How to Identify and Measure a Woodruff Key

If you’ve ever pulled apart a pulley, flywheel, or gear assembly, you’ve probably seen a small half-moon-shaped piece of metal tucked into a shaft — that’s the Woodruff key.

It might look simple, but getting the size right is essential. If the key doesn’t fit snugly, the connection can loosen, leading to wear or failure. Here’s how to identify and measure a Woodruff key correctly.

What a Woodruff Key Does

A Woodruff key locks a component — such as a pulley, gear, or coupling — onto a shaft. It fits into a machined slot on the shaft and a corresponding slot on the mating component.

Because of its curved, half-moon shape, the Woodruff key self-aligns in the shaft and spreads the load more evenly than a straight key. That makes it a popular choice for automotive, agricultural, and industrial machinery.

How Woodruff Key Sizes Are Defined

Each Woodruff key is described by two main measurements:

1. Width – the thickness of the key (which fits into the keyway on the shaft).

2. Diameter – the overall diameter of the circular section that forms the key.

These are often given in metric or imperial dimensions — for example, a “3mm x 6.5mm” or “1/8” x 1/2” Woodruff key.

How to Measure a Woodruff Key

If you’ve removed an old key and want to find the right replacement, follow these steps:

1. Clean the key — make sure there’s no dirt, oil, or burrs.

2. Measure the width using calipers or a micrometer. This must match the keyway slot width in the shaft.

3. Measure the diameter of the key’s curved edge. The easiest way is to measure the height from the flat base to the curve’s top and multiply by two.

4. Check fitment — if possible, test-fit the key in the shaft slot. It should sit slightly proud of the shaft to engage with the mating component’s slot.

Metric or Imperial?

Both systems are still widely used.

• Metric Woodruff keys are measured in millimetres (for example 3 mm × 3.7 mm).

• Imperial Woodruff keys use inches (for example 1/8 " × 1/2 ").

They look similar, but they’re not interchangeable — even a small size difference can stop the key seating correctly in the shaft or keyway.

At Selo, we stock a full range of metric and imperial Woodruff keys, all accurately machined and available individually — no minimum order. Whether you’re replacing one key or building workshop stock, you’ll find the right size here.

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