IEC Motor Frame and Gearbox Interface: B5, B14, B35 and Direct-Coupling Compatibility

Most R/F/K/S gear units are designed to accept a directly-coupled IEC-standard motor. The compatibility between the motor and the gearbox input depends on three dimensions — shaft diameter, flange PCD and pilot diameter — plus the bolt-hole pattern. Get any of these wrong and the motor either will not mount or will mount but transmit damage.

The IEC frame size system

IEC frame sizes are designated by a code that encodes the shaft centre height and the flange or foot dimensions. The two-digit height code runs from 63 (small) to 355 (large). The corresponding shaft diameter, flange PCD and pilot diameter are standardised in IEC 60072. A gearbox input flange is rated for a range of motor frame sizes (for example, 'IEC 80–132' means the unit accepts 80, 90, 100, 112 and 132 frame motors).

The three mounting codes: B5, B14, B35

1. B5: large flange only, no feet. The motor mounts to the gearbox face via a flange with bolt holes around a PCD defined in IEC 60072. Common for vertical and inline installations.
2. B14: small flange only, no feet. Used on small motors (typically 63 to 90 frame) where the flange PCD is below 200 mm. Common on modular geared motors where the gearbox input flange has a small bolt circle.
3. B35: large flange plus feet. The motor has both mounting options — flange to gearbox face and feet to a baseplate. Used on larger motors (typically 100 to 355 frame) where the weight requires the additional feet support.

The three critical dimensions

For direct coupling, all three of these dimensions must match between the motor shaft/flange and the gearbox input flange.

• Shaft diameter: must match within tolerance. The motor shaft is typically h6 or k6 tolerance; the gearbox input bore is typically H7. The fit is usually a light interference or transition fit that locates the coupling hub.
• Flange PCD (pitch circle diameter of the bolt holes): must match exactly. Off by even 1 mm and the bolts will not line up. Common PCDs are 165 mm (IEC 80), 215 mm (IEC 100), 265 mm (IEC 132), 300 mm (IEC 160), 350 mm (IEC 180), 400 mm (IEC 200).
• Pilot diameter (the spigot that locates the motor face): must match within 0.1 mm. This is what holds the motor concentric to the gearbox input shaft. A mismatch here will cause vibration and premature bearing failure.

What happens when dimensions don't match

Wrong shaft diameter: the coupling hub will not fit, or worse, will fit but transmit damage under load. Wrong PCD: bolts do not line up, and installers either force them (stripping the threads) or give up and machine an adapter (costly field fix). Wrong pilot: the motor face sits at an angle to the gearbox face, the input shaft sees a side load, the bearing fails within 6 to 12 months. All three failures look like 'the gearbox failed' in the warranty claim but are actually 'the motor and gearbox were mismatched at installation'.

How the gearbox is rated for motor frames

A gearbox data sheet will list a range of IEC frame sizes that the unit accepts. For example, an F37 might be rated 'IEC 71–80' meaning it accepts 71 frame and 80 frame motors via an input flange adapter. Going outside this range is a sign that the unit has been re-engineered for a non-standard application, or that the wrong input flange has been fitted. The data sheet also lists the maximum motor power the input flange can accept — exceeding this rating risks input bearing failure independent of the gearbox thermal capacity.

What to confirm when ordering a motor and gearbox pair

Confirm the gearbox input flange code (for example, IEC 132 B5) and the motor frame and mounting code (for example, IEC 132 B5). Confirm shaft diameter and length. Confirm whether a coupling is required (typical for helical-bevel and helical-worm) or whether the motor pinion mates directly with the gearbox gear (some inline units). If a coupling is used, confirm the bore size matches the motor shaft and the gearbox input shaft.

What if the motor and gearbox are from different suppliers

This is normal and supported as long as both comply with IEC 60072 dimensions for the frame size in question. The compatibility is dimension-based, not brand-based. If the motor is from a different region and uses NEMA frames (T-frame, C-face), an adapter plate is required. NEMA-to-IEC adapters are standard products but should be ordered with both motor and gearbox drawings to ensure correct pilot depth and bolt clearance.

For project-specific IEC frame compatibility confirmation, send the motor nameplate, the gearbox input flange code and any non-standard mounting requirement to sales@chinfar.com.