A key, in terms of mechanical engineering, is an element used to connect a shaft to a rotating machine piece. The key prohibits relative rotation between the two, which allows torque to be placed upon the rotating piece. In order to work, both the shaft and rotating piece must have a keyway or keyseat — a slot for the key to fit into. A scotch key is a type of keyway used to make sure a connection is possible.
Put very simply, a scotch key is drilled axially into a shaft to fit a key into. Essentially, a scotch key is a very easy keyway to make, in that it just needs to be drilled into the shaft, lengthwise, with the rotating element matched to the placement of the keyway, and a key, just as a bolt, placed through the rotating piece, into the keyway and secured in place.
Using a Scotch Key
Because of the simple nature of a scotch key, they can only be used at the end of an axle. Scotch keys are very basic, and as such are often used as a makeshift attempt at securing the rotational element in place. Generally a round key is then placed into the axial hole, as drills themselves create round passages. The major problem with a scotch key is finding a replacement part, as often the do-it-yourself nature of drilling a hole can make it difficult to match.
Creating a Scotch Key
A simple way to a scotch key is to fit the end of a shaft flush to a hub (the rotating piece). Place the end of a drill at the point of the hub precisely above the center of the shaft, and drill through the hub and into the shaft. The depth of the hole depends on the size of both the hub and the shaft, but needs to go far enough that an inserted key can secure the hub in place. Tap out the hole and use a screw or a bolt to secure the shaft and hub in place.
Other Types of Keys
In addition to scotch keys, mechanical engineers use a few other methods to secure rotational parts. The most widely used is the parallel key, in which a cross section of the gear or shaft is cut out for the attaching element to fit into. Woodruff keys, or half-moon keys, use a semi-circular pocket and are utilized for internal couplings on high-speed machines to help prevent wear. Tapered keys have a wedge that engages the hub in place.