Mechanisms - Chp 9

Rotary

turns around in a circle

Linear

moves in a straight line

Recipricating motion

moves forwards and backwards in a straight line

Oscillating motion

swinging along an arc aroung a central point

Mass

• the measure of the amount of matter in something
• measured in kilograms

Weight

the measure of the force of gravity exerted on an object

Weight calculation

mass x gravity

Work

the amount of mechanical energy transferred by a force to an object

Work Calculation

work = force x distance (metres)

Energy/work unit

joules or newton metres

Power

the amount of work done, or energy transferred, in a unit of time

Power calculation

power = work/time (seconds)

Power unit

watts
1 watt = 1 joule / second

Lever

a mechanism consisting of a rigid bar that can pivot around a fixed point called the fulcrum

Mechanical advantage

the ratio of the two forces, load and effort

Mechanical advantage calculation

mechanical advantage = load/effort

Advantages of levers

• allow us to move heavy loads by applying relatively small effort over a long distance, the greater the distance from the fulcrum the greater the mechanical advantage
• allow us to increase movement

Velocity ratio lever

the ratio betweena small movement on the side of the large load produces a large movement on the side of the small effort

Velocity ratio calculation lever

distance moved by effort speed of effort
------------------------------ = ------------------
distance moved by the load speed of load

Torque

force which causes a change in the rotational motion of an object around its fulcrum and axis, measured in n/m

Torque calculation

torque = distance form fulcrum x force

Classes of lever

F - Class 1 Fulcrum is in the centre - pliers
L - Class 2 Load is in the centre - wheelbarrow
E - Class 3 Effort is in the centre - tweezers

System

• a combination of parts of components, which work together to perform a task or activity
• a mechanical system uses one or more mechanisms to control input and output forcesp

Pneumatics

use of the power in compressed air

Hydraulics

the use of the power in compressed liquids

Advantages of using mechanisms

• reduces the amount of effort needed to do work
• are more efficient if properly designed
• are able to carry out a variety of tasks and are often the only possible way to achieve something
• reduces strain on a user's body

Disadvantages of using mechanisms

• sliding parts wear unless lubricated
• moving parts may be hazardous
• critical parts must be made with precision and can be expensive

Moment of force/Turning effect

the product of the force and the perpendicular distance between its line of action and the fulcrum

Law of the lever

states that if a lever is balanced, the clockwise moment must be equal to the anticlockwise moment

Equilibrium

a state in which opposing forces or influences are balanced.

Linkage

a mechanism that allows force or motion to be directed to where it is needed, consists of a system of levers connected to each other by pivots

Uses of linkages

• change direction of motion
• change the type of motion
• change the size of a force
• can make two things move at the same time or parallel to eachother

Reverse motion linkage

• shaped like a Z, the middle rod moves around a fixed central pivot moving the top rod one way and the bottom rod the opposite way

• used to make things move in opposite directions

Push-pull linkage

• shaped like an N, two parallel rods move around two fixed central pivot points creating identical parallel motion at the other end of the linkage
• provides mechanical advantage when moving things back and forth

Bell crank linkage

• changes the direction of movement by 90 degrees

• used in bicycle brakes

Parallel motion linkage

• has the shape of a parallelogram, the links on opposite sides remain parallel when extended and retracted, retaining a vertical line

• used to raise platforms up and down

Treadle linkage

• changes rotary motion to reciprocating motion in a push pull linkage, as the motor turns and the fixed central pivot points and move them back and forth

Spring

• an elastic object used to store mechanical energy, have the shape of a helix
• the coil of a spring changes shape when a load is applied and the springs back when the load is removed

Tension springs

stretches when a load pulls them

Compression springs

shorten as the load pushes them

Torsion springs

twist as torque turns them

Pulley

a mechanism providing mechanical advantage that makes it easier to lift heavier loads, consists of a rope or belt that runs around one or more wheels, one end of the rope is pulled which raises the other end

• provides great mechanical advantage, can transfer rotary motion, can be used to change the speed of the motion

Simple pulley system

consists of two pulley wheels each on a shaft, connected by a belt

Jockey pulley

an idler puller which helps retain tension in the pulley, otherwise they will slip, resulting in the effort and energy being lost

How does the size of the driver pulley affect speed?

• if the driver pulley is larger than the driven pulley the output speed will be greater than the input speed
• if the driver is smaller than the driven then the output speed will be less than the input

Speed of rotation unit

revolutions per minute / rpm

Velocity ratio gears

if the pulley wheels are different sizes the smaller one will spin faster than the larger one

Velocity ratio calculation gears

driver speed / driven speed
size of driven / size of driver

What is a pulley drive belt made from

usually synthetic fibres eg neoprene and polyurethane, with a v -shaped cross section

What is a movable pulley

a pulley with a free axle, which is free to move

How do you find the mechanical advantage of a pulley?

count the number of rope the weight is distributed across

How do you make the driver and the driven in a pulley system rotate in opposite directions?

cross the belt in an x shape

What are the different types of belts?

flat belt- completely flat, need to be tensioned correctly to prevent slipping

round belt- have a circular cross-section, run in a pulley with a circular groove

vee-belt- aka v-belt or wedge rope, fits in the v shape of the pulley, has better grip than the previous

toothed belt - aka notch or cog belts, have teeth which mamth with a toothed pulley, little to no slippage

Advantages of belt drives

• reduced wear

• no rusting

• no need for lubricationg

• inexpensive

Disadvantages of belt drives

• loss of efficiency due to friction

• can be damaged by oil or heat

• noise levels are high

• can slip resulting in a loss of effort and energy

Spur gears

gears that mesh together and are parallel to the axis of rotation

Gear train

an arrangement of several simple gears meshing together

Idler gears

does not affect gear ratio of output speed, ensures the driven and driver rotate in the same direction

What is the basic rule of gears?

• if a larger gear drives a smaller gear, the output speed increases

• is a smaller gear drives a bigger gear, the output speed decreases

What are compound gears?

two or more gears fixed on the same shaft rotating at the same speed

Worm and worm wheel gear

• the worm is a gear with one long tooth threaded around a shaft

• the worm wheel gear meshes with the worm at a 90^o angle

• after each rotation of the worm, the worm wheel gear only turns by one tooth, giving it a gear ratio of 50:1

• the worm can turn the worm wheel gear but not vice versa

Bevel gears

transfer motion through 90^o

Rack and pinion

changes rotary motion to linear motion or vice versa

Ratchet and pawl

as the ratcher wheel turns the pawl catches in the dip between the teeth, allows a wheel to turn in only one direction

Chain and sprocket

gear wheels joined by a linked chain

Cam and follower

can be used to change rotary motino into recipricating motion

Crank and slider

converts recipricating motion into rotary motion or vice versa

What is friction?

a force resistant to motion

How can friction be minimised?

• lubrication

  • ball bearings

What is the moment of a force?

the product of the force and the perpendicular distance between its line of action and the fulcrum