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Mechanical, Build & Pneumatics·Lesson 40 of 47

Gearboxes That Grenade and Fasteners That Vibrate Loose

Prevent stripped gears, bent mounting plates, sheared shafts, and loose hardware: shock-load math, plate stiffness, set-screw failures, and threadlocker.

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Failure 1: Stripped or chipped gear teeth

Gears die from shock loading, not steady torque. Slamming a drivetrain from full speed to reverse, or a mechanism crashing into a hard stop at speed, spikes torque far beyond nominal. Fixes: add motion profiling and slew limiting so torque rises smoothly; set motor current limits (40 A is a common drive value) so the gearbox never sees stall torque repeatedly; and add a slip clutch on mechanisms prone to crashing.

Failure 2: Bent mounting plates

A common gearbox mounting problem is insufficient structural support. The force to lift an elevator can be large, if it tries to pull the motor output shaft and a driven shaft together, it can bend a thin aluminum plate, throwing off gear mesh and chewing teeth. Fix: use both plates of a gearbox (don't cantilever a single plate), add standoffs/spacers between plates to maintain center distance under load, and gusset the mount to the frame.

Failure 3: Sheared or spun shafts

Set screws on round shafts back out and the shaft spins inside the gear/sprocket. Prefer hex shaft (1/2 in hex is standard; pair it with a heavy-duty hex bearing such as AndyMark am-2986, 1.125 in OD) so torque transmits through the flats, not a set screw. Where you must clamp, use clamping collars and a flat or knurl.

Failure 4: Fasteners vibrating loose

FRC robots are vibration machines; unsecured bolts walk out over a match. Defenses:

  • Nylock nuts wherever a nut is accessible (good for vibration but heat-limited).
  • Blue (medium-strength) threadlocker on threads that don't need frequent service; it's removable with hand tools.
  • Avoid permanent (red) threadlocker on anything you'll service.
  • Correct torque, an under-torqued bolt loosens, an over-torqued one strips aluminum threads. Use thread inserts (heli-coils) in aluminum for high-cycle fasteners.

Diagnostic workflow when something keeps breaking

  1. Inspect the failed part: stripped teeth = shock load; bent plate = stiffness; spun bore = set-screw shaft.
  2. Quantify the load path; is anything crashing into a hard stop at speed?
  3. Apply the matching fix (profiling/current limit, plate stiffening, hex shaft, locking hardware).
  4. Run 50+ cycles on the practice bot and re-inspect before trusting it in a match.

Pre-match habit

Do a fastener audit before every match: a quick wrench check on drivetrain, gearbox, and superstructure bolts catches the loose one before it falls out on the field.

Key takeaways

  • Gears strip from shock loads, not steady torque; add motion profiling, slew limiting, current limits, and slip clutches.
  • Support gearboxes on both plates with spacers to hold center distance, a thin cantilevered plate bends and ruins gear mesh.
  • Use hex shaft instead of set-screw round shaft, and lock fasteners with nylocks or blue threadlocker, then audit bolts before every match.

Go deeper

Lesson quiz

Required

Answer all 3 questions correctly to complete this lesson.

1.Which threadlocker characteristic correctly distinguishes medium-strength blue (e.g., Loctite 242/243) from high-strength red (e.g., Loctite 271)?

2.Why are nylon-insert lock (nylock) nuts a poor choice for a fastener that sits right next to a high-temperature heat source?

3.A spur-gear gearbox keeps stripping teeth under load. Which design/assembly factor most directly causes gear teeth to grenade?

Answer every question to submit.