Off the top of my head (having been elbow deep in OE50 changing out M60A1 final drives once or twice several decades ago). The double pair of toothed wheels are called Drive Sprockets. These move (drive) the tank treads upon which the Road Wheels ride when the tank is moving. In effect, the treads are a moving road for the tank to travel. Each Drive Sprocket is bolted to very thick specially crafted Drive Shaft. There are two Drive Shafts, one for each side of the tank. The Drive Shafts rotate at variable speeds which turn the tank per driver steering. The Drive Shafts are mechanically controlled by the Final Drive Transmission which is mechanically linked to the Driver Controls. Most Final Drive Transmissions are positioned inside the rear deck engine compartment underneath the tank's engine. Typically it is the Transmission which "fails". An automotive engineer is better suited to explain all the reasons that might happen. (I just rode 'em and told the gunner what to shoot at . . . . ). Modern tanks use hydraulics and electronics to enhance transmission robustness. These Drives can burn out as well as break. WW2 tank transmissions were mostly mechanical levers, sprockets, and linkages. Weak links or heat fatigued metal would break.
A tank cannot move when its Drive fails. A tank crew can change out a broken Drive Sprocket with help from an Armor Battalion's Maintenance Office mechanic, making field repairs possible. Fixing drive shafts and transmissions is supposed to be done by much more specialized mechanics at Division/Corps depot levels because you have to lift the engine completely out of the tank to access the transmission housings. You either haul the tank to a Rear Area for repairs or wait for the front to advance far enough that the Rear Area comes to you. Of all the things a tank crew doesn't want to suffer outside of combat, transmission failures are the most irksome.
