Wedge Keys

Wedge keys that sit in a recess in the shaft and hub are called sunk keys. This connection method is suitable for parts subject to strong rotational movement. Where high precision is not critical, wedge keys — a driven-in steel bar shaped like a wedge, with a 1:100 taper — are widely used in various mechanisms. The hub axis is displaced relative to the shaft axis by the amount of the fit clearance. If the hub is short, this causes the connected parts to skew, since the end face is not perpendicular to the shaft axis. A wedge key creates a strong, pre-stressed connection. The key’s main purpose is to provide a removable mounting for a gear, flywheel, or pulley on a shaft.

Wedge Keys

Individual fitting is difficult: the slope angle of the groove cut into the shaft component must match the angle of the key. The keys are therefore fitted by filing and scraping against a marking compound (bluing). Sunk wedge keys are more reliable than friction keys and flat (saddle) keys, and more practical to manufacture than tangential keys. They have wide working faces with clearances along the side faces, so during operation radial interference develops in the connection between the hub and the shaft, increasing the runout of the mounted part. For this reason, wedge keys have not become widely used in modern production. They cannot be used where high precision and critical connections are required.

Wedge keys that sit in a recess in the shaft and hub are called sunk keys. This connection method is suitable for parts subject to strong rotational movement.

For light loads, a flat key connection is used. To make disassembly easier, a sunk wedge key with a head (gib head key) is used.

The standard key is the main type. Grooves in the shaft are formed by disk milling, and grooves in the hub by end-mill broaching.

Wedge keys transmit torque through the friction force generated in the connection by the press-fit of the key, which creates working load stress.

Disadvantages:

• the keyway weakens the shaft and hub, so the shaft diameter and hub thickness must be increased;
• because centering of the wheel on the shaft is disrupted, oppositely positioned keys must be used;
• manufacturing complexity: grooves cut with an end mill require manual fitting, while grooves cut with a disk mill require securing the key with screws.

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