MECHANICAL TECHNOLOGY Past Paper FEBRUARY/MARCH 2016 Memo/Memorandum - GRADE 12 NATIONAL SENIOR CERTIFICATE

MECHANICAL TECHNOLOGY
FEBRUARY/MARCH 2016

MEMORANDUM

MARKS: 200
TIME: 3 hours

QUESTION 1: MULTIPLE-CHOICE QUESTIONS

1.1 C √ (1)
1.2 B √ (1)
1.3 D √ (1)
1.4 D √ (1)
1.5 A √ (1)
1.6 C √ (1)
1.7 D √ (1)
1.8 D √ (1)
1.9 B √ (1)
1.10 D √ (1)
1.11 D √ (1)
1.12 C √ (1)
1.13 B √ (1)
1.14 B √ (1)
1.15 A √ (1)
1.16 C √ (1)
1.17 A √ (1)
1.18 C √ (1)
1.19 D √ (1)
1.20 B √ (1)

[20]

QUESTION 2: SAFETY

2.1 Safety – Centre lathe

  • Look out for revolving work pieces √
  • Do not remove shavings by hand √
  • Be careful not to run the cutting tool into the chuck √
  • Do not make any adjustments on the work piece while the machine is running √
  • Do not leave tools on the machine while in operation √ (Any 2 x 1) (2)

2.2 Safety – Tensile Tester

  • Use safety goggles √
  • Do not apply excessive pressure √
  • Test piece to be well secured for testing √
  • Check hydraulic fluid level √ (Any 2 x 1) (2)

2.3 Safety – Spring Tester

  • Spring tester should be in a good condition √
  • Spring tester must be fitted correctly and firmly √
  • Ensure that the spring cannot slip out of position before applying the load √
  • An uniform load must be applied √
  • Release the load carefully and uniformly √ (Any 2 x 1) (2)

2.4 Safety – Cylinder Leakage

  • Clean the area around the spark plug, before removing the spark plug √
    • To prevent dirt from falling into the cylinder. √
  • Be careful when removing radiator cap √
    • The water may be hot and under pressure. √
  • Do not exceed the specified pressure to test the cylinder √
    • To prevent damage to the seals and tester. √
  • The tester must fit properly and be well tightened in the spark hole or injector hole √
    • To prevent damage to the tester and spark hole or injector hole. √
      Any 2 x 2 (4)

[10]

QUESTION 3: TOOLS AND EQUIPMENT

3.1 Compression testing

3.1.1

  • Wet compression test √
  • Dry compression test √ (2)

3.1.2

  • Worn cylinders √
  • Worn piston rings √
  • Worn piston √
  • Leaking inlet valve √
  • Leaking exhaust valve √
  • Leaking cylinder head gasket √ (Any 2 x 1) (2)

3.2 Oil pump

  • Oil pressure meter or oil pressure tester √ (1)

3.3 Cooling system test

  • Remove the radiator cap and fit the tester √
  • Pump air at the prescribed pressure into the system √
  • Note the reading √ and if the reading drops, it indicates a leaking system √
  • To test for a leaking cylinder-head gasket, √ the engine is started. √
  • If the reading increases while the engine idles, it indicates on a leaking cylinder-head gasket √ (7) [12]

QUESTION 4: MATERIALS

4.1 Iron-carbon properties

4.1.1 Pearlite:

  • Good ductility √
  • Hard √
  • Strong and tough √
  • Resistant to deformation √ (Any 2 x 1) (2)

4.1.2 Cementite:

  • Intensely hard √
  • Brittle. √ (2)

4.2 Critical points

4.2.1 AC1 – lower critical point

  • The lowest temperature to which steel must be heated to be hardened. √√
  • The lowest temperature where the structure starts to change. √√ (Any 1 x 2) (2)

4.2.2 AC3 – high critical point

  • The highest temperature to which the steel can be heated to obtain maximum hardness. √√
  • The temperature where the steel completely loses its magnetic properties. √√
  • The temperature where the steel's structure is at its finest. √√ (Any 1 x 2) (2)

4.3 Carbon content determines the hardness of steel. √ (1)

4.4 Cementite structure in steel determines the hardness. √ (1)

4.5 Ferrite structure in steel determines the ductility. √ (1)

4.6
Austenite is a solid solution of iron and carbon also called iron carbide. √ The structure is at its finest. √ (2)
[13]

QUESTION 5: TERMINOLOGY

5.1 Gear calculation

mectecm51

5.2

mectecm52

5.3 Screw thread cutting

  • Set up the work piece in the lathe and turn the part to be threaded to the major diameter of the thread. √
  • Set the compound slide to the correct angle (30°) to the right and set the tool up accurately in the post. √
  • Consult the index plate of the quick-change gearbox for 2 mm pitch and move the levers accordingly. √
  • Start the lathe and set the cutting tool so that it just touches the work piece. Set graduated dials to zero (cross feed and compound slide) √
  • Move cutting tool a short distance off end of work piece and feed compound slide say 0,06 mm inwards. √
  • With lathe turning, engage half nuts at the correct line on the chasing dial, putting the first cut in progress. √
  • Withdraw the cutting tool quickly and disengage the half-nut lever. Return the carriage to the starting point of the thread. √ OR Stop the machine, leave half nut engaged, back off slide past zero and return carriage to start position in reverse √
  • Check with thread gauge to see if thread pitch is correct. √
  • Repeat with successive cuts until thread is complete. (Remember to bring cross-feed collar back to zero for each cut) √
  • Each successive cut is set by means of the compound slide. Check thread with ring gauge for correct fit. √

(10)
[30]

QUESTION 6: JOINING METHODS

6.1 MIG/MAGS welding equipment

6.1.1 MIG/MAGS welding equipment √ (1)

6.1.2 Labels

A = Shielding gas cylinder √
B = Regulator √
C = Gas flow meter √
D = Continuous wire reel √
E = Welding gun √
F = Arc √
G = Earth clamp √ (7)

6.1.3 Purpose

Prevents oxygen √ to come in contact with the molten metal.√ (2)

6.2 Weld defects

6.2.1 Defect: Slag inclusion
Causes:

  • Included angle is too narrow. √
  • Rapid chilling. √
  • Weld temperature is too low. √
  • High viscosity of molten metal. √
  • Slag from previous run weld not removed. √ (Any 2 x 1) (2)

6.2.2 Defect: Undercutting

Causes:

  • Faulty electrode manipulation. √
  • Current too high. √
  • Arc length too long. √
  • Speed of weld too fast. √ (Any 2 x 1) (2)

6.3 Welding defects

6.3.1
Defect: Lack of fusion
Preventions:

  • Adjust the electrode angle and prepare the V groove properly.√
  • Weave must be sufficient to melt sides of the joint. √
  • Proper current will allow fusion. √
  • Adjust welding speed to ensure fusion. √ (Any 2 x 1) (2)


6.3.2 Defect: Weld craters
Preventions:

  • Use lower current. √
  • Use proper welding technique. √
  • Use correct electrode √ (Any 2 x 1) (2)


6.4 Dye penetration test

  • Clean the weld that needs to be tested. √
  • The dye is sprayed onto the clean surface. √
  • Allow the dye to penetrate the weld joint. √
  • Excess dye is cleaned away with a cleaning agent. √
  • Allow surface to dry thoroughly. √
  • Spray a developer onto the surface to bring out the dye trapped in the crack. √
  • The dye will show all the surface defects√ (7)
    [25]

QUESTION 7: FORCES

7.1 Resultant

mectecm71

mectecm73

mectecm74

QUESTION 8: MAINTENANCE

8.1 Routine maintenance.

  • Tear on the belt.√
  • Misalignment of belt drive. √
  • Overheating of components. √
  • Belt slip. √
  • Belt wear. √
  • Pulley wear. √
  • Financial loss due to the damage suffered.√
  • Loss of valuable production time. √ (Any 2 x 1) (2)

8.2 Cutting fluid

  • To allow it to flow easily √
  • Dissipate excess heat √
  • Prevent excessive load on pump √ (Any 2 x 1) (2)

8.3 Flash point

Is the lowest temperature at which the oil gives off a vapour which will ignite. √√ (2)

8.4
'API'
American Petroleum Institute √√ (2)

8.5 Automatic transmission fluid

  • Transmit power in the torque convertor √
  • Let hydraulic fluid transmit energy in order to move various parts such as the servo unit. √
  • Acts as heat transfer medium to transfer heat within the transmission to outside and assist in cooling it down. √
  • Acts as a lubricant for gears and bearings. √(Any 2 x 1) (2)

8.6 Replace belt in a drill press

  • Machine should be switched off the locked out. √
  • Tension on the belt to be released by loosening an adjusting screw or releasing the belt tensioner. √
  • Remove the belt. √
  • Replace with new belt of the correct type and size. √
  • The belt should be re-tensioned and aligned. √ (5)
    [15]

QUESTION 9: SYSTEMS AND CONTROL

9.1 Gear drive

mectecm91

9.1.1 Rotational frequency of the electric motor:


9.1.2
Speed ratio of gear train:

mectecm922

9.3 The volume of gas can be changed by the altering of ...

  • its pressure √
  • its temperature √
  • both its pressure and temperature √ (Any 2 x 1) (2)

9.4 Definition of Boyle's law

The volume of a given mass√ of gas is inversely proportional to the pressure√ on it, if the temperature remains constant√ (3)


9.5
Hydraulics
9.5.1

mectecm95

QUESTION 10: TURBINES

10.1 Water turbine blades
To supply water pressure √ to the turbine √ (2)

10.2 Reverse flow

  • Deriaz √
  • Francis √ (2)

10.3 Supercharger

  • Roots√
  • Twin screw√
  • Centrifugal √
  • Vane √ (Any 2 x 1) (2)

10.4 Turbocharger

  • Exhaust gases drive the turbine √
  • The turbine drives a compressor via a common shaft√
  • The compressor forces √ compressed air above atmospheric pressure into the cylinder √
  • Exhaust gases leave system through the exhaust pipe √ (5)

10.5 Supercharger over turbocharger

  • Do not suffer lag √
  • More efficient at low revolution per minute. √
  • Does not require extensive exhaust modification.√
  • No special shutdown procedure is required. √ (Any 2 x 1) (2)

10.6 Lag

Lag is the delay √ between pressing the accelerator pedal √ and feeling the pressure building up.√ (3)

10.7 Supercharger drive

  • Belt drive √
  • Gear drive √
  • Chain drive √ (Any 2 x 1) (2)

10.8 Gas turbine disadvantages

  • Cost is much greater than for a similar-sized reciprocating engine since the materials must be stronger and more heat resistant. √
  • Manufacturing operations are also more complex. √
  • Usually less efficient than reciprocating engines, especially at idling speed. √
  • Delayed response to changes in power settings. √
    (Any 2 x 1) (2)
    [20]

TOTAL: 200