mechanisms notes 2012

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COMMONWEALTH SECONDARY SCHOOL

DESIGN AND TECHNOLOGY2012

Name: ________________ (

) MECHANISMS

Class: _______

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.

Transport

Communication

Leisure

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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.

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COMMONWEALTH SECONDARY SCHOOL

DESIGN AND TECHNOLOGY2012

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. 2. 3. 4. To reverse the direction of rotation To increase or decrease the speed of rotation To move rotational motion to a different axis 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.Gear teeth

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.

bevel gear

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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.Worm

Gear Wheel

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?

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COMMONWEALTH SECONDARY SCHOOL

DESIGN AND TECHNOLOGY2012

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.I am driven by you!

I am the driver. I move first!

2.1.3 Gear Ratio Calculations To calculate gear ratio, all we need to know is the number of teeth in each gear.

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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 Bs 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.

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COMMONWEALTH SECONDARY SCHOOL

DESIGN AND TECHNOLOGY2012

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

Flat belt

vee belt

Two pulleys rotate in the same direction

Two pulleys rotate in the opposite direction

2.2.2 Compound Pulley Systems 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.

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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 =

Output movement Input movement

How many degrees will the driven pulley rotate through for a single revolution of the driver pulley? Answer:Diameter of driver pulley angular movement of driven pulley = Diameter of driven pulley angular movement of driver pulley

= Velocity Ratio = = Output movement Input movement1800 3600

200 3600 400

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.

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COMMONWEALTH SECONDARY SCHOOL

DESIGN AND TECHNOLOGY2012

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?

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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.

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COMMONWEALTH SECONDARY SCHOOL

DESIGN AND TECHNOLOGY2012

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.

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link

link link link

crank

brake pedal

leg pedal of a sewing machine

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

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