Mechanotechnology N3 Lecturer s Guide ISBN: 978-1-4308-0617-2 R Cameron & LL Maraschin This Lecturer s Guide accompanies the following Student s Book: Title: Mechanotechnology N3 Author: R Cameron & LL Maraschin ISBN: 978-1-4308-0048-4 Web PDF ISBN: 978-1-4308-0235-8
Mechanotechnology N3 Lecturer s Guide R Cameron & LL Maraschin, 2012 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, photocopying, recording, or otherwise, without the prior written permission of the copyright holder or in accordance with the provisions of the Copyright Act, 1978 [as amended]. Any person who does any unauthorised act in relation to this publication may be liable for criminal prosecution and civil claims for damages. First published in 2012 by Troupant Publishers [Pty] Ltd PO Box 4532 Northcliff 2115 Distributed by Macmillan South Africa [Pty] Ltd ISBN: 978-1-4308-0617-2 It is illegal to photocopy any page of this book without written permission from the publisher. While every effort has been made to ensure the information published in this work is accurate, the authors, editors, publisher and printers take no responsibility for any loss or damage suffered by any person as a result of reliance upon the information contained herein. The publisher respectfully advises readers to obtain professional advice concerning the content. While every effort has been made to trace the copyright holders and obtain copyright permission from them, in some cases this has proved impossible due to logistic and time constraints. Any copyright holder who becomes aware of infringement on our side is invited to contact the publisher. To order any of these books, contact Macmillan Customer Services at: Tel: (011) 731 3300 Fax: (011) 731 3535 Email: customerservices@macmillan.co.za
Contents Module 1: Power transmission...1 Summative assessment...1 Module 2: Brakes...10 Summative assessment...10 Module 3: Bearings...12 Summative assessment...12 Module 4: Water pumps, cooling and lubrication...14 Summative assessment...14 Module 5: Hydraulics and pneumatics...23 Summative assessment...23 Module 6: Internal combustion engines...25 Summative assessment...25 Module 7: Cranes and lifting machines...29 Summative assessment...29 Module 8: Material and material processes...31 Summative assessment...31 Module 9: Industrial organisation and planning...34 Summative assessment...34 Module 10: Entrepreneurship...37 Summative assessment...37
Module 1: Power transmission Summative assessment 1. This answer is unavailable and we apologise for any inconvenience caused. 2. This answer is unavailable and we apologise for any inconvenience caused. 3. D N d n d n D N 2002400 500 960 mm 4. Hard rubber out er casing Cords t o prevent excessive st ret ching Included angle 40 0 Soft rubber 5. Efficiency: the drive is positive and no slippage occurs. This results in a high efficiency since little or no energy, due to friction, is wasted. Longevity. In a well maintained chain drive wear and fatigue take place over a relatively long period; and heat, dust and dirt do not have a big effect, resulting in a relatively long chain life. Suitable to hostile environments; this is due to its ability to cope with the effects of heat, dust and dirt. Long shelf life. Chains do not suffer the same degrading effects that time has on belt materials. No heat generated (no flexing). 1
6. 6.1 SR 960 570 1,68 :1 6.2 ANS Service factor = 1,3 6.3 Design power = 881,3 114,4 kw ANS 6.4 Diameter of pulley from table = 280 mm 7. It is a positive drive They transmit power directly Gear drives require very little maintenance 8. Spur gears In spur gears the axes of the shafts are parallel and the teeth are cut straight across the blanks. Although several teeth of a spur gear are in mesh at any time, because of inaccuracies in machining there is always shock as the load is taken by successive teeth. Gear Pinion Applications Extensive use in motor vehicles Where medium speed reductions are required 2
Helical gears If a stepped spur could be made up of an infinite number of laminations, each displaced a minute amount from its neighbour, extremely smooth gear transmission could be obtained. This, in effect, is produced by machining each gear tooth in the form of a helix, so that several teeth mesh simultaneously. Because the teeth in helical gears are set at an angle, axial thrusts are set up. Applications Where quiet operation is required Where heavy loads are transmitted Double helical gears To counteract the effects of the axial thrust set up in a helical gear, two sets of teeth are cut on the same blank. The angles are cut opposite to one another. The teeth are separated by a groove to avoid excessive pressure build-up of oil between the two set of teeth. These are also known as herringbone gears. Groove Application Where end thrust is to be eliminated 3
Worm and wheel Worm gear reducers are also used quite frequently in the industrial plant. Their compactness of design and construction allows them to be placed in a relatively small space. Most worm gear reducers are used in fractional or small power applications. However, worm gear reducers driven by large power motors are also supplied to many industries. The compactness of the worm gear reducer is a direct result of the positioning of the worm (input gear) on its shaft and the output gear that is driven by the worm. The worm performs the same job as the high speed pinion gear in other reducers. Note that the force transmission through the worm gear reducer is at right angles at all times. This right angle force transmission combined with the worm design allows the reducer to operate in only one direction. Reverse rotation of the unit cannot occur. Wheel Casing Out put shaft Bearings Input shaft Worm Worm and worm wheel gearbox Most single reduction worm gears vary in ratio from 5:1 to 60:1. The ratio is determined by the number of threads or starts cut on the worm and the number of teeth on the driven gear. Usually, there is only one thread on the worm. This single thread is counted as one tooth as it contacts the driven gear. If more than one thread is 4
cut (two, four, six or eight) on the worm, then the ratio would be changed to reflect the different number of threads. The ratio is obtained by dividing the number of teeth in the gear by the number of threads (or starts) in the worm - just as a spur gear ratio is found by dividing the number of gear teeth by the number of pinion teeth. For example, if a single thread worm is used with a worm gear having 50 teeth, the gear ratio or reduction is 50:1. If the worm has two threads and the number of worm gear teeth remains at 50, the reduction becomes 50:2 or 25:1. Applications Dividing head of a milling machine Elevators Bevel gears Bevel gears are shaped like sections of cones. Bevel gears are spur gears where the intersecting angle that the shaft makes is 90. Application Used in the raising and lowering of tables in drilling, milling machines, etc. Rack and pinion A flat piece of material with gear teeth cut into it is called a rack. When it meshes with a spur gear the system is known as a rack and pinion. The spur gear can run back and forth over the rack or otherwise the spur gear can remain stationary and the rack driven back and forth. 5
Pinion Rack 9. 10. Where space is limited Where power must be transmitted Where the direction of the power transmitted must be changed Gears connecting parallel shafts: Spur gear Gears where the centre lines intersect: Bevel gear Gears where the shafts are at an angle to each other: Worm and worm-wheel gear 11. Advantages: No concentrated loads at the end of the teeth Meshing of gears takes place gradually and smoothly They can take heavier loads Disadvantages: They are difficult and expensive to manufacture Thrust bearings have to be fitted to counteract side thrust There is less resistance to corrosion and wear 12. Advantages: You can use it effectively to reduce speed The direction of the drive can be changed The drive is silent during operation Disadvantages: Wear takes place due to friction Drive is not possible from the worm-wheel to the worm Due to high friction a lot of power is used 13. On the simple gear drive each gear is fitted to its own shaft, while on the compound gear drive a single shaft can have two or more gears fitted to it. 6
14. 15. Check the oil seals for leakage Always use the best quality type of oil in gearbox Check and top up the oil to the required level Check the condition of the gear tooth regularly Spigot flange Key Recessed flange Key Shaft A Shaft B Bolt and nut Flange coupling 16. The Oldham coupling is used where the axes of two shafts to be joined are not in a straight line. It consists of a floating middle piece with two tongues at right angles to each other on opposite sides of the disc. 17. Flange A Flange B Key Driven shaft Driving shaft Leat her or rubber washer Clearance t o count er fault y alignment Coupling bolt Pin-type flexible coupling Clut ch 7
18. 19. Where engagement should take place without a shock effect No need required for a large electric motor to reach full speed Where it can act as a safety factor in overload situations Permanent coupling: Flange coupling Flexible coupling: Pin type flexible coupling Self-aligning coupling: Universal coupling 20. By the sliding movement of the jaws over each other 21. 22. Flange coupling Marine coupling Pin type flexible coupling Chain coupling 23. 23.1 Bibby flexible coupling 23.2 Heavy loads 23.3 High and low speeds 23.4 Drive takes place in both directions 24. 25. Long shafts are more difficult and expensive Long heavy shafts are difficult to transport Difficult to handle and install long shafts A D E B C F A SHAFT B KEY C CROSS PIECE D KEY E SHAFT F YOKE 8
26. 27. 28. 29. 30. 31. 32. 33. Slip occurs between the engaging surfaces A considerable axial force is needed to keep the clutch engaged It must have the correct viscosity Provide effective lubrication It must not foam in the clutch It must not mix with water Where engagement should take place without a shock effect No need required for a large electric motor to reach full speed Where it can act as a safety factor in overload situations Input shaft Output shaft Drive unit (impeller) The diameter of the clutch is smaller If one disc fails the clutch can still function on the remaining discs Less slippage takes place Greater power can be transmitted Cylindrical drum A star wheel Friction blocks Positive Friction Centrifugal Hydraulic Cause: Grease or oil on the contact surfaces Remedy: Remove grease or oil on the contact surfaces to make it dry Cause: Faulty operations to control mechanisms Remedy: Test regularly for wear on pins, bushes and other components and adjust where required. Cause: Glazing of frictional surface due to slippage Remedy: Make contact surface rough using sandpaper 9