Servo - December 2009

frame while adding the o-ring belt.

Now all that remained was to

cram in all the electronics and bolt on the top.

It’s done! It’s combat-ready and even under weight! SV

PARTS IS PARTS:

Screws For Y u!

● Original Text by Professor Marco Antonio Meggiolaro; Summarized by Kevin M. Berry

For this month’s Parts Is Parts ection, we dip once again into the bottomless well that is known as the RioBotz Combot Tutorial. Professor Marco Antonio Meggiolaro authored the tutorial. His book is available free for download at www.riobotz.com. br/en/ tutorial.html. All information here is provided courtesy of Professor Meggiolaro and RioBotz.

Screws are joining elements which have helical threads around their perimeter. They are used in countless applications to apply forces, to fasten joints, to transmit power (in worm gears), or to generate linear motion. The helical threads (in general, wrapped according to the right hand rule) are inclined planes that convert the applied torques in the screws into axial forces. The main types of screws are presented in Figure 1.

Screws used in a robot structure should have hex a (hexagonal) or Allen head because they allow the highest tightening torques. Screws used in the electronics can be flathead or Phillips types.

• Hex head — Easily tightened with open-ended wrenches. Always use the 8. 8 or 10. 9 class types (made out of hardened steel); they have twice the strength of regular (mild steel) screws. Stainless steel screws have higher strength than mild steel ones, but much lower strength than hardened steel screws. They should not be used in structural parts (besides, they are much more expensive).

• Allen — The highest strength screws. Use the 12. 9 or 10. 9 class types (made out of hardened alloy steel), as they have three times

the strength of regular screws.

Despite their higher impact toughness, don’t use stainless steel screws. Their low yield strength will let them bend easily during combat, making it difficult to disassemble the robot. Stainless steel Allen bolt heads are also easier to strip than hardened steel ones. Figure 1 shows the button, standard, and flathead (flush head) types. The flathead types are good for thick plates used in the robot’s exterior because they are embedded flush to the plate surface so have less chance of being knocked off by spinners.

Avoid using flathead screws to fasten thin sheets. In this case, the button head ones should be used; they also work well against spinners. Flathead screws require that the plates are countersunk, which reduces joint strength. As a general rule, at least 0.5 mm (0.02 in) of the plate thickness should not be countersunk.