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Quiz: Bearings, Shafts and Shaft Seals

Test your understanding of fan shafts, bearings, bearing monitoring and shaft seals with these review questions.

1. What does an L-10 bearing life rating describe?

  1. The number of hours after which every bearing in a population is expected to have failed
  2. The number of hours by which 10% of a large population of identical bearings, at a given speed and load, are expected to have failed
  3. The guaranteed minimum life of one individual bearing, stated in years of operation
  4. The number of hours a bearing can run without lubrication before 10% of its raceway wears away
Show Answer

The correct answer is B. Bearing life is a statistical estimate of fatigue failure, and L-10 life is the hours by which 10% of a large population of identical bearings at a given speed and load are expected to have failed. It is not the point where every bearing fails (A), it is not a guarantee for one bearing (C), and FanPedia's life figures assume proper lubrication rather than describing running without it (D).

Concept Tested: Bearing Life

See: Bearing Life

2. Which shaft seal does FanPedia describe as its standard type?

  1. The lip shaft seal with a Buna-N elastomer lip
  2. The stuffing box seal with Graphoil rings
  3. The double carbon ring seal with a lantern ring
  4. The friction shaft seal, also called a tacky cloth seal
Show Answer

The correct answer is D. The friction shaft seal is FanPedia's standard type: a ring of soft seal material, most often tacky cloth, clamped by a cover plate so that it rubs lightly on the shaft. The lip seal (A) is the next step up when leakage must be reduced. Stuffing box (B) and double carbon ring (C) seals limit leakage further but are more expensive and are chosen for demanding constructions such as nominally leak-tight fans.

Concept Tested: Friction Shaft Seal

See: Lip and Friction Shaft Seals

3. Which statement correctly distinguishes a bearing seal from a shaft seal?

  1. A bearing seal keeps dirt out of the bearing and lubricant in; a shaft seal on the housing wall limits air or gas leakage along the shaft
  2. A bearing seal limits air leakage out of the fan housing; a shaft seal keeps grease inside the bearing
  3. A bearing seal is used only on ball bearings; a shaft seal is used only on spherical roller bearings
  4. A bearing seal and a shaft seal are two names for the same part on the drive side of the fan
Show Answer

The correct answer is A. A bearing seal is built into the bearing housing and protects the bearing from dust and moisture while keeping lubricant in; its type also sets the bearing's speed limit. A shaft seal is mounted on the fan housing where the shaft passes through the wall, and its job is the airstream, not lubrication. Option B reverses the two jobs, a common mix-up. Options C and D confuse two separate parts.

Concept Tested: Bearing Seal

See: Bearing Seals

4. Why does a hot fan normally use one fixed bearing and one floating bearing rather than two fixed bearings?

  1. A floating bearing runs cooler because it carries no radial load
  2. Two fixed bearings would let the impeller slide axially into the inlet funnel
  3. The shaft grows longer as it heats, and two fixed bearings would turn that growth into thrust load on the bearings
  4. A floating bearing lets the shaft be removed without pulling the impeller
Show Answer

The correct answer is C. Only one bearing should control the shaft's axial position. A shaft that warms by hundreds of degrees gets measurably longer, and if both bearings clamp it, the growth has nowhere to go except into the bearings as thrust load. The floating bearing still supports the shaft radially (A is wrong) and simply lets it slide. Two fixed bearings hold the shaft more firmly, not less (B), and the floating bearing has nothing to do with shaft removal (D).

Concept Tested: Fixed Bearing

See: Fixed Bearings

5. A fan has bearings rated at an L-10 life of 40,000 hours and runs 12 hours a day, 5 days a week. About how many calendar years is that L-10 life?

  1. 6.4 years
  2. 9.6 years
  3. 12.8 years
  4. 25.6 years
Show Answer

The correct answer is C. Use Y = L-10 ÷ (hours per day × days per week × 52). The fan runs 12 × 5 × 52 = 3,120 hours a year, and 40,000 ÷ 3,120 ≈ 12.8 years. 9.6 years (B) is the answer for 16 hours a day, 5 days a week, the chapter's own example. 6.4 years (A) is roughly the figure for 24 hours a day, 5 days a week. 25.6 years (D) comes from counting only half the hours.

Concept Tested: Bearing Life

See: Bearing Life in Calendar Years

6. An Arrangement 1, horizontally mounted fan handles 450°F air on split spherical roller pillow blocks. How should the two bearings be ordered?

  1. Both bearings fixed, because the fan is horizontal
  2. Fixed bearing on the drive end and floating bearing on the inboard end near the impeller
  3. Floating bearing on the drive end and fixed bearing on the inboard end near the impeller
  4. Both bearings floating, because the airstream is above 300°F
Show Answer

The correct answer is B. At 301°F and above, every bearing type uses one fixed and one floating bearing, so two fixed bearings (A) are ruled out; split roller bearings use one of each even below 300°F. On horizontally mounted fans the fixed bearing goes on the drive side, so the floating bearing sits on the inboard end. Option C is the layout for vertically mounted fans. Two floating bearings (D) would leave nothing to locate the shaft axially.

Concept Tested: Fixed Bearing

See: Fixed Bearings

7. A fan for an aggregate drying plant will run in heavy dust on split spherical roller pillow blocks. Which bearing seal choice best follows the chapter's guidance?

  1. A lip seal, because contact seals give the tightest fit at any speed
  2. A standard labyrinth seal, because it is the only non-contact option
  3. No bearing seal, because the shaft seal on the housing already keeps the dust out
  4. A taconite seal, after checking the fan speed against its speed limit and allowing for the wider bearing it needs
Show Answer

The correct answer is D. Taconite seals are designed for dirty or abrasive environments on split pillow block spherical roller bearings. Two checks follow: the standard taconite seal has a lower speed limit than a labyrinth (a non-contact Canadian type exists for higher speeds), and it makes the bearing wider, which can mean a longer shaft or a larger bearing support. Lip seals (A) are for ball bearings at low to moderate speeds. The shaft seal (C) protects the airstream, not the bearing.

Concept Tested: Bearing Seal

See: Bearing Seals

8. A specification says: "Provide provisions for vibration monitoring in the horizontal and vertical directions at each fan bearing. Sensors by others." What does the fan order need on a two-bearing fan?

  1. Four drilled and tapped holes in the bearing housings, with no sensors shipped
  2. Four accelerometers shipped loose, with sensor cords supplied by Twin City Fan
  3. Two tapped holes, one per bearing, because each sensor reads both directions
  4. Bearing RTDs installed through a T-fitting on each grease fitting
Show Answer

The correct answer is A. "Provisions" means drilled and tapped holes in the bearing housings, not the sensors themselves. Two bearings times two directions gives four holes, and "sensors by others" means nothing is shipped loose; the standard tap is 1/4"-28 UNF with a 1-inch spot face unless the plant asks for 1/4"-18 NPT. Option B ignores "by others," and sensor cords are supplied by others in any case. Option C undercounts the directions. Option D describes temperature sensors, not vibration sensors.

Concept Tested: Bearing Vibration Sensor

See: Monitoring the Bearings

9. A nominally leak-tight Arrangement 8 fan handles a gas that must not escape. Why is a double carbon ring seal chosen over a single carbon ring seal?

  1. The double ring uses larger carbon rings, so it seals with zero clearance and no leakage at all
  2. The lantern ring between its two rings lets customer-supplied purge gas at a slightly higher pressure fill the seal, so any leakage is purge gas flowing into the fan
  3. The double ring needs no purge supply, while the single ring must be purged with nitrogen
  4. The single ring cannot be installed without first removing the impeller from the shaft
Show Answer

The correct answer is B. The double ring seal places a lantern ring at a purge hole between two split carbon rings. When clean air or inert gas is supplied there slightly above the gas pressure in the fan, leakage across the inner ring is purge gas entering the fan rather than process gas escaping. FanPedia's leak-tight list includes the double ring but not the single ring. No seal reaches zero leakage (A), the purge belongs to the double ring (C), and both seals use split rings that install around the shaft (D).

Concept Tested: Carbon Ring Seal

See: Carbon Ring Seals

10. FanPedia's nominally leak-tight construction says not to use a shaft sleeve and cap. What is the underlying reason?

  1. Shaft sleeves cannot be made from the same material as the airstream parts
  2. Shaft sleeves are allowed only on Arrangement 4 fans
  3. A sleeve makes the shaft too large to fit the shaft seal's bore
  4. The joint between the sleeve and the shaft is one more path for gas to leak along
Show Answer

The correct answer is D. A shaft sleeve shields the shaft from an airstream of a different material, but the gap between sleeve and shaft creates an additional leak path, which defeats the purpose of leak-tight construction. Option A is not a reason the chapter gives; the sleeve exists to shield a shaft whose material differs from the airstream parts. Sleeves are typical on Arrangements 1, 8, 9 and 10, while Arrangement 4 fans can use a protruding hub instead (B is reversed). Seal fit (C) is a machining detail, not the reason FanPedia gives.

Concept Tested: Shaft Sleeve

See: Shaft Features