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Quiz: Fan Bases and Vibration Isolation

Test your understanding of fan bases, vibration, isolator deflection and installation accessories with these review questions.

1. What distinguishes a unitary (channel) base from the other FanPedia base types?

  1. It is filled with concrete by the customer to add mass
  2. It always requires flexible connectors at the inlet and outlet
  3. It is designed for use without isolators and is anchored directly to the floor
  4. It is built only for fans hung from an overhead structure
Show Answer

The correct answer is C. A unitary base uses structural channel to support the fan assembly and is designed for use without isolators. It is bolted or anchored directly to the floor or foundation, where vibration is not a concern. Concrete fill describes an inertia base (A). Required flex connectors belong to the isolation and inertia bases, which sit on isolators (B). Hung fans use isolation hangers, not a base (D).

Concept Tested: Unitary Base

See: Unitary Base

2. What does a zero speed switch do?

  1. It detects slowing or stopping of the fan shaft
  2. It prevents the fan from turning backward when it is idle
  3. It measures airflow through the inlet funnel
  4. It stops the motor whenever vibration exceeds a limit
Show Answer

The correct answer is A. FanPedia describes the zero speed switch, also called a speed sensor, as a device that detects slowing or stopping of the fan shaft. Its usual job is to tell a control or alarm system that a fan has stopped even though its starter is still energized, as when a belt breaks. Preventing reverse rotation is the job of an anti-backspin device (B). Airflow is measured with a piezometer ring (C). Vibration monitoring uses bearing vibration sensors (D).

Concept Tested: Zero Speed Switch

See: Installation Accessories

3. Which statement best describes what vibration isolation does?

  1. It balances the impeller so the fan no longer vibrates
  2. It stiffens the floor so it can carry the fan's weight
  3. It increases the fan's speed so it runs above resonance
  4. It limits the force the operating fan transfers to its supporting structure
Show Answer

The correct answer is D. FanPedia defines vibration isolation as preventing or limiting the force an operating fan transfers to its supporting structure. It does not reduce the fan's own vibration; it keeps that vibration from travelling into the building. Balancing (A) reduces the unbalance force itself, which is a different remedy. Isolation uses resilient mounts, not a stiffer floor (B). Changing the fan's speed (C) is not what isolators do.

Concept Tested: Vibration Isolation

See: Vibration Isolation

4. A belt-driven fan runs at 900 RPM, driven by a motor at 1,750 RPM. Which forcing frequency sets the isolation requirement?

  1. 29.2 Hz, from the motor shaft
  2. 15 Hz, from the fan shaft
  3. 44.2 Hz, the sum of the two shaft frequencies
  4. 14.2 Hz, the difference between the two shaft frequencies
Show Answer

The correct answer is B. Forcing frequency is \( f = N/60 \). The fan shaft gives 900/60 = 15 Hz and the motor shaft gives 1,750/60 ≈ 29.2 Hz. The lowest forcing frequency is the hardest to isolate, so 15 Hz sets the requirement. An isolator that handles 15 Hz handles 29.2 Hz even better. The chapter never combines shaft frequencies by adding them (C) or subtracting them (D).

Concept Tested: Vibration

See: Vibration

5. An isolator compresses 0.25 in. under its actual load. Using \( f_n \approx 3.13/\sqrt{\delta} \), what is the natural frequency of the mounted assembly?

  1. About 6.3 Hz
  2. About 12.5 Hz
  3. About 3.1 Hz
  4. About 0.8 Hz
Show Answer

The correct answer is A. \( \sqrt{0.25} = 0.5 \), so \( f_n = 3.13/0.5 \approx 6.26 \) Hz. Option B divides by 0.25 instead of its square root, which is the result for a 0.0625 in. pad. Option C is the natural frequency at 1 in. of deflection. Option D multiplies by 0.25 instead of dividing. Deflection sits under a square root, so quadrupling deflection only halves the natural frequency.

Concept Tested: Isolator Deflection

See: Isolator Deflection

6. Why do neoprene pads (about 0.0625 in. deflection) make vibration worse under a 600 RPM fan?

  1. Neoprene softens with time, so the pads lose their deflection
  2. The pads add mass, which raises the fan's forcing frequency
  3. The pad's natural frequency (about 12.5 Hz) is above the 10 Hz forcing frequency, so the frequency ratio is below about 1.4 and force is amplified
  4. Pads isolate only horizontal vibration, and a slow fan vibrates vertically
Show Answer

The correct answer is C. The pad's natural frequency is \( 3.13/\sqrt{0.0625} = 12.52 \) Hz. At 600 RPM the forcing frequency is 10 Hz, so \( r = 0.80 \). Below \( r \approx 1.41 \), transmissibility exceeds 1, and here the pad passes about 2.8 times the force a rigid mount would. Isolation depends on the frequency ratio, not on material aging (A), forcing-frequency changes (B) or direction (D). Under a 1,170 RPM fan the same pad gives about 30% isolation.

Concept Tested: Vibration Isolation

See: Isolator Deflection

7. An inertia base has a pour cavity 6 ft long, 4 ft wide and 6 in. deep. The fan, motor and steel base weigh 2,500 lb. Using 150 lb/ft³ for concrete, what total weight must the spring isolators be selected for?

  1. 2,500 lb
  2. 3,700 lb
  3. 1,800 lb
  4. 4,300 lb
Show Answer

The correct answer is D. The volume is \( 6 \times 4 \times 0.5 = 12 \) ft³, so the concrete weighs \( 150 \times 12 = 1{,}800 \) lb. The total is 2,500 + 1,800 = 4,300 lb. Option A sizes springs for the bare fan, and those springs would bottom out once the customer pours the concrete. Option C is the concrete alone. Option B uses 100 lb/ft³ instead of 150 lb/ft³ for the concrete.

Concept Tested: Inertia Base

See: Inertia Base

8. A specification calls for "Arrangement 9F, SWSI, belt driven, spring isolators mounted under the fan." What is FanPedia's stated alternative?

  1. Keep Arrangement 9F and place neoprene pads under the fan instead of springs
  2. Use an Arrangement 1 fan with a separate isolation base, with isolators under the base
  3. Use a unitary base under the Arrangement 9F fan and set it on springs
  4. Mount the isolators directly under the 9F motor frame only
Show Answer

The correct answer is B. FanPedia states that Arrangement 9F is not suitable for mounting vibration isolators directly under the fan. When isolators are required, it says to use an Arrangement 1 fan with a separate isolation base, with the isolators under the base's mounting brackets and flex connectors at the inlet and outlet. A unitary base is designed for use without isolators (C). Pads (A) or partial mounting (D) do not change the 9F limit.

Concept Tested: Isolation Base

See: Isolation Base

9. A fan sits on an isolation base while its detached inlet box stands on grade. What does FanPedia require between the fan and the inlet box?

  1. A rigid bolted flange so the box steadies the fan
  2. A seismic snubber between the box and the base
  3. A flex connector (slip connection)
  4. Nothing, because the inlet box is not part of the airstream
Show Answer

The correct answer is C. When the fan and the inlet box are on different mounting planes, they can move relative to each other. FanPedia then requires a flex connector, or slip connection, between them. This follows the general isolation rule that rigid duct connections are not allowed on an isolated fan. A rigid flange (A) would short-circuit the isolators. A snubber limits excessive motion but does not seal the joint (B). The inlet box carries the airstream (D).

Concept Tested: Inlet Box Mounting Planes

See: Inlet Box Mounting Planes

10. An engineer changes from 1 in. springs to 2 in. springs under a 1,170 RPM fan. What happens, and why is the gain modest?

  1. The natural frequency falls by a factor of √2 (3.13 Hz to about 2.21 Hz); the frequency ratio is already high, so isolation improves only slightly
  2. The natural frequency is halved, so transmitted force is cut in half
  3. The natural frequency doubles, so the springs move closer to resonance
  4. Nothing changes, because deflection does not affect natural frequency
Show Answer

The correct answer is A. Natural frequency varies with \( 1/\sqrt{\delta} \), so doubling deflection divides it by √2, not 2 (B). Deflection lowers the natural frequency rather than raising it (C), and it does affect it (D). With 1 in. springs at 1,170 RPM, \( r \) is already 6.23 and isolation is about 97.4%, so 2 in. springs add only a little. As the 600 RPM example shows, the extra deflection matters most on slow fans.

Concept Tested: Isolator Deflection

See: Isolator Deflection