Woodruff Key Size Chart — ASME B17.2 Dimensions & Keyseat Reference

A Woodruff key is a half-moon (semicircular) key used to transmit torque between a shaft and hub. Unlike parallel keys, Woodruff keys self-align in their circular keyseat pocket, making them the standard choice for tapered shafts, small-diameter shafts, and applications where axial movement of the key is acceptable. This page is a complete reference for Woodruff key dimensions, keyseat depths, cutter sizes, and numbering conventions per ASME B17.2. If you’re looking for standard parallel key and keyway dimensions, see our Keyway Dimensions & Size Chart (ANSI B17.1 & Metric). For tolerance data on parallel keyways, see our Keyway Tolerance Chart.

How Woodruff Key Numbers Work

Every Woodruff key has a standard identification number that encodes its dimensions. The system is simple once you know the rule: Last two digits = nominal diameter (B) in eighths of an inch Preceding digits = nominal width (W) in thirty-seconds of an inch Examples:

• #404 = 4/32 (1/8″) wide x 4/8 (1/2″) diameter
• #808 = 8/32 (1/4″) wide x 8/8 (1″) diameter
• #1212 = 12/32 (3/8″) wide x 12/8 (1-1/2″) diameter

Key numbers with a -1 suffix (e.g., 617-1) have the same nominal width but a shorter actual length and greater distance below center, making them less tall than the standard version with the same number.

Woodruff Key Dimensions Chart — ASME B17.2

All dimensions in inches. Height C is measured at the full radius (top of the key). Height D is measured at the flat bottom. Length F tolerance is +0.000 / -0.010.

Note: Height C is measured at the full radius (curved top). Height D is measured at the flat bottom. For keys smaller than 1/2″ diameter (key numbers ending in 02, 02.5, and 03), heights C and D are identical because the key is so small that both profiles are effectively the same.

Woodruff Key Width Tolerances

Woodruff key width tolerance per ASME B17.2:

All standard Woodruff keys use a single width tolerance of +0.001 / -0.000 across all sizes. The key is always at nominal or slightly oversize on width.

Woodruff Keyseat Dimensions (Shaft)

The shaft keyseat for a Woodruff key is a circular pocket, not a rectangular slot. It’s cut with a Woodruff keyseat cutter sized to the key number. Keyseat width tolerance is +0.005 / -0.000.

Hub keyseat: The hub keyseat (keyway) for a Woodruff key is a standard rectangular axial slot, just like a parallel key hub keyway. Hub keyseat width tolerance is +0.002 / -0.000 for most sizes.

When to Use a Woodruff Key vs a Parallel Key

Use a Woodruff key when:

• The shaft is tapered (the most common reason). The semicircular pocket lets the key tilt to match the taper.
• The shaft diameter is small (under ~1″) and a parallel key would weaken the shaft too much.
• Axial adjustment is needed. The key can “float” slightly in its circular pocket.
• Assembly ease matters. Woodruff keys are self-aligning and drop into their keyseat.
• The application is light to moderate torque. Automotive accessories, small pump shafts, instrument drives.

Use a parallel key when:

• High torque or reversing loads are present. Parallel keys have more shear area and a tighter fit.
• The shaft is straight (not tapered). No reason to use a Woodruff key.
• The keyway needs to resist axial movement. Woodruff keys don’t resist axial forces.
• The shaft is large enough to accommodate a rectangular keyseat without excessive stress concentration.

Key limitation of Woodruff keys: The circular keyseat pocket removes more material from the shaft than a rectangular keyseat of equivalent torque capacity. This creates a larger stress concentration. On shafts subject to fatigue loading, this matters. For high-cycle fatigue applications, a parallel key with a well-radiused keyseat is usually the better choice.

Woodruff Keyseat Cutter Sizes

The shaft keyseat for a Woodruff key is cut with a dedicated Woodruff keyseat cutter. The cutter number matches the key number. The cutter diameter is slightly larger than the key diameter to allow for resharpening.

Woodruff keyseat cutters are standard, off-the-shelf tooling available from most industrial cutting tool suppliers. The cutter creates the circular pocket in the shaft; the rectangular hub keyway is cut separately using standard keyway methods.

Metric Woodruff Keys (ISO 3912)

ISO 3912 covers metric Woodruff keys. The designation system uses width (b) x height (h) x diameter (D) in millimeters. Metric Woodruff keys are used primarily in European and Asian manufactured equipment.

Note: ISO 3912 also defines a “Whitney” form key as an alternative profile. The standard Woodruff form is the default; the Whitney form is specified only when called out on the print.

Woodruff Key FAQ

How do you measure a Woodruff key? Measure the width (W) across the flat faces, and the diameter (B) across the curved portion. Match these to the standard chart above. The key number encodes both dimensions. If you have a worn key and need a replacement, measuring width and diameter to the nearest 1/32″ and 1/8″ respectively will get you the correct key number. What’s the difference between a Woodruff key and a half-moon key? They’re the same thing. “Half-moon key” is the informal name for a Woodruff key, referring to its semicircular shape. In engineering drawings and specifications, always use the Woodruff key number to avoid ambiguity. Can a Woodruff key be used in a blind bore? Not typically. Woodruff keys sit in a circular pocket in the shaft (external), not in a bore (internal). The hub keyway for a Woodruff key is a standard rectangular slot, which can be a blind or through keyway. If your application requires an internal semicircular keyseat, that’s non-standard. What material are Woodruff keys made from? Standard Woodruff keys are AISI 1018 to 1045 cold-drawn steel. The key is designed to be softer than both the shaft and hub, so it acts as a shear pin under extreme overload. Don’t harden the key unless the application specifically requires it and engineering has signed off. How do I choose the right size Woodruff key for my shaft? Match the shaft diameter to a key diameter (B) that’s slightly smaller. The key should fit within the shaft cross-section without the circular keyseat pocket exceeding roughly 40% of the shaft diameter. Beyond that, you’re weakening the shaft excessively. For typical applications, select a key where B is approximately 25-30% of the shaft diameter.

Need Internal Keyways Cut for Your Hub?

The shaft keyseat for a Woodruff key is cut externally with a standard Woodruff cutter, but the hub keyway is an internal rectangular slot, the same type of operation NMT’s keyseat millers are designed for. If you need precision internal hub keyways for Woodruff key applications, including blind bores, non-standard widths, or difficult materials, we manufacture custom tooling to your specifications. For standard parallel key and keyway dimensions, see our Keyway Dimensions & Size Chart. For tolerance specifications, see our Keyway Tolerance Chart. Call: 513-541-6682 Email: nationalmachinetoolco@gmail.com National Machine Tool Co. — Cincinnati, OH — Over 100 years manufacturing custom keyseat millers. Request a Quote →