Welcome to the Magic of Engineering!

Transcription

1 The Magic of Engineering card 1 Welcome to the Magic of Engineering! Did you know that using modern materials you can make a small buggy capable of going faster than most people can run? Or, if you scaled it up into a full-size car, it would have a top speed of hundreds of miles per hour? This pack invites you to design and make a high-speed buggy using real engineering materials and components. As well as getting pleasure from making a precision product, you will learn many important things about engineering - including why familiar things such as cars and mountain bikes look the way they do. You will discover that engineering is creative, fascinating and - above all - can be great fun!

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3 The Magic of Engineering card 2 electric power Cordless appliances such as electric drills have now become very common as small motors have become more powerful and batteries are able to store more energy. Many toys and models, for example, are also powered by small motors and batteries. Electric Motors An electric motor connected to a battery turns chemical energy into mechanical movement. Miniature DC electric motor The three main parts of the motor are an armature, fixed magnets and a commutator. Electrical energy is supplied to coils of wire on the armature through the commutator - a kind of switch. The armature coils become magnetic and push against the fixed magnets causing the armature to turn. As the armature turns, the commutator switches the flow of electrical energy so that the armature continues to turn at very high speed. Case Magnets Armature Commutator End plate Brushes Exploded view of DC motor

4 The buggy motor turns at 3,000 revolutions per minute (RPM) when connected to a 6 volt battery. When it is made to do some work, such as turning the wheels of the buggy, it slows down and takes more energy from the battery. If you try to make the motor do too much work, it stops and gets very hot. This is called stalling. A small motor works best (most efficiently) when it is running at high speed. However, it does not have much turning power or torque. You can easily slow down a small motor by pinching the spindle between your fingers. We can use a pulley or gears to give higher torque. In a portable drill, a gearbox connects the motor to the chuck. In your buggy, a pair of pulleys can be used to connect the motor to the axle. In these examples, the motor turns rapidly and the chuck or axle more slowly - but with higher torque. Gearbox Motor

5 card 2 continued... Batteries Batteries produce electrical energy by means of chemical changes inside. Batteries have an anode (+ connection), cathode (-ve connection) and an electrolyte. The electrolyte changes as the battery is used. Some batteries can be re-charged, but ordinary batteries undergo permanent chemical changes and eventually go flat. Outer jacket Cathode Cathode collector Paste insulator and container Positive cap Insulating washer Separator Electrolyte Anode Zinc Carbon Cell Batteries are available in different sizes - e.g. AAA, AA, C, D sizes. All these batteries are 1.5 volts; however, it is important to remember that the larger ones provide more electrical energy than smaller ones. It is always better to use larger batteries for powering electric motors if they are not too heavy. D cell C cell AA cell AAA cell 42.5 cc cc cc cc 3

6 If batteries are connected in + series (+ve to -ve to +ve to -ve) the total voltage is the sum of all the 1.5 volt batteries used. Two batteries connected in series give 3 volts, 3 batteries v v 3 v give 4.5 volts and so on x 1.5 v batteries in series - Some batteries such as pp3 give 9 volts because they consist of 6 smaller 1.5 volt batteries (or cells) connected in series. It is a mistake to think that a small 9 volt battery will power a motor. The motor may run quickly at first, but the battery will go flat very rapidly. 1.5 v cell PP3 battery Battery Safety Never mix re-chargeable batteries with ordinary ones. Take great care not to short-circuit batteries especially rechargeable ones. Never put batteries in a pocket where they may touch coins, keys or other metal objects. Never mix used batteries with new ones. (The new ones try to charge up the old ones and may cause them to explode.)

7 The Magic of Engineering card 3 Drive Systems A small vehicle or buggy powered by an electric motor may use a drive or transmission system connecting the motor to the driving wheels. This connects the motor to the wheels and allows it to run efficiently (as quickly as possible). All cars, for example, have a transmission system which normally includes a gearbox. Pulley Drive System Pulleys are wheels with a V groove around the edge to hold a drive belt. A pulley drive system is very easy to make and it will work even if the various parts are not quite lined up. The pulleys supplied with the buggy packs are approximately 6mm diameter (to fit onto the motor spindle) and 36mm diameter (to fit onto an axle). When these pulleys are connected by a belt, the motor pulley has to turn six times to make the larger pulley turn just once. This is because the larger pulley is six times the diameter of the smaller one. We say that such a drive system has a speed reduction ratio of 6 to 1 or 6:1. This ratio allows the motor to turn quite rapidly but also give high torque or turning power to the axle. To get even higher torque, we can increase the size of the larger pulley or decrease the size of the motor spindle pulley. However, this will also reduce the speed of the axle.

8 To calculate the pulley ratio, divide the diameter of the smaller pulley into the larger one. E.g. 36/6 = 6:1. What is the ratio if the larger pulley is changed to 48mm in diameter? To calculate the speed of the output shaft, divide the motor speed by the larger number in the ratio. E.g. 3,000rpm/6 = 500 rpm. This is not quite the right answer! Because of friction and other energy losses we have to derate the speed of the motor by about 50% and assume it actually runs at only 1,500 rpm. What then is the correct answer in the above example? To calculate the speed of a buggy, multiply the output shaft speed by the circumference of the drive wheels. The circumference is found by multiplying the diameter by Pi, e.g. 75mm = 235mm. (Pi is a special figure in maths which makes many engineering calculations possible. If you measure the circumference of a circle, it will always be times longer than the circle s diameter.) E.g. (1) 1500rpm/6 = 250rpm. (2) 250rpm 235 = 58,750mm/minute or m/minute

9 card 3 continued... Propeller Drive System A propeller is an unusual form of propulsion for a wheeled vehicle. Past attempts to propel cars and trains using a propeller have not been very successful. However, the drive system is very simple indeed and extremely high speeds can be achieved for a buggy running on a flat smooth surface. Experimental rail car The blades of a propeller are shaped so that as it turns air moves through it. The faster the propeller turns, the greater the movement of air. This movement of air creates a high pressure area just behind the propeller and a low pressure area just in front. This causes forward movement of the propeller and whatever is attached to it. High pressure Low pressure Movement of propeller

10 For the sake of safety and increased efficiency, propellers are sometimes contained in a circular enclosure called a duct and a complete unit is called a ducted fan. A simple duct can be made for the buggy propeller by folding metal or other material into a cylinder around it. Propeller Safety Avoid finger contact with a rotating propeller. Never hold a rotating propeller near anybody s eyes.

11 The Magic of Engineering card 4 Wheels, Shafts and Bearings Different vehicles, including models, require different types of wheel - some without tyres (trains) and some with (bicycles). The jet propelled car, about to make the world record bid, is unusual in using solid aluminium wheels. Whatever the vehicle, it is essential that its shafts or wheels turn as freely as possible. This normally means that special bearings are necessary. spoked wheels with rubber tyres Solid aluminium wheels Wheels The buggy pack contains 75mm and 40mm diameter moulded wheels both with a 2.8mm diameter centre hole. This allows a 3mm diameter steel shaft to be interference fitted. Interference fits are used widely in engineering for joining things and rely on one component being just slightly smaller than another. Because the plastic wheels are soft, the steel shaft might be inserted out of true. It is best to use either a special jig or, for example, a drilling machine (making sure the power is turned off). This will ensure that the shaft is fitted at exactly 90 to the face of the wheel. Jet powered world record car Fitting a shaft

12 The buggy wheels may be used as supplied or fitted with tyres. The use of solid wheels reduces rolling resistance when a vehicle is moving (but gives a bumpy ride over rough surfaces). The rolling resistance of a racing cycle with narrow high pressure tyres, for example, is much less than that for a mountain bike. The 40mm diameter wheels can be clipped together and covered with slices of foam rubber pipe insulation. Miniature moulded tyres made for scale models can also be used. Either option looks attractive, but will probably increase rolling resistance. Remember that larger driving wheels will give higher overall speed; smaller wheels will give more power but at the expense of speed. Two small wheels clipped together Foam tyre If larger wheels are used, it is a good idea to balance them. Car wheels are given small weights to prevent undue vibrations due to one side of the wheel having slightly more mass than the other (because of uneven moulding of the tyres). The larger moulded plastic wheels may have more mass on one side of the centre hole than the other which could set up vibrations at high speed. (This is noticeable if you hold the buggy off the ground and run the wheels under power.) A very simple balancing method is to put an axle through the wheel and place this on the edge of two thin metal plates. If the wheel is heavier on one side, this will always come to rest at the bottom when the wheel is gently pushed along. The wheel can be balanced by carefully drilling out material from the heavy side. Simple wheel balancing method

13 card 4 continued... Shafts and Axles A shaft is fixed to the centre of a wheel and turns in a bearing. An axle is fixed and the wheel turns on it. A stub axle is a short axle fixed in place at one end. If the wheels in the buggy pack are to run on an axle, they must be drilled out to 3.1mm for a running fit. If necessary, a 4mm screw can be used as a stub axle for the buggy because of the very light loading. (This is not normally good engineering practice because the screw thread is not a good bearing surface and will eventually wear into the soft wheel.) Stub axle Bearings The purpose of a bearing is to reduce wear and friction to a minimum. Most powered vehicles and bicycles use ball bearing races, but these can be expensive and are unnecessary for many smaller applications. The alternative is to use plain bearings. Plastic (plain) bearing bushes are provided in the buggy pack. These fit tightly into a hole made with the TEP punch tool and allow a 3mm diameter shaft to turn freely within them. A bearing bush should always be used in an aluminium chassis because it is not good engineering practice to use aluminium itself as the bearing material. Bearing bush Bearing balls Ball bearing race

14 Pulley and Wheel Fitting The large yellow wheels are a tight fit on the 3mm diameter steel shaft, but the larger pulley is a loose fit. However, these can be coupled together so that the pulley drives the wheel. The boss of the pulley is pressed into the centre of the wheel as shown. Pulley 3 mm shaft Large wheel

15 The Magic of Engineering card 5 Pre-coloured Sheet Metals During the last few years there has been a revolutionary change in the way products such as white goods are made. ( White goods is the name given to domestic appliances like washing machines, refrigerators etc.) Instead of making the outer cases of these products and painting them when formed, the metal is now pre-coloured and then post-formed. This technique saves on costs and is environmentally friendly. TEP pre-coloured metal and chassis construction The pre-coloured metal sheet supplied for the buggy is 0.5mm thick aluminium coated with a hard but flexible polyester film. This metal can be cut, punched and folded without damage. It is covered with a protective plastic which should be left on until all making is complete. Designing with Sheet Metal Sheet metal can be folded into many different shapes to provide strength and stiffness. Food packages made from card are good examples of how a flexible sheet material can be made into strong containers to support loads and resist forces. We say that such containers possess strength because of their geometry or shape. In engineering, this folding principle is used to create very strong structures from very thin metal sheet. A car is made from thin steel sheet folded in complex ways to give a stiff but relatively light body shell.

16 Making Chassis Sections A strong and stiff buggy chassis can be made from pre-coloured metal sheet by folding along two sides. The fold also provides a location for wheel axles. Other more complex sections can be made by joining simple sections together using adhesives or mechanical fastenings such as screws and nuts. 1. Decide on the size and shape of your chassis and make a full-size dimensioned drawing that best describes it. 2. Measure and mark out the net or development of the chassis outline on the precoloured metal and also mark out positions for any punched holes.

17 card 5 continued Cut out the shape on a guillotine. 4. Punch out holes for fastenings and bearing bushes using the TEP punch tool. 5. Position one edge of the chassis in the metal folding unit and screw down the clamping bar. Fold as required (usually at 90 ).

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19 The Magic of Engineering card 6 Fastenings One of the big challenges in engineering is joining components together safely. Many fastening techniques used on a larger scale in cars, washing machines etc. can also be applied to your buggy. These include mechanical fastenings such as nuts/ screws, rivets and adhesive bonding. The development of pressure sensitive adhesives (adhesives on tape) has given engineers new freedom in designing joints. Pan head screw Nut Plastic rivet Locking ring A typical car contains thousands of different joints ranging from welding to adhesive bonding Adhesive tape Screws and Nuts Screws and nuts are still one of the most common types of fastening. They enable parts to be joined together permanently, but allow taking apart if necessary. They offer a strong joint because a very high locking force can be applied to tightening them up - e.g. using a pair of pliers and screwdriver. Forces applied to a screw and nut

20 Screws can also be used to make a loose coupling when two nuts are tightened against one another. This is called a lock-nut joint. The screws and nuts supplied in the buggy pack are 4mm metric - a standard thread now used throughout the world. The screws have a slotted pan head. Nuts tightened together on screw When nuts and screws are assembled, it is normal to use a washer behind the nut so that when it turns it does not damage the material it is tightened against. Sometimes a crinkle or locking washer is used. This cuts into the two mating surfaces and prevents the nut coming loose. Crinkle washer Rivets There are many different types of rivet for joining parts together. These include, for example, DIY pop rivets and the plastic rivets that hold the interior panels of cars to the doors. The rivets supplied in the buggy pack are made from polythene and lock into a small ring (locking ring). Two - part TEP rivet Pop rivet Two - part bifurcated rivet When a rivet has been assembled, any excess length can be cut off with scissors or a craft knife. A joint made by this method is only as strong as the rivet itself and the tightness of the locking ring. The big advantage, though, is speed of assembly and the fact that the rivet can be taken off easily and re-used. It can also be used to make loose joints where one or more of the joined parts have to move. Trimming a plastic rivet

21 card 6 continued... Adhesive Bonding Pressure sensitive adhesive (PSA) is the name given to tapes coated in adhesive which sticks on contact. Sellotape is the most common example of a pressure sensitive adhesive. The tapes used in industry are double-sided so the tape itself (called the substrate) is really just a convenient way of carrying and storing the adhesive. Double-sided tapes available in the shops have a rubberbased adhesive and are described as either low-tack (relatively weak) or high tack (relatively strong). The tapes used in industry now use much stronger adhesives based on acrylic and other polymers. These have such high bonding strength that they are used, for example, to join aircraft wing parts together and the outer panels onto buses. Panels of bus bonded to frame with adhesive tape Ordinary double-sided tapes can be used to join buggy parts together permanently if: the surface area is large (e.g. 10cm2 minimum) the surfaces are clean (e.g. wiped with methylated spirits) the surfaces are flat Some examples of suitable and unsuitable joints are shown. BAD GOOD

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23 The Magic of Engineering card 7 Steering methods The steering (and suspension) of a modern vehicle is based on a complex system of linkages or connected parts. These ensure that the vehicle s wheels can be turned in a controlled way and without undue wear on the tyres. Most vehicles, such as cars, use a system called ackermann steering. As the wheels are turned each one takes up a slightly different angle to allow for the fact that one wheel is moving over a larger radius. Car suspension system Ackermann steering principle A simpler method of steering, commonly used on small towed trucks and toys uses a centre-pivot steering system. This does not compensate for the different radius of curvature of the wheels. Some vehicles, including small cars, use only a single front or back wheel for steering. Centre pivot steering on tow truck

24 Buggy Steering The three methods of steering described above can be used so that the buggy can be set to go in a straight line or a circular path. 1. Centre-pivot steering. This can be achieved, for example, using a folded metal axle-frame attached to the main chassis using a screw and nut. A rubber or fibre washer(s) trapped between the frame and the chassis is one method of creating a tight joint - but one that can be turned for adjusting the steering angle. If pre-set steering angles are required, a wire linkage rod can be used to plug into a series of holes in the chassis plate. (Working out the positions for these holes to give a range of steering angles is an interesting engineering exercise!) Screw Friction washer Locknuts Linkage rod 2. Single wheel steering. As in two-wheel steering, the single wheel may be at the front or rear of the buggy. The wheel can be contained in an axle-frame or set to one side on a stub-axle. For a rapid prototype a screw could be used for the stub-axle, but it is not good engineering practice in the longer term to use a screw thread as a bearing surface. Screw to hold axle frame to chassis Frame Axle Screw as stub axle Screw to hold axle frame to chassis 'L' axle frame

25 card 7 continued Ackermann steering. It is possible to make an ackermann steering system using two folded plates of metal connected by a linkage rod. Each plate is fitted with a stub-axle. For the purposes of a prototype, this type of steering system can be assembled using plastic rivets. Different steering geometries or layouts can be examined rapidly using rivets in punched metal card or plastic. TEP rivets Wire linkage Simple Ackermann steering system Bushes Hole drilled to size Stiff card Rivets Wire link Experimenting with card or plastics

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27 The Magic of Engineering card 8 Putting It All Together: Designing and Making a Buggy When engineers design something, they take many decisions based on what they know will work. To design your buggy, you will need to refer to the cards in this series and make decisions that will lead to more detailed design work. You can use pencil and paper - or even a computer - to help you work out ideas and show them to other people. The following is a guide to making design decisions about your buggy and making it. It will help to organise your thinking - but, remember, you have to keep thinking about the complete buggy when thinking about any of the parts. Specification: Make a clear statement of what the buggy has to do. You can use this to judge how well the buggy performs. Basic Design Decisions: How many wheels - 3 or 4? What sort of chassis - one part or more joined together?

28 Method of propulsion: propeller or drive to the wheels? Type of steering - single wheel, centre-pivot, ackermann? Make rough sketches of how you think the buggy will look. You may prefer to draw views looking from the top and side rather than trying to draw in 3D. Detailed Design Decisions: For example, how will the electric motor be mounted onto the chassis for a propeller drive? If you want to put a body on the chassis, what will it look like and how will it fit on? What will the steering parts look like? Detailed Drawings Measure the various parts in the buggy pack and then make more detailed drawings of your buggy using drawing instruments. It is helpful if these are full-size. At this stage you can work out on paper if and how things will fit together. For example, if you use a propeller, how far above the ground must you mount the motor? If you use the larger wheels, will the propeller be clear of them?

29 card 8 continued... Make a final working drawing that you can refer to when you mark out the various parts. Check that you have enough material and components in the pack. Will you need more things from school or the local model shop? Manufacturing Mark out the metal parts accurately. Use a fineliner pen to mark out on the plastic protective film Cut out the metal parts Punch any holes needed Fold the metal parts Join any metal parts together Insert bearing bushes Fit the motor and other parts Evaluation Does the buggy work and live up to your expectations? How does it compare with the specification you wrote for it? Are there any obvious problems that can be corrected or modifications made to make it work better?