Keys and Keyways: Types, Sizing and Alternatives
Short answer
- A key and keyway transfers torque from a shaft to a hub, and the right key is chosen by taking width from the ANSI/ASME B17.1 table for the shaft diameter and setting length from shear and bearing stress checks.
- Four key families are common: parallel square and rectangular keys, gib-head keys, Woodruff keys and taper keys, while a sled-runner keyway profile cuts the stress-concentration factor from as high as 2.5 to roughly 1.5.
- Four families dominate: parallel square and rectangular keys, gib-head keys, Woodruff keys and taper keys.
When a shaft must drive a hub, the most common way to transfer torque is with a key and keyway. The problem you face is not whether a key will work—keys are reliable—but how to choose the right geometry, size the key correctly, and decide when a different locking method is more appropriate.
What key types are used for shaft-hub torque transmission?
Four families dominate the market for mechanical torque transmission:
- Parallel (square) and rectangular keys – the classic shape cut to a uniform depth across the hub and shaft. Square keys have equal width and height (e.g., 0.250" × 0.250"); rectangular keys are taller than they are wide (e.g., 0.250" × 0.375").
- Gib‑head keys – a square or rectangular section with a reduced‑height “gib” that slides into a shallow recess in the hub. The gib provides a positive stop while allowing a shorter keyway in the hub.
- Woodruff keys – semi‑circular keys that sit in a semi‑circular pocket cut into the shaft end. They are ideal where a full‑depth keyway would weaken a short shaft.
- Taper keys – keys that are machined with a slight taper (typically 1:100). The taper forces a tight fit when the hub is pressed onto the shaft, reducing backlash.
All of these are produced to ANSI/ASME B17.1 dimensions, which means you can select a key based on shaft diameter alone—no need to calculate a custom width for each torque case.
How do you size a key for a given shaft?
The width (or height) of a key is taken directly from the standard table that matches the shaft diameter. For example, a 2‑inch shaft uses a 0.250‑inch square key; a 4‑inch shaft steps up to a 0.500‑inch key. The table also gives the key depth (usually 0.5 × key width for square keys) and the recommended keyway depth in the hub (typically 0.5 × key width). Because the width is fixed, the designer adjusts the length of the key to meet the torque requirement.
Two stress checks are required:
- Shear in the key: τ = T / (w · l · d) where T is the transmitted torque, w is key width, l is key length, and d is key depth. The allowable shear for medium‑carbon steel is roughly 0.6 × ultimate shear strength (≈140 MPa for typical 1045 steel). Solve for l to ensure τ stays below that limit.
- Bearing stress on the shaft: σ = T / (0.5 · D · l) where D is shaft diameter. The bearing stress must stay below the material’s compressive limit (often taken as 0.5 × ultimate tensile strength). Again, l is the variable that reduces σ.
In practice, designers start with the minimum length recommended by the standard (usually 1 × shaft diameter) and increase it in 0.5‑inch increments until both stresses are acceptable. A common rule of thumb is to keep the key length under 1.5 × shaft diameter to avoid excessive weakening of the hub.
How does keyway geometry affect stress concentration?
Cutting a keyway introduces a notch that concentrates stress at the fillet. The stress‑concentration factor (K_t) for a sharp‑corner keyway can be as high as 2.5, meaning the local stress is more than double the nominal bearing stress. To mitigate this, modern practice includes:
- Providing a fillet radius of at least 0.05 × key width at the keyway corners.
- Using a sled‑runner (also called a “relief” or “rounded”) keyway profile, where the bottom of the keyway follows a gentle curve rather than a flat cut. This reduces K_t to roughly 1.5.
- Choosing an end‑mill cut only when the part geometry or production volume makes a sled‑runner impractical. End‑mill cuts are faster but leave sharper corners.
When a high‑speed rotating shaft is involved, the reduced stress concentration from a sled‑runner can extend service life by a noticeable margin, especially if the hub is made from a softer alloy.
When is a simple key not the right choice?
Keys are versatile, but several alternative locking methods outperform them under specific conditions:
- Taper‑lock bushings and QD (quick‑detach) bushings: A tapered bushing is pressed onto the shaft, and a matching hub with a corresponding taper slides over it. The taper creates a self‑locking fit that eliminates backlash. QD bushings add a set‑screw that holds the hub in place for rapid removal. Both are covered in our bushings and hubs catalog.
- Keyless locking assemblies (e.g., clamp‑type or spline‑lock collars). These use a split hub that tightens around the shaft with a torque‑controlled bolt, providing uniform pressure and zero backlash. They are often used on high‑speed gear drives where precision is critical.
- Shrink discs: A thin steel disc is heated, slipped over the shaft, then allowed to contract, creating an interference fit. This method is popular for reversible drives that must maintain zero backlash under load.
- Splines: Multiple parallel keyways machined into the shaft and hub. The increased contact area distributes load, making splines the go‑to solution for very high torque (e.g., >10 kNm) or where repeated assembly/disassembly is required.
- Set screws: A single M6‑type screw pressed into a threaded hole in the hub. This is acceptable only for light, non‑reversing loads where the torque is well below the shaft’s capacity. Set screws are often found on small motor pulleys or fan brackets.
Choosing among these alternatives depends on three factors: torque magnitude, required backlash, and maintenance frequency. For example, a motor driving a high‑speed gearbox that must be removed weekly is a good candidate for a keyless locking assembly rather than a traditional key.
How do keys fit into the rest of the power-transmission train?
Even when you select a key, the surrounding components affect the overall reliability. A mismatched hub‑to‑shaft fit can cause excessive bearing loads on adjacent shaft couplings and collars. Likewise, the choice of fastener for the hub (e.g., a set screw or retaining bolt) falls under fasteners and hardware. Finally, if the hub carries a chain drive, the sprocket geometry must accommodate any hub offset introduced by the keyway; see our sprockets guide for alignment tolerances.
In practice, verify that the hub’s bore clearance (typically +0.001" to +0.003" for a press fit) does not interfere with the keyway depth. A too‑tight bore can cause the key to bear unevenly, accelerating fatigue. Conversely, an overly loose bore will allow the hub to shift under load, increasing the effective shear on the key.
How do you select and verify a key step by step?
Use the following steps to decide whether a key is appropriate and, if so, how to size it:
- Identify shaft diameter and material. Look up the standard key width and depth from the ANSI table.
- Calculate the required key length using the shear and bearing stress formulas. Keep length < 1.5 × shaft diameter unless the hub design forces a longer key.
- Choose a keyway profile. For high‑speed or fatigue‑critical applications, specify a sled‑runner profile with a 0.05 × key width fillet radius.
- Check the hub bore clearance and ensure the keyway does not create a stress concentration that exceeds the material’s fatigue limit.
- If torque exceeds 1.5 × shaft diameter · allowable shear, or if zero backlash is required, evaluate alternatives such as taper‑lock bushings, keyless locking assemblies, or splines.
- Document the selected fastener type (set screw, retaining bolt, etc.) and verify compatibility with the surrounding shaft couplings and collars.
Remember that any work on rotating equipment must be performed by qualified personnel following lock‑out/tag‑out procedures. Improper key installation can lead to sudden shaft failure, which is a serious safety hazard.
If you need help locating the appropriate key blanks, bushings, or alternative locking hardware, feel free to reach out for a cross‑reference or quotation.
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Request a QuoteFrequently asked questions
What are the main types of keys used on shafts?
Four families dominate: parallel square and rectangular keys, gib-head keys, Woodruff keys and taper keys. Parallel keys have a uniform depth across hub and shaft, gib-head keys add a reduced-height gib that gives a positive stop, Woodruff keys are semi-circular and sit in a pocket in the shaft end, and taper keys use a typical 1:100 taper to reduce backlash.
How do you size a key for a shaft?
Key width comes directly from the standard table that matches the shaft diameter; for example, a 2-inch shaft uses a 0.250-inch square key and a 4-inch shaft uses a 0.500-inch key. Because width is fixed, length is adjusted to meet torque, checking shear in the key and bearing stress on the shaft. Designers start at 1 × shaft diameter and keep length under 1.5 × shaft diameter.
How can you reduce stress concentration in a keyway?
A sharp-corner keyway can have a stress-concentration factor as high as 2.5. Using a fillet radius of at least 0.05 × key width at the corners and a sled-runner profile lowers that factor to roughly 1.5. End-mill cuts are faster but leave sharper corners, so they suit cases where a sled-runner is impractical because of part geometry or production volume.
When should you use something other than a key?
Alternatives outperform keys under specific conditions. Keyless locking assemblies and shrink discs provide zero backlash, splines suit very high torque above 10 kNm or repeated assembly and disassembly, and set screws are acceptable only for light, non-reversing loads. The choice depends on three factors: torque magnitude, required backlash and maintenance frequency.