KEY

551 522 81 SPLINE is usually what keeps a gear, a pulley, or a coupling from spinning freely on a shaft. Think of it as the mechanical lock that makes sure when the shaft rotates, the component mounted on it rotates right along with it. It fits snugly into a keyway on both parts, transmitting torque directly. Without it, you'd just have a shaft turning inside a loose hub, which obviously isn't going to get much done on a drill rig.

When this little guy starts to give up the ghost, you'll usually notice some slop developing between the shaft and whatever it's supposed to be driving. At first, it might just be a bit of inconsistent power delivery, but pretty soon, you'll start hearing a distinct clicking or even a grinding noise, especially when the load changes or you're trying to put some serious torque through it. If it shears completely, well, then you've got a free-spinning shaft and a stationary component, and your rig's not doing much of anything.

When you're replacing one of these, don't just swap it out and call it a day. Make sure you thoroughly inspect both the shaft's keyway and the keyway in the mating component – the gear, the coupling, whatever it is. Often, if the KEY failed, those keyways took a beating too, and if they're chewed up, your new KEY won't last. A common oversight is not cleaning out the keyways properly before fitting the new KEY; any burrs or debris will cause issues down the line.