1. FRC3096 Analysis of Frame Bending Impact on Chain Tension 2010 Kitbot Frame

2. Problem Statement Beginning at the end of the initial event, the chassis began throwing chains intermittently. Outside horizontal braces were added during the fix-it window. During the second event the problem became chronic. Outside diagonal braces were added during the event, but this did not impact the chain loss. Driving on the ramp in front of the goal and on the bump seemed to catalyze the failure.

3. Root Causing Used Fishbone process highlighted in Chris Fultz’s white paper Continuous Improvement Process for FIRST TeamsContinuous Improvement Process for FIRST Teams

4. Approach Frame flexing determined to be most likely cause. Do not iterate in hardware. –Expensive in material & time Use analytical techniques (FEA) to iterate Validate analytical baseline to physical results to date –Video –Teardown Use results from analysis for hardware Validate hardware

5. Analytical Technique Finite Element Analysis (FEA) –Apply loads to CAD geometry to calculate stresses and deflections –Autodesk Inventor was used –More info on FEA available at http://en.wikipedia.org/wiki/Finite_element_ method http://en.wikipedia.org/wiki/Finite_element_ method

6. FRC3096 Analysis Info Boundary Conditions –Held fixed at holes in center of each rail Load –300 lbf at axle in direction of normal vector of wheel bracket closed face Material –Wheel Bracket – 5052 Aluminum Yield Stress – 255 Mpa – Stress Image Conventions –White lines signify original position –Safety Factors <1 signify permanent bending

7. FRC3096 Wheel Bracket - Deflection

8. FRC3096 Wheel Bracket - Stress Peak stress

9. FRC3096 Wheel Bracket – Safety Factor Permanent Bending

10. FRC3096 Wheel Bracket w/ Horizontal Support Deflection

11. FRC3096 Wheel Bracket w/ Horizontal Support Stress Peak Stress

12. FRC3096 Wheel Bracket w/ Horizontal Support Safety Factor

13. FRC3096 Wheel Brackets w/ Horizontal & Diagonal Supports - Deflection

14. FRC3096 Wheel Brackets w/ Horizontal & Diagonal Supports - Stress Peak Stress

15. FRC3096 Wheel Brackets w/ Horizontal & Diagonal Supports – Safety Factor

16. FRC3096 Wheel Bracket w/ Horizontal Support – both sides - Deflection

17. FRC3096 Wheel Bracket w/ Horizontal Support – both sides - Stress Peak Stress Model is restrained here for analysis. This is a false peak called a singularity

18. FRC3096 Wheel Bracket w/ Horizontal Support – both sides – Safety Factor Singularity Functional Peak Stress

19. FRC3096 Wheel Brackets w/ Diag Supports – both sides - Deflection

20. FRC3096 Wheel Brackets w/ Diag Supports – both sides - Stress This is not focused on a point and is a real stress Yup, still here

21. FRC3096 Wheel Brackets w/ Diag Supports – both sides – Safety Factor

22. FRC3096 Wheel Brackets w/ Horizontal & Diagonal Supports – Both Sides - Deflection Peak displacement is no longer at axle location Axle displacement is much less

23. FRC3096 Wheel Brackets w/ Horizontal & Diagonal Supports – Both Sides - Stress Peak Stress is much lower

24. FRC3096 Wheel Brackets w/ Horizontal & Diagonal Supports – Both Sides – Safety Factor

25. Video Analysis Brick Wall Test –Start in contact with wall –Drive forward Top View –http://www.youtube.com/watch?v=BlOqd3tBavkhttp://www.youtube.com/watch?v=BlOqd3tBavk –Notice how wheel in upper portion of view ducks under Clippard tank when power is applied Side View –http://www.youtube.com/watch?v=WbxNld3Oaeohttp://www.youtube.com/watch?v=WbxNld3Oaeo –Slack that develops in chain as pushing starts –Clearance develops around sprocket as pushing continues until chain jumps off sprocket

26. Teardown Analysis Permanent bending where wheel riser attaches to frame rail

27. Teardown Analysis Permanent bending where riser attaches to frame rail

28. Conclusions FEA results correlate well with physical conditions. –Magnitude may be debatable, but direction and concentrations were correct –Magnitude is difficult to determine based upon momentary spikes in load due to impact with bumps or other robots. Decided to apply FEA solution of horizontal & diagonal braces to both sides of riser. –This effectively turns the sides of the frame into a truss Applied hardware survived numerous practice sessions and the MARC event without a chain failure.

29. Additional Info – FRC815 Riser Stress cracks at top bend of riser