The short answer is mechanical:

Mounting Feet Flange Shaft orientation Typical role
IM B3 Yes No Horizontal Base-mounted drives using a coupling, belt or gearbox
IM B5 No Yes Horizontal Flange-mounted pumps, gearboxes and compact machines
IM B35 Yes Yes Horizontal Installations that need feet plus a flange interface
IM V1 No Yes Vertical, shaft down Vertical pumps and other machines mounted above the driven equipment

Those descriptions identify the arrangement. They do not make two motors interchangeable. The IEC mounting code must be checked together with the frame size, flange dimensions, shaft dimensions, permitted bearing loads and installation orientation.

What the mounting code tells you

IEC 60034-7 provides the mounting-arrangement designation used on motor drawings and nameplates. The familiar short codes describe how the machine is supported and the direction of its shaft.

IM B3: feet, horizontal shaft

A B3 motor sits on two feet fixed to a base. It has no mounting flange. Alignment therefore comes from the motor feet, shims and base rather than from a flange pilot.

B3 is common for horizontally coupled pumps, fans, compressors and gearboxes. Belt drives also use foot-mounted motors, but belt pull adds radial load to the shaft and bearings. Confirm the allowable radial load at the actual pulley position instead of treating the mounting code as a bearing-load rating.

IM B5: flange, no feet

A B5 motor has a flange at the drive end and no feet. The flange supports and locates the motor on the driven machine. A locating pilot, the bolt circle and the mating face therefore determine alignment.

“B5” does not state the actual flange dimensions. Two B5 motors can still have different pilot diameters, bolt circles, holes or shaft extensions. Use the dimension drawing for the exact frame and manufacturer.

IM B35: feet and flange

B35 combines the B3 feet with a B5-style flange. The feet can support the motor while the flange locates or connects it to the driven equipment.

The presence of both interfaces does not mean they should be tightened against two misaligned structures. The base and flange faces must agree geometrically. Forcing one interface into position after the other is fixed can distort the frame or leave the shaft misaligned.

IM V1: vertical flange, shaft down

V1 is a flange-mounted vertical arrangement with the shaft end pointing downward. It often appears on vertical pumps, where the driven equipment is below the motor.

Vertical service changes the bearing problem. Rotor weight and driven-equipment thrust act axially, so the supplier must know the magnitude and direction of the external thrust. Drain positions, condensation control, cooling airflow, terminal-box access and lifting also need review in the installed orientation.

B3, B5, B35 and V1 are not dimension standards by themselves

The mounting code answers “how is the motor installed?” It does not answer “will this motor fit?” A replacement review needs both the mounting arrangement and the interface dimensions.

Interface What to compare Why it matters
Frame IEC or NEMA frame designation and manufacturer drawing Establishes the dimensional family, but not every special feature
Feet Shaft height, foot-hole spacing, foot width and base level Controls shaft centreline and base fit
Flange Outside diameter, pilot/register diameter, bolt circle, hole size and thickness Controls location, fastening and mating fit
Shaft Diameter, extension, shoulder, key and keyway Controls coupling or hub fit
Bearings Permitted radial and axial loads, including direction and load point Prevents a mechanically compatible motor from overloading its bearings
Orientation Horizontal or vertical, shaft up or down Affects bearings, drainage, cooling and lifting
Services Terminal box, cable entry, drains, grease points and sensors Determines whether the installed motor can be connected and maintained

For frame references, use the NEMA-to-IEC frame chart and the explanation of motor frame size versus power rating. Neither replaces the certified dimension drawing for the motor being ordered.

A replacement check that prevents most mounting errors

  1. Photograph the complete installation and the nameplate. Include the base, flange, coupling or pulley, shaft direction and nearby obstructions.
  2. Record the full mounting code. Do not reduce IM B35 to “flange motor” or describe IM V1 only as “vertical.”
  3. Measure the mating interfaces. Record dimensions from the equipment drawing where possible; use field measurements to confirm rather than replace that drawing.
  4. Check the shaft centreline and extension. A motor can bolt down and still leave the coupling at the wrong height or axial position.
  5. Define external loads. Give the supplier coupling weight, pulley diameter and belt forces, or pump thrust and its direction.
  6. Confirm the installed orientation in writing. Ask whether the quoted bearing, drain and cooling arrangement is approved for that orientation.
  7. Approve one motor-specific drawing. The purchase order and the installation team should work from the same revision.

When a pulley is involved, follow the mounting review with the belt-drive alignment and overhung-load check and the effect of belt tension on motor bearing life. For coupled equipment, use the shaft-dimension and coupling-fit review.

Common mistakes

Calling every flanged motor B5

B5 is a specific footless horizontal arrangement. B35 has a flange and feet; V1 is vertical with the shaft down. The difference affects support, alignment, bearings and service access.

Assuming the same frame number guarantees the same flange

A frame designation narrows the dimensional family. It does not remove the need to compare the flange and shaft drawing, especially across manufacturers or when the original motor was customized.

Turning a horizontal motor upright

A motor that physically fits in a vertical position is not automatically suitable for it. Bearing axial capacity, thrust direction, grease distribution, drains, fan and cowl arrangement, and lifting points may all change.

Supporting a B35 motor from misaligned feet and flange

The two interfaces must be coplanar and concentric as the design requires. Tightening the feet to pull the flange into alignment, or tightening the flange to pull the feet down, transfers assembly error into the frame and bearings.

Selecting from photographs alone

A photograph can identify the likely arrangement, but it cannot establish a pilot diameter, bolt circle, shaft fit or bearing capacity. Use it to prepare the measurement list, not to approve the replacement.

What to send with an RFQ

For a new or replacement motor, provide:

  • required mounting code: IM B3, IM B5, IM B35 or IM V1;
  • motor frame and the original dimension drawing if available;
  • driven-equipment drawing and mating-flange dimensions;
  • shaft, coupling, pulley or hub details;
  • radial load, axial load and the point and direction of application;
  • horizontal or vertical orientation, including shaft-up or shaft-down;
  • terminal-box and cable-entry position;
  • ambient, ingress-protection and hazardous-area requirements;
  • photographs and measured dimensions for a replacement project.

LEADGO’s three-phase induction motor range includes foot, flange and combined mounting arrangements across the applicable series. Final availability depends on the frame and rating. Send the interface drawing with the enquiry so the quotation can identify the exact mounting and dimensions rather than only a general series.

Sources

  • IEC 60034-7, Rotating electrical machines — Part 7: Classification of types of construction, mounting arrangements and terminal box position.
  • IEC 60072-1, Dimensions and output series for rotating electrical machines — Frame numbers 56 to 400 and flange numbers 55 to 1080.