Analyses
The requirements for the hardtop are to:
Keep the profile of the modular hardtop as close to an original hard top profile as possible.
Have the capability to bolt a roof rack directly to the hard top with an option of going through the hardtop and bolting a roof rack to the roll cage.
Be able to handle a 150-200 lb. load (with a roof rack).
Have the tensile strength of the hardtop equal the tensile strength of an OEM hardtop.
Adapt existing windows for convenience.
Reduce noise level by 15%
Reduce heat loss by 20%
Be able to deconstruct the product into a storage space of 2.5 ft by 3 ft by 2 ft.
Be able to withstand water seeping through the connecting sections for a time period of 10 minutes of a constant supply of water.
Description of Analyses:
Analysis: (Design Issues, Calculated Parameters, Best Practices)
1:
Design Issue:
The Issue presented in the 1st Green Sheet was the design of a square tubular frame for the side panels that could handle a 400 lb. point load.
Calculated Parameters:
The parameters of the frame are that it must withstand a point load of 400 lb. With this value, the maximum shear and moment that would be applied can be found. The max. shear and moment values allow for the maximum stress, minimum stress, mean stress, alternating stress, and stress ratio to be calculated. The stress values give parameters to then find a material with material properties that suffice for the frame design.
Best Practices:
From the Machine Elements in Mechanical Design 6th Edition the material properties can be found in Table 15-15 for Hollow Tubing. The properties given are; the outside diameter, inside diameter, wall thickness, area, weight/ft, and section properties. These values are then applied in rougefab.com/tube-calculator to find materials that would work for the project. From the website, there were three suggestions for materials that should be used. The three materials are 7075-T6 Aluminum, DOM 1020, and 4130 N. Of these three, DOM 1020 will be used due to it being a material that would be easier to weld.
2:
Design Issue:
Bolt size needed for a shear force load of 400 pounds at the center of the panel where panels Top Front and Top Rear connect.
Calculated Parameters:
From calculating the moment of inertia, Q and using the shear force given, the shear flow can be calculated. This gives the shear force per nail in the spacing they are orientated in. With this force, the bolt size requirement can be found.
Best Practices:
Using 4140 annealed steel will allow you to use ¼” bolts. Depending on the bolts used, the size of the bolts can vary. The other option is to use more bolts which will allow you to use a smaller diameter bolt.
3:
Design Issue:
Calculating the critical load the side panel tubular frame can handle before it would buckle.
Calculated Parameters:
By calculating the radius of gyration, slenderness ratio, column constant and applying the fixidity of the beam structure you then have to choose the Johnson formula to calculate the critical load.
Best Practices:
With the value calculated there should not be a problem with the tubular frame buckling.
4:
Design Issue:
Calculating the angle of deflection at a load to then calculate the flexural rigidity of the composite layup.
Calculated Parameters:
By applying the equation for beam deflection under a 3-point load you can find the angle of deflection with a certain load applied.
Best Practices:
To get more practical values a test specimen will be made to verify the value and then apply it to the flexural rigidity equation.
5:
Design Issue:
Evaluating the things to consider when applying the lay-up process.
Calculated Parameters:
Layers Involved: (top to bottom)
1.Plastic 2. Breather Material 3. Teflon Paper 4. Peel Ply 5. Fiberglass layup (three layers if e-glass is used) in the orientation of 0 degrees, 45 degrees, -45 degrees. 6. Foam Core 7. Fiberglass layup (three layers if e-glass is used) in the orientation of 0 degrees, 45 degrees, -45 degrees. 8. Peel Ply 9. Teflon Paper 10. Breather Material 11. Plastic. Make sure there are spots to attach a vacuum system. Use sealant to seal the upper and lower plastic layers.
Best Practices:
Remove excess resin
The epoxy to resin ratio is; for every 1 oz. of fabric that is used, 2.5 oz. of resin should be used.
If E-Z Epoxy is used, a 24-hour cure time is needed at 77 degrees Fahrenheit.
6:
Design Issue:
Material Quantity
Calculated Parameters:
Fabric needed: Top Front: 7.8 yards, Top Rear: 10.2 yards, Rear: 1.4 yards, RH/LH: 8 yards Total: Approx. 27.4 yards (3 layers top and bottom for each part)
Epoxy Needed: 2.9 Gallons
Square Tubing: 180”
Best Practices:
1:2.5 Ratio for fabric to resin
7:
Design Issue:
Top Rear Panel bolt size requirement/number of bolts
Calculated Parameters:
From calculating the moment of inertia, Q and, using the shear force given, you can calculate the shear flow. This allows you to calculate the shear force per nail in the spacing they are orientated in. With this force you can then find the bolt size required.
Best Practices:
12” spacing. 5/32” Bolts. 3 Bolts on each side would be necessary for this size of bolt.
8:
Design Issue:
Proper adhesive for installing the windows.
Calculated Parameters:
Shear force of the glued area.
Best Practices:
Standard 3M 08693 Auto Glass Urethane Windshield Adhesive will suffice. The tensile property needed in this case is 22 PSI, the tensile properties of 3M is 1200 PSI.
9:
Design Issue:
Weight added on top of panels in storage.
Calculated Parameters:
Critical load that can be applied to a panel in storage.
Best Practices:
An overload of weight is unlikely.
10:
Design Issue:
Welded Joints in the side panel tubular structure.
Calculated Parameters:
Weld Thickness
Best Practices:
1/8-1/4” welds are recommended for the welds.
11:
Design Issue:
Heat loss of the hard top compared to the soft top on the Jeep currently.
Calculated Parameters:
Heat transfer with a soft top versus heat transfer with the modular hardtop.
Best Practices:
Use a infrared thermometer to measure the actual heat transfer through the materials.
12:
Design Issue:
Noise Reduction calculation compared to the soft top on the Jeep currently.
Calculated Parameters:
Db level of each top.
Best Practices:
Use a decibel reader.
Scope of Testing and Evaluation:
The scope of the testing and evaluation of the parts made/constructed can be found in the testing/methods section.
Optimizations:
One method used to optimize the strength of the side panels so that they could handle the load requirement was by welding the nuts onto the frame made for the side panels. By doing so, the foam core structure would make the side panels even stronger. Welding nuts onto the frame eliminated the need to make an access to both sides of the frame. While the strength of the square tubing is designed to handle the 200 lb load that would be applied to the roof of the hardtop, the steel frame does little to support a side load if someone were to step on the panel in storage. By using a foam core on the side panels, the panels become as strong as the other panels and they would be able to handle a 200 lb side load.
By testing for these requirements a few small issues were brought to light. One issue during the waterproof test was that there was a leak in the rear top corners of the side panels. This indicated that a seal in the corners was necessary. The assumption that the panels did not need a seal in the corners were made initially. This mistake was easily fixed with some seal material. The next issue was that, when adapting the original windows to the side panels, the windows only had an 1/8” clearance between the window and the panels. This made for a tight fix. With this tight fit, the windows needed more caulk to ensure a good adhesion between the panel and the window. Even though the windows fit, the realization that the measurements should have been checked earlier was made. This would have decreased the cost of the project by $18. The last issue that came to light was that a few of the bolts had slight alignment issues that increased the time to assemble the panels. While a time requirement was not made, this issue could affect whether someone would want to assemble/disassemble the panels themselves. Overall, the hardtop performed well and fulfilled the requirements stated above.
