What is a motor flange

What Is a Motor Flange? (Definition)

A motor flange is a flat, machined ring or plate on the drive end (DE) of a motor that replaces — or supplements — the traditional mounting feet. Instead of resting on a base, a flange-mounted motor is fastened face-to-face onto the driven equipment. Because the flange and its spigot are machined in the same operation as the bearing housing, the motor shaft ends up automatically concentric with the mating bore, so no field alignment is needed.

Every motor flange is built from three functional features, and understanding them is the key to specifying the correct part:

1. The spigot (pilot / register / rabbet)

The spigot is a short cylindrical shoulder machined concentric to the shaft. It slots into a matching recess in the gearbox or pump housing and does the actual centering work — the bolts only clamp. IEC calls it the spigot or pilot; NEMA calls it the rabbet or register (dimension AK). Its concentricity to the shaft is tightly toleranced under IEC 60072-1.

2. The mounting holes and pitch circle diameter (PCD)

The bolt holes sit on a pitch circle diameter (PCD), also called the bolt circle diameter (BCD) or NEMA AJ. The holes are either clearance (through) holes — you pass a bolt through the flange into a tapped machine housing (IEC FF / NEMA D-flange) — or tapped (threaded) holes — you screw a bolt from the machine side into the motor face (IEC FT / NEMA C-face).

3. The flange outer diameter and thickness

These set the envelope and the load path. On a large FF/B5 flange the outer diameter is bigger than the motor body; on a small FT/B14 flange it is usually equal to or smaller than the body. The flange must carry the full cantilevered weight and torque reaction of the motor, so its thickness is a structural, not cosmetic, dimension.

How a Flange Mount Works: Step-by-Step

1. The spigot enters the housing bore. The machined pilot diameter is a close (often h7/H8) slip fit into the mating recess of the gearbox, pump adapter, or bracket. This locates the motor radially and forces the shaft axis to coincide with the driven-shaft axis.
2. The faces seat flat. The machined flange face contacts the machine face squarely, setting axial position and squareness (perpendicularity of the shaft to the mounting plane).
3. The coupling engages. Because the shaft is now pre-aligned by the spigot, a rigid or short flexible coupling — or a direct splined/keyed input into a gearbox — can be joined without dial-indicator alignment.
4. The bolts clamp. Bolts on the PCD are torqued to develop a clamp load that resists the motor’s weight and the reaction torque. Clamp force, not friction from a loose fit, keeps the joint rigid: F = T / (K·d), where T is bolt torque, d bolt diameter, and K the nut factor (~0.2 for dry steel).
5. Load transfers through the flange. Weight and vibration flow from the motor, through the flange ring, into the machine structure — which is why flange concentricity and stiffness directly affect bearing life and vibration.

The result is a compact, self-aligning, floor-space-free drive. That is exactly why close-coupled pumps, speed reducers, and OEM machines overwhelmingly use flange motors instead of foot-mounted ones.

Motor Flange Types Compared (IEC 60034-7)

IEC 60034-7 assigns each mounting arrangement an IM (International Mounting) code. The four flange-related codes below cover almost every industrial flange motor. The most important — and most confused — distinction is B5 vs. B14: B5 has clearance holes (bolt passes through the flange), while B14 has tapped holes (bolt threads into the flange).

Vertical variants (shaft down = V1, shaft up = V3/V18) use the same flanges but need thrust-rated bearings and, outdoors, a rain canopy. For a foot-mounted comparison, see our guide to AC vs. DC motors.

IEC Flange vs. NEMA Flange (Global Sourcing)

North American equipment uses NEMA MG 1 terminology instead of IM codes. Functionally they map cleanly, but the dimensions and frame numbers differ, so an IEC B5 motor will not bolt to a NEMA D-flange housing without an adapter.

FF and FT Flange Designations Explained

Under IEC 60072-1, a flange is named by its type letters plus the pitch circle diameter in millimetres. So FF265 means a through-hole (FF) flange with a 265 mm PCD; FT115 means a tapped (FT) flange with a 115 mm PCD. The table pairs common IEC frame sizes with their standard flange.

Spigot diameter “N” is the pilot register the housing bore must match. Values follow IEC 60072-1; always confirm against the specific manufacturer datasheet before machining a mating part.

Motor Flange Engineering Data: Tolerances, Loads & Formulas

A flange looks simple, but its performance is governed by tight geometric tolerances and a real mechanical load path. Getting these numbers right is what separates a vibration-free drive from premature bearing failure.

Concentricity and run-out tolerances (IEC 60072-1)

IEC 60072-1 defines two tolerance grades for the spigot and face: Normal (N) and Reduced (R) for precision applications. The spigot must be concentric to the shaft, and the mounting face perpendicular to it, within these limits (total indicator reading, TIR):

Exceed these and the shaft runs eccentric to the driven bore, injecting a once-per-revolution radial force that shortens bearing L10 life and raises vibration.

Key formulas for flange-mounted drives

• Bolt clamp load: F = T / (K · d) — clamp force F from torque T, bolt diameter d, nut factor K (≈0.20 dry, ≈0.15 lubricated). Under-torqued flange bolts let the joint fret and loosen under vibration.
• Reaction torque on the flange: T = P × 9550 / n (N·m), with power P in kW and speed n in rpm. This is the torque the flange bolt pattern must react at start-up and stall.
• Bolt shear from reaction torque: F_bolt = 2T / (n_b · PCD) — tangential load per bolt, with n_b bolts on the pitch circle diameter. Larger PCD = lower per-bolt load.
• Cantilever bending on the flange: M = W · L_cg — motor weight W acting at its center-of-gravity distance L_cg from the flange face; the reason large motors prefer B35 (foot + flange) over B5 alone.
• Bearing L10 life: L10h = (10⁶ / 60n) · (C/P)³ for ball bearings — misalignment from a poor flange fit raises the equivalent load P and cuts life cubically (see SKF bearing calculator).

Worked example

A 7.5 kW, 1,450 rpm B5 motor on an FF265 flange (PCD 265 mm, 4 bolts): reaction torque T = 7.5 × 9550 / 1450 ≈ 49 N·m. Tangential load per bolt F_bolt = 2 × 49 / (4 × 0.265) ≈ 92 N — small, so the bolts are sized by clamp/vibration, not shear. This confirms why correct preload torque matters more than bolt count on most flange motors.

Materials and typical thermal limits

Flanges are cast iron (heavy-duty rigidity), aluminium (light OEM frames), or fabricated/stainless steel (washdown). Insulation class governs the surface heat the flange transmits into the driven machine: Class B 130 °C, Class F 155 °C, Class H 180 °C per IEC 60034-1 — relevant when a plastic pump adapter meets a hot motor flange.

Best Applications for Flange-Mounted Motors

How to Select the Right Motor Flange: Step-by-Step

1. Confirm the standard. IEC (metric, IM codes) or NEMA (inch, C/D-face)? This is set by your existing machine, not preference.
2. Read the mating housing. Measure the recess bore (this must equal the motor spigot Ø N / AK) and the bolt circle (PCD / AJ). These two numbers define the flange.
3. Identify the hole type. Do bolts pass through the flange into a tapped housing (B5 / D-flange, FF) or thread into the motor face (B14 / C-face, FT)? This is the #1 mistake — verify with a photo or thread gauge.
4. Match the frame & shaft. The flange PCD is tied to frame size; also confirm shaft diameter, length, and key so the coupling fits.
5. Check load direction. Heavy motor, vibration, or overhung load? Step up to B35 (foot + flange) rather than trusting the flange bolts alone.
6. Specify tolerance grade. Precision spindles or high-speed pumps warrant the Reduced (R) concentricity grade, not Normal (N).
7. Select material & enclosure. Cast iron for industrial rigidity, stainless for washdown, aluminium for light OEM; then pick IP/enclosure rating.

Common Engineering Mistakes with Motor Flanges

• Confusing B5 and B14. They look similar but B5 has clearance holes and B14 has tapped holes. A B14 ordered as B5 will not bolt on. This is the most frequent flange ordering error.
• Matching PCD but ignoring the spigot. Two flanges can share a bolt circle yet have different pilot diameters — the motor won’t center. Always match spigot Ø N (AK) and PCD (AJ).
• Mixing IEC and NEMA. An IEC B5 flange will not fit a NEMA D-flange housing; the metric/inch dimensions differ. Use an adapter plate or order to the correct standard.
• Overloading a bare B5 flange. Using flange-only mounting for a heavy or overhung load overstresses the flange bolts and bends the mounting face. Move up to B35.
• Under-torquing the bolts. Insufficient clamp load lets the joint fret, loosen, and shift under vibration. Torque to spec using F = T/(K·d).
• Forcing a vertical mount. Turning a horizontal B5 motor shaft-down without a thrust-rated bearing (and rain canopy outdoors) leads to early bearing failure.
• Paint or burrs on the spigot/face. Contamination breaks the flat face contact and concentric fit, re-introducing the misalignment the flange was meant to remove.

Motor Flange Troubleshooting Table