Technical Foundation
What Is a Key Accessorie?
A machine key is a small metal piece inserted into matching keyways cut into a shaft and a mating hub (pulley, gear, coupling), preventing relative rotation between them while transmitting torque — a mechanical interlock rather than relying on friction alone. Square and rectangular keys are the most common general-purpose type, sized proportionally to shaft diameter per ANSI B17.1; woodruff keys are semicircular, self-aligning and commonly used on tapered or smaller shafts where a straight key might rock; gib head keys add a protruding head for easier removal with a puller on blind or hard-to-access keyways.
Key size must match both the shaft diameter (per standard proportion tables) and the keyway width/depth actually cut into the shaft and hub — an undersized key adds unwanted play, while an oversized key won't seat and can crack the hub during installation. Keyway broaches and cutters are used to machine new internal keyways into a bore (broaching) or shaft (milling), and key material (typically low-carbon steel, sometimes stainless for corrosive environments) should have adequate shear strength for the torque being transmitted, since the key is often the intentional 'weak link' or wear point in the drivetrain.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Key Type | Square, rectangular, woodruff, gib head, taper | Determines fit method, self-alignment, and ease of installation/removal for the specific shaft/hub design |
| Key Size (Width x Height) | Proportioned to shaft diameter per ANSI B17.1 | Must match both shaft diameter and the actual cut keyway dimensions, not just nominal shaft size |
| Key Length | Cut to fit the hub's keyway length, or stock keystock cut to size | Length affects torque transmission area — too short a key can shear under load |
| Material | Low-carbon steel (standard), stainless steel (corrosive service) | Must have adequate shear strength for the torque transmitted; often intentionally the drivetrain's weak point |
| Keyway Dimensions | Width, depth per standard for the mating shaft/hub | Broaches and cutters must be sized to produce a keyway matching the key's cross-section |
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Applications
Where Key Accessories Get Used
Machine keys and key accessories are specified anywhere a shaft needs to transmit torque to a mounted component without relying on friction alone.
Power Transmission & Pulley Assemblies
Square and rectangular keys locking pulleys, sprockets and gears to shafts in belt and chain drive systems.
Electric Motor & Coupling Installation
Keys and keyways transmitting torque between motor shafts and driven equipment through flexible couplings.
Machine Tool Manufacturing & Repair
Keyway broaching and precision key fitting during machine tool assembly and spindle repair.
Automotive & Small Engine Repair
Woodruff keys securing flywheels, pulleys and timing components to crankshafts on small engines.
Pump & Compressor Drive Assemblies
Machine keys locking impellers and drive components to pump and compressor shafts.
Custom Fabrication & Equipment Rebuilding
Keystock cut to custom length for one-off shaft-to-hub connections in fabrication and rebuild work.
FAQ
Key Accessories Questions, Answered
How do I determine the right key size for my shaft?
Key cross-section (width and height) is proportioned to shaft diameter per standard tables (ANSI B17.1 for inch sizing), but the key must also match the actual keyway width and depth already cut into the shaft and hub — measure the existing keyway directly rather than relying solely on shaft diameter, since keyway dimensions can vary from the standard on older or custom equipment.
What's the difference between a square key and a woodruff key?
A square (or rectangular) key is a straight, uniform bar seated in matching straight keyways on both shaft and hub, suited to general power transmission; a woodruff key is a semicircular disc that seats in a matching curved keyway machined into the shaft, which is self-aligning and commonly preferred on tapered shafts or smaller-diameter shafts where a straight key's rigidity could cause it to rock under load.
Why would a key be designed to shear intentionally?
In some drivetrains, the key is deliberately sized as the weakest link so that a sudden overload (like a jam) shears the key rather than damaging the more expensive shaft, hub, or gear — this is a form of built-in mechanical fuse, though it's a design decision specific to the equipment rather than a universal rule, so always check the equipment manufacturer's spec before assuming a key is meant to be sacrificial.
What's a gib head key and why does it have a protruding end?
A gib head key has an angled, protruding head at one end, giving a surface a puller or drift can grip to remove the key from a blind keyway (one that doesn't go all the way through the hub) — without the protruding head, a key seated flush in a blind keyway would be very difficult to extract without damaging the shaft or hub.
How do I cut a new keyway if my part doesn't have one?
Internal keyways (in a bore/hub) are typically cut with a keyway broach pushed or pulled through the bore in a broaching press or arbor press, while external keyways (on a shaft) are typically milled; both require selecting a broach or cutter sized to the target keyway width, and the process should follow standard proportion tables unless the application calls for a custom keyway size specified by an engineer.
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