ANALYSIS REPORT

Computer Aided Design

Fall 2016

Final Design Project

Vertical Loads on a bike frame

Shahoon Amin

Nicholas Ninan

I. Background:1. When studying real life 3D problems, it becomes necessary to analyze the situation

from multiple perspectives. The most commonly used method is Finite Element Analysis. Although the avenues available to do this are much higher than previous years, they are still limited, and program results cannot be directly compared to hand calculations in all cases. At best, Engineers can usually only compare their computer analysis with the results predicted by established mechanical formula. At worst, Engineers must rely on their experience and intuition to guide them towards a workable “right” answer.

2. One of the programs that can be used is PTC Creo Parametric, which is the program that we utilized for this study. Although Creo is not the most widely used software in the industry, neither is it the most efficient method to study 2D models and beams. But for the purposes of this report we will be using Creo to study a problem and the analysis related to it.

II. Problem Statement1. Vertical loads: When an adult rides the bike, the nominal load is estimated as a

downward load of 900N at the seat position and a load of 300N at the pedal crank location. When a dynamic environment is simulated using the static analysis, the static loads are often multiplied by a dynamic load factor, G. In this design project, use G = 2. Use ball-joint boundary conditions for the front dropout (location 1) and sliding boundary conditions for the rear dropouts (locations 5 and 6).

2. Choose aluminum tubes of various diameters for the various members of the frame such that the bicycle is as light as possible. The minimum outside diameter is 12mm and the wall thickness is 2mm. In addition to the dynamic load factor, use a safety factor of 1.5. For compression members, include buckling as additional criterion. Use a safety factor of 1.5 for buckling also.

3.

4.

Figure 1: 3D Bicycle frame

III. 2D Model

1.

Table 1: Key Points2.

Table 2: Connectivity Table

IV. Design

1. For our first round of analysis we created the entire bike frame using revolved line segments with the minimum cross section mentioned in the problem. That led to design issues as the tubes of the frame were either too short or too long to be assembled properly with the points. Due to this issue the model would not mesh correctly and so we decided that a new method needed to be used.

2. In order to correct this we created separate parts for each tube using a Revolve to create the rounded edge. We obtained help from another student to make the parts thin and hollow using Sweep and then Sweep surface instead of Solid Sweep. These parts were connected to form the bike frame and meshed together with intersect and trim. As per the problem statement Point one is the front dropout which is a ball-joint boundary, this was fixed with no degrees of freedom. Points 5 and 6 are the rear dropouts which are sliding joints, these were fixed with translation about the X-axis. For the material properties we selected Aluminum and input the values given from the Statement. The given yield strength was utilized to ensure that the model is valid in the elastic limit. A Static Analysis was run, The issue with this method is that it took hours to run a multi pass analysis. We tried running an optimization study but it took seven to eight hours to complete, and even after multiple runs convergence was not obtained for the optimization study. We visited Dr. Singh to obtain a solution to this issue and were told that this method would not lead to an optimization.

3. After visiting with Dr. Singh we determined that we would need to do a beam analysis to optimize the frame. Our first step was to create a point to point beam connection for the frame. Once this connection was complete we assigned material properties, loads and constraints as given by the problem statement. This beam study was not optimal because we cannot run optimization studies on ideal beams. After making educated guesses for the ideal cross section we were able to make an optimal frame with the allowable Von Mises stresses. The figures obtained of Von Misses Stresses due to the deflection are shown below.

V. Hand Calculations1. By using our Creo results we decided to do hand calculations on the beam connecting

key points 2 and 3. We simplified the problem by analyzing the tube as a cantilever beam with a force of -1200 N in the Y direction and 180N in the x direction at 500mm away from the fixed position. Since the Axial force is negligible we did not need to include it when doing the analysis.

2. The max bending moment at the fixed point of the cantilever was found to be 600Nm

3. The section modulus of the initial cross section of the beam is 136mm4

S= π (D4−d4)

32(D)=π (124−84)

32(12)=136.135mm4

4. The maximum principle stress in this beam is −4402.147MPa

σ=MS

=−600000136.135

=−4402.147MPa

5. NOTE: Since this is a very simplified calculation of the beam, the analytical calculations are significantly different from the true values. The calculations are more involved and extremely tedious so one must use a Matlab code to find the displacement and the max stresses in each beam. Dr. Singh gave us a Matlab code in order to find the max displacement for the beams but unfortunately we were not able to figure out how to use it to find the stresses in each beam.

VI. Steps of Simulation in PTC Creo for Initial Analysis1. We created datum points at key points of the part file so we can do an ideal beam

analysis.

2. According to the given problem, we created Ideal beams with the given minimum diameter and thickness. The beams were connected using the datum points created.

3. Once the beams were created, a point load of -1800N was applied to the seat position of the bicycle frame. The load was initially -900N but since a dynamic factor was given, the load was multiplied by 2.

4. A point load of -300N was applied to each of the pedal position of the bicycle frame. The load was initially -300N but since a dynamic factor was given, the load was multiplied by 2.

5. A ball joint boundary was applied using a fixed point constraint on Datum PNT0.

6. A sliding boundary was applied using a fixed point constraint on Datum PNT0.

7. Once the bike frame was created with the beams and the loads and constraints applied, a material was assigned to it and material properties changed to the proper values as defined by the problem.

8. A Multi pass analysis with a convergence of 5% was run on the model.

9. Results:a) Von Mises Fringe Model: This model indicates the load, constraints and the stress

on the bike. The maximum Von Misses stress is 262.531 MPA which occurs along the indicated position. This stress is greater than the yield strength of the material.

b) Maximum Deformation about the Y axis: This model shows the deformation of the model compared to its original shape. The maximum deformation along the Y axis is 5.42643 mm.

c) Mass of the initial design as per the problem statement. The mass of the frame given the initial dimensions is .569597 Kg.

VII. Redesign1. In order for the aluminum frame to have a safety factor of 1.5 and to use the Von

Misses failure stress criterion for yielding, we changed the outer diameter of the front section of the frame to 16mm.

2. A Multi pass analysis with a convergence of 5% was run on the model.

3. Results:a) Von Mises Fringe Model: This model shows that a safety factor of 1.5 is applied

on the model and the Von misses stress are below 140 MPA. The maximum Von Misses stress is 136.874 MPA which occurs along the indicated position.

b) Maximum Deformation about the Y axis: This model shows the deformation of the model compared to its original shape. We notice that the deformation has significantly been reduced due to the change in outer diameter. The maximum deformation in the Y axis is 2.28944 mm at the indicated tube of the frame.

c) Mass of the redesigned model: The mass of the new model is .64047 Kg.