To design a safe yet economic scissor jack for a Toyota 2015 Avalon XLE.
Background
The design approach for this project was to select a vehicle, select a material, design the arms, design the bracket, design the brace, design the power screw, and design the input crank. Calculations and iterations will be performed on MATLAB.
Design Process
The vehicle selected was Toyota’s 2015 Avalon XLE. Its mass and ground clearance have been given by Toyota. A maximum height for the jack was also predefined.
The material selected for this design was AISI 1020 Hot Rolled Steel. The jack is designed for use in emergency situations. Jacks for vehicles are not used daily. Therefore, stainless steel was not chosen. Instead, 1020 steel was chosen due to its availability. The tensile yield strength and the ultimate tensile strength can be obtained.
When designing the upper and lower arm/link, it is important to note that the maximum force occurs at the minimum angle and minimum height. The design approach for the arms of the scissor jack was to design the lower arm first. Because the upper arm would be slightly wider in width compared to the lower arm, it would be stronger than the lower arm. Thus, the larger dimensions have accounted for the forces it would withstand.
The length of the arm was set to 6.5 inches, where 𝜃𝑚𝑎𝑥 and 𝜃𝑚𝑖𝑛 represent the maximum and minimal angles. The maximum and minimal angles can be obtained.
The design must be focused on the minimum angle. If a force, F, were applied along each member of the link/arm, then the vertical load, 𝐿, would equal to 2𝐹𝑠𝑖𝑛𝜃 = 𝐿. As 𝜃 decreases, the vertical load, 𝐿, increases.
The following figure studies one link and shows the free body diagram of the arm.
Analyzing with static equilibrium, 𝐴𝑥, 𝐴𝑦, and 𝐵𝑥 can be found.
Dimensions and sizing can be determined. The figure below shows the dimensions required to be solved. As stated earlier, thicknesses are placed on the outside for CAD purposes. Thickness is highlighted below with grey lines.
The holes for the pins located in this arm must be accounted for when designing for stress. This is a Box-Channel design with holes. The maximum stress will be accounted for with the appropriate stress-concentration factor. Knowing this information, iterations are performed to solve for the dimensions of the lower arm using this derived equation:
The dimensions of the upper arm are calculated similarly to the lower arm. The dimensions of the pins, top and bottom plate, power screw, locking nut, and input crank are obtained using the same logic.
Testing and Evaluation
Tests were performed at minimum height (5.6 inches) and maximum height (12 inches). Note that the input crank handle was excluded from the FEA analysis.
Compression
The figures below show the compression FEA tests. The value of the weight was the load. The load was placed on the top plate facing downward while the bottom plate was fixed. For the stress analysis, majority of the analysis was in the blue. The pins appeared to be marked red, indicating the critical points of the design.
Even though the design focused around the minimum height and the minimum angle, the following figures showed FEA performed at the maximum height.
Tensile
A tensile FEA test was applied to the power screw. The horizontal force was applied to both ends of the screw. The results were the same as the compressive FEA test (which is expected), indicating the accuracy and reliability of the FEA.
Buckling
The figures below display the FEA Buckling Test. The design warps and rotates around the power screw.
Final Design
These dimensions differ from the calculations due to the fact that the calculations focused on what were the minimum dimensions in order for safe operation. Moreover, the calculation section overlooked the fitment of parts. Parts were expanded (instead of subtracting material) in order to ensure their necessary degrees of freedom were not obstructed by other pieces/parts. Therefore, with the different dimensions applied to the SolidWorks assembly, FEA gave different results compared to the calculations.
