What Is a Key and Keyway?
A keyway is a slot machined into a shaft, and a matching slot machined into the bore of a component — a pulley, sprocket, gear, coupling, or flywheel. A small precision piece of metal called a key is inserted into these two aligned slots. The key locks the shaft and the component together rotationally, so that torque applied to one is transmitted to the other without slippage.
Without a key, a component fitted to a shaft can rotate freely around the shaft when torque is applied — which is exactly what happens when a key shears or a keyway wears. The shaft spins inside the bore while the pulley or sprocket sits still. The drive is lost.
Keys and keyways are one of the oldest and most reliable methods of shaft-to-hub torque transmission in mechanical engineering. They are simple to fit, cheap to replace, and standardised globally — meaning a pulley from one manufacturer and a shaft from another will accept the same key if both conform to the same standard.
Types of Keys
Parallel Key (Square or Rectangular)
The most common key type in industrial use. A parallel key has a constant cross-section — it is the same width and height along its entire length. Square parallel keys are used on smaller shafts; rectangular (flat) parallel keys are used on larger shafts where the shaft-to-key height ratio would make a square key too deep. Parallel keys sit half in the shaft keyway and half in the hub keyway, transmitting torque through the sides of the key.
Parallel keys can be retained in one of two ways: a close fit in both the shaft and hub keyways (used where the key must locate precisely, such as in gearboxes), or a sliding fit in the hub with a close fit on the shaft — this allows the component to slide axially on the shaft while still transmitting rotation. The latter arrangement is called a feather key.
Feather Key
A feather key is a parallel key fitted tightly in the shaft keyway but with a sliding clearance fit in the hub keyway. This allows the hub component to slide axially along the shaft (in and out) while still transmitting torque. Feather keys are used in gearboxes (sliding gear selectors), variable-position sprockets, and adjustable-position components. The key is often secured to the shaft with one or two socket head cap screws through the key body to prevent it moving axially with the hub.
Woodruff Key
A Woodruff key is semicircular in cross-section — shaped like a half-disc. The curved portion sits in a circular milled pocket in the shaft; the flat top protrudes into a conventional keyway in the hub. Woodruff keys are self-aligning (the curved base automatically centres in the shaft pocket) and work well in tapered shaft applications, such as motor shafts, small engine crankshafts (lawnmowers, chainsaws, whipper snippers), and machine tool spindles.
Woodruff keys are widely stocked at consumer hardware and automotive stores — this is why searches for "woodruff key bunnings" and "woodruff key supercheap" are common. Small engine crankshaft keys are almost always Woodruff type. In industrial applications, Woodruff keys are less common than parallel keys because the deep shaft pocket weakens the shaft more than a parallel keyway.
Gib-Head (Taper) Key
A taper key is driven in axially — it wedges between the shaft and hub, locking everything together by friction from the taper. A gib-head key has a head (like a nail head) on one end to allow extraction with a puller or screwdriver. Taper keys generate a clamping force that holds the component on the shaft even without any other retention. They are used in older machinery and some heavy industrial applications. Disadvantage: the wedging action can force the shaft eccentric relative to the bore if not fitted carefully.
Scotch Key (Flat Saddle Key)
A Scotch key (also called a saddle key or flat key) fits into the hub keyway only — there is no keyway in the shaft. The key bears on the top of the shaft. Scotch keys transmit only small torques via friction and are a field expedient when a shaft keyway is not present or has been damaged. They are not suitable for heavy-duty torque transmission. The search term "scotch key" has notable volume in Australia, reflecting their use as a quick-fix solution in agricultural and field maintenance applications.
Round Key (Pin Key)
A round key or pin key is a cylindrical pin driven into a hole drilled half through the shaft and half through the hub. Simple to cut and fit. Used in light-duty applications and some hand tools. Not suitable for high-torque transmission.
Key Steel: What It Is and How to Use It
Key steel (also called keyway steel or key stock) is bright steel bar supplied in standard cross-sectional dimensions matching common key sizes — 3×3, 4×4, 5×5, 6×6, 8×7, 10×8, 12×8, 14×9, 16×10, and so on. It is supplied in straight 300mm and 1-metre lengths and is cut to fit the specific keyway length needed.
Key steel is typically manufactured from carbon steel (C45 or equivalent — Australian Grade AS1442 Grade 1045), which provides adequate strength for most drive applications. The bright (cold-drawn) finish holds dimensional tolerances that allow a correct sliding or press fit in machined keyways without further finishing in most cases.
How to Cut Your Own Key from Key Steel
Fitting a key from bar stock is a standard workshop skill, particularly for replacing worn or sheared keys in pulleys and sprockets:
- Measure the keyway. Measure the width and depth of the shaft keyway and the hub keyway separately. Width should be consistent. Depth in the shaft is typically equal to the key height divided by two (half the key sits in the shaft, half in the hub).
- Select the correct key steel. Choose bar stock matching the keyway width (and nominally matching the key height). The standard keyway dimensions table below will confirm the correct key section for the shaft diameter.
- Cut to length. The key should be 0.5–1.5mm shorter than the keyway length to allow for thermal expansion and to ensure the key does not bottom out axially. Cut with a hacksaw or cold saw.
- Check the fit. The key should be a snug sliding fit in the shaft keyway — it should push in by hand or with light mallet taps, with no side play. In the hub, a normal fit has a small clearance (the hub can slide axially). For a close fit (in precision applications), the hub fit should also be snug with no measurable play.
- Chamfer the leading edge. Lightly file or grind a chamfer on the leading end of the key to guide entry into the hub keyway during assembly.
Standard Keyway Dimensions — Parallel Keys (ISO 773 / AS 1654)
The following table gives standard key and keyway dimensions for parallel keys. The shaft keyway depth (t1) is the depth of the slot cut in the shaft. The hub keyway depth (t2) is the depth of the slot in the bore of the component. Key height = t1 + t2 (approximately — with small fitting allowances). This standard applies to both the AIMS key steel range and to components (pulleys, sprockets, couplings) supplied in metric sizes.
| Shaft Diameter (mm) | Key Width × Height (mm) | Shaft Keyway Depth t1 (mm) | Hub Keyway Depth t2 (mm) |
|---|---|---|---|
| 6–8 | 2 × 2 | 1.2 | 1.0 |
| 8–10 | 3 × 3 | 1.8 | 1.4 |
| 10–12 | 4 × 4 | 2.5 | 1.8 |
| 12–17 | 5 × 5 | 3.0 | 2.3 |
| 17–22 | 6 × 6 | 3.5 | 2.8 |
| 22–30 | 8 × 7 | 4.0 | 3.3 |
| 30–38 | 10 × 8 | 5.0 | 3.3 |
| 38–44 | 12 × 8 | 5.0 | 3.3 |
| 44–50 | 14 × 9 | 5.5 | 3.8 |
| 50–58 | 16 × 10 | 6.0 | 4.3 |
| 58–65 | 18 × 11 | 7.0 | 4.4 |
| 65–75 | 20 × 12 | 7.5 | 4.9 |
| 75–85 | 22 × 14 | 9.0 | 5.4 |
| 85–95 | 25 × 14 | 9.0 | 5.4 |
| 95–110 | 28 × 16 | 10.0 | 6.4 |
| 110–130 | 32 × 18 | 11.0 | 7.4 |
Dimensions per ISO 773 / AS 1654 — Rectangular and Square Parallel Keys and Their Corresponding Keyways. Nominal dimensions shown; refer to the standard for full tolerance specifications.
⚠️ Imperial keyways: Some older Australian equipment and imported machinery (particularly from North America) uses inch-sized keyways. A 1-inch shaft in imperial standard takes a 1/4" × 1/4" key. These dimensions do not directly correspond to metric equivalents — a 25mm shaft takes a 8×7mm key, not a 1/4"×1/4" key. If you are fitting a key to older equipment or imported machinery, confirm whether the keyway is metric or inch before cutting from bar stock.
Key Fits: Loose, Normal, and Close
Not all key applications use the same fit between key and keyway. Three standard fits apply:
| Fit Type | Shaft Keyway Fit | Hub Keyway Fit | Application |
|---|---|---|---|
| Loose (Transit) | Slight clearance | Moderate clearance | Guide and sliding applications; components that must slide freely |
| Normal | Close sliding fit | Slight clearance | Standard general-purpose drives — pulleys, sprockets, couplings |
| Close (Precision) | Press/interference fit | Close sliding fit | Precision drives, gearboxes; no movement permitted in either direction |
For most AIMS customer applications — V-belt pulleys, chain sprockets, conveyor drives — a normal fit is correct. The key slides into the shaft keyway with hand pressure and sits with a small clearance in the hub, allowing the component to be pushed on axially before the set screw is tightened.
Keys in Pulleys, Sprockets, and Couplings
Pulleys and sprockets are the most common applications where AIMS customers encounter keys and keyways. Understanding how the system works helps diagnose failures and order the correct replacement parts.
Standard Bore Configurations
Pulleys and sprockets are supplied in several bore configurations:
- Pilot bore: A small pre-bored hole (usually 10–20mm) with no keyway. The customer bores and broaches the finished bore and keyway to suit their specific shaft diameter in their own workshop.
- Finished bore: Bored to a specific finished diameter with a machined keyway and set screw hole, ready to fit directly onto a shaft of that diameter.
- Taper lock bore: Uses a taper lock bush — described below.
When ordering a finished-bore pulley or sprocket, specify: (1) the finished bore diameter, (2) the keyway size (which will be standard for that bore diameter if the supplier follows ISO 773), and (3) whether a set screw is required.
Set Screws and Their Role
Most finished-bore pulleys and sprockets have one or two threaded set screw holes in the hub — one typically positioned over the keyway, one on the opposite side. The set screw locks the component axially on the shaft (preventing it from sliding off) but does not transmit rotational torque. Torque is transmitted by the key. A common mistake is over-tightening set screws and expecting them to hold the component without a key — set screws alone are not designed for that function in a keyed bore.
Taper Lock Bushes: An Alternative to Conventional Keyways
Taper lock bushes (or taper-lock bushings) are a system that eliminates the need for precision keyway fitting in the field. The bush is a split tapered sleeve with its own internal keyway. The component (pulley, sprocket, coupling) has a tapered bore to match. When the bush is drawn into the component taper using cap screws, it clamps onto the shaft by compression, gripping the shaft far more securely than a conventional key-and-set-screw arrangement.
Advantages over conventional keyed bores: faster installation and removal, no precision keyway broaching required in the field, the same pulley can be used on different shaft sizes by changing the bush, and the clamping force distributes load more evenly around the shaft circumference. Taper lock bushes are available in standard sizes (1008, 1108, 1210, 1215, 1310, 1610, 2012, 2517, 3020, 3525, 4030, and so on) and take a standard parallel key in the bush keyway.
If you are regularly fitting and removing pulleys or sprockets, or working with shafts that require frequent repositioning, taper lock is worth specifying. See the AIMS pulleys range and sprockets range for taper lock bore options.
Why Keys Shear — and Why That's Intentional
A sheared key is one of the most common failures encountered in drive systems. It often comes as a surprise to operators — but key shear is, in many applications, an intentional design feature rather than a failure.
The key is the weakest link in the drivetrain by design. When a drive system is subjected to a sudden shock load — a jam, a jam in a conveyor, striking a rock on a mower deck, or a sudden mechanical stop — the key is designed to shear before the shaft twists, the pulley cracks, or the gearbox is destroyed. A key costs a few dollars. A shaft, gearbox, or pulley costs orders of magnitude more.
This is why replacement key steel is routinely stocked by maintenance teams, and why knowing the key dimensions for critical drive shafts is part of good machinery maintenance practice.
What Causes Premature Key Failure (Before Genuine Overload)
While key shear under overload is by design, premature key failures indicate problems that need addressing:
- Incorrect key size: Using a key smaller than the standard dimension for the shaft — under-dimensioned keys shear at loads the drive should handle easily. Always use the correct ISO 773 key size for the shaft diameter.
- Poor keyway fit (sloppy keyway): If the key has side play in either the shaft or hub keyway, impact loads concentrate at one end of the key rather than distributing along its length. A sloppy keyway in a sprocket or pulley will shear keys repeatedly. The root cause is the worn keyway, not the key.
- Missing or loose set screw: Without axial retention, the hub can move along the shaft and cause the key to bear load on one corner rather than its full face area.
- Notch in keyway corner: Sharp corners at the ends of a keyway are stress concentration points. In machined keyways, end mills leave a rounded profile — fitting a square-ended key into a radiused-end keyway creates a stress riser at the corner. Either use a key with chamfered ends or ensure the keyway corners match the key geometry.
Sloppy Keyways in Pulleys and Sprockets: What to Do
A worn or oversized keyway in a pulley, sprocket, or hub is a common problem on older or heavily used machinery. Symptoms: repeated key shear, metallic rattling from the drive when loaded, visible chatter marks on the key face, or the component rotating slightly relative to the shaft at low load.
Options for a sloppy keyway:
- Oversize key: If the keyway has only minor wear (0.1–0.2mm oversize), an oversize key cut from key steel can restore the fit. File the key to a snug fit in the worn keyway. This is a temporary repair.
- Repair with Loctite 638 or similar retaining compound: For slight looseness, applying retaining compound to the key surfaces before assembly can take up the clearance and restore torque capacity. This is a repair, not a permanent fix for a heavily worn keyway.
- Replace the component: If the keyway in the pulley or sprocket is significantly worn or wallowed out, the component should be replaced. A worn keyway will continue to damage keys and eventually damage the shaft keyway as well.
- Machine a new keyway: If the shaft keyway is sound but the hub is worn, a new keyway can be broached or milled at 90° or 180° to the worn one. This requires a machine shop.
- Upgrade to taper lock: Converting a worn keyed-bore pulley to a taper lock configuration permanently solves the problem and provides superior holding power going forward.
How to Fit a Key
Proper key fitting is straightforward but a few details matter:
- Clean all surfaces. Remove any burrs, scale, or debris from the shaft keyway, the hub keyway, and the key itself. Burrs prevent the key from seating fully and create false tight spots.
- Fit the key to the shaft first. The key should be a close sliding fit in the shaft keyway — entering smoothly by hand or with light mallet taps, no rocking or side play. If the key is tight, check for burrs. Never force a key that does not want to enter — forcing causes galling and may split the hub when assembled.
- Check the key height. The key should protrude above the shaft surface by exactly t2 (the hub keyway depth). A simple way to verify: compare the key protrusion against the hub keyway depth using a depth gauge or a feeler gauge at the hub face.
- Slide on the component. Align the hub keyway with the key and push the component on axially. It should enter smoothly. Resistance indicates misalignment or the key is sitting too high (check for burrs under the key).
- Fit the set screw. Apply a small amount of medium-strength Loctite (blue, 243) to the set screw thread and tighten to the manufacturer's specification. The set screw provides axial retention only — do not over-tighten expecting it to do the key's job.
Key Steel Sizes Stocked by AIMS Industrial
AIMS Industrial stocks bright key steel bar in standard ISO 773 metric sizes, supplied in 300mm and 1-metre lengths. Common sizes include 5×5, 6×6, 8×7, 10×8, 12×8, 14×9, and 16×10mm. Key steel can be cut to length with a hacksaw and fits directly into standard metric keyways without further surface treatment in most applications. Browse the full range at aimsindustrial.com.au/collections/key-steel.
For pulleys, sprockets, and taper lock bushes, see:
Frequently Asked Questions
What is a keyway used for?
A keyway is a slot machined into a shaft and into the bore of a component (pulley, sprocket, gear, coupling). A key inserted into both slots locks the two parts together rotationally, transmitting torque from the shaft to the component without slippage.
What is the difference between a parallel key and a Woodruff key?
A parallel key is a rectangular or square bar that sits in a straight slot machined along the shaft. It is the standard key type for industrial pulleys, sprockets, and couplings. A Woodruff key is semicircular — it sits in a circular milled pocket in the shaft. Woodruff keys are self-aligning and commonly used on tapered shafts, small engine crankshafts, and motor shafts. They are less suitable for high-torque industrial drives because the deep shaft pocket weakens the shaft.
What is key steel and what sizes does it come in?
Key steel is bright carbon steel bar (typically AS1442 Grade 1045 or equivalent) manufactured to standard parallel key cross-section dimensions. Common metric sizes: 3×3, 4×4, 5×5, 6×6, 8×7, 10×8, 12×8, 14×9, 16×10, 18×11, 20×12mm. Supplied in 300mm and 1-metre lengths for cutting to fit. The bright drawn finish maintains the close tolerances needed for a correct keyway fit.
How do I know what key size to use on a shaft?
Key size is determined by shaft diameter. ISO 773 (and the equivalent AS 1654) specifies the key width × height for each shaft diameter range — for example, a 25mm shaft takes an 8×7mm key. See the keyway dimensions table above. If the existing key is worn or missing, measure the keyway width with a vernier caliper — the key width matches the keyway width.
Why do keys shear?
Key shear under overload is intentional — the key is the weakest link in the drivetrain by design, protecting the shaft and more expensive components from damage during jams or shock loads. Premature key shear (at normal operating loads) indicates a problem: incorrect key size, a sloppy/worn keyway, missing set screw allowing axial movement, or a stress concentration from a sharp keyway corner. Repeated key shear in the same location always has a root cause — find and fix it rather than just replacing the key.
How tight should a key fit in a keyway?
In the shaft keyway, the key should be a close sliding fit — entering smoothly by hand or with light mallet taps, no detectable side play. In the hub keyway (for normal-fit applications such as pulleys and sprockets), a small clearance is correct — the hub should slide on easily. A key that requires heavy driving into the shaft keyway is likely oversize or has burrs.
What is a feather key?
A feather key is a parallel key fitted tightly to the shaft keyway but with a sliding clearance fit in the hub. This allows the hub component to slide along the shaft axially while still transmitting torque rotation. Used in sliding gears, variable-position drives, and machine tool feed mechanisms. The key is usually fastened to the shaft with cap screws to prevent it moving with the hub.
What is a taper lock bush and when should I use one?
A taper lock bush is a split tapered sleeve that clamps onto a shaft when drawn into a matching tapered bore in a pulley or sprocket. It requires no precision keyway broaching in the field, provides superior clamping force compared to a set screw, and allows easy removal and repositioning. Specify taper lock for new installations, for applications requiring frequent removal, or to solve repeated key/keyway problems in existing equipment.
My pulley keeps shearing keys. What is the real problem?
Repeated key shear in the same pulley indicates a worn or oversize keyway in the pulley bore — the key has side play and impact loads concentrate at the key ends rather than distributing along its length. Check the keyway width with a vernier caliper against the key width. If the clearance exceeds 0.1–0.2mm, the pulley keyway is worn. Replace the pulley, or upgrade to a taper lock bore configuration to permanently resolve the problem.
Can I cut a key from mild steel flat bar?
Technically yes, but bright key steel is the correct material and is not significantly more expensive. Mild steel flat bar (Grade 250) has lower strength than key steel (Grade 1045) and will shear at lower loads than a correctly specified key. For any drive application, use proper key steel in the correct ISO 773 dimensions. Substituting mild steel flat bar risks a key shearing before it should — potentially at inconvenient or unsafe moments.
What is the difference between a keyed bore and a plain bore?
A keyed bore has a machined keyway slot — it is designed to be driven by a key on a keyed shaft. A plain bore has no keyway — it is used for components that rotate freely on the shaft (idler pulleys, bearing housings) or that are retained by other means (shrink fit, spline, set screw alone for very light duty). Taper lock bore is a third option, using a taper lock bush rather than a conventional key.
How do I measure a worn keyway to know what key size to order?
Measure the keyway width with an outside micrometer or digital vernier caliper across the slot opening. The nominal key width should match this measurement. For worn keyways, measure both ends of the slot — wear is often uneven. If the slot width varies by more than 0.15–0.2mm, the keyway is too worn for a standard key and the component should be replaced or the keyway repaired.