Keyway sizes, explained

Keyways look like they should be calculated; they are actually looked up. Standards assign every shaft-diameter range a parallel key section and two depths, so torque capacity, interchangeability and off-the-shelf keystock all line up. The metric table is DIN 6885-1 (ISO 773 aligned); inch practice uses ANSI B17.1, whose old rule of thumb — a square key about d/4 — still gets quoted in every shop.

The anatomy: b × h, t1, t2

Four numbers define the joint: key width b (the working, side-bearing dimension), key height h, the keyseat depth in the shaft t1, and the keyway depth in the hub t2. The depths split the key so it drives on its sides: t1 + t2 runs slightly over h, leaving top clearance — parallel keys are not supposed to bear on top.

Worked example — 40 mm shaft

A 40 mm shaft falls in the DIN 6885 range that takes a 12 × 8 key:

b × h = 12 × 8   t1 = 5.0 mm (shaft)   t2 = 3.3 mm (hub)

The measuring trick: across the shaft from keyseat bottom to the opposite surface reads d − t1 = 35.0 mm, and the hub bore-plus-keyway reads d + t2 = 43.3 mm — both directly caliper-able. The keyway dimension calculator returns the full set for any metric shaft, with the fit classes (normal, close, free) that control how the key slides; the keyway-by-shaft-size lookup and size chart cover the table itself.

Fits: why keys are loose or tight

The same b × h comes in different width fits: a normal fit for general transmission, a close (interference-leaning) fit where reversing loads would hammer a loose key, and a free fit for hubs that must slide. Rocking under load and the wallowed-out keyway that follows is usually a fit choice problem, not a size problem.

Common mistakes

• Cutting t1 into the hub. The deep dimension (t1) belongs to the shaft; hubs get the shallow t2. Swapping them leaves the key proud and the hub un-seatable.
• Assuming square keys. Above ~22 mm metric shafts, keys are rectangular — measure both b and h before ordering keystock.
• Sizing torque by the key alone. The standard section suits the shaft's torque capacity, but length still matters: the key bears and shears over its engaged length — check it for short hubs (the spline capacity calculator covers the high-torque alternative).
• Ignoring the stress concentration. A keyseat roughly doubles the local shaft stress in fatigue — the shaft torsion guide and shaft stress calculator carry that side of the design.

Frequently asked questions

How are keyway sizes determined?

By shaft diameter, from a standard table — not by calculation. DIN 6885-1 (metric) assigns each shaft range a key width × height and the two depths: a 40 mm shaft takes a 12 × 8 key with t1 = 5.0 mm cut into the shaft and t2 = 3.3 mm into the hub. ANSI B17.1 does the same for inch practice, where the classic rule of thumb is a square key about one quarter of the shaft diameter.

What are t1 and t2 in a keyway?

t1 is the keyseat depth in the SHAFT, t2 the keyway depth in the HUB; together they exceed the key height slightly so the key drives on its sides with top clearance. For the 40 mm / 12 × 8 case: t1 = 5.0, t2 = 3.3 (5.0 + 3.3 = 8.3 against the 8 mm key).

How do I measure a shaft keyseat for depth?

Measure across the shaft from the keyseat bottom to the opposite surface: that dimension is d − t1 (35.0 mm for a 40 mm shaft), which calipers read directly. Measuring the little remaining sliver next to the keyway is the error-prone way.

Are shaft keys square or rectangular?

Metric DIN keys are square only on small shafts (up to ~22 mm); above that they go rectangular (wider than tall, like 12 × 8). Inch practice keeps square keys to larger diameters. Assuming square from one measurement is a classic reverse-engineering mistake — measure width AND height.