SAASTE Technology/Modules/Mechanical Systems Grade 9

Mechanical Systems
Help the guard stay out of the rain

Forms of assessment:
scenario
 * Technological process
 * Test
 * Practical

Specific Outcome:

 * Understand and apply the technological process to solve problems and to satisfy needs and wants.
 * Apply a range of technological knowledge and skills ethically and responsibly.

Resources required:

 * Ruler, pencil, strips of cardboard, drawing pins or thumb-tacks.

You will be assessed on the following:

 * Your knowledge and understanding of Systems and Control (Levers)
 * Your ability to identify problems, needs and wants.
 * Your ability to consider a range of possible solutions.
 * Your ability to make informed choices.
 * Your ability to develop a design brief. * Your ability to communicate.

What is a mechanism?
It is a device that has the ability to do Mechanical work. The machine will change (convert) or pass on (transmit) energy. Machines are made from a number of working parts called MECHANISMS. All mechanisms are based on what ancient philosophers called the big five machines. The five simple machines form the basis of all other machines developed throughout history.



Machines cannot do work on their own. They need energy and someone or something to operate them. Some machines are controlled by computers. (e.g. automatic washing machine). The energy, which is used by the machine, is called the INPUT. The result of this energy input is called OUTPUT.



Three major mechanisms include: 1. LEVERS - A rod that pivots around a point.

2. GEARS - (Wheel & Axle) A wheel with teeth.

3. PULLEY - (Wheel & Axle) A wheel with a groove.

Understanding levers:
A lever can therefore be described as a mechanism designed to lift loads or create (or transfer) movement. It uses a stiff rod that pivots (turns) about a point. The point is called the FULCRUM.



The following symbols, represents the different elements in a lever system.



Three classes of levers are identified, which are determined by the placement of the FULCRUM (pivot) relative to the EFFORT and the LOAD.



The load is at one end and the effort at the other, with the fulcrum situated somewhere in between. Eg. scissors



The load is placed between the effort and the fulcrum. Eg. Wheelbarrow



The fulcrum is at one end, the load at the other and the effort somewhere in between the two. Eg. Tweezers

Results: Which example provided the greatest output?

Conclusion: What effect does the position of the fulcrum have on the output distance?

Which one of the three examples is the best demonstration of distance multiplication?



Mechanisms are used to make work easier. (e.g. input motion less than the output motion). This is called a Mechanical Advantage. There are two types of Mechanical Advantage e.g. Distance Multiplication and Force Multiplication. The mechanical advantage of a lever can be calculated. The greater the mechanical advantage, the easier it is to lift an object.

The formula used to calculate mechanical advantage is: MA =load/effort

Study the following example and complete the calculations:

Always convert mass into force, 50 kg x 10 m/s² = 500 N

An effort of 20 N is needed to lift a load of 50 kg with a wheelbarrow. What is the mechanical advantage created by the wheelbarrow?

Mechanical advantage is: MA =load/effort

MA=______

TEST 1
Grade 9.....B..........

Name .snethemba.....................

1. What is the fulcrum of a lever system?
 * ............................................................................................................/3
 * ............................................................................................................/3

2. Which symbol is used to indicate the fulcrum? ...................... /1

3. How does the effort differ from the load in a lever system? /3



4. Using the picture below, identify the position of the FULCRUM, LOAD. /2



5. Name the class of levers represented in Figures 1, 2 and 3. /6



6. Calculate the mechanical advantage you would get by using the lever in load /5 Formula: MA =load/effort



TOTAL [20]

Activity 2 LINKAGES / LINKED LEVERS
LINKAGE: A mechanism made by connecting levers together.

Linkages are designed to:
 * change the direction of a force or motion,
 * allow two parts to move at once,
 * make objects move identical to each other

The following graphic symbols can be useful in understanding how a linkage system works.



The following drawings indicate different combinations of levers in a linkage system: Fill in I for input and O for output




 * complete the practical activity on the next page.

PRACTICAL ACTIVITY
This activity must be done in groups. (2/3 students)

MAKE, LOOK AND DRAW WORKSHEET.
Use the mechanisms kits provided by your teacher and construct all the linkage systems as illustrated on the previous page. Discuss in your group the input and output direction of motion. Complete the table below.

Introduction to PULLEYS
A pulley is a wheel with a groove, e.g.




 * Use your mechanisms kit to build the above examples.
 * Draw exactly what you have done.
 * Use a block diagram to indicate the Input and Output direction.
 * Provide 2 examples of where pulleys are used in the building industry.

Introduction to GEARS
A gear is a wheel with teeth, e.g.



Use the mechanisms kit and build at least 3 examples of gears (you could use more than 2 gears in a gear system).
 * Draw exactly what you have done.
 * Use a block diagram to indicate the Input and Output directions in each of your examples.
 * Provide 2 or 3 examples of gears in used in children's toys.

PROJECT PORTFOLIO
work in groups of 3

SITUATION
You are employed as an engineer at a city office block, which has boom gates on entering and leaving. You have noticed that at peak time on rainy days the security guards have to stand in the rain to lift and lower the boom gates. Use one or a combination of the 3 simple mechanical systems to lift and lower the boom gate, which can be operated from inside the security office.


 * Discuss the words you do not understand.
 * Circle the most important words.



1. Write in your own words what you think the problem is. Try and do it in two sentences

2. Write down a design brief, a few specifications and two constraints for the problem identified in the last sentence of the paragraph.

3. In your group, think of possible solutions that will solve the problem identified in the situation above. Draw freehand sketches of two different ideas.

4. From the two solutions you identified and sketched (drew), select the one you believe to be the best solution. Give one reason for your choice.

5. Produce a two-dimensional working drawing for presentation. The working drawing should include the dimensions and the kind of materials that must be used in arriving at the solution.

6. Produce a model. You will be assessed on the effectiveness of the mechanisms used in your model.

7. Prepare a formal presentation to sell your idea to the class. Your presentation should explain to the class how the mechanism that you have designed in Activity 3 works.

HINTS

 * The product can be 2 dimensional on an A4 card.
 * Divide the card in half (one half for in the booth and the other half outside)
 * Corrugated cardboard is a versatile and cheap resource
 * One-sided corrugated cardboard can be used for gear teeth and on racks.
 * Hot melt glue guns are very effective (glues strongly and quickly)
 * Paper fasteners are quick and easy to use (e.g. fixed and loose pivots)

Glossary
Note: Related terms are grouped and therefore do not appear in alphabetical order.

LEVER : A rigid bar having load, fulcrum and effort used for mechanical advantage

LINKAGES / LINKED LEVERS : Two or more levers linked to form a system of levers

INPUT : The action that starts a system. (e.g. force, switch etc)

OUTPUT : The product that results from the action of a system (e.g movement light glowing etc)

MECHANICAL ADVANTAGE : Mechanical advantage is used in machines to make work easier. Types: change of force, distance & speed

FORCE MULTIPLICATION : Occurs where the output force of a mechanism is greater than the input force.

DISTANCE MULTIPLICATION : Occurs where the output distance of a mechanism is greater than the input distance of that mechanism.

GEAR : A mechanism for transferring and changing movement, force & speed

GEAR TRAIN : Two or more gears connected in a system

DRIVER : The gear that provides the input in a gear system

DRIVEN : The output gear on a gear system

IDLER GEAR : A gear normally placed between the driver and driven so that they turn in the same direction.

BEVEL GEAR : Changes the plain (angle) of movement in gear systems (e.g. windmill turns in vertical plain and the shaft for pumping water turns in a horizontal plain i.e. a change of 900 in direction of movement.)

WORM GEAR : this is a normal gear (worm wheel) that meshes with a worm screw.

WORM SCREW : A cylinder like gear with a screw thread (The worm screw engages / meshes with the worm gear)

RACK AND PINION : A rigid bar with teeth that will engage / mesh with the teeth of a pinion (gear wheel)

MESH : The engagement of gear teeth. (The gears mesh when they are in contact with each other.)

WHEEL : Any circular thing that turns on an axle

AXLE : The shaft that supports the wheel and the movement of that wheel.

CRANK : An axle (e.g. projecting from a wheel) that is bent. (The crank is turned to make the shaft / move.)

CAM : It is a wheel that is not circular. Used to change the direction of movement of something (the follower) that lies against it. (e.g. The oval cam spins while the follower moves up and down &shy reciprocating movement)

PULLEY : A wheel with a groove in the rim for ropes used to lift weights or apply a force.

HYDRAULIC SYSTEM : A closed system that works with fluid pressure

MASTER CYLINDER : The input cylinder in a hydraulic system

SLAVE CYLINDER : The output cylinder in a hydraulic system

An example of the type of graphic expected from learners in a linkage mechanism (Inputs and outputs must be clearly indicated. The convention for fixed and loose pivots must be followed)



Author/s: Alison Fowkes & Deon Khan (Editor &shy Johnny Freese)

The authors acknowledge: Osman Sadeck &mdash; for the use of the data sheet from: www.wcape.school.za/sadeck/ NdoE &mdash; for Adaptation to the Grade 9: CTA (Technology) - 2003