Fan Bases and Vibration Isolation

Summary

This chapter covers how fans are installed and isolated from the building. It introduces fan installation, unitary, isolation, and concrete inertia bases, vibration and vibration isolation, isolator deflection, neoprene pads, rubber-in-shear isolators, isolator mounting methods, internally isolated fans, and seismic restraint. It also covers weather covers, zero speed switches, inlet box mounting planes, and piezometer rings for measuring airflow. After completing it, readers can choose a base and isolation approach for a fan installation.

Concepts Covered

This chapter covers the following 17 concepts from the learning graph:

Concept	Concept Impact Score
Fan Installation	91
Fan Base	25
Vibration	43
Zero Speed Switch	1
Weather Cover	2
Unitary Base	1
Isolation Base	19
Vibration Isolation	39
Inertia Base	16
Isolator Deflection	11
Neoprene Isolation Pad	2
Rubber-in-Shear Isolator	2
Seismic Restraint	3
Piezometer Ring	3
Isolator Mounting Method	4
Internally Isolated Fan	1
Inlet Box Mounting Planes	1

Prerequisites

This chapter builds on concepts from:

• Chapter 1: Air, Pressure and the Fan
• Chapter 2: Airstreams, Total Pressure and Standard Air
• Chapter 3: Basic Fan Anatomy
• Chapter 4: Centrifugal, Axial and Mixed Flow Fans
• Chapter 8: Centrifugal and Axial Impeller Types
• Chapter 14: Fan Motors and Enclosures
• Chapter 18: Inlets, Outlets and Flow Control
• Chapter 19: Damper Details and Duct Connections

The Fan Has to Sit on Something

A perfectly selected fan can still hum through an office ceiling, walk off its alignment, or shake a mezzanine. The difference is usually the base and the isolators, which are a few lines on the order that nobody notices until they are wrong. By the end of this chapter you will be able to pick the base and isolation for a job and defend the choice. Let's follow the airflow.

Up to now this book has treated the fan as a machine on its own: its impeller, housing, arrangement, motor and accessories. A fan in service is also a load on a building, and a source of force that the building has to carry. This chapter covers the parts of an order that connect the fan to that building. The product statements below come from the FanPedia accessory pages on base types and vibration isolation (pp. 131–135), inlet boxes (p. 143), shaft accessories (p. 129) and piezometer rings (pp. 158–160), and from the matching technical descriptions (pp. 163–197).

Fan Installation

Fan installation is everything that puts a selected fan into service in a particular place: the structure that carries it, the base under it, any isolation between the base and the structure, the connections to ductwork, protection from weather, and the devices that let operators monitor it. The fan selection answers "what fan?"; the installation answers "how does this fan live in this building?"

Installation choices appear on the order as accessories and options, so they have to be decided before the order is entered, not on the job site. The questions an application engineer works through are always the same, even when the answers change:

1. Support. What carries the fan: a slab on grade, a steel mezzanine, a roof curb, an overhead structure? FanPedia notes that not every fan needs isolators, but every fan needs adequate support from its mounting structure (p. 133).
2. Base. Is the fan mounted by itself, or with its motor and drive on a common base? If a base is used, which kind?
3. Isolation. Is vibration a problem for the people, process or structure near the fan? If so, which isolators, at what deflection, mounted how?
4. Connections. Do the inlet and outlet need flexible connectors? (They do on any isolated fan.)
5. Environment. Is the fan outdoors, in a seismic area, or next to a detached inlet box?
6. Monitoring. Does the control system need to know the fan's speed or airflow?

Worked Example: Reading an Installation From a Job Description

A contractor asks for a belt-driven Arrangement 10 centrifugal fan to exhaust a print shop. The fan will sit on the roof of a two-story building, directly above a conference room. The building automation system must raise an alarm if the fan stops. (This job is illustrative.)

Question	What the job says	Line on the order
Support	Roof structure over an occupied room	Confirm the roof structure and curb with the contractor
Base	Arrangement 10 has an adjustable motor base inside the bearing pedestal (FanPedia p. 167)	No separate motor base needed
Isolation	Conference room directly below; vibration and hum are a nuisance there	Vibration isolators, sized later in this chapter
Connections	Isolated fan	Flexible connectors at inlet and outlet
Environment	Outdoors on a roof	Optional weather cover: FanPedia rates Arrangement 10 suitable for roof or outdoor installations when supplied with it (p. 167)
Monitoring	Alarm if the fan stops	Zero speed switch on the fan shaft

Six questions produced five order lines. Every one of them is missed regularly, because none of them shows up on the fan's performance curve.

Vibration

Vibration is the repeated back-and-forth motion of a machine or structure about its resting position. Every operating fan vibrates a little. The main source is unbalance: no impeller is perfectly balanced, so the rotating mass pulls outward slightly harder on one side, and that pull swings around once per revolution. Misaligned shafts and couplings, worn belts and bearings, and aerodynamic forces in the airstream add to it.

The force that drives vibration repeats at a definite rate called the forcing frequency (or disturbing frequency). For unbalance it is simply the rotating speed expressed in cycles per second, or hertz (Hz):

\[ f = \frac{N}{60} \]

where:

• \( f \) is the forcing frequency in hertz (cycles per second)
• \( N \) is the rotating speed in revolutions per minute (RPM)

A machine can have several forcing frequencies. A belt-driven fan has one at the fan shaft speed and another at the motor speed, and the lowest one is the hardest to isolate, as the next sections show.

Vibration matters for three reasons that FanPedia spells out (p. 133). It can be a nuisance, as in an office where it is distracting. It can upset a process, as in electronics manufacturing. And in an extreme case it can be destructive enough to damage the supporting structure. Inside the fan, the same vibration wears bearings and loosens fasteners, which is why bearing vibration sensors exist (FanPedia p. 177).

Worked Example: Forcing Frequencies of a Belt-Driven Fan

A belt-driven fan runs at 1170 RPM, driven by a motor that runs at 1770 RPM. What forcing frequencies does the installation have to deal with?

1. Fan shaft: \( f = 1170 / 60 = 19.5 \) Hz.
2. Motor shaft: \( f = 1770 / 60 = 29.5 \) Hz.
3. The lower of the two, 19.5 Hz, sets the requirement for isolation. An isolator that handles 19.5 Hz handles 29.5 Hz even better.

Had the same duty been met by a slower, larger fan at 600 RPM, the fan's forcing frequency would drop to 10 Hz. Slower fans are quieter in many ways, but they are harder to isolate, and you will see why in the section on isolator deflection.

Fan Bases

A fan base is a structural frame that supports the fan, and usually its motor and drive, as one assembly. A base does three jobs. It gives the fan and motor a common, rigid support so the belt drive or coupling stays aligned; it spreads the load onto the floor or structure; and it gives the installer one assembly to set, level and anchor instead of several loose pieces. FanPedia describes three base types for centrifugal fans: unitary, isolation and inertia (pp. 131–132). They differ in one respect above all: whether the base is meant to sit on isolators.

Unitary Base

A unitary base, also known as a channel base, uses structural channel to support the fan assembly and is designed for use without isolators (FanPedia pp. 131, 176). It is bolted or anchored directly to the floor or foundation. A unitary base is the right choice when the fan is on a solid slab or foundation and nothing nearby is bothered by vibration, as in many industrial plants. Some arrangements ship on one as standard: FanPedia states that an Arrangement 7SI assembly is mounted on a unitary base as standard, with an inertia base available as an option (p. 165).

Isolation Base

An isolation base provides a common support for the fan, motor and drive, including guards, and uses heavy-duty structural channel. It is built to sit on spring or rubber-in-shear (RIS) isolators, which limit the forces an operating fan transmits to the support structure, and it requires flexible connectors at the inlet and outlet (FanPedia pp. 131, 176). FanPedia also lists neoprene pads among the mounting options for an isolation base (p. 133). Twin City Fan engineering confirms that neoprene pads can be used under an isolation base, though it is uncommon; springs or RIS isolators are the usual choice. The heavier channel matters: once the base sits on soft isolators at its corners, the base itself, not the floor, has to keep the motor and fan in line.

Some arrangements cannot take isolators directly under the fan, and the isolation base is FanPedia's stated fix:

• Arrangement 3F SWSI (extended angle frame for the motor) is typically not suitable for mounting vibration isolators directly under the fan (p. 163).
• Arrangement 9F SWSI (extended structural frame for a motor too large for the pedestal) is not suitable for mounting vibration isolators directly under the fan. If isolators are required, FanPedia says to use an Arrangement 1 fan with a separate isolation base (p. 166).
• Arrangement 3F DWDI is the contrast case: its integral extended base has brackets to accept vibration isolators (p. 167).

Worked Example: A Spec That Asks for Springs Under an Arrangement 9F

A specification reads: "Arrangement 9F, SWSI, belt driven, spring isolators mounted under the fan." The fan serves a mechanical room on a steel mezzanine.

1. Check the arrangement against the isolation request. FanPedia states that Arrangement 9F is not suitable for mounting isolators directly under the fan (p. 166). The spec as written cannot be built.
2. Apply FanPedia's stated alternative: an Arrangement 1 fan with a separate isolation base, motor mounted on the base (p. 166).
3. Put the isolators under the isolation base's mounting brackets, not under the fan (p. 133).
4. Add flexible connectors at the inlet and outlet, which every isolation base requires (p. 131).
5. Send the revised arrangement back to the specifier with the reason, rather than quietly substituting it. The arrangement change moves the motor, so the drawings change too.

Inertia Base

An inertia base is an isolation base with reinforcing rods (rebar), filled with concrete by the customer. Like the isolation base, it supports the fan, motor and drive including guards on heavy-duty structural channel, and it sits on spring isolators with flexible connectors required at the inlet and outlet. FanPedia notes that inertia bases are typically used on longer, direct-drive fans to mitigate assembly deflection, keeping the motor, coupling, shaft and bearings in alignment, and that an optional bottom pan makes the concrete easier to pour in the field (pp. 132, 175).

The concrete does two things. It stiffens the base, so a long coupled assembly does not sag or twist between its motor and its bearings. And it adds mass. For the same unbalance force, a heavier assembly moves less on its isolators, roughly in proportion to the total mass once the fan runs well above the isolators' natural frequency (defined in the next section). The concrete also lowers the center of gravity, which steadies the assembly on its springs.

Worked Example: How Much Does the Concrete Add?

An inertia base for a direct-drive fan has a pour cavity 8 ft long, 4 ft wide and 6 in deep. The fan, motor and steel base weigh 3,000 lb. Normal-weight concrete weighs about 150 lb per cubic foot. (Dimensions and weights are illustrative.)

\[ W_c = \rho_c \times L \times B \times D \]

where:

• \( W_c \) is the weight of the concrete fill in pounds
• \( \rho_c \) is the unit weight of concrete, about 150 lb/ft³ for normal-weight concrete

• \( L \), \( B \) and \( D \) are the pour cavity's length, width and depth in feet

• Volume: \( 8 \times 4 \times 0.5 = 16 \) ft³.

• Concrete weight: \( 150 \times 16 = 2{,}400 \) lb.
• Total supported weight: \( 3{,}000 + 2{,}400 = 5{,}400 \) lb.
• Effect on motion: with the same unbalance force, motion on the isolators scales roughly with \( 3{,}000 / 5{,}400 \approx 0.56 \), so the assembly moves a little more than half as much.
• Consequence for the order: the spring isolators must be selected for 5,400 lb, not 3,000 lb. Springs sized for the bare fan would bottom out once the customer pours the concrete.

The following table summarizes the three base types described above.

Base	Construction	Sits on isolators?	Flex connectors	Typical use
Unitary (channel)	Structural channel	No, designed for use without isolators	Not required by the base	Fan on a solid floor where vibration is not a concern
Isolation	Heavy-duty structural channel, common support for fan, motor, drive and guards	Yes: spring or rubber-in-shear usual; neoprene pads possible but uncommon	Required at inlet and outlet	Isolated fan and motor that must stay aligned
Inertia	Isolation base with rebar, concrete filled by the customer	Yes, spring	Required at inlet and outlet	Longer direct-drive fans, where stiffness and mass hold alignment

Diagram: Fan Base Types

Run Fan Base Types Fullscreen

Fan Base Types

sim-id: ch22-fan-base-types-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the unitary, isolation and inertia bases, the spring isolators, the reinforcing rods (rebar) and the bottom pan on FanPedia's base drawings, given each part's function.

Content: Six numbered markers: unitary base, isolation base, spring isolators, inertia base, reinforcing rods (rebar), and bottom pan.

Provenance: FanPedia pages 131-132, "Base Types: Centrifugal Fans" (unitary and isolation bases, and the inertia base with optional bottom pan), with the printed labels removed. Drawing: FanPedia, Twin City Fan, pp. 131-132.

Ask Who Pours the Concrete

An inertia base ships as steel and rebar; the concrete is customer supplied. Put that sentence in the quote and the submittal, and offer the optional bottom pan if the base will be filled in place. It saves a phone call from a contractor staring at an empty frame.

Vibration Isolation

Vibration isolation prevents or limits the force that an operating fan transfers to its supporting structure (FanPedia p. 133). It does not reduce the fan's own vibration; it stops that vibration from travelling into the building. The working parts are vibration isolators, resilient mounts placed between the fan or its base and the structure. FanPedia's isolator families are neoprene pads, rubber-in-shear mounts (themselves a pad-type isolator) and springs, each covered below.

Not every fan needs isolation. A fan bolted to a heavy slab in a plant, far from people and sensitive equipment, usually needs only adequate support. Isolation earns its cost when the fan sits on a light or flexible structure such as a roof or mezzanine, near occupied space, or near a vibration-sensitive process. Two FanPedia rules come with it (p. 133): when isolation is used, rigid duct connections are not allowed and flex connectors are required, and some fans, such as commercial roof ventilators, are internally isolated and need no other isolation. Chapter 19 covers the flex connectors themselves.

How well an isolator works depends on how its own bounce compares with the fan's forcing frequency. Any mass resting on a spring has a natural frequency, the rate at which it bounces if you push it once and let go. The ratio of the forcing frequency to that natural frequency decides the outcome. Transmissibility is the fraction of the fan's force that gets through the isolator to the structure. For an undamped isolator it is:

\[ T = \frac{1}{\left| r^2 - 1 \right|}, \qquad r = \frac{f}{f_n} \]

where:

• \( T \) is the transmissibility (1.0 means all of the force gets through)
• \( r \) is the frequency ratio
• \( f \) is the forcing frequency in hertz
• \( f_n \) is the isolated assembly's natural frequency in hertz

Engineers usually quote the result as percent isolation, \( (1 - T) \times 100 \). The formula says three things, and the table below lists them.

Frequency ratio \( r \)	What happens	Result
Below about 1.4 (\( \sqrt{2} \))	T is greater than 1: the isolator passes more force than a rigid mount would	Amplification
Exactly 1	The fan drives the assembly at its own bounce rate	Resonance: large motion, possible damage
Above about 1.4	T falls below 1 and keeps falling as r rises	Isolation

Soft Enough Is Relative to Speed

Notice that the formula never asks how soft an isolator is in absolute terms. It asks how far apart the fan's speed and the mount's bounce are. A pad that isolates a fast fan well can make a slow fan worse than no isolator at all.

Worked Example: Percent Isolation for a Spring Under a 1170 RPM Fan

The fan from the earlier example runs at 1170 RPM (19.5 Hz). It sits on springs whose natural frequency, loaded, is 3.13 Hz (the next section shows where that number comes from).

1. Frequency ratio: \( r = 19.5 / 3.13 = 6.23 \).
2. Transmissibility: \( T = 1 / (6.23^2 - 1) = 1 / 37.8 = 0.026 \).
3. Percent isolation: \( (1 - 0.026) \times 100 = 97.4\% \).

About 97% of the fan's unbalance force never reaches the roof. These are idealized numbers: real isolators have some damping, and a flexible roof is not the rigid floor the formula assumes. They are good enough to compare isolator choices, which is how they are used here.

Isolator Deflection

Isolator deflection is how far an isolator compresses under the weight it carries. FanPedia puts it this way: vibration isolators normally compress to dampen the vibration during normal operation, and that compression is known as deflection (p. 134). Most isolators Twin City Fan uses have deflections under 1 in, 1 in or 2 in; higher deflection is available at additional cost, after engineering review (p. 134).

Deflection is the number that matters because it sets the natural frequency directly. A mass that sags a distance \( \delta \) on its springs bounces at:

\[ f_n = \frac{1}{2\pi}\sqrt{\frac{g}{\delta}} \approx \frac{3.13}{\sqrt{\delta}} \]

where:

• \( f_n \) is the natural frequency in hertz
• \( g \) is the acceleration of gravity, 386.1 in/s²
• \( \delta \) is the static deflection in inches under the actual load

Doubling the deflection does not halve the natural frequency; it divides it by \( \sqrt{2} \). And the deflection only counts if the isolator is loaded to it: an isolator rated for 1 in but carrying a fraction of its rated load deflects less and isolates less.

Worked Example: Choosing a Deflection for a Slow Fan

A large fan runs at 600 RPM, so \( f = 10 \) Hz. Compare a neoprene pad (about 0.0625 in), a rubber-in-shear mount at 0.3 in, and springs at 1 in and 2 in.

Isolator (deflection)	\( f_n \) (Hz)	\( r \)	\( T \)	Percent isolation
Neoprene pad (0.0625 in)	12.52	0.80	2.76	none: force is amplified about 2.8 times
Rubber-in-shear (0.3 in)	5.71	1.75	0.48	51.5%
Spring (1 in)	3.13	3.19	0.11	89.1%
Spring (2 in)	2.21	4.52	0.05	94.8%

The pad sits below resonance, so it makes the installation worse. Rubber-in-shear helps a little. The 1 in spring is the first choice that does real work, and 2 in buys the last few percent. Run the same pad under the 1170 RPM fan from the last example and it gives about 30% isolation, which matches FanPedia's description of pads as minimal isolation at low cost (p. 134).

Pads Under a Slow Fan

A common trap is adding neoprene pads to a slow fan "for a little isolation." A 0.0625 in pad resonates near 750 RPM and amplifies the force at any speed below about 1,060 RPM. Check the frequency ratio first, and move to rubber-in-shear or springs when it comes out under about 1.4.

The explorer below lets you repeat these calculations for any combination of fan speed and deflection before committing to an answer.

Diagram: Isolator Transmissibility Explorer

Run Isolator Transmissibility Explorer Fullscreen

Isolator Transmissibility Explorer

Type: chart sim-id: ch22-isolator-transmissibility-explorer
Library: Plotly
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the transmissibility of an undamped isolator from the fan speed and the isolator's static deflection, to within 5% of the model value.

Prerequisites: forcing frequency, natural frequency, frequency ratio, transmissibility, percent isolation, static deflection, resonance (all defined in "Vibration", "Vibration Isolation" and "Isolator Deflection" above).

Evidence of Mastery: For three challenges, the learner types a transmissibility before the curve and answer are revealed. An answer is correct when it is within ±5% of the model value. Mastery is 3 of 3. Changing speed and deflection in exploration mode is not evidence.

Misconceptions: (1) Any isolator reduces transmitted force; in fact T > 1 when r < 1.414. (2) An isolator that works on one fan works on any fan; the result depends on the fan's speed. (3) Doubling deflection halves the natural frequency; it divides it by √2.

Instructional Rationale: Apply-level calculation needs practice with a check, so the learner commits a number before the curve shows it. The plotted curve afterwards connects each calculation to the amplification, resonance and isolation regions, which confronts misconceptions 1 and 2.

Content:

Adjustable quantities:

Quantity	Min	Max	Step	Default	Unit
Fan speed N	300	3600	30	1170	RPM

Static deflection δ is chosen from six values, default 1.0 in: 0.0625, 0.2, 0.3, 0.5, 1.0, 2.0 in.

Labels for the deflection options: 0.0625 in "Neoprene pad"; 0.2, 0.3 and 0.5 in "Rubber-in-shear"; 1.0 and 2.0 in "Spring".

The chart plots T against frequency ratio r from 0 to 10, marks the current operating point, shades r < 1.414 as "amplification" and r >= 1.414 as "isolation", and marks r = 1 as "resonance". Readouts: f (Hz), f_n (Hz), r, T, percent isolation.

Challenges, fixed order:

#	Fan speed	Deflection	Model T	Feedback when wrong
1	1170 RPM	1.0 in	0.026	f = 19.5 Hz, f_n = 3.13 Hz, r = 6.23, T = 1/(6.23² − 1) = 0.026, about 97.4% isolation.
2	1170 RPM	0.0625 in	0.70	A pad's f_n is 12.52 Hz, so r = 1.56 and T = 1/(1.56² − 1) = 0.70: only about 30% isolation.
3	600 RPM	0.0625 in	2.76	r = 10/12.52 = 0.80 is below 1.414, so T = 1/|0.80² − 1| = 2.76: the pad amplifies the force.

Provenance: Deflection options from FanPedia p. 134 (pads about 0.0625 in; rubber-in-shear 0.2 to 0.5 in; springs 1 in or 2 in). Formulas are the standard undamped single-degree-of-freedom model. Fan speeds are illustrative; the sim labels them "illustrative".

Rules: f = N/60. f_n = 3.13/√δ (from (1/2π)√(386.1/δ)). r = f/f_n. T = 1/|r² − 1|. Percent isolation = (1 − T) × 100; when T > 1 the readout shows "amplified ×T" instead of a negative percentage. When |r − 1| < 0.05 the readout shows "at or near resonance" and the plotted T is capped at 10. Values rounded to 3 significant figures for display; answers judged against the unrounded model.

Learner Activity:

1. The learner reads challenge 1 and types a transmissibility.
2. The learner presses Check. The operating point appears on the curve with the model value and the learner's answer side by side.
3. After the three challenges, exploration unlocks: the learner changes speed and deflection, and the point, readouts and region label update at once.
4. The learner should notice that at 420 RPM with 0.2 in the readout shows resonance, and that every pad setting below about 1,060 RPM lands in the amplification region.

Feedback: Three challenges, fixed order, two attempts each. Correct: "Correct: T = , ." Hint after a first wrong attempt: "Not quite." plus the slip the typed value points to (a negative T; the percent isolation instead of T; |r² − 1| not inverted; the − 1 dropped or added; r not squared; RPM not converted to hertz; f_n not 3.13/√δ), or else the step for that challenge (1: work in hertz, f = N/60 and f_n = 3.13/√δ, then r and T = 1/|r² − 1|; 2: a pad's f_n is much higher, recompute it for the pad; 3: find r first and note which side of 1.414 it falls on). The hint never states f, f_n, r or T, and the curve and readouts stay hidden. After the final attempt: the "Feedback when wrong" text with the worked calculation and the model value, and the challenge counts as missed. Running count "Challenges correct: n of 3".

Starting State: Challenge 1 shown with an empty answer box and the curve hidden. The prompt reads "What fraction of the fan's force gets through a 1 in spring at 1170 RPM?"

Chapter Anchors: 1170 RPM → 19.5 Hz; 600 RPM → 10 Hz; f_n = 3.13 Hz at 1 in; T = 0.026 and 97.4% isolation at 1170 RPM on 1 in springs; the four-row table at 600 RPM (pad T 2.76; RIS 0.3 in 51.5%; 1 in 89.1%; 2 in 94.8%); pad at 1170 RPM about 30%; amplification below r ≈ 1.4; pads resonate near 750 RPM and amplify below about 1,060 RPM.

Neoprene Isolation Pads

A neoprene isolation pad is the simplest isolator FanPedia lists: molded, ribbed neoprene pads with a metal plate mounted between two of them, and a neoprene washer between the pad and the fan structure (pp. 134, 195). Typical deflection is about 0.0625 in. FanPedia's own verdict is that pads offer minimal vibration isolation and are low cost. As the worked example showed, they suit fast fans on solid floors, where they mostly stop noise and fretting at the mounting feet, and they should not be used where real isolation is required.

Rubber-in-Shear Isolators

A rubber-in-shear (RIS) isolator is itself a pad-type isolator: it carries its load through a rubber element loaded in shear rather than straight compression, which makes it softer than a plain neoprene pad of similar size. FanPedia's floor-mounted RIS pads consist of two steel load plates embedded in a rubber pad; ceiling-hung RIS mounts sit in a formed metal surround (pp. 134, 196). Typical deflection is 0.2 to 0.5 in for both. RIS is the middle step between plain neoprene pads and springs: enough deflection for a fast fan, not enough for a slow one. Twin City Fan engineering reports that RIS mounts are most often used under isolation rails, with the fan sitting on the rails. RIS and springs are the usual isolators for an isolation base.

Springs are the third family and the subject of Chapter 23. For now, the summary below sets the three side by side.

Isolator	Construction (FanPedia p. 134)	Typical deflection	Isolation
Neoprene pad	Metal plate between two ribbed neoprene pads, neoprene washer	About 0.0625 in	Minimal, low cost
Rubber-in-shear	Steel load plates in rubber (floor); formed metal surround (ceiling hung)	0.2 to 0.5 in	Moderate
Spring (open, housed, restrained, seismic)	Steel spring, with or without housing	1 in or 2 in (ceiling-hung open: 1 in)	High

Isolator Mounting Methods

An isolator mounting method is where the isolators go between the fan and the structure. FanPedia lists five (p. 133):

• Directly under the fan: isolators under the foundation holes in the fan structure, using springs, RIS or neoprene pads. This is usually limited to fans with impellers 36.5 in in diameter or less; swingout housed centrifugal fans of any impeller size are an exception, with flat pads only.
• Isolation rails: isolators between two flat bars, the top bar bolted to the fan's foundation holes and the bottom bar to the floor. Springs or RIS; Twin City Fan engineering says RIS is most often used here, with the fan sitting on the rails.
• Mounting angle: an angle between the fan and the isolators, usually used when the fan inlet lacks a structural support. Springs or RIS.
• Isolation base: the fan, possibly the motor and other components, on a structural base with isolators under the base's mounting brackets. Springs, RIS or neoprene pads (p. 133). The base-type page (p. 131) calls for spring or RIS isolators. Twin City Fan engineering confirms neoprene pads can be used under an isolation base, though it is uncommon; springs or RIS are the usual choice. RIS mounts are themselves a pad-type isolator.
• Isolation hangers: the fan suspended from an overhead structure with spring or RIS hangers between it and the structure.

Diagram: Isolator Mounting Methods

Run Isolator Mounting Methods Fullscreen

Isolator Mounting Methods

sim-id: ch22-isolator-mounting-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the five isolator mounting methods (directly under the fan, isolation rails, mounting angle, isolation base and isolation hangers) on FanPedia's drawings, given a description of each.

Content: Five numbered zones: directly under the fan; isolation rails (RIS and spring versions); mounting angle; isolation base; isolation hangers.

Provenance: FanPedia page 133, "Vibration Isolation: Overview and Mounting" (directly under fan, isolation rails in RIS and spring versions, mounting angle, isolation base, isolation hangers), with the captions removed and the drawings rearranged into two rows. Drawing: FanPedia, Twin City Fan, p. 133.

Chapter 23 works through each method's hardware.

Internally Isolated Fans

An internally isolated fan carries its own isolators inside the unit, between the rotating assembly and the fan's outer structure, so no other isolation is required. FanPedia gives commercial roof ventilators as the example (p. 133). On an order, that means the isolation question is already answered by the product. Check the current catalog to confirm a model is internally isolated before leaving external isolators off.

Seismic Restraint

A seismic restraint is a device that holds an isolated fan in place during an earthquake, when ground motion would otherwise throw it off its soft mounts. Isolators are deliberately free to move; a restraint limits how far. FanPedia shows two forms (pp. 134–135). Seismic springs are like restrained springs, except that the housing nearly surrounds the entire spring so it can withstand the loads of a seismic event. Snubbers work alongside the isolators on a fan or base as shock absorbers that prevent excessive movement in any direction, and are often used when a fan must withstand seismic loading. Whether a job needs seismic restraint is set by the building's location and code, so ask for it at the quotation stage. Chapter 23 covers both devices.

Lots of Mounts, One Decision

Bases, isolator families, deflections and mounting methods can feel like a pile of unrelated parts. They are really one decision taken in order: does this fan need isolation, how much deflection does its speed demand, and what does the arrangement allow underneath it. Work the selector below one scenario at a time and the pattern shows itself.

The selector below gives you seven installations and asks you to recommend the base or mounting for each, using only the FanPedia rules covered so far.

Diagram: Fan Base and Isolation Selector

Run Fan Base and Isolation Selector Fullscreen

Fan Base and Isolation Selector

Type: microsim sim-id: ch22-fan-base-isolation-selector
Library: p5.js
Status: Specified
Bloom Level: Evaluate
Bloom Verb: recommend
Learning Objective: The learner will recommend a base or mounting for a described fan installation and justify it with the FanPedia rule that governs the case.

Prerequisites: unitary base, isolation base, inertia base, isolator mounting methods, internally isolated fan, flexible connectors, Arrangements 1, 7SI, 8 and 9F (defined above and in Chapters 11 and 19).

Evidence of Mastery: For each of 7 scenarios the learner assigns one of 5 recommendations and then picks one of 3 justifications. A scenario is correct when both match the answer table. Mastery is 6 of 7 on the first attempt.

Misconceptions: (1) Every fan needs isolators. (2) Isolators can go under any arrangement. (3) A unitary base can be set on springs. (4) An internally isolated roof ventilator needs external isolators.

Instructional Rationale: Evaluate-level recommendation requires a judgment against criteria, so the learner commits to an option and then to the rule that justifies it; the justification step separates a correct guess from a correct judgment.

Content:

Recommendations (5): Unitary base; Isolation base; Inertia base; Isolators directly under the fan; No added base or isolation.

#	Scenario shown to the learner	Correct recommendation	Correct justification (shown with 2 distractors)
1	Arrangement 7SI fan, slab on grade in a plant, no isolation requested	Unitary base	7SI ships on a unitary base as standard; unitary bases are designed for use without isolators.
2	Arrangement 1 belt-driven SWSI fan with motor, on a mezzanine above offices; isolation required	Isolation base	Isolation bases give fan, motor and drive a common support and sit on spring or RIS isolators.
3	Spec reads "Arrangement 9F with springs under the fan"	Isolation base	9F is not suitable for isolators directly under the fan; FanPedia says use Arrangement 1 with a separate isolation base.
4	Long direct-drive Arrangement 8 fan on springs; motor, coupling, shaft and bearings must stay aligned	Inertia base	Inertia bases are typically used on longer direct-drive fans to mitigate assembly deflection and hold alignment.
5	Commercial roof ventilator on a roof curb	No added base or isolation	Commercial roof ventilators are internally isolated; no other isolation is required.
6	Arrangement 4 fan with a 30 in impeller; isolation wanted with no separate base	Isolators directly under the fan	Direct mounting under the fan's foundation holes is usually limited to impellers 36.5 in or less; 30 in qualifies.
7	Fan and detached inlet box mounted together on a base on a plant floor; no isolation	Unitary base	Fan and inlet box can share one base on the same plane; without isolators that base is a unitary base.

Distractor justifications for every scenario are drawn from the other six rows' justifications, two per scenario, fixed by the seed 22.

Provenance: FanPedia pp. 131–133 (base types, mounting methods, internally isolated fans, 36.5 in limit), p. 143 (inlet box mounting planes), p. 165 (7SI on unitary base), p. 166 (9F). Scenarios are illustrative; the sim labels them "illustrative".

Rules: Exact match on both recommendation and justification. Scenario order shuffled once with seed 22 and then fixed.

Learner Activity:

1. The learner reads a scenario and selects a recommendation.
2. The learner selects a justification from three choices and presses Submit.
3. Feedback appears; the learner moves to the next scenario.
4. After all seven, a summary shows each scenario, the learner's answer and the correct one.

Feedback: Seven scenarios, two attempts each. Correct: "Correct: ." Incorrect recommendation for scenarios 3 or 6: "Check the arrangement and impeller size against the mounting rules first." Incorrect recommendation otherwise: "Ask whether isolation is needed, then what the arrangement allows." Correct recommendation with wrong justification: "Right choice, wrong reason. Which FanPedia rule decides this case?" After the second wrong attempt the answer and justification are shown and the scenario counts as missed. Score "n of 7 first-attempt correct".

Starting State: Scenario 1 of 7 with no selection made. The prompt reads "What should this fan sit on?"

Chapter Anchors: 7 scenarios; 5 recommendations; 36.5 in impeller limit; Arrangement 9F → Arrangement 1 with isolation base; 7SI on unitary base as standard; commercial roof ventilators internally isolated.

Inlet Box Mounting Planes

An inlet box (Chapter 18) can be attached to the fan or detached from it and supported separately. When it is detached, the installer has to know whether the fan and the box share one mounting surface. Inlet box mounting planes describes that relationship. FanPedia shows two cases (p. 143):

• Same plane: the fan and inlet box both stand on grade (legs to grade), or both stand on one base, unitary or isolation (legs to base). They move together.
• Different planes: the fan sits on an isolation base or a unitary base while the inlet box stands on grade. Now the fan and box can move relative to each other, and FanPedia requires a flex connector (slip connection) between the fan and the inlet box.

The practical rule follows from the isolation section: if the fan floats on isolators and the inlet box does not, the joint between them must be flexible, exactly as at the duct connections.

Installation Accessories: Weather Cover, Zero Speed Switch, Piezometer Ring

Three smaller accessories complete most installations.

A weather cover (FanPedia lists it with the motor cover, p. 126) is a cover that protects the motor from rain, snow and sun when the fan is installed outdoors. It is an option that makes some arrangements suitable for outdoor use: FanPedia describes Arrangement 10 fans, including the lab exhaust version, as suitable for roof or outdoor installations when supplied with the optional weather cover (pp. 167, 175). On spark-resistant construction, the weather cover is one of the mild-steel parts FanPedia lists (p. 109).

A zero speed switch, also called a speed sensor, detects slowing or stopping of the fan shaft (FanPedia p. 129). Other designs are provided when the customer requests them. Its usual job is to tell a control or alarm system that a fan has stopped even though its motor starter is still energized, as when a belt breaks.

A piezometer ring is part of an airflow measuring system based on the principle of a flow nozzle, with the fan's own inlet funnel serving as the nozzle (FanPedia pp. 158, 191). Tubing around the throat of the funnel forms the ring, and a tap on the face of the funnel provides the second pressure. The pressure drop between them is measured by a differential pressure transducer: the face tap goes to the high-pressure side and the ring to the low-pressure side. The tubing is nylon for temperatures up to 180°F and copper above. Piezometer rings are available on plenum fans and on SWSI and DWDI housed centrifugal fans (p. 158), and FanPedia also shows inline centrifugal and mixed flow mountings (p. 160). Turning that pressure reading into airflow is covered in Chapter 23.

Accessory	What it does	Order note
Weather cover	Protects the motor outdoors	Makes some arrangements, such as Arrangement 10, suitable for roof or outdoor use
Zero speed switch	Detects slowing or stopping of the fan shaft	Other designs on customer request
Piezometer ring	Measures inlet funnel pressure drop for airflow	Plenum, SWSI and DWDI centrifugal fans; transducer optional

Key Takeaways

• Fan installation is a set of order decisions (support, base, isolation, connections, environment, monitoring) that must be made before the order, because none of them appear on the fan curve.
• Vibration is driven mainly by unbalance at the forcing frequency \( f = N/60 \); the lowest forcing frequency sets the isolation requirement.
• A unitary base is for use without isolators; an isolation base usually sits on spring or RIS isolators (neoprene pads are possible but uncommon) with flex connectors; an inertia base is an isolation base with rebar and customer-poured concrete for longer direct-drive fans.
• Vibration isolation works only when the frequency ratio exceeds about 1.4; below that an isolator amplifies the force, and at 1 it resonates.
• Isolator deflection sets the natural frequency, \( f_n \approx 3.13/\sqrt{\delta} \): pads (about 0.0625 in) give minimal isolation, rubber-in-shear (0.2 to 0.5 in) moderate, springs (1 or 2 in) high.
• The five isolator mounting methods are directly under the fan, isolation rails, mounting angle, isolation base and isolation hangers; some arrangements, such as 9F, cannot take isolators directly under the fan.
• Internally isolated fans, such as commercial roof ventilators, need no other isolation; seismic restraints keep isolated fans in place during an earthquake.
• When a fan and a detached inlet box sit on different mounting planes, a flex connector is required between them.
• Weather covers, zero speed switches and piezometer rings finish the installation: outdoor protection, stop detection and airflow measurement.

You Can Put a Fan in a Building

You can now read a job, pick a unitary, isolation or inertia base, and check a deflection against a fan speed with the frequency ratio. That last calculation is the one that separates an isolator that works from one that makes things worse.

See Annotated References