Technical Foundation
What Is a Machine Key?
A machine key is a precision-fit piece of steel that sits in a matching slot (keyway) cut into both a shaft and the hub of a pulley, gear, coupling or sprocket mounted on it, transmitting torque directly through the key's shear strength rather than relying on friction between the shaft and hub alone. This is the standard method for connecting rotating power transmission components wherever torque loads exceed what a friction (set-screw only) connection could reliably hold without slipping.
Square and rectangular keys are the most common profiles, sized by standard tables (imperial per ANSI B17.1, metric per DIN 6885) directly against shaft diameter, so the correct key size is generally determined by the shaft size rather than chosen independently. Woodruff keys use a semicircular profile that seats in a matching curved keyway, offering better self-alignment on tapered shafts and easier assembly/disassembly, common on smaller shaft diameters and light-duty applications. Key material and hardness must be adequate for the transmitted torque — an undersized or under-hardened key can shear under load, which is by design a safer failure mode than damaging the shaft or hub in some applications, since keys are comparatively inexpensive to replace.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Key Profile | Square, rectangular, Woodruff, gib head | Determines keyway shape required and suitability for parallel vs. tapered shaft mounting |
| Size Standard | ANSI B17.1 (imperial), DIN 6885 (metric) | Standard key size is determined directly by shaft diameter per published tables |
| Material | Low-carbon steel, alloy steel | Must be adequate for the transmitted torque; keys are often intentionally the sacrificial weak link |
| Key Length | Cut to length or sold as bulk key stock | Must match the hub's keyway length exactly, typically shorter than the full shaft keyway to allow assembly clearance |
| Fit Tolerance | Class 1 (clearance) to Class 3 (interference) | Determines how tightly the key fits the keyway — affects vibration resistance and ease of assembly |
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Applications
Where Machine Key Get Used
Machine keys are specified anywhere torque must be reliably transmitted between a shaft and a mounted hub component without relying on friction alone.
Power Transmission Assemblies
Connecting pulleys, sprockets and couplings to shafts in belt and chain drive systems.
Gearbox & Motor Coupling
Torque transmission between motor shafts and connected gearboxes or driven equipment.
Pump & Fan Drive Assemblies
Securing impellers and fan wheels to drive shafts against the torque loads of fluid or air handling.
Conveyor Systems
Keying drive pulleys and sprockets to head and tail shafts throughout conveyor drive trains.
Machine Tool Spindles & Drives
Keyed connections in gear trains and drive components within machine tool power transmission paths.
General Fabrication & Repair
Replacement keys for maintenance and rebuild of keyed shaft-hub connections across MRO applications.
FAQ
Machine Key Questions, Answered
How do I determine the correct key size for a given shaft diameter?
Standard key sizing tables (ANSI B17.1 for imperial, DIN 6885 for metric) specify the key cross-section directly against shaft diameter ranges — for example, a 1 inch shaft has a standard corresponding square key size per the table. This means key size is generally not an independent design choice; it follows directly from the shaft diameter unless a non-standard application specifically calls for something else.
What's the difference between a square key and a Woodruff key?
A square (or rectangular) key has a straight profile fitting a straight keyway, suited to parallel shafts and generally higher torque capacity for its size. A Woodruff key has a semicircular profile seated in a matching curved pocket, which self-aligns better on tapered shaft ends and is easier to assemble and remove — common on smaller-diameter, lighter-duty shafts, particularly tapered ones like some motor shafts.
Why would a key be designed to shear under overload?
In some applications, the key is intentionally the weakest link in the torque path so that a sudden overload (a jam, a stall, a shock load) shears the key rather than damaging the more expensive and harder-to-replace shaft or hub. This is a deliberate design choice in some drivetrains, though not universal — check whether your application relies on this behavior before assuming any sheared key indicates a design or material defect.
What's a gib head key and when is it used?
A gib head key has an angled protruding head at one end, allowing it to be driven in and, critically, pulled back out with a puller or wedge tool without needing access to the opposite end of the keyway — useful where the hub or shaft geometry makes standard key removal difficult. It's functionally the same as a square or rectangular key otherwise, just with this added removal feature.
Does key fit tolerance (clearance vs. interference) matter for my application?
Yes — a looser (clearance) fit is easier to assemble but can allow slight movement leading to wear or noise under reversing or vibrating loads, while a tighter (interference) fit resists this movement better but is harder to install and remove. Match the fit class to the application's vibration and reversing-load characteristics rather than defaulting to whatever fits easiest by hand.
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