Direct-coupled fan motor assembly: motor, flexible coupling and shared base for an industrial fan
Motor, flexible coupling and shared base that drive an industrial fan · foundation and shims, soft foot under 0.002 in, coupling alignment, thermal growth allowance
A direct-coupled fan motor assembly is the drive end of an industrial fan in which the motor shaft is joined to the fan shaft through a flexible coupling, with motor and fan bearings mounted on one base, an inertia base or a concrete foundation. Whether it runs smoothly is decided less by the motor itself than by how it is set: the stiffness and mass of what it sits on, the shims under its feet, the coupling alignment, and the allowance for the motor growing as it warms up.
Foot-mounted motors coupled directly to industrial fan shafts on a shared base, inertia base or foundation, with their shims, flexible coupling and alignment. Motor types, efficiency classes and sizing are covered on the Electric motor page; fan-and-drive modules inside air handling units are covered on the Fan assembly for air handling units page; fan selection and performance are covered on the Centrifugal fan page.
CNFX editorial research · Sources checked October 12, 2026 · 3 sources
Stand the motor and fan on a base heavy and stiff enough, and shim each foot flat
Aerovent's centrifugal fan manual sets out the foundation it expects: a mass at least three times that of the supported assembly, a stiffness of 1 × 10⁶ to 4 × 10⁶ lb/in depending on fan size and speed, extending 6 in beyond the fan and driver but no more than twice the area the equipment needs, with 1 to 1½ in of height allowed for shimming, grouting, levelling and fixings. It recommends an inertia base for arrangement 8 fans, which its arrangement chart lists as direct drive, to keep rotor and drive aligned. [1]
Where a structural steel base or platform carries the fan, Aerovent asks that no natural frequency of the structure fall within 30% of the fan speed. [1] Editorial calculation: for a fan running at 1,480 rpm (24.7 Hz), that keeps structural natural frequencies out of roughly 17.3 to 32.1 Hz.
Under the motor feet, Ludeca's soft-foot guide advises no more than three, or at most four, clean precut stainless steel shims per foot; with the 13 standard thicknesses, three or fewer shims reach any thickness up to 0.150 in. Clean flat precut stainless shims compress by at most 0.5% to 1% of their thickness, against as much as 5% to 6% for hand-cut brass, and each air gap between shims allows about 0.00025 to 0.0005 in of movement. [2]
Soft foot is checked by loosening one foot at a time with the other three tight and reading shaft or foot movement; Ludeca treats any reading above 0.002 in as soft foot to be corrected before the shafts are aligned. [2] Aerovent makes grouting the last installation step, after all shims are checked and levelling nuts are backed off, using a commercial non-shrink grout that also fills the anchor bolt sleeves. [1]
Ask the assembler to record soft-foot readings for each motor foot and the number and material of shims under each foot.
02
Align the coupling hubs to the fan maker's limit and set the motor low for thermal growth
Aerovent tells installers to check factory-mounted couplings, which can be knocked out of line in shipping and installation, and realign them to within 0.002 in offset and parallel. To allow for the motor's thermal growth it sets the motor 0.001 in low for each inch of shaft, up to 0.005 in, and it refers to the coupling maker's data for alignment limits and gap. [1]
The table is Rexnord's misalignment capacity for its Falk Wrapflex elastomer couplings; a chart in the same catalogue shows that combined angular and parallel misalignment must fall inside a triangle under each size's line, so the two limits cannot both be used in full at once. [3] Editorial calculation: the fan maker's 0.002 in realignment target is one-tenth of the 0.020 in installation limit for a 5R or 10R coupling, so the fan's own limit governs the setting, not the coupling's.
Editorial calculation on Aerovent's thermal growth rule: a motor with a 2 in shaft is set 0.002 in low, a 3 in shaft 0.003 in low, and any shaft of 5 in or more is capped at 0.005 in. [1]
For coupling sizing, the same Rexnord catalogue gives a service factor of 1.0 for constant-torque blowers and 1.5 for forced draft fans, and an operating range of −40 to 95 °C for the polyurethane element, asking for engineering review where the service factor is above 1.5 and temperatures exceed 79 °C. [3]
Falk Wrapflex size
Installation limit: parallel offset
Installation limit: angular
Operating limit: parallel offset
Operating limit: angular
2R–4R
0.010 in
0.25°
0.020 in
1.00°
5R–10R
0.020 in
0.25°
0.040 in
1.00°
20R–80R
0.040 in
0.25°
0.080 in
1.00°
State the coupling make and size, its alignment limits, the hub gap, the motor shaft diameter, and whether the alignment report is taken cold with the thermal offset applied.
03
Data for a fan motor assembly enquiry
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Fan: [arrangement; speed; shaft diameter at coupling]
Motor: [power; frame; speed; shaft diameter]
Base: [shared steel base, inertia base or foundation; natural frequency check]
Shims: [stainless precut; maximum per foot]
Soft foot: [limit per foot, e.g. 0.002 in]
Coupling: [type and size; service factor; alignment limits]
Alignment: [offset and parallel target; thermal offset]
Records: [soft-foot and alignment report]
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
Share this page
Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
How closely should a fan motor be aligned to the fan shaft?
Aerovent asks for 0.002 in offset and parallel for factory-mounted couplings, tighter than the 0.010 to 0.040 in installation limits Rexnord gives for its Wrapflex couplings. [1][3]
Why is the motor set lower than the fan?
To allow for its thermal growth: Aerovent sets the motor 0.001 in low per inch of shaft, up to 0.005 in. [1]
How many shims can go under a motor foot?
Ludeca advises no more than three, or at most four, clean precut stainless steel shims per foot. [2]
Sources and what still needs a direct check
Sources checked 2026-10-12. Foundation, base, alignment and grouting rules come from Aerovent's centrifugal fan installation manual IM-140; shim and soft-foot practice from Ludeca's soft-foot paper; coupling misalignment limits and service factors from Rexnord's Falk Wrapflex catalogue.
