NEMA MG-1 Part 4

NEMA motor shaft dimensions by frame

Shaft diameter, extension length and keyseat for every NEMA frame we build, with the MG-1 diameter tolerance each part loads with and the finished size limits it produces. The same numbers the configurator stamps on the drawing.

NEMA MG-1 Part 4 fixes the shaft extension of a standard motor frame: the diameter U, the length N-W from the mounting face to the shaft end, and the keyseat (or, on the smallest frames, the flat) that drives the coupling or sheave. The frame number alone therefore says what the shaft is, which is why a question like "what is the shaft diameter of a NEMA 184T" has one answer.

The table below lists the 71 frames in the catalog, from 42 at 3/8″ to 449T at 3-3/8″, grouped the way the motor families are sold. Beyond the dimensions, it carries the shaft-extension diameter tolerance each part loads with and the finished maximum and minimum diameter that tolerance produces, which is the number a coupling or bearing bore is checked against.

Pick a frame in the configurator and the part opens with exactly these dimensions and this tolerance on the extension; material, finish and added features stay editable, and the drawing title block stamps the source frame so any deviation is visible on the print.

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Shaft dimensions and applied tolerance by frame

Inch dimensions per NEMA MG-1 Part 4. U is the shaft diameter; N-W the extension beyond the mounting face. The tolerance is the one on the part's shaft segment; max and min are the finished diameter limits it produces.

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Frame families in the catalog

Summarised from the same data: frame count, diameter and extension span, and what the shaft end carries in each family.

The MG-1 diameter tolerance, as applied

Every frame carries a unilateral tolerance on the extension diameter: the upper deviation is zero, so the shaft is never over nominal, and the lower deviation depends on size. Read off the parts themselves, the rule is +.0000/-.0005 on shafts through 1.500″ and +.0000/-.0010 above it. In this catalog that means 39 frames from 3/8″ to 1-3/8″ carry +.0000/-.0005, and 32 frames from 1-5/8″ to 3-3/8″ carry +.0000/-.0010.

The tolerance sits on the shaft segment, not on the part defaults, so it shows as the actual deviation in the configurator and on the drawing rather than as a preset name. ISO 286 h-classes were not used because none tracks the MG-1 band across the whole size range; a buyer who needs an ISO class on a motor shaft can set it on the diameter and the configurator computes those limits instead.

Frame families and variants

The families table summarises the groups the data uses. T-series and large T-series frames carry a single keyseat and a plain shaft end; TS short-shaft frames are the direct-coupled variant and are smaller in diameter as well as shorter than the same frame number's T shaft, so they are distinct parts. C-face (TC) frames share the T shaft of the same number; the flange is on the motor, not the shaft. JM and JP close-coupled pump frames add an axial tapped hole in the shaft end for impeller retention, JP with the longer shaft for a packing gland and sleeve.

Tags come from the data as well: 56C is tagged C-face, 56J is tagged J-mount, 449T is tagged Largest. Submersible, hydraulic-splined and pool-pump frames are not in the catalog because they need features the configurator does not expose yet.

Related: Motor shafts · Keyway and keyseat dimensions · Shaft fits and tolerances.

Frequently Asked Questions

These references are for specifying a part; every value here is one the configurator applies when you build one. Numbers are representative of the standard; check them against your part and its mating components before ordering. See all guides on the guides page.