Key Steel

What is key steel?

Key steel is precision-ground bar stock used to machine the keys that transfer torque between a rotating shaft and its hub — pulleys, sprockets, gears, couplings and fans. The key sits in a matched keyway cut into both shaft and hub, locking the assembly against slip under load. AIMS Industrial stocks metric and imperial key steel in square and rectangular sections, in zinc-plated carbon steel and 304 stainless, from Goliath, Precision Brand and Finer Power Transmissions.

What is a keyway?

A keyway is the rectangular slot machined into a shaft or hub that houses the key. The shaft keyway is cut along the shaft axis to half the key's height; the hub keyway is broached through the bore to the other half. With the key seated in both keyways, torque transfers from shaft to hub through the side faces of the key.

How do you choose the right key steel size?

Key cross-section is determined by shaft diameter — for example, a 12-17 mm shaft uses a 5x5 mm square key or a 5x3 mm rectangular key. Longer shafts and larger diameters take progressively larger key sections. Length is then cut to suit the hub width, typically with a small clearance to allow for the hub face chamfer.

Key steel is precision-ground bar stock used to machine the keys that transfer torque between a rotating shaft and its hub — pulleys, sprockets, gears, couplings, fans and other drive components. The key sits in a matched keyway cut into both shaft and hub; correctly sized to AS 2938 / ISO 773, it locks the assembly against slip under load and forms one of the most fundamental joints in mechanical power transmission. AIMS Industrial stocks metric and imperial key steel in square and rectangular sections, in zinc-plated carbon steel and 304 stainless, from Goliath, Precision Brand and Finer Power Transmissions.

Key Size by Shaft Diameter — AS 2938 / ISO 773 Quick Reference

The cross-section of a parallel key is dictated by shaft diameter. Use the table below to select the correct square or rectangular section per AS 2938 / ISO 773 — the dominant Australian and international standard for parallel keys and keyways. [VERIFY:] AS 2938 current edition year before quoting in tender documents.

[VERIFY:] Sizing values above against latest AS 2938 / ISO 773 tables — the boundary shaft diameters and keyway depths should be cross-checked against a current copy of the standard for production work.

Australian and International Standards for Key Steel

Parallel keys and keyways are among the most heavily standardised components in mechanical engineering. The dominant standards stocked by AIMS conform to are:

• AS 2938 — Australian Standard for parallel keys and keyways for shafts. The current edition is harmonised with ISO 773 and covers metric square and rectangular parallel keys, with dimensional tables for keys and keyways for shafts from 6 mm to 500 mm diameter. [VERIFY:] current edition year.
• ISO 773 — International Standard for rectangular or square parallel keys and keyways. The metric reference used worldwide; AS 2938 is the Australian adoption.
• DIN 6885 — German parallel key standard. Widely referenced on imported European drive components (German pumps, gearboxes, motors). Dimensionally compatible with ISO 773 / AS 2938 for the same key size.
• DIN 6886 — Taper keys (1:100 taper, used in older machinery and some heavy-duty applications).
• DIN 6887 — Gib-head taper keys (taper key with a projecting head for easier removal).
• DIN 6888 / ISO 3912 / AS 1402 — Woodruff keys (semi-circular key seating in a milled pocket).
• BS 4235 / ANSI B17.1 — Imperial parallel keys and keyways. Required when fitting older Australian machinery, imported US equipment or imperial-shaft drive systems.

For Australian drive work, AS 2938 (metric) and BS 4235 (imperial) cover the overwhelming majority of keyed connections. The DIN 6885 reference appears on European OEM data sheets; the key dimensions are interchangeable with AS 2938 / ISO 773 for the same nominal size.

Material Grades — Selecting the Right Steel for the Application

Key steel is supplied in several grades. Selection is driven by torque, environment and corrosion exposure.

Carbon Steel (1045 / EN8) — Zinc-Plated

Medium-carbon steel — typically AISI 1045 / EN8 equivalent with a tensile strength around 580 - 650 MPa — is the workhorse grade for general drive applications. Zinc plating provides mild corrosion resistance for indoor and undercover applications. This is the standard stocked grade for shaft-to-pulley and shaft-to-sprocket assemblies on conveyors, fans, compressors, gearbox output shafts and general industrial drives.

Alloy Steel (4140 / EN19) — High-Strength

AISI 4140 (EN19) chromium-molybdenum alloy steel is selected where torque levels demand higher shear strength. Tensile strength is typically 850 - 1,000 MPa in the annealed condition, higher again when heat-treated. Used in heavy mining drives, large gearbox couplings, marine propulsion shafts and high-load reversing applications. [VERIFY:] heat-treatment condition (annealed vs quenched and tempered) when ordering to match the design calculation.

Stainless Steel Key Steel — 304 & 316

Stainless steel key steel covers the corrosion-exposed end of the duty range. 304 stainless handles food-grade and general washdown applications (dairy, brewing, beverage, packaging, pharmaceutical, general outdoor). 316 stainless handles marine and chloride-aggressive environments (coastal plant, seawater spray, chlorinated wash-downs, swimming pool plant rooms, mining slurry). AIMS holds a working stocked range of 304 stainless key steel in the most common metric sections; 316 stainless and larger / non-standard 304 sections are available on indent — typically 1-3 week lead time depending on section and length. Call us with the section, length and quantity for a quote.

Stainless 304

304 stainless key steel is selected for corrosive or wash-down environments — food processing, beverage, pharmaceutical, packaging and general outdoor work. 304 resists most organic acids, dairy and brewing chemistries and freshwater corrosion. Tensile strength is lower than carbon steel (typically 500 - 700 MPa); upsize the key one section or specify shorter hub engagement at high torque.

Stainless 316

316 stainless is the marine, chloride and aggressive-chemical grade — coastal installations, seawater spray, chlorinated wash-downs, swimming pool plant rooms, mining slurry work. The molybdenum content provides better pitting resistance than 304 in chloride environments. Available on indent at AIMS — supply by section and length, 1-3 week lead time typical.

Brass — Non-Sparking

Brass key stock is used in hazardous areas where a steel key contacting a steel shaft could generate a spark capable of igniting gas, fuel vapour or combustible dust. Common in gas plant, fuel handling, grain handling and flour milling environments. Lower mechanical strength than steel — limited to lighter-duty drives and only where the explosion-protection requirement applies.

Cross-Section Types — Square, Rectangular and Round

Square Keys

Square cross-section keys (width equals height) are the most common in light to medium duty drives — pulleys, sprockets and small couplings up to roughly 40 mm shaft diameter. Square keys are easier to machine in the hub keyway and are interchangeable end-for-end at assembly.

Rectangular Keys

Rectangular keys (width greater than height — typically W:H ≈ 5:3 or 4:3) are used on larger shafts (typically above 40 mm) and where the keyway needs to be shallower to preserve shaft strength. The wider face spreads torque across a greater contact area while keeping the shaft keyway depth proportionally smaller. AS 2938 specifies rectangular keys for all shaft sizes above 38 - 44 mm.

Round Keys

Round keys — essentially dowels seating in a half-round groove milled across the shaft / hub joint — are uncommon in modern drive work but appear in some legacy machinery, light instrument shafts and specialised applications where the keyway must not interrupt shaft cross-section significantly. Not stocked as standard.

Key Sizing — Width, Height and Length Methodology

Key sizing follows three rules — width comes from the standard (AS 2938 table), height comes from the cross-section (square or rectangular), and length comes from the hub.

Width — Approximately ¼ of Shaft Diameter

As a quick rule of thumb, key width is approximately one-quarter of shaft diameter. AS 2938 prescribes the exact width for each shaft band — see the Quick Reference table above. Never machine a wider key than the standard prescribes; the keyway will weaken the shaft.

Height — Square or Rectangular

For square keys, height equals width. For rectangular keys, height is reduced — typically to 5/8 to 3/4 of width — to keep the keyway depth in the shaft to a minimum.

Length — 1.5× Shaft Diameter Minimum

Key length is governed by the hub width and torque. As a starting point, key length should be at least 1.5 times shaft diameter to develop full torque capacity. In practice, length is sized to the hub width minus a small clearance — see "Cutting Key Stock to Length" below. For very high torque, two keys can be fitted at 90° or 120° around the shaft.

Worked Example

For a 40 mm shaft driving a pulley with a 60 mm wide hub:

• Width × Height — per AS 2938 (38 - 44 mm band): 12 × 7 mm rectangular (or 12 × 12 mm square if the hub keyway allows).
• Length — minimum 1.5 × 40 = 60 mm. With a 60 mm hub, cut the key to 55 - 57 mm (allow 3 - 5 mm clearance — see below).
• Material — 1045 zinc-plated for standard drive, 4140 if reversing or high shock load, 304 stainless if washdown.

Selection Criteria — What to Specify When Ordering

When sizing key steel for a specific application, work through:

1. Shaft diameter — drives cross-section per AS 2938 (or BS 4235 for imperial shafts).
2. Hub width — sets the cut length (hub width minus clearance).
3. Standard match — confirm whether the original equipment uses AS 2938 / ISO 773 / DIN 6885 (all dimensionally compatible) or BS 4235 / ANSI imperial.
4. Torque transmitted — high torque or shock load points to 4140 alloy; standard duty to 1045 carbon.
5. Environment — corrosion exposure drives 304 or 316 stainless selection; hazardous areas require brass.
6. Tolerance class — standard commercial key stock is fine for most general drives; precision-ground key stock (e.g. Precision Brand) is selected for high-speed, reversing or OEM matched-fit work where backlash must be minimised.

Cutting Key Stock to Length

Important clearance rule — cut the key 3 - 5 mm shorter than the hub width. This gives clearance for the hub face chamfer at each end and prevents the key from over-extending past the hub face, where it can foul against adjacent components or trap on the assembly. A key cut hard to hub width binds on the chamfer and can prevent the hub from seating fully against the shoulder.

Cutting tools — small sections (up to 12 mm) cut cleanly with a hacksaw or abrasive cut-off wheel. Larger sections benefit from a horizontal bandsaw or cold saw. Always deburr the cut ends with a file or grinder; a sharp burr will gall the keyway during assembly.

Custom Keys and Non-Standard Keyways

When existing equipment has a non-standard keyway — common in older Australian machinery, custom-broached hubs and one-off OEM work — the supplied key may need to be machined from oversized stock. Buy key steel one section above the keyway dimensions and machine to fit. For Woodruff key applications, supplied semi-circular Woodruff keys (per AS 1402 / DIN 6888 / ISO 3912) are sized by a single number that indicates both diameter and thickness — these are not cut from key steel bar but are dedicated stocked components.

Common Applications

• V-belt pulleys — mounted to motor shafts and driven shafts via parallel keys (or via taper-lock bushes — see below)
• Sprockets — for roller chain drives on conveyors, gearbox output shafts, agricultural and mining equipment
• Couplings — flexible and rigid couplings between motor and driven shaft
• Gears — on gearbox output and idler shafts
• Fans, blowers and impellers — for HVAC, ventilation and process equipment
• Pump shafts — driven impellers on centrifugal and positive-displacement pumps
• Machine tool spindles — drive pulleys and gear arrangements on lathes, mills and drills
• Automotive and mobile driveline — auxiliary drive shafts and transmission components

Companion Components — Taper-Lock vs Keyed Connections

A keyed connection isn't the only way to lock a hub to a shaft. The most common alternative is a taper-lock bush — a tapered bushing with a slit that compresses around the shaft when bolts in the hub are tightened, gripping the shaft by friction alone (no keyway required). Taper-lock systems are dominant on modern V-belt pulleys, sprockets and couplings where:

• The shaft must not be weakened by a keyway (smaller or hollow shafts)
• The drive component must be removed and refitted regularly
• The shaft is a standard imperial size and the bush handles the metric conversion

Keyed connections remain dominant where high-shock or reversing torque exceeds taper-lock friction grip, on direct-drive couplings, on large mining and process drives, and on legacy equipment with pre-existing keyways. For a full taper-lock walkthrough including sizing, installation and removal, see our taper-lock bush guide.

AIMS Notes on Key Steel Selection and Stocking

To match the right key steel section, grade and length we need:

• Shaft diameter and whether the shaft is metric or imperial
• Hub width (drives cut length)
• Drive type — V-belt pulley, sprocket, coupling, gear, fan etc. — and any nameplate torque
• Service environment — indoor / outdoor / washdown / marine / hazardous area
• Standard reference on the existing equipment if known (AS 2938, DIN 6885, BS 4235 etc.)
• Quantity and length — single key, batch of 10, or 300 mm / 1,000 mm / 3,000 mm bar stock

For shafts above 100 mm or specialised metallurgy (e.g. heat-treated 4140 for mining drives, monel or duplex stainless for severe-service applications), we'll quote against project requirements.

Companion Ranges at AIMS

Key steel sits within our broader raw materials range and works alongside our power transmission range:

• Key stocks — pre-cut metric and imperial key stock in standard sizes
• Taper-lock bushes — keyless alternative for pulleys, sprockets and couplings
• Locking assemblies — high-torque keyless shaft-to-hub locking elements
• V-belt pulleys — mounted via key or taper-lock bush
• Timing pulleys — for synchronous belt drives
• Sprockets — for roller chain drives
• Couplings — flexible and rigid shaft couplings
• Bearings — for the rotating shaft
• Shims & shim stock — for hub alignment and float adjustment

Companion Resources

For background on key types, keyway design, parallel vs Woodruff keys and full sizing tables, see our keyways, keys and key steel size chart guide. For taper-lock alternatives and removal technique, see the taper-lock bush guide.

Need a specific section, length, grade or precision tolerance? Call (02) 9773 0122 or contact our team — we'll match shaft size, drive load and service environment to the right key steel grade.