NEMA T-frame motor shafts span 0.625 in (frame 56) to 3.375 in (frame 449T), with keyway width scaled from 3/16 in up to 1 in across the same range [S3][S7].
The frame number is a direct code, not a model number: the first two or three digits divided by 16 give the shaft centerline height in inches, and a trailing letter (T, TS, C, U, Y, Z) sets the shaft and mounting family [S2][S5].
How to Read a NEMA Frame Number
The first two or three digits encode the "D" dimension, which is the shaft height from the bottom of the feet to the shaft centerline. Divide that number by 16 to recover inches: a 56 frame is 3.5 in, a 143T is 8.94 in (143 ÷ 16), a 256T is 16.0 in, and a 449T is 28.06 in [S2][S6]. Two competing conventions appear in print; some guides state the first two digits divided by 4, which only works for the older 3-digit pre-T system [S2]. For modern T frames the divide-by-16 rule is the one that matches the nameplate [S5][S6].
The trailing letter sets the mechanical family. "T" has been the standard since 1964, replacing the "U" frame introduced in 1952 [S2]. A "C" suffix flags a C-face flange mount with threaded bolt holes on the face, used heavily on pumps and gearmotors. A "TS" suffix is a T frame with a short shaft for direct-coupled loads, where the shaft length is roughly half of the standard T dimension. A "Y" suffix marks non-standard mounting, and a "Z" suffix flags a non-standard shaft, which can be longer, larger in diameter, threaded, or with an extended keyway [S2][S5].
The middle digit, on a three-digit frame like 145T or 184T, encodes the front-to-back bolt spacing. A 143T has 3.00 in between the centers of the mounting-bolt holes front-to-back, while the 145T stretches that to 5.00 in for the same shaft height [S2]. Same 145T centerline height, longer foot, more room for a coupling on the back end.
Shaft Diameter and Keyway by Frame
Standard NEMA T-frame shaft diameters and keyseat dimensions follow a fixed table [S3][S7]. The keyway is typically a square cross-section, with the key width equal to roughly one quarter of the shaft diameter.
The standard 143T and 145T frames share the 0.875 in shaft and 3/16 in keyseat, so a coupling or shaft key stocked for one fits both. Keyways are cut to a square section, with the depth of cut measured from the shaft surface, which means the effective shaft diameter at the keyway is reduced by roughly the keyway depth on the top side [S7]. When you fit a shaft collar or a shaft coupling over a keyed region, account for that reduced clearance.
C-Face, D-Face, and Short-Shaft Variants
A C-face motor keeps the standard T-frame shaft and keyway, but adds a flange on the face with a bolt circle and threaded holes. The "C" suffix on a frame like 56C or 145TC does not change the shaft diameter, but the face mount changes the shaft protrusion (N-W dimension) so a coupling or guard can sit behind the flange without interference [S2][S4]. The flange bolt circle (AJ), register diameter (AK), and hole pattern are fixed by frame size and are listed in any standard NEMA quick-reference chart [S3].
The "TS" suffix is the other variant worth knowing. A 286TS keeps the 1.875 in shaft and 1/2 in key of the 286T, but the usable shaft length is cut from 4.625 in to roughly 3.375 in, with the "N-W" exposed length shortened for direct-coupled service [S3]. Mechanics reach for a TS frame when the driven equipment has its own short input shaft and a coupling hub, so the long T-frame shaft extension would just add overhang and critical-speed risk.
The pre-1964 "U" frames overlap the T dimensions in most respects, but the keyseat widths on certain U frames are not interchangeable with the T frame of the same numeric family. A 256U and a 256T share the 6.25 in shaft height but the U-frame shaft can differ in length and key proportion, so when you are replacing a U-frame unit, verify U, V, and the keyseat from the nameplate, not from the frame number alone [S2][S3].
Matching Keys, Couplings, and Mounting Hardware
The shaft key is the simplest part and the most often misordered. A NEMA 145T expects a 3/16 in square key, roughly 1.5 in long; a 286T expects a 1/2 in square key, longer to suit the 4.625 in shaft. Square keys are the default, but rectangular keys (flat stock) show up on older U-frame and special-shaft machines, so measure before ordering [S3][S7]. Forged steel keys (ASTM A686 or A668 grades) are the standard material; stainless keys appear in food-grade and washdown enclosures.
The shaft fastening choice above the key is just as consequential. A standard parallel key with a setscrew on the hub is acceptable for moderate torque, but for reversing service or high cyclic loads the hub should be taper-bored or the coupling should be a clamping style. A drive motor on a conveyor that reverses every cycle will eat a setscrew-hub keyseat in a few months if the key was not also fully engaged along its length.
The companion ac motor frame selection rule of thumb: pick the T frame that delivers the horsepower and speed you need, then confirm that the driven equipment's input bore and keyseat match the T frame's U and keyway dimensions. If the driven unit was made for an IEC metric shaft, you will need a spacer sleeve or a metric-to-Imperial coupling hub, since NEMA is inches and IEC is millimeters [S2][S9].
What Goes Wrong, and What to Verify
The most common field problem is a mismatch between the nameplate frame and the actual shaft. "Z" suffix motors, special-shaft builds, and C-face kits with stub shafts all deviate from the standard table. A 145T nameplate with a 1.125 in shaft is possible on a custom build; do not assume the table applies. Measure U, V, and the keyseat with calipers before ordering a replacement coupling or guard [S2][S5].
A second problem is keyseat depth creep. Repeated impact loading or reversing service can wallow out the keyseat, especially on the smaller 3/16 in and 1/4 in keyways. The standard 143T/145T keyway is 3/16 in wide by 3/32 in deep, leaving only the bottom half of the key bearing on the shaft [S7]. If you can rock the hub on the shaft with the setscrew loose, the keyseat is worn and the shaft (not just the key) needs replacement.
A third issue is metric-versus-Imperial confusion. IEC frames use the metric shaft height in millimeters (so an IEC 80 frame is 80 mm, roughly 3.15 in, close to NEMA 56 but not the same), and the shaft diameters and keyways are metric, not fractional inch. A coupling or guard stocked for NEMA 56 will not drop onto an IEC 80B14 without an adapter [S2][S9].
For a trackable next step: when you are about to order a drive motor, pull the existing nameplate, write down the frame, U, V, key width, and key depth, and cross-check against the Baldor NEMA quick reference chart before you spec a replacement ac motor [S3]. If the equipment is metric IEC, also confirm the bore and keyseat against the IEC dimension table, not the NEMA one. A second signal to watch: NEMA TC-frame TENV and TEBC builds for inverter duty are increasingly shipped with the same 1.875 in / 1/2 in keyseat as the 286T family regardless of frame length, so the key seat is no longer a reliable indicator of frame size on those builds [S3][S4].
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