Keyway Size Chart — Key Width, Height and Depth by Shaft Diameter

Metric Keyway Size Chart — DIN 6885-1 Parallel Keys

DIN 6885-1 (high form) fixes the key section b × h from the shaft diameter, the keyway depth t₁ in the shaft and t₂ in the hub. ISO 773 and BS 4235 list the same sections and depths, so a German, a British and an ISO drawing agree on the numbers. The d − t₁ column is the check dimension a caliper reads, given at the top of each diameter step; the last column is the key-length range the standard lists for that section.

Ranges read “over – to”: a 30 mm shaft is the top of the 22–30 step and takes 8 × 7; a 22 mm shaft is the top of 17–22 and takes 6 × 6. Back clearance = t₁ + t₂ − h, the gap between the key top and the hub keyway bottom — the key carries on its flanks, never on its back. DIN 6885-3 (low form) uses shallower depths and is not interchangeable with this chart.

Standard key lengths run in one series for every section: 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40, 45, 50, 56, 63, 70, 80, 90, 100, 110, 125, 140, 160, 180, 200, 220, 250, 280, 320, 360, 400, 450, 500 mm. A key is cut from this series; a length between two steps — 33 mm, say — is a machined key, not a stock one.

Inch Keyway Size Chart — ANSI B17.1 Square and Rectangular Keys

ANSI B17.1 sizes the key from the shaft diameter in the same way, but the keyseat depth is defined differently from the metric chart: each member is cut half the key height deep, so a 1/4 in square key sits 1/8 in in the shaft and 1/8 in in the hub. Square keys are the preferred form through 6-1/2 in shafts; rectangular keys are used where the hub wall is thin. In the usual Class 2 fit the keyseat width is held to the key width +0.002 in / −0.000 in, and the key stock itself is bought slightly oversize and fitted.

Metric and inch keys do not mix: 1/4 in = 6.35 mm, 3/8 in = 9.525 mm, 1/2 in = 12.7 mm. A 3/8 in key in a 10 mm keyway rocks by 0.475 mm and hammers the flanks out; a 10 mm key does not enter a 3/8 in keyseat at all. When a machine has one of each, the shaft is cut for the key that exists — see the repair table below.

Keyway Tolerances — Which Fit to Write on the Drawing

The key is the constant and the keyway carries the fit: the key width is h9 in every case, and the three fits differ only in the tolerance class written on the shaft and hub keyway widths. That is why an N9 keyway and a P9 keyway take the same key from the same box, and why writing just “8 mm keyway” on a drawing leaves the fit to chance.

Keyway depths t₁ and t₂ carry a plus tolerance only — the standard adds 0.1 to 0.2 mm above nominal depending on the section — because a keyway that is too deep only increases back clearance, while one that is too shallow lifts the hub off the shaft. The width is where a wrong number costs money: a keyway cut 0.05 mm wide on an 8 mm N9 flank is a key that hammers.

What Belongs on a Keyway Drawing — and What Is Usually Missing

The check dimension d − t₁, not only t₁. Keyway depth is defined from a theoretical shaft crest that has been milled away; nobody can measure it directly. A 25 mm shaft with t₁ = 4.0 mm is dimensioned 21.0 mm from keyway bottom to the opposite surface, and that is the number checked on the machine with a caliper or a depth micrometer.

A symmetry tolerance of the keyway to the shaft axis. Without it, a keyway cut 0.1 mm off centre still meets the width and depth — and the hub sits off-centre, the key bears on one flank only, and a coupling that was balanced runs out. The same applies to the hub keyway relative to the bore.

Length, position and end form of the keyway. Where the keyway starts from the shaft end, how long it is, and whether it ends round (milled with an end mill, for a Form A round-ended key) or runs out (cut with a side-and-face cutter or on a keyseater, for a Form B square-ended key). A round-ended keyway is milled in one set-up; a run-out keyway on a shoulder cannot be.

The key material and length. Stock keys are C45 (1045) in the length series above; a key that must be 316 stainless, or 33 mm long, or stepped for a repaired keyway, is a made part — see custom machine keys, parallel, Woodruff, gib-head and oversized for worn keyways.

Materials for Keyed Shafts, Hubs and Keys

Where the Chart Stops — Worn Keyways, Odd Shafts and Keys No Catalogue Lists

EKINSUN LTD is a custom parts manufacturer: keyed shafts to a sketch or a worn sample, keyed hub bores in replacement gears and gearbox parts, shaft collars bored to your shaft and the keys themselves are machined to drawing or measured from the part, one piece up. The chart above is the standard case; the table below is honest about where a stockist serves you better and where a stockist cannot help.

Typical requests read like this: “I need a 30 mm shaft in 316, 180 mm long, with an 8 mm keyway 45 mm long starting 10 mm from the end, diameter h6 — quantity 2.” Or: “The 10 mm keyway in this pump hub is hammered out — can you make a stepped key or recut it?” A sketch with the shaft diameter, the key size and the keyway position is enough to quote; the worn part itself is better still. German-speaking engineers use the Passfedernut-Tabelle DIN 6885, the same chart with the drawing notes in German.

Quick Answers by Shaft Diameter

What size keyway for a 20 mm shaft?

A 20 mm shaft takes a 6 × 6 mm parallel key to DIN 6885-1: the shaft keyway is 3.5 mm deep, the hub keyway 2.8 mm deep, and the check dimension d − t₁ is 16.5 mm. Standard key lengths for the 6 × 6 section run 14–70 mm. Write the keyway width as 6 N9 for a normal fit or 6 P9 for a tight one.

What size keyway for a 25 mm shaft?

A 25 mm shaft takes an 8 × 7 mm key: shaft keyway 4 mm deep, hub keyway 3.3 mm deep, check dimension d − t₁ = 21 mm. The 8 × 7 section is listed in lengths of 18–90 mm. EKINSUN LTD mills this keyway to N9 or P9 in shafts turned to h6, one piece up.

What size keyway for a 30 mm shaft?

A 30 mm shaft still takes the 8 × 7 mm key — it sits at the top of the 22–30 mm step — with a 4 mm deep shaft keyway, a 3.3 mm deep hub keyway and a check dimension of 26 mm. Only above 30 mm does the section step up to 10 × 8.

What size keyway for a 40 mm shaft?

A 40 mm shaft takes a 12 × 8 mm key to DIN 6885-1: shaft keyway 5 mm deep, hub keyway 3.3 mm deep, check dimension d − t₁ = 35 mm, key lengths 28–140 mm. Note that 10 × 8 (30–38 mm) and 12 × 8 (38–44 mm) share the same depths and differ only in width.

What size keyway for a 50 mm shaft?

A 50 mm shaft takes a 14 × 9 mm key: shaft keyway 5.5 mm deep, hub keyway 3.8 mm deep, check dimension 44.5 mm, key lengths 36–160 mm. At this size a P9 keyway in 42CrMo4 is the usual specification for reversing drives.

What size key for a 1 in shaft?

A 1 in shaft takes a 1/4 in square key to ANSI B17.1, seated 1/8 in deep in the shaft and 1/8 in deep in the hub; the rectangular alternative is 1/4 × 3/16 in, seated 3/32 in deep. In millimetres that key is 6.35 × 6.35 mm — not an 8 × 7 and not a 6 × 6.

What size key for a 1-1/2 in shaft?

A 1-1/2 in shaft takes a 3/8 in square key, 3/16 in deep in shaft and hub; the rectangular key is 3/8 × 1/4 in at 1/8 in depth. The 1-3/8 to 1-3/4 in step covers 35–44 mm shafts, where the metric chart would use 10 × 8 or 12 × 8.

What size key for a 2 in shaft?

A 2 in shaft takes a 1/2 in square key seated 1/4 in deep in each member; the rectangular key is 1/2 × 3/8 in, 3/16 in deep. A 1/2 in key is 12.7 mm wide — it will not enter a 12 mm metric keyway, and a 12 mm key rocks 0.7 mm in a 1/2 in keyseat.

Related charts: limits and fits — ISO 286 tolerance chart for the h6 / H7 seats a keyed shaft runs in, dowel pin size and tolerance chart, and metric bolt torque chart for the clamp screws on a split hub.

Keyway FAQ