When working with shafts, gears, pulleys, or couplings, one small feature plays a huge role in transmitting torque effectively — the keyway. Whether you are in mechanical engineering, manufacturing, or repair work, knowing the correct keyway size chart is essential for ensuring proper fits, safety, and durability of mechanical assemblies.
This guide will walk you through what a keyway is, why it matters, the different types, how sizing works, and provide you with a detailed keyway size chart in both metric and imperial standards. Along the way, we’ll also look at real-world examples, common mistakes, and tips for correct measurement.
What is a Keyway?
A keyway is a slot or groove cut into both a shaft and the mating part (gear, pulley, hub, or coupling). A small piece of metal called a key is inserted into this slot to lock the two parts together. The purpose is simple yet powerful: to allow torque (rotational force) to transfer from the shaft to the component without slippage.
Imagine you have an electric motor connected to a conveyor pulley. The motor shaft spins rapidly, and if the pulley isn’t firmly locked in place, it would simply slip around the shaft. By machining a keyway and inserting a key, the shaft and pulley rotate as one unit.
Keyways have been used in machines for centuries, from old textile mills to modern aerospace equipment. Their simplicity makes them reliable and cost-effective, which is why they remain one of the most common methods of power transmission.
Why Keyway Sizing Matters
It’s tempting to think a small groove can’t make much difference, but the wrong keyway size can lead to costly problems:
Strength: If the key or slot is too small, it may shear under heavy torque. If oversized, it weakens the shaft.
Alignment: Incorrect sizing causes wobbling or misalignment between shaft and component.
Safety: Failure in a rotating assembly can lead to dangerous accidents.
Efficiency: A proper fit reduces vibration and energy losses.
Durability: Correct keyways extend the life of both shaft and hub by distributing forces evenly.
In short, a keyway is small, but its role in performance and safety is huge. Engineers always consult the keyway size chart before machining to make sure they select the correct dimensions.
Standard Keyway Dimensions
Keyway sizes are not random. They are standardized by organizations such as:
ANSI (American National Standards Institute) – Used widely in the USA.
ISO (International Organization for Standardization) – Common globally.
DIN (Deutsches Institut für Normung) – German standard, often aligned with ISO.
The standards are based on shaft diameter. For every shaft size, there is a recommended key width, height, and depth. This ensures components made in different factories will still fit together.
For example:
A 20 mm shaft typically uses a 6 × 6 mm key, with a keyway depth around 2.8 mm.
A 1-inch shaft typically uses a 1/4 × 1/4 inch key, with a depth of 1/8 inch.
This consistency is crucial in global industries where machines are assembled from parts made in different countries.
Keyway Size Chart (Metric & Inch)
Below are standard reference charts. Always double-check the latest standards for critical applications.
🔹 Metric Keyway Size Chart (ISO Standard)
| Shaft Diameter (mm) | Key Size (Width × Height mm) | Keyway Depth (mm) |
|---|---|---|
| 6 – 8 | 2 × 2 | 1.0 |
| 8 – 10 | 3 × 3 | 1.4 |
| 10 – 12 | 4 × 4 | 1.8 |
| 12 – 17 | 5 × 5 | 2.3 |
| 17 – 22 | 6 × 6 | 2.8 |
| 22 – 30 | 8 × 7 | 3.3 |
| 30 – 38 | 10 × 8 | 3.3 |
| 38 – 44 | 12 × 8 | 3.3 |
| 44 – 50 | 14 × 9 | 3.8 |
| 50 – 58 | 16 × 10 | 4.3 |
| 58 – 65 | 18 × 11 | 4.9 |
| 65 – 75 | 20 × 12 | 5.4 |
| 75 – 85 | 22 × 14 | 6.4 |
| 85 – 95 | 25 × 14 | 6.4 |
| 95 – 110 | 28 × 16 | 7.4 |
| 110 – 130 | 32 × 18 | 8.4 |
| 130 – 150 | 36 × 20 | 9.4 |
| 150 – 170 | 40 × 22 | 10.4 |
🔹 Imperial (Inch) Keyway Size Chart (ANSI Standard)
| Shaft Diameter (inches) | Key Size (Width × Height in) | Keyway Depth (in) |
|---|---|---|
| 1/4 – 3/8 | 1/16 × 1/16 | 1/32 |
| 3/8 – 1/2 | 1/8 × 1/8 | 1/16 |
| 1/2 – 3/4 | 3/16 × 3/16 | 3/32 |
| 3/4 – 1 | 1/4 × 1/4 | 1/8 |
| 1 – 1 1/4 | 5/16 × 5/16 | 5/32 |
| 1 1/4 – 1 1/2 | 3/8 × 3/8 | 3/16 |
| 1 1/2 – 1 3/4 | 7/16 × 7/16 | 7/32 |
| 1 3/4 – 2 | 1/2 × 1/2 | 1/4 |
| 2 – 2 1/2 | 5/8 × 5/8 | 5/16 |
| 2 1/2 – 3 | 3/4 × 3/4 | 3/8 |
| 3 – 3 1/2 | 7/8 × 7/8 | 7/16 |
| 3 1/2 – 4 | 1 × 1 | 1/2 |
These tables are used every day in workshops and design offices. Many machinists even hang laminated versions of these charts near lathes and milling machines for quick reference.
Types of Keyways
Not every application uses the same type of keyway. The most common ones include:
Parallel Keyway
Straight slot along the shaft.
Most widely used in industrial machinery.
Easy to machine, strong, and reliable.
Woodruff Keyway
Uses a half-moon shaped key.
Common in automotive engines, especially crankshafts.
Advantage: Automatically aligns itself, reducing assembly errors.
Tapered Keyway
Uses a tapered key that wedges into place.
Prevents loosening due to vibration.
Often found in heavy-duty machinery.
Spline Keyway
Uses multiple keyways spaced evenly around the shaft.
Allows high torque transmission.
Common in gearboxes, aircraft, and automotive drive systems.
Choosing the correct type depends on application requirements like torque load, precision, and ease of assembly.
How to Measure a Keyway
If you’re checking an existing shaft or hub, here’s a step-by-step process:
Measure Shaft Diameter
Use a caliper or micrometer.
Identify the range in the keyway chart.
Measure Keyway Width
Place caliper jaws inside the slot to measure width.
Measure Keyway Depth
Measure from the shaft surface to the bottom of the slot.
Check Against Standards
Compare measurements to ISO or ANSI charts.
⚙️ Pro Tip: Always measure with precision tools. Even a 0.1 mm error can affect torque transmission and wear.
Real-World Example
Let’s say you’re repairing a factory conveyor system. The motor shaft has a diameter of 30 mm.
From the ISO chart, a 30 mm shaft requires an 8 × 7 mm key.
Keyway depth should be 3.3 mm.
If you mistakenly install a 10 mm key, it won’t fit properly. Forcing it could damage the hub slot, causing vibration and eventual breakdown. If you choose a smaller 6 mm key, it may shear under heavy load.
This simple example shows why referencing the chart is essential — it saves time, money, and prevents machine failures.
Common Mistakes in Keyway Sizing
Oversized keys: Cause stress concentrations and cracks.
Ignoring tolerances: Loose fits lead to wobbling and wear.
Wrong key type: Using a parallel key when a woodruff is required.
Material mismatch: Using soft aluminum hubs with shallow keyways leads to deformation.
In industries like automotive or aerospace, even a small mistake can cause catastrophic failures.
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Keyway Tolerances
Standards include tolerance ranges to ensure smooth fits:
Keys are usually slightly undersized in width.
Keyways are slightly oversized.
This creates a clearance fit that makes assembly possible.
For example, a 10 mm key might be made 9.97 mm wide, while the keyway slot is 10.03 mm. This ensures the key can slide in without excessive force but still transmit torque effectively.
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Conclusion
The keyway size chart may look like a simple table, but it’s the backbone of countless machines worldwide. By selecting the right keyway dimensions, you ensure:
Safe operation
Long-lasting machinery
Accurate torque transmission
Cost savings on repairs
Whether you are a machinist cutting a shaft, an engineer designing equipment, or a mechanic repairing a gearbox, the keyway size chart is your go-to reference.
A small groove may not look important, but in the world of mechanics, it truly is the key to reliable performance.
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