

third order lever

Fulcrum one end, load opposite, effort in the middle e.g. Tweezers

Push and pull linkage

It maintains the direction of the input motion- meaning that the output motion goes in the same direction.

Bell crank

It changed the input motion by 90 degrees. Making vertical motion horizontal or horizontal vertical.

Circulatory cam

Steady rise and fall

Pear cam

Rapid rise and fall with long dwell

Snail cam

long dwell followed by steady rise and sudden drop can only turn in one direction

Heart cam

Slight rise and fall with no dwell period

Torque

The force that causes rotation

Cam (rotary system)

Mainly used to change rotary motion into reciprocating motion through use of a follower.

Follower (rotary system)

The thing moving up and down.

Crank (rotary system)

Used to rotate the shaft, which rotates the cam.

Gears

Toothed wheels that mesh to transfer motion from one part of a machine to another.

Gear train

Transmit rotary motion and torque (causes rotation). Different sized gears increase/ decrease speed of rotation.

Driver gear

The driver gear (input) turns the driven gear (output). the gears turn in opposite direction.

Idler gear

Used to change direction of rotation so driver gear turns in the same direction as driver gear. Size does not affect speed.

Velocity ratio

Speed of driver gear / gear ratio

Best velocity ratio

Large driver gear with small driven gear.

Gear ratio

Number of teeth on driven gear/ number of teeth on driver gear e.g. 20: 40 = 1:2

Belts

A drive belt (loop of flexible rubber) connects two or more pulleys together and the belt transfers power from the pulley to receiving system.