IEC 60072-1 ties shaft end diameter, key, and keyway geometry directly to motor frame size, so a frame 80 specifies a 19 mm shaft with a 6x6 mm key, frame 132 a 38 mm shaft with a 10x8 mm key, and frame 315 an 80 mm shaft with a 22x14 mm key, all dimensioned to the B.S. 4235 / ISO 773 metric parallel-key family [S1][S2].
That standardization is what lets a replacement motor from a different maker bolt straight into the existing coupling, pulley, or gearbox on the driven machine without re-machining the shaft seat or the hub bore [S4][S5].
Why Frame Size and Shaft Diameter Are Coupled
Shaft end diameter is not a free choice on IEC motors, since IEC 60072-1 locks the diameter (dimension "D") to the frame number, and the tolerance class steps with the diameter: j6 up to 28 mm, k6 from 38 to 50 mm, m6 above 50 mm [S3]. The logic is mechanical, not administrative: torque per cross-section is roughly constant, so larger-frame motors with higher rated torque get larger shafts, and the loose transition fit on a 24 mm shaft becomes an interference-grade m6 on a 60 mm shaft to hold the larger coupling hub concentric under load.
Because frame numbers carry the shaft geometry with them, two motors marked 132M from different vendors will accept the same coupling, the same key, and the same keyway dimensions; an M-frame simply means the stator stack is longer within the same shaft envelope, not that the shaft has changed [S4][S5]. This is the practical reason spec sheets for fans, gearboxes, and shaft couplings are written in frame sizes, not bare kilowatts.
Reference Table: IEC Frame vs Shaft Diameter, Key, and Keyway
The table below compiles the standard IEC 60072-1 frame-to-shaft mapping [S3][S5] against the parallel-key family in ISO 773 / B.S. 4235 Part 1 [S1][S2]. Key seat depth in the shaft (T1) and key height (h or T) come from the same B.S. 4235:1972 column D, so a keyseat is not just a slot, it is a depth-controlled seat sized to leave the full shoulder of the shaft intact for fatigue life.
Frame / Shaft D (mm) / Key W x T (mm) / Keyway W x T1 (mm): 56 / 9 / 3 x 3 / 3 x 1.4; 63 / 11 / 4 x 4 / 4 x 1.8; 71 / 14 / 5 x 5 / 5 x 2.3; 80 / 19 / 6 x 6 / 6 x 2.8; 90 / 24 / 8 x 7 / 8 x 3.3; 100 / 28 / 8 x 7 / 8 x 3.3; 112 / 28 / 8 x 7 / 8 x 3.3; 132 / 38 / 10 x 8 / 10 x 3.3; 160 / 42 / 12 x 8 / 12 x 3.3; 180 / 48 / 14 x 9 / 14 x 3.8; 200 / 55 / 16 x 10 / 16 x 4.3; 225 / 60 / 18 x 11 / 18 x 4.4; 250 / 65 / 18 x 11 / 18 x 4.4; 280 / 75 / 20 x 12 / 20 x 4.9; 315 / 80 / 22 x 14 / 22 x 5.4 [S1][S2][S3][S5]. Note that frame 90 and frame 112 share the 28 mm shaft end family on IEC metric small-frame tables, and frame 225 and frame 250 share the 60 mm / 18x11 family; hub bore selection must follow the actual shaft D, not the frame number alone.
How Keyway Depth Is Drawn from Shaft Diameter
Keyway width is selected first, then depth follows from the key seat table. For a 38 mm shaft, the key is 10 mm wide by 8 mm tall, the keyway in the shaft is cut 3.3 mm deep, and the remaining 4.7 mm of key height rides up into the hub keyway of the coupling [S1][S2]. On a 75 mm shaft, the same pattern scales: 20x12 key, 4.9 mm shaft seat, 7.1 mm in the hub, leaving the shaft material below the seat thick enough to avoid the classic "keyway fracture" that kills undersized shafts at the shoulder fillet.
This is also why loose-tolerance imperial references such as ANSI B17.1 show keyseat tolerances on the order of +0.000 / +0.002 in for the depth and +0.000 / +0.001 in for the width on a 1-1/2 in shaft [S9], and why Rexnord-style flexible coupling manuals cap machined keyway sides and bottom surface finish at 250 microinches Ra to keep stress concentrations inside the fatigue allowables [S6]. A keyway cut 0.2 mm too deep drops the effective shaft section at the key seat by a measurable amount and shortens fatigue life disproportionately on smaller shafts [S7][S8].
Selection Criteria: Rectangular vs Square vs Woodruff
For IEC motors in this size band, rectangular parallel keys (DIN 6885 / ISO 773 / B.S. 4235) are the default, and they come in two practical variants. Square keys (W = T, e.g. 6x6, 8x8) are used on short, lightly loaded hubs and on the smaller shaft sizes up to about 22 mm, where manufacturing convenience outweighs the slight loss of hub contact area. Rectangular keys (T less than W, e.g. 10x8, 22x14) are used on longer hubs and higher torque, because the key sits lower in the shaft seat and the hub keyway still gets a full 4 to 5 mm of engagement above the shaft shoulder [S1][S2].
For each shaft diameter, the standard list gives exactly one key size and one keyway depth, so a spec that calls out a 38 mm shaft with a 12x8 key is non-standard and will not line up with stock coupling bores. Comparison of the three common key geometries on the same 38 mm shaft: square 10x10 mm key is convenient but leaves 1 mm less of key above the shaft seat than a 10x8 rectangular key, so torque capacity per unit hub length is lower; rectangular 10x8 mm key, 3.3 mm shaft seat, is the IEC / ISO 773 default and is the practical choice for a standard flexible coupling; Woodruff keys (half-moon, sized by cutter number) are not part of the IEC 60072-1 standard shaft end, since they reduce shaft fatigue life and are reserved for short, light-duty applications outside the IEC frame envelope [S1][S2][S9].
Common Failure Modes and Spec Pitfalls
Most field failures on keyed IEC shaft ends come from one of three spec errors, not from the motor itself. First, a key that is too short for the hub length transmits the torque through a small contact patch and shears; ISO 773 key length is typically 0.9 times the hub length, and undersizing it is the classic cause of a milled keyseat in the bore. Second, a keyway that is too wide for the key (loose fit) causes fretting wear and eventual backlash, especially on reversing drives such as conveyors and packaging machinery, where the same shaft coupling will see both directions of torque every cycle. Third, mixing a metric key into an ANSI B17.1 hub, or vice versa: a 3/8 in (9.525 mm) imperial key does not seat properly in a 10 mm keyway, and a 10 mm key does not seat in a 3/8 in hub, so even a 0.5 mm difference shows up as a hammering fit on first start-up [S1][S2][S6][S8].
On the shaft side, the IEC tolerance jump from j6 to m6 above 50 mm is also where the assumption "the coupling will just press on" needs to be checked. An m6 shaft at 60 mm has an interference band of roughly -0.011 to -0.030 mm relative to an H7 hub, so a standard keyed sleeve coupling will still slide on for assembly, but a taper-lock bushing or shrink disk needs to be sized with that fit in mind [S3]. Hubs with a single set screw plus key are the most common production combination, and on those the key only needs to carry the reaction torque in the absence of the screw, so the key is essentially a backstop, not the primary torque element.
Who This Standardization Is For, and Where It Does Not Apply
IEC 60072-1 dimensional standardization is the right reference for general-purpose industrial induction motors from 56 frame upward, which covers everything from small fan and pump motors up to multi-megawatt industrial drives. It is also the right reference for any retrofit where the driven equipment (gearbox, shaft coupling, pulley) was already built to IEC dimensions, and for any specification where spare-parts interchangeability across vendors matters more than the last few percent of torque density [S3][S4][S5].
It is not the right reference for NEMA T-frame motors, which use inch-based shaft diameters and a different key family (typically 3/16 in through 3/4 in keys for 1-3/8 in through 3-3/8 in shafts, per ANSI B17.1) [S7][S9]. It is also not the right reference for high-speed servos, explosion-proof stepper motors in hazardous locations, or any application where the shaft geometry has been modified for a custom coupling. For servos and steppers, the shaft end is typically a smooth, keyless, or metric-only design per the servo maker's own standard, and adapting it to a parallel-key connection means re-machining rather than re-specifying [S3]. Engineers sourcing construction machinery and equipment drives that mix European IEC motors with American NEMA gearboxes should expect to do a coupling, not a key, swap at the interface.
Verifiable Signals to Track
Two nodes are worth a check before locking a spec. First, confirm whether the motor in question is actually an IEC 60072-1 standard shaft end, since some vendors (and many servo lines) ship non-standard shaft diameters within a nominal frame size, and the only reliable check is the dimension "D" on the nameplate datasheet rather than the frame number alone. Second, when replacing a failed motor in a 132 to 280 frame range, hold a physical key gauge against the new shaft before mounting the existing coupling: 10x8 vs 12x8, 18x11 vs 20x12, and 22x14 vs 25x14 are common near-misses on motors that differ by a single frame size step [S1][S2][S5].
For the relevant spec sheets and selection criteria, see shaft key.
See also our earlier report, IEC 60204-1 Emergency Stop Categories 0, 1, and 2: Spec Rules and Pitfalls.