Technical Foundation
What Is a Step Key Stock?
Key stock is bar material — square or rectangular in cross-section — cut to length and installed into matching keyways machined into both a shaft and the hub of a gear, pulley, or coupling mounted on it, transmitting rotational torque between the two without relying on friction fit alone. The key's cross-sectional dimensions are set by standard tables (ANSI B17.1 in the US) based on shaft diameter, ensuring a keyway sized for a given shaft accepts a correspondingly sized standard key.
Key stock is sold in three common tolerance grades: bar stock (loose, general-purpose tolerance), precision (tighter tolerance for a closer fit in ground keyways), and oversized/undersized stock, used when a keyway has been slightly enlarged during rework or repair and a standard-size key would fit too loosely. Material selection — typically low-carbon or medium-carbon steel, with stainless steel for corrosive environments — should match or exceed the shaft and hub material's hardness to avoid the key itself deforming under torque, which is often called out on the equipment's original spec sheet.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Cross-Section Size | Width x height, sized per ANSI B17.1 to shaft diameter | Standard tables match key size to shaft diameter range — undersized keys shear under load |
| Shape | Square vs. rectangular cross-section | Rectangular keys are often used on larger shafts where a full square key would require too much shaft material removal |
| Tolerance Grade | Bar stock, precision, oversized/undersized | Precision and oversized stock address repair situations where a standard key would fit too loosely |
| Material | Low/medium carbon steel, stainless steel | Key material should match or exceed shaft/hub hardness to avoid the key deforming under torque rather than protecting the shaft |
| Length | Cut-to-length or stock bar lengths | Length is matched to the hub's keyway length, not the full shaft length |
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Applications
Where Step Key Stock Get Used
Key stock is specified anywhere a shaft needs a positive, non-slip torque connection to a mounted gear, pulley, sprocket or coupling.
Power Transmission & Gearbox Assembly
Key stock transmitting torque between shafts and mounted gears, sprockets, and couplings in gearboxes and drives.
Pump & Motor Coupling
Keys transferring rotational torque between motor shafts and pump or driven equipment couplings.
Conveyor Drive Systems
Key stock securing pulleys and sprockets to conveyor drive shafts.
Machine Tool Spindle & Drive Assemblies
Precision key stock used in machine tool drivetrains where fit tolerance affects overall machine accuracy.
Equipment Repair & Rebuild
Oversized key stock used when repairing worn or reworked keyways during equipment rebuild.
FAQ
Step Key Stock Questions, Answered
How do I determine what key stock size I need?
Key size is standardized against shaft diameter under ANSI B17.1 — look up your shaft diameter in the standard's key size table, or measure the existing keyway width and depth directly if replacing a key on an already-machined shaft.
What's the difference between bar stock and precision key stock?
Bar stock key material has a looser, general-purpose tolerance suitable for most standard applications; precision key stock is ground to a tighter tolerance for a closer fit, typically specified where fit accuracy matters more, such as in precision machine tool assemblies.
When would I need oversized key stock instead of standard size?
Oversized key stock is used when a keyway has been slightly enlarged during rework, repair, or wear over time, and a standard-size key would fit too loosely and allow play or premature wear in the connection.
Should I use square or rectangular key stock?
This is generally dictated by the standard size table for your shaft diameter — smaller shafts typically use square keys, while larger shaft diameters often call for rectangular keys to avoid removing excessive material from the shaft for an oversized square keyway.
Does key stock material need to match the shaft material?
Not necessarily match exactly, but the key material should be hard enough relative to the shaft and hub to transmit torque without deforming — using a key softer than intended for the application can result in the key itself shearing or deforming rather than protecting the shaft and hub.
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