Lesson 12.4: The Design Margins That Survive a Tournament
Technical Context
Margin allows the robot to keep working as battery voltage, game elements, alignment, temperature, and wear vary during an event.
The standards below turn that margin into values the team can check during design and testing.
The standards
| Standard | Value | What it protects against |
|---|---|---|
| Static safety factor on a load bearing mechanism | at least 2x | Acceleration, impact, a heavier element, a low battery |
| Sustained motor load | under 60% of stall torque | Heat; past 60% it is slow and cooking |
| Drivetrain limit | traction limited, not motor limited | Stalled motors draw full current and overheat |
| Voltage drop on a run | under 3% | Motors weakening exactly when working hardest |
| Weight budget reserve | 10% | Fasteners, wiring, and the second version of a part |
| Trials before calling something reliable | 25 or more | Intermittent failures invisible in three tries |
| Stage overlap on a slide at full extension | at least 60 mm | Binding and deflection at the worst position |
| Spares in the pit | one of anything that has broken before | Your own failure history is the packing list |
Why 2x and not 1.2x
Static torque is only what holds the mechanism still. Everything real adds to it:
- Accelerating the mechanism rather than holding it
- Catching it after a collision
- A game element heavier than the one on your bench
- A battery at 11.5 V instead of 12.6 V
- Friction that was not there when the mechanism was new
A safety factor of 2 covers those. A factor of 1.2 covers none of them, and it is exactly the number a spreadsheet produces when someone wants the answer to be yes.
A safety factor of 6 is not caution, it is a different mistake. Every unit of extra reduction slows the mechanism proportionally, so an arm with enormous margin takes seven seconds to raise and costs you the cycle. Aim for 2 to 3 and spend the rest on speed.
Schedule margin
Time. A mechanism finished the night before an event has never been tested at match weight, on a fresh battery, by the actual driver, twenty-five times.
The design margins above are worthless if the schedule has no margin. Finishing a week early and spending that week testing is worth more than any factor of safety.
Fill-in-the-Blank Practice
- Aim for a static safety factor of at least
__________. - Sustained motor load should stay under
__________percent of stall torque. - Reserve about
__________percent of the weight budget for unmodeled hardware and later revisions.
Show answers
- 2 (two times)
- 60
- 10
Exercise
Take your most loaded mechanism and compute its actual safety factor at the worst case. Then check its sustained load against 60% of stall. Write both numbers in the notebook, whatever they say.
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