Woodruff keys are some of the most common components in machining, machine engineering, and maintenance. However, not many people know how to adequately size and measure a woodruff key. In this article, we'll provide you with an explanation of how to size and measure woodruff keys, as well as a standardized size chart that would make your selection process easier.

How Are Woodruff Keys Sized and Measured?

Woodruff keys are often confusing to adequately measure, especially if they're measured in inches. The best possible approach to measuring a half-moon key is to either use the sizing chart (provided below) or to take a caliper and take the measurements yourself and cross-reference your measurements with those displayed in the standardized ANSI chat.

Before we dive into how to measure each type, it is important to talk about measuring units. Machinists worldwide recognize two different unit systems for measuring length: metric and imperial

• Metric woodruff keys are measured in millimeters (ISO standard) or in micrometers in more precise applications, though the latter is very rare and usually only applies when manufacturing a woodruff key yourself.
• ANSI woodruff size charts express measurements in inches and nominal key sizes with higher numbers corresponding to bigger keys. For example, an ANSI 202 measures 1/16 x 1/4, while ANSI 1012 measures 5/16 x 1 1/2.

Regardless of which unit of length you're using, you'll need to measure the following: width, height, and length.

Width is a pretty straightforward measurement to take, as you only need to measure the thickness of the machine key. Measuring height is also pretty straightforward, as you only measure from the lowest point to the flat top face.

Length is a bit trickier, considering that the overall height of the standard woodruff key is less than half its nominal diameter, so those two measures shouldn't be confused. What you measure as the length of your standard key isn't the same measurement as its nominal diameter. This applies to both metric and inch woodruff keys.

Full-Radius Woodruff Keys

Full-radius half-moon keys are a full half-circle in size, which fully interface with the shaft's key seat. When measuring and ordering full-radius woodruff keys, you'll only need to take into account the width of the key and its height. The length can be derived from the key's height.

Flat-Bottom Woodruff Keys

Flat-bottoms have a similar profile as the full-radius keys, but the bottom section is actually flat instead of being rounded. The dimension of the bottom is also a standardized measurement and depends on the key number, though several part numbers may share the same bottom measurements.

Using thickness and bottom measurements is perfectly fine when ordering a new key. Top length can also be used, but it's more a part of the naming convention rather than a directly measurable dimension.

Hi-Pro Woodruff Keys

High-profile woodruff fasteners have a full-radius or a flat-bottom profile with added feet (or lips) and are used to prevent the key from wobbling in the key seat. When measuring, you need to account for the overall thickness of the part, the overall length (the overall top profile), and lip height.

Are All Woodruff Keys the Same?

You can distinguish woodruff keys by the type of key and by the type of material. Full-radius and flat-bottom woodruff keys are the most commonly used keys in the manufacturing and maintenance of equipment. They're usually made of high-carbon steel, though some applications require the use of stainless-steel woodruff keys.

Regardless of the type and material, their overall function remains the same, to interlock between a keyseat and the keyway and thus prevent the rotating shaft and hub from rotating independently from one another while also acting as fail-safes that prevent shearing of pricey parts.

The fundamental advantage of woodruff keys is that they do not require machining at the shaft shoulders, which reduces stress concentrations at the shaft's end. As a result, they are perfect for high-speed rotating assemblies because they effectively reduce the danger of damage.

Woodruff Key Chart

This is a standardized ANSI woodruff key sizing chart. Please refer to the included image for measurement. All dimensions are given in inches unless stated otherwise.

Key No.	ANSI Key No.	Nominal Key Size W x D	Width (W)		Diameter (B)		Actual Length		Height				Distance Below Center
									Full Radius ( C)		Flat Bottom (D)
			Min.	Max.	Min.	Max.	Min.	Max.	Min.	Max.	Min.	Max.	Dec.	Frac.
201	202	1/16 x 1/4	0.0625	0.0635	0.240	0.250	0.238	0.248	0.104	0.109	-	-	0.016	1/64
206	202.5	1/16 x 5/16	0.0625	0.0635	0.302	0.312	0.301	0.311	0.135	0.140	-	-	0.016	1/64
207	302.5	3/32 x 5/16	0.0938	0.0948	0.302	0.312	0.301	0.311	0.135	0.140	-	-	0.016	1/64
211	203	1/16 x 3/8	0.0625	0.0635	0.365	0.375	0.364	0.374	0.167	0.172	-	-	0.016	1/64
212	303	3/32 x 3/8	0.0938	0.0948	0.365	0.375	0.364	0.374	0.167	0.172	-	-	0.016	1/64
213	403	1/8 x 3/8	0.1250	0.1260	0.365	0.375	0.364	0.374	0.167	0.172	-	-	0.016	1/64
1	204	1/16 x 1/2	0.0625	0.0635	0.490	0.500	0.481	0.491	0.198	0.203	0.188	0.194	0.047	3/64
2	304	3/32 x 1/2	0.0938	0.0948	0.490	0.500	0.481	0.491	0.198	0.203	0.188	0.194	0.047	3/64
3	404	1/8 x 1/2	0.1250	0.1260	0.490	0.500	0.481	0.491	0.198	0.203	0.188	0.194	0.047	3/64
4	305	3/32 x 5/8	0.0938	0.0948	0.615	0.625	0.602	0.612	0.245	0.250	0.234	0.240	0.062	1/16
5	405	1/8 x 5/8	0.1250	0.1260	0.615	0.625	0.602	0.612	0.245	0.250	0.234	0.240	0.062	1/16
6	505	5/32 x 5/8	0.1562	0.1572	0.615	0.625	0.602	0.612	0.245	0.250	0.234	0.240	0.062	1/16
61	605	3/16 x 5/8	0.1875	0.1885	0.615	0.625	0.602	0.612	0.245	0.250	0.234	0.240	0.062	1/16
7	406	1/8 x 3/4	0.1250	0.1260	0.740	0.750	0.730	0.740	0.308	0.313	0.297	0.303	0.062	1/16
8	506	5/32 x 3/4	0.1562	0.1572	0.740	0.750	0.730	0.740	0.308	0.313	0.297	0.303	0.062	1/16
9	606	3/16 x 3/4	0.1875	0.1885	0.740	0.750	0.730	0.740	0.308	0.313	0.297	0.303	0.062	1/16
91	806	1/4 x 3/4	0.2500	0.2510	0.740	0.750	0.730	0.740	0.308	0.313	0.297	0.303	0.062	1/16
10	507	5/32 x 7/8	0.1562	0.1572	0.865	0.875	0.856	0.866	0.370	0.375	0.359	0.365	0.062	1/16
11	607	3/16 x 7/8	0.1875	0.1885	0.865	0.875	0.856	0.866	0.370	0.375	0.359	0.365	0.062	1/16
12	707	7/32 x 7/8	0.2188	0.2198	0.865	0.875	0.856	0.866	0.370	0.375	0.359	0.365	0.062	1/16
A	807	1/4 x 7/8	0.2500	0.2510	0.865	0.875	0.856	0.866	0.370	0.375	0.359	0.365	0.062	1/16
13	608	3/16 x 1	0.1875	0.1885	0.990	1.000	0.982	0.992	0.433	0.438	0.422	0.428	0.062	1/16
14	708	7/32 x 1	0.2188	0.2198	0.990	1.000	0.982	0.992	0.433	0.438	0.422	0.428	0.062	1/16
15	808	1/4 x 1	0.2500	0.2510	0.990	1.000	0.982	0.992	0.433	0.438	0.422	0.428	0.062	1/16
B	1008	5/16 x 1	0.3125	0.3135	0.990	1.000	0.982	0.992	0.433	0.438	0.422	0.428	0.062	1/16
152	1208	3/8 x 1	0.3750	0.3760	0.990	1.000	0.982	0.992	0.433	0.438	0.422	0.428	0.062	1/16
16	609	3/16 x 1 1/8	0.1875	0.1885	1.115	1.125	1.104	1.114	0.479	0.484	0.469	0.475	0.078	5/64
17	709	7/32 x 1 1/8	0.2188	0.2198	1.115	1.125	1.104	1.114	0.479	0.484	0.469	0.475	0.078	5/64
18	809	1/4 x 1 1/8	0.2500	0.2510	1.115	1.125	1.104	1.114	0.479	0.484	0.469	0.475	0.078	5/64
C	1009	5/16 x 1 1/8	0.3125	0.3135	1.115	1.125	1.104	1.114	0.479	0.484	0.469	0.475	0.078	5/64
19	610	3/16 x 1 1/4	0.1875	0.1885	1.240	1.250	1.230	1.240	0.542	0.547	0.531	0.537	0.078	5/64
20	710	7/32 x 1 1/4	0.2188	0.2198	1.240	1.250	1.230	1.240	0.542	0.547	0.531	0.537	0.078	5/64
21	810	1/4 x 1 1/4	0.2500	0.2510	1.240	1.250	1.230	1.240	0.542	0.547	0.531	0.537	0.078	5/64
D	1010	5/16 x 1 1/4	0.3125	0.3135	1.240	1.250	1.230	1.240	0.542	0.547	0.531	0.537	0.078	5/64
E	1210	3/8 x 1 1/4	0.3750	0.3760	1.240	1.250	1.230	1.240	0.542	0.547	0.531	0.537	0.078	5/64
22	811	1/4 x 1 3/8	0.2500	0.2510	1.365	1.375	1.352	1.362	0.589	0.594	0.578	0.584	0.094	3/32
23	1011	5/16 x 1 3/8	0.3125	0.3135	1.365	1.375	1.352	1.362	0.589	0.594	0.578	0.584	0.094	3/32
F	1211	3/8 x 1 3/8	0.3750	0.3760	1.365	1.375	1.352	1.362	0.589	0.594	0.578	0.584	0.094	3/32
24	812	1/4 x 1 1/2	0.2500	0.2510	1.490	1.500	1.474	1.484	0.636	0.641	0.625	0.631	0.109	7/64
25	1012	5/16 x 1 1/2	0.3125	0.3135	1.490	1.500	1.474	1.484	0.636	0.641	0.625	0.631	0.109	7/64

 

Summary

We hope that our guide provided you with some insight on how to size and measure woodruff keys. We strongly suggest that you cross-reference your measurements with the provided chart, and when in doubt, always go with a bigger key — a bit of filing should size it to its appropriate measurements. If you're interested, you can learn more about woodruff keys here.