FRC bolts back out from vibration, not neglect. Here's which Loctite grade goes where, nylock vs jam nuts, real torque specs, and the pre-match fastener check that catches it.
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A bolt that backs out on your robot almost never does it because someone forgot to tighten it. It does it because the joint sees thousands of tiny repeated slips, from chain and belt tension pulsing, motors spinning at thousands of RPM, drivetrain impacts, and defense, and pure friction between threads can't survive that forever. The fix is the right thread locker for the joint, the right nut, and a check routine that catches a backing-out bolt before it ends your match.
A threaded fastener holds because friction between the threads resists rotation, and that friction comes from clamping force, which isn't constant. Every time the joint sees a shock, like a wheel landing after a field seam, a chain snapping taut, a gear tooth meshing, the surfaces under the bolt head separate by a microscopic amount and slide against each other. Each cycle lets the bolt rotate a fraction of a degree in the loosening direction. One cycle does nothing. Thousands of them over a match, a practice day, and a season add up to a bolt that's visibly backed out, or a nut spinning freely on a shaft that used to be solid.
That's why the parts that loosen on FRC robots aren't random. They're the parts closest to a vibration source: swerve module mounting bolts, wheel axle screws, gearbox covers, shaft set screws, and pivot bolts on intakes and shooters that cycle every match. A frame rail bolted to another frame rail, sitting under static load, almost never has this problem. A joint next to a spinning motor or a chain does.
Loctite and equivalent anaerobic thread lockers come in color-coded strength grades, and picking the wrong one is either useless or turns a five-minute repair into a drilled-out screw. Loctite's own guidance describes blue as removable with normal hand tools and red as intended to be permanent, needing localized heat around 500°F to break loose. Purple and green fill in around those two:
| Grade | Color | Strength | Removable? | Good FRC use |
|---|---|---|---|---|
| 222 | Purple | Low | Yes, by hand | Small screws you expect to adjust often, like encoder or limit switch mounts |
| 242 / 243 | Blue | Medium | Yes, with hand tools | The default for FRC: gearbox screws, module bolts, standoffs, anything you might service mid-season |
| 271 / 277 | Red | High | Only with heat, ~500°F | Rarely needed on a robot you'll be rebuilding constantly; save it for joints you truly never plan to revisit |
| 290 | Green | Medium-high, wicking | Limited | Applied after assembly on a fastener you already torqued and forgot to lock, since it wicks into the seated threads by capillary action |
Blue is the workhorse. REV's MAXSwerve assembly instructions recommend Loctite Blue 242 (or equivalent) on the encoder, bearing retention, fork mounting, wheel axle, and drive spur gear screws, and call for a star-tightening pattern plus a full 24-hour cure on the steering drive screws specifically. Even the manufacturer treats some bolts as more critical than others.
One warning that matters a lot in FRC: standard blue and red thread lockers attack polycarbonate. The methacrylate chemistry is a known stress-cracking agent for polycarbonate and similar plastics, and manufacturer literature says to test compatibility before use on those materials. That matters because a lot of robots carry polycarbonate somewhere, chassis panels, electronics board covers, spacers cut from Lexan. For any fastener that touches poly, use a plastic-safe formula like Loctite 425, or skip chemical thread locker entirely.
That's also why a growing number of FRC teams use Vibra-Tite VC-3 instead of Loctite on swerve modules. It isn't an anaerobic adhesive, it's a resin coating that stays flexible rather than curing hard, so it can be removed and retightened without reapplying, and it's safe on plastics. WCP's maintenance documentation for the Swerve X2 and X2S modules specifies Vibra-Tite VC-3 on every bolt in the module, both at first assembly and at rebuild.
These solve the same problem, self-loosening under vibration, in different places on the robot.
A nylock nut (nylon-insert lock nut) has a ring of nylon molded into the top. The bolt cuts its own thread through that nylon as it's driven in, and the nylon's elastic grip on the bolt resists backing out, independent of clamping force. That makes nylocks the right default for most bolted joints: standoffs, plate-to-plate connections, anywhere a bolt goes through a clearance hole into an accessible nut. The catch is that the insert wears a little every reuse. Most engineering guidance treats a nylock as good for only a handful of reuses on a non-critical joint, and as effectively single-use on anything safety-critical. A quick field test: if you can spin the nut past the nylon with just your fingers, it's done its job for the last time.
A jam nut is a thinner secondary nut run down against a primary nut to lock the assembly by friction between the two nuts rather than the thread itself. In FRC this shows up on threaded rod and turnbuckle-style adjusters: chain and belt tensioners built from all-thread, adjustable linkage rod ends, anywhere you're fixing the position of a threaded shaft rather than clamping two plates together. Run the thin jam nut on first and snug it to roughly a quarter to half of final torque, then run the thick nut down against it while holding the jam nut still with a second wrench. Do it backwards, thick nut first, and the jam nut does almost nothing.
Use a nylock for a normal bolt-through-plate joint where you want a mechanical lock that doesn't depend on chemistry. Use a jam nut to lock the position of threaded rod or an adjuster. Thread locker works alongside either one, or on its own when there's no accessible nut, like a screw threading directly into a tapped hole in aluminum.
Most FRC hardware is small, #6-32 up to 1/4-20, or M3 to M6, and it's easier to strip a thread or shear a screw head than to under-tighten. The general rule: thread locker resists vibration, not brute torque. Get the joint fully seated, snug it down, then stop. Don't chase "tighter" once resistance starts ramping up fast.
For joints where it actually matters, use the numbers the manufacturer publishes instead of guessing. REV's MAXSwerve assembly instructions call for roughly 15 to 19 inch-pounds on the fork and wheel axle screws, with a note not to overtighten. CTRE sells preset torque wrenches for their own hardware, 0.6 Nm for CAN terminal screws and 0.9 Nm for power terminal screws on Kraken, Minion, and Talon FXS devices, useful if you've chased a flaky CAN bus connection back to an undertightened lead. WCP sells a similar wrench for #10-32 structural bolts at 2.5 Nm, about 22 inch-pounds. Without a published number, "hand-tight plus a modest additional turn with a driver" is the realistic default, not a full-arm crank.
WCP's maintenance guidance for its swerve modules is blunt about frequency: check the bolts periodically during practice, and after every match at events, using a printed checklist rather than trusting memory. Copy that model for the whole robot, not just the drivetrain.
Build a short list of the fasteners that actually see vibration, not every bolt on the robot:
Check by hand, not by eye. A bolt backed out a sixteenth of a turn looks identical from three feet away. Put a wrench or driver on it and feel whether it moves. A paint-pen line across the bolt head and onto the adjacent part, drawn right after final torque, turns this into a two-second visual check later: if the line is broken, the bolt moved.
This article is AI-assisted: drafted from primary sources, then reviewed and edited by hand. Errors still get through. When one is reported we fix it and write down what changed — publicly, in the corrections log.
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