Fasteners and Hardware

Screws, nuts, shafts and bearings hold your robot together, and they take a beating in every match. Robot combat puts sudden shock loads on hardware, often sideways (shear) loads, which is very different from most machines. This page covers choosing and using fasteners, threadlocker, shafts and bearings. For parts names, see also NameThatPart.

Screws

Sizes

Small robots mostly use metric M2 to M4 screws (M3 is very common in 1 lb and 3 lb robots), or imperial #2-56 to #8-32. Larger robots use M5 and up. Pick one or two sizes for your whole robot if you can, so you need fewer tools and spares at events.

Head styles

Socket head cap screws are the strongest and easiest to tighten.
Button head screws are lower-profile but have a smaller hex socket, so they strip more easily and can't be tightened as much.
Flat head (countersunk) screws sit flush with the surface. Exposed screw heads are "spinner bait" – easy for a weapon to catch – so countersinking screws on outer armor is a good idea.[1] Note that imperial flat heads are usually 82° and metric ones 90°, so match your countersink. Don't countersink carbon fiber (see Armor and Materials).

Materials

Material	Notes
Alloy steel (e.g. class 12.9, usually black)	Very strong (about 1,200 MPa tensile strength) and cheap. Very hard screws can be less tolerant of shock loads than slightly softer grades.
Stainless steel	Tougher but weaker than alloy steel (about 700 MPa for common A2-70).
Titanium (Grade 5)	About 56% of the weight of steel and fairly strong, but expensive.
Aluminum	Very light but much weaker; only for low-load parts like covers.

Team Run Amok's advice is that medium-strength steel screws are usually the best choice, and that you can make up for lower strength by using a few more screws.[2]

Threads in different materials

Aluminum and other metals: tapped threads work well. As a rule of thumb, a screw should engage at least about 1× its diameter in steel and 2× in aluminum.[3]
UHMW and HDPE: don't tap fine machine threads into polyethylene – they strip easily. Use coarse self-tapping or Plastite-style screws in a correctly sized pilot hole. The plastic grips the threads tightly, so they don't vibrate loose.[4]
3D printed parts: use heat-set inserts (brass threaded inserts melted in with a soldering iron set about 10–20 °C above the plastic's printing temperature), or nuts captured in pockets. Check the insert maker's recommended hole size.[5] See 3D Printing.

Nuts and threadlocker

Nyloc (nylon-insert lock) nuts resist vibrating loose and can be reused a few times.
Threadlocker keeps screws in metal from loosening:
- Blue (medium strength, e.g. Loctite 243 or 242) is the usual choice – removable with hand tools.
- Red (high strength, e.g. Loctite 271) needs heat (around 250 °C) to remove, so save it for parts you never want to take apart.
- Purple (low strength, e.g. Loctite 222) suits small screws.
- Standard threadlockers only cure between metal parts, and can crack plastics – Loctite's own data sheet doesn't recommend 243 on plastics.[6] For plastic parts, use a plastic-compatible product such as Loctite 425 or Vibra-Tite VC-3, or a nyloc nut.

Shafts and bearings

Weapon shafts take enormous loads. Use a solid shaft of tough steel, supported at both ends – standoffs are just spacers and aren't strong enough. Many small spinners use a high-grade steel bolt or shoulder bolt as a dead shaft (the shaft stays still and the weapon spins on bearings around it).[7][8]
With a dead shaft, ball bearings with a spacer tube between their inner races let you clamp the shaft tight, so it becomes part of the frame. Add a strong spacer block between the frame plates to take the clamping force.
Bronze (oilite) bushings have a little more friction than ball bearings but survive huge shock loads. Space bearings well apart for stability.
Shaft collars stop shafts sliding; clamping collars hold much better than set-screw ones. The tiny set screws supplied with small hubs often aren't up to combat loads (see Wheels).
Support drive motor shafts too (see Drive Systems).

Where to buy

McMaster-Carr – huge range, fast shipping, and free CAD models of most parts.
Bolt Depot and Fastenal – fasteners in small and large quantities.
Repeat Robotics and FingerTech – small hardware, shafts, bearings and parts made for combat robots.
See also Suppliers.

Fasteners and Hardware

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