A local council has issued an expression of interest for designers to embellish a natural park and make it safe and attractive for children. As part of the embellishment, the council wants a set of moving figures which will be situated on the bank of a stream of water. The moving figures are to be powered by the force of flowing water and cannot be larger than 2 metres × 2 metres × 2 metres. The local council demanded that these moving figures should require the least maintenance possible. You need only to design and make one moving figure with a scale of 1:4, but still use the same material as the full-sized mechanism.
Design and make a 1:4 scale model of a moving figure that will be situated on the bank of a stream of water. The figures movement must be powered by the force of flowing water and must not be larger than 2m x 2m x 2m (scaled down to 1:4 size). The scale model must use the same materials that will be used in the full-sized mechanism. The model must also be attractive and safe for children, and also require little to no maintenance.
The model must be safe and attractive for children. It must move.
It will be situated on the bank of a stream of water. Its movement must be powered by the stream of water.
It cannot be larger than 2m x 2m x 2m (scale 1:4 = 50cm x 50cm x 50cm)
It must require as little maintenance as possible. Only one moving figure is required to be made. It must take no longer than 14 weeks to finish.
Woods
Woods can be split into 3 main categories. These are: Hardwoods, Softwoods and Manufactured Woods. Hardwoods are not necessarily hard, however they share some properties; their trees are broad-leaved and lose their leaves in autumn. They also generally grow more slowly than softwood trees. Some more noticeable properties are that the grain is very compact and twisted. Since hardwood trees grow slowly (typically taking anywhere from 60 to a 100 years to mature), they are consequently more expensive than other woods, however they are more durable. Hardwoods are generally used to make high quality furniture and some types are also used to make models.
Softwoods are not necessarily soft, but all softwood trees are coniferous (cone-bearing), have needle-like leaves and are evergreen. Softwoods are generally low-cost because they are fast growing (20-30 years to mature) and they are often commercially grown in plantations. However, they have loosely spaced grain which makes them much less durable than hardwoods and can be split easily. They are also lighter than hardwoods and are sold as planks or sheets. Softwoods are also used to make paper and cheaper furniture due to their inexpensiveness, availability and being relatively easy to cut and shape.
Manufactured Woods are becoming increasingly more popular and are available in a variety of sizes and finishes. They are generally made from the excess and the usually wasted pieces of wood when trunks and branches are cut into planks and sheets. The more expensive of these woods have a good quality wood or veneer on the surface but a lower quality in the inside. The internal wood can be blocks (block board), wooden chips (chip board) or small fibre (MDF).Plywood is made by gluing together thin veneers with the grain of each veneer running in alternate directions. Manufactured woods (mainly chipboard and sometimes MDF) are becoming more popular to make furniture like beds, drawers and other pieces of furniture that do not require much durability, due to them being so cheap.
Tight grained hardwood.
Loosely spaced softwood grain.
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Plastics
Plastics are made from crude oil and are mainly split into Thermoplastics and Thermosetting Plastics (also known as Thermosets). Thermoplastics are the most commonly used plastics nowadays. Thermoplastics soften when heated (even at relatively low temperatures, which make them unsuitable for high temperatures). Because of this, thermoplastics can be formed into a shape using a wide variety of methods. As long as the shape is not heated up too much, the shape it was given remains permanent, however, when heated, the shape will soften and the plastic is able to be reshaped. This property allows thermoplastics to be recycled easily. Some examples of thermoplastics are acrylic, polypropylene, polystyrene, polythene and PVC.
Thermosetting plastics can only me moulded into a shape once due to the chain reaction that happens at a molecular level. Since they do not soften on heating, thermosetting plastics can be used in temperatures that are relatively high. However, when these thermosetting plastics are heated excessively, they simply break down. Some examples of thermosetting plastics are melamine, Bakelite, polyester and epoxy resins.
There are also composite plastics. Composites are made by mixing materials together to get enhanced properties. Polyester resin is mixed with glass fibre to make GRP used for boatbuilding and fishing rods. Epoxy resin mixed with carbon fibre is stronger than steel but lighter and as such is used in many machines, even spacecraft.
Some plastic forming methods include vacuum forming, injection moulding, extrusion and also blow moulding.
Metals are very important materials in todays world as they are used in many situations; from wiring in a house, to machines going out into the far reaches of space.
Metals can be split into 2 categories; Ferrous and Non-Ferrous. The word ferrous comes from the Latin word for "containing iron". This can include pure iron, such as wrought iron, or an alloy such as steel. Due to them containing iron, which is itself magnetic, ferrous metals are often magnetic, but this is not always the case. Also, because they contain iron, most ferrous metals rust very easily unless they are treated by methods such as galvanising, special coatings or just by painting them. Non-ferrous metals contain no iron and therefore most of them do not rust. Some of examples of Non-ferrous metals include aluminium, copper and some precious metals like silver and gold.
Alloys are metals made from mixtures of at least two and sometimes more metals and are generally made to obtain metals with better or more properties than their base constituent metals. It is for this reason that specially designed alloys are used in situations needing very tough materials like in jet engines. However, alloys do not retain the properties off their constituent metals. An example of an alloy is steel, because it is a mixture of iron and carbon. This makes steel a ferrous metal and an alloy. However alloys can be ferrous and non-ferrous. An example of a commonly used alloy is solder, which is non-ferrous as it contains lead and tin. Another non-ferrous alloy that was very important to the early development of man is bronze, made from copper and tin. It is an interesting fact that alloys with different properties can be made using the same metals, but in different ratios.
Mechanisms are generally split into main 4 types; gears (and other rotary systems), cams and cranks, levers, and linkages. All of these mechanisms are very commonly used in most of our machinery these days, and although we do not always see them, they are still some of the most important tools we use.
Whormwheel gear Bevel gear
Gears
Gears are used to transmit forces without any loss of energy (without any slip). The shafts of individual meshed gears are generally parallel with each other. Two meshed gears will always turn in opposite directions. To get two gears to rotate in the same direction, idler gears can be used. Idler gears are placed in between the two gears and they do not affect the gear ratio.
Gear Ratio
The gear ratio is used to calculate the change in speed of rotation of the gears. To calculate the gear ratios, the number of teeth on the driven gear are divided by the number of gears on the driven gear. When the gear ratio is larger than 1, there will be a velocity decrease and the mechanical advantage would be higher whilst when the gear ratio is less than 1, there will be a velocity increase but a decrease in the mechanical advantage.
Different types of gears
There are many types of gears. Some of these include Simple gear trains, Compound gear trains, Bevel and wormwheel gears and Rack and pinion gears. Simple gear trains only have 1 gear on each parallel shaft. It doesn’t matter the number of meshed gears, the gear ratio will still be worked the same. Compound gear trains have at least 2 or more gears on a shaft. This means that larger changes in speed of rotation can be achieved. To work out the gear ratio, the individual gear ratios of each shaft are multiplied together. Bevel and wormwheel gears do not have parallel shafts and they are generally perpendicular (the shafts). Rack and Pinion shafts are able to transfer rotary motion into linear motion.
Similar rotary systems
There are also other rotary systems that are very similar to gears, and use many of the same principles. Some of these are; chains and sprockets, often used in bicycles and motorcycles, and belts and pulley which are often used in engines and other machinery.
Levers
Levers are simple mechanisms that create a mechanical advantage for the user. Because of this, they are used to lift heavy weights with the least amount of effort. There are 3 main parts that make up a lever; the load, the fulcrum and the effort. The load is the force that the object being moved makes, the effort is the force made by the user and the fulcrum/pivot is the point at which the lever acts. There are 3 classes of levers:
The most common types of levers are First-Class and Second-Class because they give you a Mechanical Advantage. This means you can move a large load using a small effort. Third-Class levers are used less often because their mechanical advantage is less than 1. This means the force needed to use them is greater than the force they can move. To work out the mechanical advantage (MA) the load is divided by the effort. However, with First and second class levers you have to move the effort a great distance to move the load a short distance. The velocity ratio (VR) can be found by dividing the distance moved by the effort by the distance moved by the load. To find the efficiency of a system, the mechanical advantage is divided by the velocity ratio, then multiplied by 100%.
Linkages
Linkages are very useful mechanisms because they allow forces and motion to be transmitted from on area to another.
The linkage to the left is a reverse motion linkage. The pivot is in the middle of the simple lever so the output travels the same distance as the input. If the pivot was not in the middle but slightly offset, then the distances travelled would not be equal.
The linkage to the right is a push pull linkage. It produces input and output motion in the same direction. By moving the fixed pivots the size of motion can be changed.
Cams
Cams are also some very useful and commonly used mechanisms that convert rotary motion into reciprocating/ linear motion. It is often used in conjunction with other mechanisms, with the reciprocating end being used to power another mechanism like a valve. A common example is the camshaft of a car, which takes the rotary motion of the engine and translates it into the reciprocating motion necessary to operate the valves of the cylinders.
A cam mechanism is made up of 2 parts; the cam itself and the follower. Some cams can have different shape, in order to slightly change the final motion produced. Followers can also have many different shapes, which either change the direction of the resultant motion or simply change how it follows the cam (if the head is flat, the motion will be slightly different ie: moves more quickly).
