Optimal Electric Motor Mounting Positions Explained

Optimal Electric Motor Mounting Positions Explained: IM Codes, Shaft Orientation and Bearing Loads

The Two Axes Hidden in “Mounting Position”

A 4 kW motor was taken off a horizontal pump skid and bolted upright onto a vertical inline pump. Power, voltage, speed and frame size all matched. The drive-end bearing failed in eleven weeks. Nothing about the motor was undersized; what changed was the direction of gravity relative to the bearing, and no rewinding specification would have caught it.

The list most buyers arrive with is five items long: horizontal, vertical, flange, foot, face. Those five are not alternatives. They mix two independent axes into one set, so a reader can never work out which combination a given machine actually has. Horizontal and vertical describe the shaft axis. Flange, foot and face describe how the frame is attached to the machine. A motor can be flange-mounted with a horizontal shaft or flange-mounted with a vertical shaft, and those two machines have almost nothing in common mechanically.

IEC 60034-7:2020 resolves this by encoding construction type, mounting arrangement and terminal box position in a single designation, the IM code ^[1]. On a drawing, that code is what a mounting position means: one designation fixing both axes at once. Once it is there, the attachment and the axis are fixed together, and the bearing, lubrication and cooling questions become answerable.

The flange as a component is a separate subject. B5, B14, FF and FT designations and NEMA C-face dimensions are set out in what is a motor flange. Reading the mounting field off an existing nameplate is in how to read a motor nameplate. What follows here is the code itself, and the mechanical consequences of the orientation it fixes.

How to Read an IM Code

The 2020 third edition of IEC 60034-7 provides two classification systems. Code I is an alphanumeric designation for machines with end-shield bearings and a single shaft extension; Code II is an all-numeric designation covering a wider range of machine types, including those covered by Code I ^[1]. Both start with the letters IM, for international mounting.

Code I: the alphanumeric form

The letter is the fastest read. B denotes a horizontal shaft axis and V a vertical one. The digits that follow describe the arrangement of feet, flanges and bearing end-shields. IM B3 is the plain foot-mounted machine; IM B5 has a drive-end flange and no feet; IM V1 has the same flange but the shaft points downward. The full digit definitions and the permitted combinations are tabulated in the standard, not in a catalogue summary.

The 2020 edition added a subclause on marking shaft inclination or declination ^[1]. It matters on inclined conveyors and tilted mixer drives: a shaft is not always exactly horizontal or exactly vertical, and an installation outside those two cases needs the angle recorded rather than rounded to the nearest familiar code.

Code II: the all-numeric form

Code II replaces the letter and digits with a four-digit number, so the same machine can appear as IM B3 on one drawing and IM 1001 on another. The digits encode the type of construction, the mounting arrangement, the shaft position and the shaft extension form. Terminal box position is part of the classification too. Two otherwise identical motors can therefore be non-interchangeable in a tight cabinet.

Common Arrangements Side by Side

Nine arrangements account for the large majority of industrial enquiries. The last column is the one that decides the order, because it names the constraint a power-based comparison never shows. The list below is a selection aid; the normative tables are in IEC 60034-7.

The four V rows are where the trap sits. The same motor frame can be supplied for shaft-up or shaft-down duty, but the bearing arrangement, the lubrication path and the drain hole position are not identical. Ordering by frame size alone and specifying the orientation later is the most common way to receive a machine that fits and does not last.

Vertical Duty: Where the Load Goes

In a horizontal machine the rotor weight is carried radially by two bearings and the axial load is close to zero unless the coupling or belt introduces one. Turn the same machine upright and the entire rotor mass becomes a continuous axial load on one bearing. Add the hydraulic thrust of a pump impeller and the number on the bearing is no longer a rounding error.

Rotor weight becomes an axial load

Take a 4 kW, 1,450 rpm machine with a 28 kg rotor, originally supplied as IM B3 for a horizontal pump. Mount it as IM V1 on a vertical inline pump with an impeller thrust of 600 N, downward. The arithmetic is short:

A deep-groove ball bearing will tolerate some axial load, but 875 N applied for every hour of a continuous duty is a different specification from an occasional thrust during start-up. The vertical variant needs a bearing arrangement rated for that load, an explicit allowed axial load figure from the motor maker, and a lubrication path that still works when gravity points along the shaft. ISO 281 defines the basic rating life as the life associated with 90 percent reliability, and it provides a separate modified rating life that accounts for lubrication condition and contamination ^[5]. Both are calculated from the load the installed orientation actually produces.

Bearing arrangement and lubrication

On V1 and V3 machines the usual answer is an angular-contact bearing or a paired bearing set at the drive end, with the non-drive-end bearing located or floating as the design requires. Grease migration and oil distribution also change: a lubricant that stays in the bearing pocket on a horizontal shaft can drain away on a vertical one. SKF’s failure-cause reference lists inadequate lubrication and contamination among the dominant early-failure mechanisms, and both are sensitive to orientation ^[9].

Put one question to a supplier and judge the reply: what is the allowed continuous axial load for the V1 variant, which bearing arrangement delivers it, and what is the relubrication interval in that orientation. A supplier who answers with a frame number has not answered.

Overhung Load and the Cube Law

Horizontal mounting brings a different problem: the radial load from a belt, chain or gear acting on the shaft extension. That load is called overhung load, and its magnitude depends on how far the pulley sits from the drive-end bearing as much as on transmitted power. Tsubaki’s selection data gives an OHL coefficient by transmission type (1.0 for chain, 1.25 for a toothed belt, 1.5 for a V-belt) and a location factor Lf that rises with the ratio of the load distance ℓ to a reference length Q ^[10].

The worked example

Consider a 3 kW, 1,450 rpm IM B3 motor driving through a V-belt. Let the theoretical radial force from the transmitted torque be 800 N, the reference length Q be 80 mm, and the bearing’s basic dynamic load rating C be 20,000 N. For a ball bearing the ISO 281 exponent is 3, and basic rating life in hours is L10h = (106 / (60 × n)) × (C / P)3 with n in rpm ^[5].

The load doubles and the life falls to one eighth: 53,200 hours becomes 6,650 hours. A pulley spacer added to clear a guard does exactly that to a bearing sized without it, which is how a ten-year bearing becomes an eighteen-month one. L10 is a 90 percent reliability figure, so roughly one bearing in ten fails before it; correcting for lubrication and contamination with the modified rating life usually shortens the real number further ^[5].

A motor that passes every electrical test and still eats bearings on one particular machine is usually sound: the problem is on the shaft, not in the winding, and the same arithmetic finds it. Where a gearbox or a belt stage sets the ratio, the mounting and the overhung load belong in the same conversation as the gearbox selection itself, which is covered in selecting a reduction gearbox and in shaft-mounted gearbox applications.

Alignment, Soft Foot and Vibration Limits

Mounting decides how well a motor can be aligned, and alignment decides how much vibration the bearings see. On a foot-mounted B3 machine the baseplate is part of the alignment system; on a flange-mounted B5 the spigot fit does most of the locating work. Mixing the two on a B35 without checking flatness preloads the frame before the coupling is even fitted.

Soft foot

Soft foot is the condition where one foot does not sit flat on the base. Tightening its bolt then distorts the frame, moves the bearing centres and produces a vibration signature that looks like unbalance. It is a machining and shimming fault, not a motor fault, and it is found by loosening one bolt at a time with a dial indicator on the foot. Any vibration investigation that skips this step will blame the rotor.

A factory vibration value is recorded with the motor uncoupled; a field measurement includes the base, the coupling, the driven machine and any vibration transmitted into it from outside. Using one to judge the other produces false rejections of good motors and false acceptance of bad installations. Where airborne noise is the complaint rather than structural vibration, the sources and remedies are separated in how to reduce electric motor noise.

Cooling, Condensation and Drain Holes

Orientation changes where heat goes and where water collects. A standard TEFC motor is designed around an axial airflow path from the fan end over the fins. Put that machine in a narrow vertical bay with the fan cowl close to a wall, or under a canopy that recirculates its own discharge air, and the winding runs hotter at the same load. IEC 60034-1:2026 defines rating and performance, including the thermal classes and temperature rises against which that condition is judged ^[7].

Drain holes and condensation

Water that never enters from outside is the harder problem. IEC 60034-5:2020 classifies degrees of protection provided by the enclosure, but its scope explicitly excludes moisture produced by condensation, corrosive dust and vapour, fungus and vermin; the fifth edition also adds a clarification on the term open drain hole ^[6]. A higher IP number therefore does not solve a machine that breathes humid air, cools down overnight and condenses water on the winding.

What solves it is an installation detail: drain holes at the lowest point of the enclosure in the installed orientation, kept open, plus anti-condensation heaters on machines that stand idle in humid conditions. The lowest point is not a fixed property of the motor; it moves when a B3 becomes a V1. Drain plugs that were correctly positioned on a horizontal frame can end up on the side of a vertical one.

Dimensions That Must Match

The IM code states the arrangement; a dimensional standard states whether it physically fits. IEC 60072-1:2022 covers foot-mounted machines with shaft heights from 56 mm to 400 mm and flange-mounted machines with flange pitch circle diameters from 55 mm to 1080 mm, and it integrates the relationships between frame size, shaft extension, rated output and flange number ^[2]. A frame number on its own is therefore not a complete interchangeability statement.

IEC and NEMA dimensions are not interchangeable

North American machines follow ANSI/NEMA MG 1, which uses its own frame series and its own C-face and D-flange dimensions ^[8]. An IEC B5 flange and a NEMA C-face can look similar in a photograph and still have a different bolt circle, a different spigot diameter and a different shaft extension. An adapter plate solves the mechanical mismatch, but it adds a stack-up of tolerances that affects coupling alignment.

For a replacement, decode the existing nameplate first, then confirm those four items against the supplier drawing. The nameplate procedure is in how to read a motor nameplate, the flange dimensions in what is a motor flange, and a compact IEC example in the 2 HP square flange motor data sheet.

A Step-by-Step Selection Sequence

Work outward from the driven machine. This sequence is the one that prevents rework; run it backwards and a correctly rated motor ends up on an incorrectly specified frame.

1. Fix the shaft axis first. Horizontal, vertical shaft down, vertical shaft up, or an inclined angle that must be marked under IEC 60034-7:2020.
2. Fix the attachment. Feet, flange, or both, according to what the driven machine offers. Select the Code I designation that describes the result.
3. Get the loads, not just the kilowatts. Ask for the radial overhung load at the shaft extension, its distance from the drive-end bearing, and any continuous axial thrust. The torque calculation itself is set out in how to calculate motor torque.
4. Check the bearing rating life against the installed geometry. Use the cube law for ball bearings and the modified rating life where lubrication or contamination is a concern ^[5].
5. Confirm the dimensional series. Shaft height, flange pitch circle, spigot, bolt pattern, shaft extension and terminal box side. Mark IEC or NEMA explicitly.
6. Confirm the thermal and moisture arrangement. Air path in the installed position, drain hole at the lowest point, heaters for idle machines in humid air, insulation class and temperature rise ^[7].
7. State the vibration acceptance basis. Factory uncoupled limits under IEC 60034-14 and in-service evaluation under ISO 20816-1 are different tests ^{[3] [4]}.
8. Put all seven items in the RFQ. A supplier who receives only a kilowatt rating and a frame number cannot quote the machine that will survive the installation.

A pump drive brings the hydraulic duty into that interface, which is developed further in choosing the right motor for your pump. For general industrial drives, the service conditions and enclosure categories that bound the selection are in what is a general purpose motor.

Five Mistakes That Reach the Factory

1. Treating horizontal, vertical, flange, foot and face as five alternatives. They are two axes. A flange-mounted motor can be horizontal or vertical, and the bearing specification changes between them.
2. Ordering a horizontal motor and rotating it on site. The rotor weight becomes an axial load, the lubrication path changes and the drain hole moves. The bearing was not selected for it.
3. Specifying power and speed but not overhung load. The pulley position is a bearing-life input, not a mechanical afterthought.
4. Believing an IP rating covers condensation. IEC 60034-5:2020 excludes moisture produced by condensation from its scope; drains and heaters are separate decisions ^[6].
5. Judging an installed machine against a factory vibration sheet. IEC 60034-14 values are recorded uncoupled; a field reading includes the base, coupling and driven machine ^[3].

Troubleshooting: Symptom, Cause, Action

Why Greensky Power

We are a motor manufacturer, and on mounting questions we would rather be judged on data a buyer can check than on a claim about rank. For a non-standard arrangement we will state the IM designation on the drawing, the allowed continuous axial load for the vertical variant, the allowed radial overhung load at a stated distance from the drive-end bearing, the bearing arrangement and the relubrication interval, the flange and shaft extension dimensions against IEC 60072-1 or NEMA MG 1, and the factory vibration record measured uncoupled under IEC 60034-14.

With those figures on the table a replacement conversation takes one exchange. If an installation needs a specific orientation, a sealed or heated enclosure, a particular terminal box side or a flange that is not in the standard series, that is custom work rather than a catalogue item; the range of what we build is on our custom electric motor page and in the brushless DC motor catalogue.

Mounting is one layer of a wider application map that starts at electric motor applications; company background is on the electric motor manufacturers page. If you already have a drawing, send it through the contact page and we will answer against the eight RFQ items above.

References

1. IEC. (2020). IEC 60034-7:2020, Rotating electrical machines – Part 7: Classification of types of construction, mounting arrangements and terminal box position (IM Code), Edition 3.0. IEC Webstore. https://webstore.iec.ch/en/publication/64225
2. IEC. (2022). IEC 60072-1:2022, Rotating electrical machines – Dimensions and output series – Part 1: Frame numbers 56 to 400 and flange numbers 55 to 1080, Edition 7.0. IEC Webstore. https://webstore.iec.ch/en/publication/67088
3. IEC. (2018). IEC 60034-14:2018, Rotating electrical machines – Part 14: Mechanical vibration of certain machines with shaft heights 56 mm and higher – Measurement, evaluation and limits of vibration severity, Edition 4.0. IEC Webstore. https://webstore.iec.ch/en/publication/59889
4. ISO. (2026). ISO 20816-1:2026, Mechanical vibration – Measurement and evaluation of machine vibration – Part 1: General guidelines, Edition 2. ISO. https://www.iso.org/standard/89921.html
5. ISO. (2007). ISO 281:2007, Rolling bearings – Dynamic load ratings and rating life, Edition 2. ISO. https://www.iso.org/standard/38102.html
6. IEC. (2020). IEC 60034-5:2020, Rotating electrical machines – Part 5: Degrees of protection provided by the integral design of rotating electrical machines (IP code) – Classification, Edition 5.0. IEC Webstore. https://webstore.iec.ch/en/publication/64224
7. IEC. (2026). IEC 60034-1:2026, Rotating electrical machines – Part 1: Rating and performance, Edition 15.0. IEC Webstore. https://webstore.iec.ch/en/publication/89961
8. NEMA. (2021). ANSI/NEMA MG 1, Motors and Generators. ANSI Webstore. https://webstore.ansi.org/standards/nema/ansinemamg2021
9. SKF. Bearing failures and their causes. SKF Group. https://www.skf.com/group/support/bearing-failures-and-their-causes
10. Tsubaki. Technical data for selection: service factor, inertia ratio and allowable overhung load. Tsubaki Power Transmission Products. https://en.tt-net.tsubakimoto.co.jp/tecs/engd/gen/engd_gen_ggm_sry.asp

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