bb
CAD LEVEL 3 ENGINEERING DIPLOMA UNIT 2/3 TASK FOR APPLICATIONS OF COMPUTER AIDED DESIGN. LO.1 LO.2 LO.3 LO.4
RUN DATA FROM BLOODHOUNDSSC ON FIRST FULL REHEAT TEST RUN.
The CAD drawings on the BLOODHOUNDSSC website show all the major parts of the car and will be used to isolate any run problems so modifications can be carried out.
The EJ200 fuel tank is held in place by twelve M20x125 stainless steel bolts, washers and nuts giving all round fixing.
These are used to attach the webs on the tank to brackets fitted to the chassis. The direct mounting of the tank helps with some stiffness across chassis rails.
Scenario 1.
The car is run for several passes on a UK runway to test the systems are working correctly when the EJ200 run at relatively low speeds. The car is taken back to the workshop for an appraisal and check of the effects of the runs. The team find that 10 of the 12 fixing bolts for the tank have significantly loosened. They were correctly tightened during the assembly process. The tank support brackets are found to be unsuitable and a new design will be needed. A decision also had to be made about the type of fastening used to hold the tank in place before the car went for faster runs.
The bracket design showed below has proved to have insufficient stiffness as the three sided rails flex in action thus loosening the fastenings.
TASKS
Research the type of fixings that could be used to hold the tank in place in all conditions. Choose a suitable fastener. You must make a case for your choice with an accurate specification. There are some types below to get you started:-
Change to studs
Use spring washers
Use interior shake-proof washers
Use nyloc nuts
Create a 3D CAD drawing of all parts of your fixing to show why it will be secure. Your drawing should show full detail and be rendered.
Engineers work in teams in the BLOODHOUNDSSC project. Threads and bolt sizes are notoriously unreliable in CAD software as many of the drawings were created in Imperial in the US and then converted in Europe. The dimensions are averaged by the conversion software and can be out by significant factors in pitch and diameters. To ensure you are able to work accurately work in teams of 3 and create a library of hex bolts and nuts from 3- 10mmx 125 that you can share. You may work off your 20mm drawing.
Cap headed HT bolts are often used with Torx or Spline drives on high performance vehicles. Draw 3 bolts of your choice, size, thread type and thread length with a cap head and suitable drive socket.
Redesign the bracket to give a solid fixing with adjustment capability. Draw your design in a 3D CAD package to demonstrate how you have solved the problem.
SUGGESTED ANSWER
The Nordlock Washer System has a pair of washers with an interlocking step on one side and a milled upper surface.
The thread pitch is less than the gradient of the two piece washer making it very unlikely to work loose.
The Aerotight nut has a slotted ridge that is 0.2mm smaller than the thread diameter below it. When the nut is tightened the slotted ridge is forced out gripping the bolt thread very tightly. The larger the thread diameter , the more effective the grip.
Thread Size Nom. / Pitch mm / A/F Dia. Max mm / N Dia. Max mm / H Max mm / F Min. mmM 2.5 / 0.45 / 5.0 / 4.75 / 2.97 / 1.49
M 3 / 0.5 / 5.5 / 5.25 / 3.52 / 1.89
M 3.5 / 0.65 / 7.0 / 6.65 / 4.79 / 2.69
M 4 / 0.7 / 7.0 / 6.65 / 4.79 / 2.69
M 5 / 0.8 / 8.0 / 7.65 / 6.02 / 3.49
M 6 / 1.0 / 10.0 / 9.58 / 7.95 / 4.63
M 8 / 1.25 / 13.0 / 12.53 / 10.00 / 6.12
M 10 / 1.5 / 17.0 / 16.53 / 12.03 / 7.62
M 12 / 1.75 / 19.0 / 18.47 / 14.48 / 9.62
M 16 / 2.0 / 24.0 / 23.47 / 18.75 / 12.62
M 20 / 2.5 / 30.0 / 29.47 / 22.35 / 15.62
A 3D PRINTED BRACKET DESIGN SOLUTION FROM A CAD PROGRAMME AT 0.6 SCALE
Gerry Heather
November 2010