Gears require a lot of precision, even if you’re only mating two of them together. If the gears are even a millimeter too far apart or too close together, they may either slip or bind.
Flexible power trains allow mechanical power or movement to be transferred from one place to another using some form of bendable material, and so they tend to be easier to implement.
Akin to the gear are pulleys, belts, sprockets, and roller chains. Pulleys are used with belts, and sprockets are used with roller chain. The pulley and sprocket are functionally identical to the gear. The only difference is that pulleys and sprockets use belts and roller chain, respectively, to transfer power. With gears, power is transferred directly.
A benefit of using pulleys-belts or sprockets-chain is that you don’t need to be as concerned with the absolute alignment of the mechanical parts of your robot. When using gears you must mount them with high precision. Accuracies to the hundredths of an inch (and better) are desirable to avoid “slop” in the gears as well as the inverse—binding caused by gears that are meshing too tightly. Belts and roller chain are designed to allow for slack; in fact, if there’s no slack you run the risk of breaking the chain!
Pulleys come in a variety of shapes and sizes. You’re probably familiar with the pulleys and belts used in automotive applications. These are likely to be too bulky and heavy to be used with a robot. Instead, look for smaller and lighter pulleys and belts used for copiers, fax machines, VCRs, and other electronic equipment. These are available for salvage from whole units or in bits and pieces from surplus outlets.
Pulleys can be either flat, V type (the pulley wheel has a V-shaped groove in it), or the cog type. Cog pulleys require matching belts. With these you need to ensure that the belt is not only the proper width for the pulley you are using but also has the same cog pitch.
Sprockets and roller chain are preferred when you want to ensure synchronism. For large robots you can use 3/8" bicycle chain. Most smaller robots will do fine with 1/4" roller chain, which can frequently be found in surplus stores.
Metal roller chain is commonly available in preset lengths, though you can sometimes shorten or lengthen the chain by adding or removing links. Plastic roller chain, while not as strong, can be adjusted more easily by using snap-on links.