Fasteners, threadlocker and things that come loose

An FTC robot has hundreds of screws, and a match is two and a half minutes of vibration, collisions and sudden stops. Every one of those screws is trying to undo itself. The robots that make it to the end of a long competition day are the ones whose builders treated fasteners as part of the design, not an afterthought.

Nylon Insert Lock Nuts - Vibration Resistant Nuts | Fasteners 101· Albany County FastenersWatch on YouTube

Reading a screw size

Most FTC hardware is metric. A metric thread is written as M, then the outside diameter in millimetres, then optionally the pitch (the distance from one thread to the next):

M4 x 0.7   →  4 mm diameter, 0.7 mm between threads
M3         →  3 mm diameter, standard pitch

The two build systems most FTC teams use standardise on one size each: REV uses M3 hardware and goBILDA uses M4. You will also see imperial sizes such as #6-32 on older or American parts. They do not mix with metric — a screw that "almost" fits will strip.

Heads and the tools that turn them

Head	What it looks like	Typical use
Socket head	Tall cylinder with a hex hole	General structure; strong drive
Button head	Low dome with a hex hole	Where a tall head would hit something; on plastic
Countersunk (flat)	Cone that sits flush	Surfaces that must be flat
Grub / set screw	No head at all	Set-screw hubs and collars
Hex head	Outside hexagon, turned with a nut driver	REV's standard M3 screw

The drive size is not the same as the thread size, so check the vendor's page. For example, goBILDA lists a 3 mm hex key for its M4 socket heads and 2.5 mm for its M4 button heads. REV's standard M3 screw has a 5.5 mm hex head driven with a 5.5 mm nut driver, and REV lists 2.5 mm, 2 mm and 1.5 mm keys for its M3 socket heads, button heads and set screws.

Colour-code your hex keys

Game Manual 0's suggestion: wrap each size of driver in a different colour of electrical tape. Grabbing the 2.5 mm key for a 3 mm socket is how heads get rounded out.

Nuts that stay on

Nylon-insert lock nut (nyloc): A nut with a nylon ring inside. The bolt has to cut its way through the nylon, and the friction stops the nut vibrating loose.

Game Manual 0's rule is simple: use nylon lock nuts wherever you use nuts. Plain hex nuts loosen under vibration. A few habits make them work:

Tighten firmly, not as hard as you can. REV's advice: firm will do — do not keep going until the screw cannot turn.
Enough screw to reach through the nylon. If the screw ends before the nylon ring, nothing is locking it.
They wear. A nyloc can be reused, but it locks a little less each time. If one spins on easily by hand, throw it away.

Threadlocker

Threadlocker: A liquid adhesive, such as Loctite, applied to threads. It cures between metal threads once the screw is tightened and stops the screw backing out under vibration.

There are two strengths you will use:

Blue (medium, removable) — holds through vibration but comes apart with normal tools. This is the one you want almost everywhere.
Red (high strength, permanent) — Game Manual 0's words: permanent, "and we mean it." Do not use it unless you never want the joint apart again.

The bottle is the wrong colour

Blue Loctite usually comes in a red bottle, and red Loctite in a blue one. "Blue" and "red" refer to the liquid. Read the label, not the bottle.

Where it helps most: motor and servo mounts, set screws, and anything that vibrates — the places Game Manual 0 specifically recommends it. Apply a small drop to the screw's threads, not to the hole, and let it cure before running the mechanism hard.

In the Lab with LOCTITE® - Proper Use of Threadlocker· LOCTITE® North AmericaWatch on YouTube

Never on polycarbonate

Threadlocker cracks polycarbonate

Do not use blue or red threadlocker on screws going into or through polycarbonate (Lexan) or other plastics. Game Manual 0 warns that it is known to make brittle plastics like polycarbonate crack. Henkel's own technical data sheets for Loctite 242 and 243 say the same: they are not recommended on plastics, particularly thermoplastics, because the plastic can stress crack. The crack often shows up later — a clear shield that was fine at the build session splits at the competition.

For plastic parts, use a nylon lock nut, or a screw with a nylon patch pre-applied to its threads, instead.

Rules that prevent most fastener failures

These come from Game Manual 0's hardware tips, and almost every new team breaks at least one of them:

Clear hole into thread. A screw should pass through a plain (unthreaded) hole in one part and into a threaded hole or nut. If both parts are threaded, the screw cannot pull them together.
Check the length. A screw that is too long hits the bottom of a threaded hole — or the bottom of a T-slot in extrusion — before the head is tight. It feels tight but the parts are loose.
Button heads, or a washer, on plastic. The edge of a socket head digs into plastic parts like servos. A button head or a washer spreads the load.
Avoid set screws where a clamping hub or hex bore will do. If you must use one, add blue threadlocker.
Throw away damaged screws. A screw with a rounded-out head or stripped thread goes in the bin, not back in the box.

Getting out a stripped screw

Try a fresh, non-ball-end hex key first — worn keys and ball ends slip. A rubber band between the key and the head adds grip. Failing that, cut a slot in the head with a hacksaw or rotary tool and use a flat screwdriver, or use a screw extractor.

Check before every match

Fasteners loosen gradually, so catch them before they fail. Make a short pre-match checklist of the screws that matter most — motor mounts, wheel hubs, collars, anything holding a mechanism to the frame — and have someone check them between matches. Some teams draw a paint-pen line across each critical screw head and the part beside it. If the line no longer lines up, the screw has turned.

Check yourself

You are screwing a polycarbonate guard onto your intake and want to make sure the screws do not vibrate loose. What should you use?

Check yourself

You are attaching a bracket to a T-slot extrusion. The screw stops turning, but the bracket still wobbles. What is the most likely problem?

Fasteners are where build systems differ most visibly. Next: REV and goBILDA side by side, and how to mix them.

Sources and further reading

Game Manual 0 — Fastener Guide (thread naming, nyloc nuts, threadlocker and the polycarbonate warning)
Game Manual 0 — Hardware Tips and Tricks
Henkel — LOCTITE 243 Technical Data Sheet and LOCTITE 242 Technical Data Sheet (plastics warning under General Information)
REV DUO — M3 Hardware and Tips and Tricks
goBILDA — M4 socket head screws and M4 button head screws