What motor flange dimensions describe

Motor flange dimensions describe five features: the bolt circle (PCD), the pilot diameter, the outer diameter, the bolt hole size and the flange thickness. Read them together with the shaft diameter and length, and you know whether a motor will bolt onto a gearbox, pump or machine frame without machining.

Motor flange dimensions refer to the agreed set of measurements on the drive-end face of an electric motor. They control how the motor centres and bolts to a driven machine. A flange is simply a projecting rim or plate used for attachment. On a motor it is machined with a tight spigot, so the shaft lines up with the load.

Which letters and numbers matter on a flange drawing?

Most IEC drawings use a short letter code. M is the bolt circle diameter, often listed as motor flange PCD dimensions in catalogues. N is the pilot (spigot) diameter, P is the outer diameter, S is the hole diameter on through-bolt flanges, and T is the flange thickness. The shaft appears as D × E, meaning diameter times length. When you scan motor flange dimensions on a drawing, these letters give the motor shaft and flange dimensions mm that a designer needs.

In day-to-day enquiries, the commonest slip is confusing M with P. A "165" flange has a 165 mm bolt circle, not a 165 mm outer diameter (that would be 200 mm). Read the letter before the number.

Why does flange size not follow motor power directly?

Because the flange is a mounting option, not a power rating. A 90 frame motor can carry an FF165 or an FT115 flange, and an 80 frame motor can also take FF165. So why do so many buyers assume that kilowatts decide the interface? Mostly because catalogues list power and frame together, which hides the separate choice of flange code. The frame sets shaft size and centre height, while the flange code sets the face. It works like a plug and a wall socket: power decides the cable, but the flange decides whether the plug fits.

Standards behind motor flange dimensions: IEC, EN and NEMA

Three documents define most of the motor flange dimensions you will meet. IEC 60072-1 covers frame and flange series. EN 50347 links output power to mounting size on three-phase induction motors. NEMA MG 1 covers North American frames. In Spain, EN 50347 is published as UNE-EN 50347, so local specifications normally cite it. Industry consensus is that these texts, not individual catalogues, are the reference when two suppliers disagree.

What does IEC 60072 say about motor frame size dimensions?

IEC 60072 motor dimensions are organised by frame size, which is the shaft-centre height in millimetres: 80, 90, 100, 112, 132 and so on. For each frame the standard fixes the foot hole spacing, the shaft extension and the available flange series. In practice, two motors of the same frame size from different makers should be interchangeable for foot or flange mounting, within tolerance. Of course, mounting is only part of the story. Overall length, terminal box position and weight still vary between makers.

Summary of the scope of IEC 60072-1 (paraphrased, not a verbatim quotation): the standard relates rated outputs of rotating electrical machines to frame numbers and fixes mounting dimensions, such as shaft extensions and flange sizes, so that machines from different manufacturers can be interchanged.

How do FF and FT codes relate to B5 and B14?

B5 and B14 are mounting arrangement codes from IEC 60034-7, whereas FF and FT are flange designations. For three-phase motor flange types FF FT, remember a simple rule. FF means free (through) holes and pairs with B5. FT means tapped holes and pairs with B14. A B35 motor adds feet to a B5 flange. The number after the letters is the bolt circle in millimetres, so one glance at the nameplate reveals the PCD.

NEMA speaks a different language. MG 1 describes C-face (tapped holes in the face) and D-flange (through holes) mounts, with suffix letters such as TC on the frame number. IEC motor flange sizes and NEMA designations never share a code, which is why mixing catalogues is risky.

Reading B5 and B14 motor flange dimensions in a size chart

To read an electric motor mounting flange size chart, find the frame size row, note the flange code, then read M, N, P and S from left to right. The code already gives the bolt circle: FF165 means a 165 mm PCD. Table 1 lists the most common IEC motor flange sizes for B5 mounting.

How do you read B5 flange motor dimensions?

Take a 90L motor with an FF165 flange. The chart gives M = 165 mm, N = 130 mm (j6), P = 200 mm and four holes of 12 mm diameter. The motor flange bolt pattern is therefore a 165 mm circle with holes spaced at 90°. The pilot must enter a recess of about 130 mm (H7) on the gearbox or machine. If that recess measures 110 mm, you are looking at an FF130 interface, and the motor will not seat.

Table 1: B5 (FF) motor flange dimensions per IEC 60072-1, in mm

Flange thickness T differs between makers, so it is not tabulated here. Take it from the datasheet.

What are B14 face mounting flange dimensions?

B14 face mounting flange dimensions use tapped holes in the motor face, so the bolts enter from the machine side. This arrangement is common on compact geared motors and on frames up to about 132. Table 2 lists the FT series.

Table 2: B14 (FT) face flange dimensions per IEC 60072-1, in mm

Compare the two charts and a useful pattern appears. FF100, FF115, FF130 and FF165 share the same M, N and P as FT100, FT115, FT130 and FT165. Only the hole type differs, through hole versus thread. That is why a machined adapter can sometimes convert one to the other, though only after an engineering check of bolt length and thread engagement.

NEMA C-face motor flange dimensions for imported machines

NEMA C-face motor flange dimensions are defined in inches by AJ (bolt circle), AK (pilot diameter), BD (outer diameter) and BF (tapped hole size), with the shaft diameter given as U. Table 3 converts the common frames to millimetres for readers used to metric drawings.

Table 3: NEMA C-face motor flange dimensions, inches (mm)

Can NEMA and IEC flanges be interchanged?

Not directly. A 182TC has a 184.2 mm bolt circle and a 215.9 mm pilot, while the nearest IEC flange, FF215, has a 215 mm bolt circle and a 180 mm pilot. Notice the trap: the number 215 appears in both, but in one case it is a pilot and in the other a PCD. Shaft sizes also differ slightly (28.6 mm against 28 mm). Adapter plates exist, but they add length and need a stiffness and alignment check.

Where do Spanish OEMs meet NEMA motors?

Typically on machines imported from North America, such as packaging lines or pumps, or when a supplier quotes a design originally built for that market. Such motors are often wound for 60 Hz, so on a 50 Hz grid speed and performance change as well. Mechanical fit is therefore only half the question. Ask the supplier for a 50 Hz rating and a dimensioned drawing.

Tolerances that control motor flange dimensions in practice

The pilot diameter normally takes a j6 tolerance up to 230 mm and js6 above, the shaft takes j6, k6 or m6 depending on its diameter, and the bolt holes are clearance holes. The mating bore should be H7, which gives a transition fit that centres the motor without force. Table 4 summarises typical values, which you should verify against IEC 60072-1.

Table 4: typical fits and tolerance classes for motor flange dimensions

What is the right fit for the flange pilot diameter motor spigot?

For a flange pilot diameter motor spigot, a j6 pilot in an H7 recess is the standard answer. It slides in by hand yet keeps the shaft centred. Too tight and paint or burrs jam it. Too loose and the flange face carries the misalignment into the bearings. Remember that paint thickness counts, and a spigot with a heavy coating can behave like a different class.

How much runout is acceptable?

IEC 60072-1 defines normal, reduced and precision classes for concentricity of the pilot and perpendicularity of the face. For pilots between roughly 100 and 230 mm, the normal class is commonly quoted at about 0.10 mm, with the reduced class near half that. Treat these as orientation values and read the standard text. With a dial indicator on the pilot and face, a visible wobble on the gearbox side can contribute to bearing noise, so check both parts.

Why do tolerances matter more with variable-speed drives?

Because inverter-fed motors often run at higher or wider speed ranges than fixed-speed ones, and vibration from misalignment grows with speed. A fit that was tolerable at 1,500 rpm may become audible at higher frequency. The same applies to high-efficiency permanent magnet and reluctance designs, which are almost always drive-controlled.

Step-by-step: how to verify motor flange dimensions

You can verify motor flange dimensions in six steps using a nameplate, a calliper and the mating machine drawing. The routine below takes about ten minutes and avoids most ordering errors.

1. Read the nameplate for frame size, mounting code (B5, B14 or B35), power and pole count.
2. Find the flange code (FF or FT) in the manufacturer's chart, or derive it from the PCD.
3. Measure the pilot N with a calliper. For the PCD, measure the outer edges of two opposite holes and subtract the hole diameter.
4. Count the holes and check their diameter or thread size with a gauge.
5. Measure shaft diameter D and length E, and confirm the key and tolerance.
6. Compare every value with the mating drawing, then check overall length, terminal box side and weight.

A frequent error is reading Spanish drawings that use a decimal comma. A hole marked 14,5 mm is 14.5 mm, but a quick glance can turn it into 145. Units and tolerance letters stay the same.

Worked example 1: replacing a B5 motor on a gearbox (illustrative scenario)

A maintenance team removes a failed 1.5 kW, 4-pole motor marked 90L, IM B5. The gearbox input recess measures 130 mm and the bolt circle 165 mm. Table 1 points to FF165 with a 24 × 50 mm shaft, and a 90L frame at 1.5 kW matches the EN 50347 power-frame assignment. The team orders an FF165 replacement, checks that the shaft key is 8 mm wide, and confirms the 12 mm holes against M10 bolts. Fit is achieved without an adapter.

Worked example 2: choosing between B5 and B14 for a compact drive (illustrative scenario)

A machine builder has a housing with a 110 mm recess, a 130 mm bolt circle and four M8 tapped holes on the machine side. That is an FT130 interface, so frame 100 or 112 with a 28 × 60 mm shaft applies. A B5 motor with FF130 would not suit because that code belongs to frame 71 with a 14 × 30 mm shaft. Choosing by frame and flange code together avoids buying the wrong mounting. If the quote is for an IE4 or IE5 motor, ask whether its frame assignment follows the EN 50347 table, because the flange code, not the power class, decides mechanical fit.

Which reading mistakes cause most returns?

Three recur. Mistaking P for M, ignoring the difference between through holes and tapped holes, and assuming a frame size implies one flange. Sound familiar? A short written checklist on the purchase order prevents all three.

Adapters, drawings and motor flange dimensions PDF files

Gearbox motor flange adapter dimensions are defined by the adapter's input side (a bolt circle and pilot matching the motor) and its output side (matching the gearbox or machine). When the two interfaces differ, an adapter plate or bell housing bridges them. Always specify both sides.

How do gearbox motor flange adapter dimensions work?

An adapter plate is machined with a pilot on one face and a recess on the other. Together with a coupling, it carries torque from the motor shaft to the input shaft. Check three things: the axial length added by the plate, the coupling bore sizes, and the bolt length so threads in B14 holes are not bottomed. Think of it as a translator between two languages. It works well, but each added part adds a little tolerance.

What should a flange mounted electric motor drawing include?

A proper flange mounted electric motor drawing shows the flange code, M, N, P, S or thread, the shaft D × E with key, the overall length, the terminal box position and the mounting code. It should also state the tolerances for the pilot and shaft. If a drawing lacks the pilot tolerance, ask for it before approving the order.

Where can you find a reliable motor flange dimensions PDF?

Use the manufacturer's current catalogue or datasheet first, because it reflects the exact housing. For general reference, use the standard texts through the national body, such as UNE in Spain, or the IEC webstore. A motor flange dimensions PDF copied from a forum may mix B5 and B14 values or omit tolerances. Inverter-integrated permanent magnet synchronous motors (IE4/IE5) and reluctance machines are often offered on standard IEC frames and flange codes, but the integrated drive can lengthen the housing, so verify the overall length as well as the flange.

Reading motor flange dimensions comes down to a repeatable habit: identify the standard, read the flange code, confirm M, N, P and S, then check the fits and the shaft. With the charts above, a calliper and the mating drawing, you can match motor flange dimensions to a gearbox or machine before any money is spent. Of course, charts are only a guide. The manufacturer's dimensioned drawing for your exact model is always the final word.