IEC Motor Mounting Configurations: B3, B5, B14, B35 Explained

The mounting configuration of an electric motor defines how it is mechanically attached to the driven machine or structure. IEC standard 60034-7 defines all standard mounting types using the IM (International Mounting) code system. Understanding these codes is essential for correct motor specification and successful interchangeability between manufacturers.

The IEC IM Code System

Every motor mounting configuration has a full IM code (e.g. IM B3, IM V1) and an equivalent numerical code (IM 1001 for B3). The letter part indicates shaft orientation and mounting surface: B = horizontal shaft, V = vertical shaft. The numeral indicates the specific configuration. In practice, engineers use the short letter-number codes (B3, B5, B14, B35) exclusively.

Most IEC motors can be ordered in multiple mounting configurations from the same frame series. The motor housing is identical; only the bearing housing covers, terminal box position and (for flange motors) the flange face change.

B3 — Foot Mounting, Horizontal Shaft (IM 1001)

B3 is the most common motor mounting worldwide. Four tapped feet on the underside of the motor housing bolt to a flat base plate or machine frame. The shaft exits horizontally from the drive end.

Dimensions and standardisation

IEC 60072-1 standardises the foot hole pattern for each frame size. Key dimensions: foot width (A), centreline height (H), shaft diameter (d) and shaft length (l). A motor with frame 132M from any IEC-compliant manufacturer will have identical foot bolt hole positions, enabling direct bolt-in replacement.

Applications

B3 is used for any application where the motor can be mounted on a horizontal surface below the driven machine or alongside it: belt-driven fans and pumps, gearboxes with B3 input, conveyors with coupled gearmotors, and general purpose machinery where layout flexibility is needed.

Variants

• B6, B7, B8: Foot mounting with the feet on the side or top of the motor rather than the bottom. Same foot dimensions, different mounting surface orientation. Used when horizontal mounting on the foot-bottom is not possible due to space constraints.

B5 — Large Flange, Horizontal Shaft (IM 3001)

B5 motors have a large circular flange on the drive-end bearing housing. The flange has a precision-machined spigot (register) and a bolt circle for direct bolting to a mating flange on the driven machine. There are no feet — the flange provides all the mechanical support.

Dimensions and standardisation

IEC 60072-1 specifies the flange diameter (D), spigot diameter and bolt circle for each frame size. The flange diameter is significantly larger than the frame size number. For example, a Frame 132 B5 flange has an outer diameter of approximately 250 mm.

Applications

B5 is the standard for close-coupled centrifugal pumps, where the motor flange mates directly to the pump casing lantern bracket. It is also used for helical gearboxes with integrated IEC motor mounts (the motor bolts directly to the gearbox input face). The absence of a coupling shaft — the motor shaft inserts directly into the pump or gearbox — eliminates shaft alignment requirements and reduces system length.

B14 — Small Flange (Face Mounting), Horizontal Shaft (IM 3601)

B14 is similar to B5 but uses a smaller flange diameter with threaded (tapped) holes instead of through-bolt holes. The flange sits flush against the driven machine face. B14 is always smaller than B5 for the same frame size.

Key difference from B5

B5 bolts pass through the motor flange from the motor side. B14 bolts screw into tapped holes in the motor flange from the driven machine side. B14 is specified for compact gearmotor assemblies where the motor must be recessed into a machine housing.

Applications

B14 is the dominant mounting for motors coupled to standard IEC gearboxes (helical, worm, planetary). Most gearbox manufacturers provide B14 motor input flanges in IEC sizes. The small flange keeps the overall motor-gearbox length to a minimum.

B35 — Combined Foot and Flange (IM 2001)

B35 combines both B3 feet and a B5 large flange on the same motor. This gives the installer maximum flexibility: the motor can be mounted on its feet (B3 style), on the flange (B5 style), or on both simultaneously for especially heavy motors.

Applications

B35 is used where future installation changes are anticipated, or where both foot support and flange coupling are needed for rigid mounting of large, heavy motors. It is also the standard specification in many general-purpose motor catalogs, as it covers more applications than either B3 or B5 alone.

V1 — Flange Mounting, Vertical Shaft Downward (IM 3011)

V1 motors are essentially B5 motors oriented with the shaft pointing vertically downward. The motor hangs below its mounting flange. Special attention must be paid to bearing lubrication — the standard end-shield bearings are designed for horizontal duty; vertical mounting may require modified grease quantities or additional thrust bearing capacity.

Applications

V1 is the standard for vertical inline pumps (sump pumps, vertical turbine pumps), vertical agitators and submersible drives. The downward shaft orientation allows direct drive of vertical pump impellers without bevel gearbox or right-angle coupling.

Shaft Extension Options

Independent of the mounting type, motors are available with:

• Single shaft extension (standard): Drive end only.
• Double shaft extension: Both drive end and non-drive end have a shaft stub. Used for driving two loads simultaneously (e.g. motor with encoder on the non-drive end).
• Hollow shaft: Common on gearmotors where the driven shaft passes through the motor centre.

Mounting Selection Summary

All motors in the INDASTRA catalog list their available mounting configurations on the product specification page. Use the electric motor filter to search by mounting type, or contact our technical team if you need a non-standard variant.