Upload
kookwf
View
151
Download
1
Tags:
Embed Size (px)
Citation preview
COMMONWEALTH SECONDARY SCHOOL DESIGN AND TECHNOLOGY2012
Koo WF 1
Name: ________________ ( ) Class: _______
MECHANISMS
Content Outline 1. Introduction 2. Movement 3. Gears 4. Pulleys 5. Cams and followers 6. Levers 7. Screw Threads 8. Linkages
1. Mechanisms are used today, as they have been for thousands of years, as a mean of
controlling movement & force. Our lives depend on mechanisms for making things work at
home and at school, in shopping & leisure centres and in factories & offices. Just imagine
how inconvenient it would be if you we had to walk to CSS from home everyday by foot.
Leisure
Transport
Communication
2
Mechanical devices are designed to produce necessary movements that help to facilitate
our lives and enable us to work more efficiently.
The four types of movements are:
Linear – straight line motion
Rotary – circular motion
Reciprocating – forward and backward motion along a straight line
Oscillating – forward and backward motion along an arc
Can you think of mechanical devices that incorporate the movements listed above?
A mechanism is usually considered as a ‘SYSTEM’, which produces an OUTPUT
movement or force, from a given INPUT movement or force.
Mechanical systems are used to convert one type of movement at the input to a different
type of movement at the output.
A glue stick, for example, uses a screw
thread to convert the rotary movement
of the knob into the linear movement of
the glue.
COMMONWEALTH SECONDARY SCHOOL DESIGN AND TECHNOLOGY2012
Koo WF 3
2. Mechanical Components
2.1 Gears
The gear wheel is a basic mechanical component. Its purpose is to transmit rotary motion
and force. Gears are generally used for one of four different reasons:
1. To reverse the direction of rotation
2. To increase or decrease the speed of rotation
3. To move rotational motion to a different axis
4. To keep the rotation of two axis synchronized
A gear wheel has teeth around its edges. These teeth can lock into the teeth of another
gear wheel. When one wheel rotates, it makes the other wheel turn as well.
2.1.1 Types of Gears
a) Bevel Gears
These are designed in such a way that two of them come in contact rotate at 90° to each
other.
Examples are hand drill and food mixers.
Gear teeth
bevel gear
4
b) Spur Gears (most common industrial gears)
Two gears are placed edge-to-edge turn in opposite directions;
If the gears are of different diameters, different speeds can be
generated;
For example, if a driver gear with 10 teeth and a
driven gear with 20 teeth are meshed, the driven
gear will rotate at half the speed of the driver.
c) Worm and Gear Wheels
They are used to change the direction of motion by 90°.
The gear wheel cannot be used to rotate the worm wheel.
Examples: Found in mechanisms for opening and closing Venetian blinds.
d) Rack and Pinion Gear
The pinion is fixed on a shaft. When the pinion turns it
makes the rack move in a straight line. Pulling or pushing
the rack makes the pinion turn.
Can you identify the rack and pinion?
Worm
Gear Wheel
COMMONWEALTH SECONDARY SCHOOL DESIGN AND TECHNOLOGY2012
Koo WF 5
e) Other Gears
There exist more other types of gears, each designed for different uses and applications.
These include the helical gears, planetary gears and more. Discover more on their uses
and functions.
2.1.2 Driver, Driven and Idler Gears
A gear system consists of two or more gears which are meshed together. A driver gear is
one that receives energy from a power source, such as an electric motor. A driven gear is
one that receives motion from the driver gear.
2.1.3 Gear Ratio Calculations
To calculate gear ratio, all we need to know is the number of teeth in each gear.
I am driven by you!
I am the driver. I move first!
6
In the figure above, the diameter of the gear on the left is twice that of the gear on the
right. The gear ratio is therefore 2:1 (pronounced "two to one"). Every time the larger gear
goes around once, the smaller gear goes around twice. You can see that if both gears had
the same diameter, they would rotate at the same speed but in opposite directions.
If two gears are in mesh, the following is established:
Speed of Gear A × no. of teeth in Gear A = Speed of Gear B × no. of teeth in Gear B
For example,
Gear A has 60 teeth and rotates and has rotational speed of 10 rpm.
Gear B has 30 teeth.
What is Gear B’s rotational speed?
Answer:
Speed of Gear A × no. of teeth in Gear A = Speed of Gear B × no. of teeth in Gear B
10 ×60 = Speed of Gear B × 30
Therefore, what is the speed of Gear B?
2.2 Pulleys
A pulley is a wheel that is used with a belt or cable for transmitting movement or force. It is
used to increase or decrease the speed of rotation. Pulleys are used in lifts & cranes for
converting rotary motion into linear motion. In a tape measure or retractable washing line,
pulleys convert linear motion into rotary motion.
COMMONWEALTH SECONDARY SCHOOL DESIGN AND TECHNOLOGY2012
Koo WF 7
2.2.1 Types of Pulleys
The two common types of pulleys are the flat and ‘vee’ pulleys.
Two common belt arrangements: open drive and crossed drive
2.2.2 Compound Pulley Systems
Flat belt ‘vee’ belt
Two pulleys rotate in the same direction Two pulleys rotate in the opposite direction
Pulleys can be combined in a compound
pulley system to help lift heavy weights.
The more pulleys in a compound system,
the easier it is to lift the weight.
8
2.2.3 Pulley Systems: Velocity Ratio (Transmission Ratio)
The calculation of the velocity ratio of pulleys is similar to that used to calculate gear ratios.
Take a look at the following example:
Velocity Ratio =
How many degrees will the driven pulley rotate through for a single revolution of the driver
pulley?
Answer:
angular movement of driven pulley = × angular movement of driver pulley
= 0360400200
×
Velocity Ratio =
= 0
0
360
180
2.3 Cams
A cam is a specially shaped piece of material fixed to a shaft. It is normally used with a
follower to create a cam and lever or cam and slider mechanism. The edge, or profile, of
the cam guides the motion of a follower. It converts this input motion into a reciprocating
output motion of the follower. The cam normally rotates with constant velocity of rotation.
Diameter of driver pulley Diameter of driven pulley
Output movement Input movement
Output movement Input movement
COMMONWEALTH SECONDARY SCHOOL DESIGN AND TECHNOLOGY2012
Koo WF 9
2.3.1 Types of Cams
There are 3 common types of cam shapes: the pear, snail, and eccentric. Most cams are
designed to have a smooth curved shape so that the motion transmitted to the follower is
smooth and without sudden jerk.
2.3.2 Types of followers
A follower can slide or roll on the edge or surface of a cam. The common types of
followers are the flat, knife-edged and roller followers.
Can you identify which is which?
2.3.3 Toy Cam Examples
Can you figure out how the following examples work?
10
2.4 Levers
2.4.1 First Class Lever
A first class lever with its various parts is labeled as shown below. A downward
force on the effort arm will cause an upward force to be applied to the load. Objects
like hand carts, scissors, pliers and nail extravagators are all examples of first class
levers.
2.4.2 Second Class Lever
The load in a second class lever lies between the fulcrum and the applied effort.
Examples of this class are wheelbarrows, and bottle openers.
2.4.3 Third Class Lever
The third class lever is characterized by having the applied effort between the
fulcrum and the load. This class of levers is used as movement amplifiers.
Examples are hammers, tweezers and fishing rods.
COMMONWEALTH SECONDARY SCHOOL DESIGN AND TECHNOLOGY2012
Koo WF 11
2.5 Linkages
A linkage is a mechanism made by connecting levers together.
To connect the levers together you can use any type of fastening which allows free
movement, for example screws, pins, paper fasteners, pop rivets etc.
It is also defined as a system of links used in machines and equipment to transform and
transmit motion from one direction into another direction.
2.5.1 Reverse Motion Linkage
Reverse motion linkages are used to change the direction of motion. A single lever
with a pivot at its centre reverses and input motion without affecting the input force.
The input force and the length of transmitted movement can be increased or
decreased by altering the position of the pivot.
2.5.2 Parallel Motion Linkage
Parallel motion linkages are used to make 2 or more parts of a mechanism move
and stay parallel to each other as the linkage move.
12
Examples of linkages Useful Links: http://technologystudent.com
http://www.flying-pig.co.uk/mechanisms/ http://www.howstuffworks.com/gears.htm
http://www.howstuffworks.com/pulley.htm
link
leg pedal of a sewing machine
link
brake pedal
link
link
crank