VEX gears come in 2 types: standard/normal strength and high-strength. As shown in the diagram below, high-strength gears have number of features that make them an improvement over the “standard” (thinner) gears.

• High-strength gears are about twice as thick as standard gears, meaning they are unlikely to crack, and the teeth are less likely to break off. (Everything does have its limits, however; high-strength gears do not come with magic included.)
• The center of the high-strength gear, out of the box, is just a big square hole, and the user chooses which of 2 inserts to put in the middle (or no insert, in the case of high-strength shafts):
  • one insert (not shown) makes the center a round hole in which a shaft can spin freely; this is useful for idler gears whose shafts are not driving a mechanism, but rather just there to hold a gear;
  • the other, shown in the image above, is a metal insert that has a square hole that a standard shaft then fits through. These metal inserts are really snug; sometimes they need to be wedged in with a screwdriver, pliers, etc., but once they're in, they're not going anywhere, and they're not going to strip. One insert goes on each face of the gear.
• The smallest (12-tooth) gear is solid metal, and is not going to crack, even under duress.

For a competition robot, high-strength gears are recommended for their reliability and durability.