LEAVING CERTIFICATE EXAMINATION 2011
Engineering project
Higher level
Examination number:
Contents:
Introduction...... 3
Analysis of Design Brief...... 4
Investigation of Solutions...... 7
Criteria for Selection of Solutions......
Production Drawings......
Electronics......
Testing and Evaluation......
Introduction:
Historically, the harsh winter climate of the northern hemisphere created
challenges in the cross country transportation of people and their
possessions. In the late 19th Century this resulted in the invention of
snowmobiles. Although originally developed as utility vehicles, advances
in design and technology since then, have provided manufactures with
the scope to produce a full range of recreational, special purpose and
competition versions of the snow mobile.
Analysis of design brief:
Design a model of a snowmobile to the general specifications outlined.
The model snowmobile should:
(a) Have the rear propulsion unit controlled by an ON/OFF switch;
(b) Incorporate front skis with a steering mechanism;
(c) Have seating capacity for the driver only.
Presentation of the completed model should ensure that:
(a) All the main operating features are clearly visible without dismantling.
(b) The longest dimension of the device does not exceed 300 mm.
(c) Electric power does not exceed 9 volts.
Key words and their dictionary definitions:
Model: A miniature representation of a thing, with the several parts in due proportion; sometimes, a facsimile of the same size.
Snowmobile: Is a land vehicle for winter travel on snow.
Rear propulsion: Propulsion - means to push forward or drive an object forward.
Controlled: to exercise restraint or direction over; dominate; command.
ON/OFF Switch: a device for turning on or off
Incorporate: unite or merge with something already in existence, have as a component.
Skis: A ski is a long, flat device worn on the foot designed to help the wearer slide smoothly over snow, plural.
Steering Mechanism: A mechanism by which something is steered.
Seating/the driver only: You have to have a seat for one person.
Handle bars: Metal tube used to steer the snowmobile.
What is a Snowmobile?
• A snowmobile, also known in some places as a snow machine, sled, or skimobile, is a land vehicle for winter travel on snow.
• Designed to be operated on snow and ice, they require no road or trail.
• Design variations enable some machines to operate in deep snow or forests; most are used on open terrain, including lakes or driven on paths or trails.
• Usually built to accommodate a driver and perhaps one adult passenger, their use is much like motorcycles and All-terrain vehicles (ATVs) intended for winter use on snow-covered ground and frozen ponds and waterways.
• They have no enclosure other than a windshield and its engine normally drives a continuous track or tracks at the rear; skis at the front provide directional control.
Investigation of solutions:
Here are some images of Snow mobile designs that I found, which heavily affected my design process.
There are various designs for Snowmobiles. They are all based around the same design principles as given in the design brief.
Here, I will talk about detailed analyses of the basic existing designs:
Basic Design of Snow Mobiles:
Tracks and Steering:
Snow mobile tracks are made of a light material for mobility and speed, while a handlebar/ski mechanism steers snowmobiles.
Tracks succeed where wheels often fail; they spread the vehicle weight over a greater surface area, allowing it to move on soft, slippery or unstable terrain where wheeled vehicles are unable to gain traction. Tracks keep a heavy snowmobile from sinking in soft snow, just as long, flat skis and wide, flat snowshoes spread a person's weight over a larger area.
Snowmobile tracks also provide traction on slippery surfaces like snow and ice, where regular wheels would slip and slide. The large surface area and the roughness of the treads create friction between the snowmobile and the surface, giving the snowmobile a better grip. Most snowmobiles add sharp studs to their tracks; these act like cleats on athletic shoes and sink a short distance into the ice or hard snow, gouging small holes that enable the tracks to grip the ice even tighter on particularly slippery terrain.
Handlebars:
You steer a snowmobile by turning the handlebars, in much the same way as you steer a bicycle or motorcycle. The handlebars connect to a stem mounted near the front of the snowmobile, which in turn connects to skis mounted on a bracket at the bottom of the snowmobile. Turning the handlebars turns the skis in the same direction.
Skis:
Snowmobile skis come in varying widths and sizes for different terrain, as well as single or double-ski models. Wider skis displace the snowmobile's weight over a larger area, enabling it to "float" higher over the snow, while narrower skis allow for sharper turns.
Shock absorbers play a large part in providing a comfortable and stable ride. Springs and dampers are used in a manner similar tomountain bikes, enabling the rider to pilot their vehicle with the mobility of skis, but the stability of a tracked vehicle.
Engine:
Heavier touring models tend to use the four-stroke engine, while the lighter, sportier models use the two-stroke.
A snowmobile engine links to a track drive, which rotates the tracks. The wheels on a snowmobile are essentially large gears with teeth spaced evenly with holes in the tracks. Every rotation of the gears powers the tracks and drives the snowmobile forward. The faster the engine, the faster the gears rotate, and the faster the tracks move.
Snowmobiles also have a clutch system that is a type of pulley-based CVT (Continuously Variable Transmission). This system consists of two pulleys, or clutches, connected by a drive belt. The primary clutch sits on the engine crankshaft. A pressure spring holds the primary's two halves apart when the engine rpm is low. As the engine begins to accelerate, the clutch weights generate enough centrifugal force to close the clutch, allowing the belt to move freely and transmit power.
The secondary clutch connects to the track drive, which turns the wheels and powers the tracks. A spring in the secondary clutch operates the cams, which are sensitive to torque. As the engine's RPM increases and the primary transmits power, these cams squeeze together and tighten the belt. The process continues as the snowmobile accelerates. Once it reaches its top speed, the primary clutch closes, moving the belt into a higher "gear." Because the snowmobile needs less power to accelerate than it did to get going, the secondary clutch opens.
Criteria for selection of solution
Possible Designs:
I chose two possible designs, A and B. I chose idea B because over all its was a simplier, cost effective and a more sufficient design. I only have a certain amount of time to complete this project, so the simpliest design would be a logical choice. It only involved the use of 8 sprockets and 4 chains, rather than Design A that involved the use of 12 sprockets and 6 chains, which just caused unecassary complication of the rear propulsion system, making it awkward and cumbersome!
The chains and gears are also very expensive, costing E9.99 for a set of 670mm chain and E4.99 for the sprockets varing on sizes, so this was also an important factor to take into account.
After my research on the designs of Snow mobiles I found one similarity which was a basic frame, I began my project by producing a frame made of aluminium, this was to hold everything into place, so it was vital! Aluminium is a light weight but yet strong material, perfect for my design. As I talk about in the ‘Manufactoring Processes’ pg. 16, I drilled holes into the side of the frame at varies sizes, this was an imporovement form design A, as they are both aluminum frames, this is a small improvement on its design, but has a great effect on the performance of the vehicle.
I could have used aluminium as the body of my frame, but i wanted to make the vehicle as light as possible, so it can travel fast. I decided to use a polymer frame instead. Not only does it look better, but its a lighter material.
Finally, I will talk about the steering system I incorporated into my design, the Pantograph system, this involved the use of 5 small rectangular pieces of aluminium, of size {..}. If they are placed into the right position and the centre leg pivited as you can see in my drawings, they demonstrate the basic pantograph system.
These small improvements can change the performanceof the drastically.
Manufacturing process
Frame:
I chose to use aluminium as the material for my frame, it is a light but strong material, which will be a great feature, as it will have to hold the seating compartment and the electronics, but also I need my snowmobile to be lightweight, as it may have to travel fast on the snowy terrain. I began with a rectangular piece of aluminium of [……] in size. I marked out my bending lines. I drilled holes into the side of the frame, this is a positive aesthetic point but also helps in the design of the snowmobile, and these holes allow the snow to pass through as it travels on the snow. The use of light weight material and drilling of the side panel holes increases the power to weight ratio which will beneficial to the performance of the machine. I drilled four holes, two on each side for the axels. As I was using bevelled gears, I had to cut out a small rectangular piece from my frame to allow the bevelled gears to spin. I rounded the corners off with a file for aesthetic appeal and safety. I then bent the frame at 90 degrees on each side.
(PHOTO)
Axels:
The axels were very simple to design; I got two lengths of 4mm […] steel bar and threaded them with a die tool. I threaded them with a 4mm die.
(PHOTO)
Body:
The body of my snowmobile was made from … black acrylic. Originally, I had made a polymer body, from a thin sheet of red polymer. I formed a mould out of clay; I designed it aerodynamically so when it travelled there would be less drag. After, I placed a sheet of red polymer on to the vacuum former. It formed a great piece, with a lovely finish but unfortunately, it would not fit on to my frame correctly, so due to time constraints I made a simple acrylic piece. I got a rectangular piece of black acrylic of size […X…] and marked out bending lines. I also centre punched the markings for where I would be drilling the hole for the steering columns, for the bulb and L.E.D fittings. After completing this, I bent the acrylic pieces into a stylish angle; I found doing this it more appealing than a box shape.
(PHOTO)
Track:
In my design, I decided to use a similar design as a track but using chains instead. I bought the chain and then assembled them in the workshop, these work equally as well as the track system, but not as expensive. The chains move by the rotation of the gears on the rear axle spun by the motor, for this to work, I attached bevel gears on to the end of the motor and on to the axis. This allowed the chains to run smoothly.
(PHOTO)
Steering:
For my steering, I had thought of a couple of designs, but eventually used the simplest and most effective design, the Pantograph system. The Pantograph is made up of
Skis:
My skis were also made from black acrylic. I designed, marked and measured out the style and size of skis I wanted on to two rectangular pieces of acrylic using a scriber and ruler. I centre punched the middle of each ski; this is where I would slot the steering column connected to my Pantograph steering system. I created a small bend at the top of my ski on the heated bender for style and functionality.
(PHOTO)
Secondary Frame:
Initially I was not going to have a secondary frame, but I needed to make one, as I had not taken into count where the skis would be fitted. This Secondary frame allows the skis to fit perfectly into my design without hitting off the moving chains and sprockets. I cut a rectangular piece of aluminium of size [….] I marked, centre punched and drilled 2 holes, these would allow me to attach the secondary frame on to my initial frame. I then drilled another two holes; these would allow me to fit on the acrylic body and steering system for my snowmobile. I marked where it was to be bent with the scriber and right angle tool. I bent the sheet on the box pan bender.
(PHOTO)
Electronics
My project required:
· One Key Switch.
· 4 chains and 8 sprockets.
· One double poll double throw switch.
· One mm28 motors (one with gearbox)
· 2 fixed L.E.D’s