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

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.

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

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

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 method method

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

FRC3096 Wheel Bracket - Deflection

FRC3096 Wheel Bracket - Stress Peak stress

FRC3096 Wheel Bracket – Safety Factor Permanent Bending

FRC3096 Wheel Bracket w/ Horizontal Support Deflection

FRC3096 Wheel Bracket w/ Horizontal Support Stress Peak Stress

FRC3096 Wheel Bracket w/ Horizontal Support Safety Factor

FRC3096 Wheel Brackets w/ Horizontal & Diagonal Supports - Deflection

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

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

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

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

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

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

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

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

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

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

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

Video Analysis Brick Wall Test –Start in contact with wall –Drive forward Top View – –Notice how wheel in upper portion of view ducks under Clippard tank when power is applied Side View – –Slack that develops in chain as pushing starts –Clearance develops around sprocket as pushing continues until chain jumps off sprocket

Teardown Analysis Permanent bending where wheel riser attaches to frame rail

Teardown Analysis Permanent bending where riser attaches to frame rail

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.

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