| Parts infobox | |
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| Image | |
| designers | Timothy Schmidt |
| date | 2021 |
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| lifecycles | |
| tools | Saws, CNC Mills |
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| git | |
In mechanical engineering, a key is a machine element used to connect a rotating machine element to a shaft. The key prevents relative rotation between the two parts and may enable torque transmission. For a key to function, the shaft and rotating machine element must have a keyway and a keyseat, which is a slot and pocket in which the key fits. The whole system is called a keyed joint. A keyed joint may allow relative axial movement between the parts.
Commonly keyed components include gears, pulleys, shaft couplers, and washers.