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Quiz: Bearing Details, Lubrication and Pedestals

Test your understanding of bearing life ratings, pillow block housings, fixed and floating bearings, lubrication, bearing and gas purge seals, and fan pedestals with these review questions.

1. Which bearing restraint does FanPedia name as the first choice for Type A spark-resistant construction?

  1. Bearing positioners (jacking screws) tightened against the housing
  2. Bearing dowel pins passing through a hole drilled in the bearing base
  3. Bearing stop blocks welded to the pedestal on each side of the bearing
  4. Slotted bolt holes in the pillow block base
Show Answer

The correct answer is B. FanPedia's Type A spark-resistant page lists restrained bearings, by dowel pins or stop blocks, with dowel pins as the first choice. A doweled bearing can also be removed and reinstalled in exactly the same position. Stop blocks (C) are the other accepted method, not the first choice. Positioners (A) adjust a bearing's location but do not restrain it, and slotted holes (D) exist so the bearing can be moved during mounting.

Concept Tested: Bearing Dowel Pins

See: Holding the Bearing in Place

2. What is the Twin City Fan standard for bolting a pillow block bearing to the pedestal on a fan with a 3-7/16 inch shaft?

  1. A two hole mount, one bolt on each side of the shaft
  2. A flange mount bolted to a plate perpendicular to the shaft
  3. A four hole mount, two bolts on each side of the shaft
  4. A single center bolt with a dowel pin
Show Answer

The correct answer is C. FanPedia gives the TCF standard by shaft diameter: a two hole mount for shafts of 2-15/16 inch and below, and a four hole mount for 3-7/16 inch and above. More bolts spread the load and resist rocking under larger shaft loads. A two hole mount (A) applies to the smaller shafts. A flange mount (B) is a different housing type, and option D is not a mounting standard in the chapter.

Concept Tested: Two and Four Hole Mount

See: Bolting the Bearing Down

3. When would a totally split roller bearing be specified instead of a standard split pillow block?

  1. On a large fan where pulling the shaft to change a bearing would be a major outage
  2. On any fan running above 300°F, because split parts expand freely
  3. On small fans where a lower-cost housing is wanted
  4. On fans whose bearings must carry no axial load
Show Answer

The correct answer is A. A totally split roller bearing splits the housing and every internal part, so the bearing can be removed with the shaft left in place. FanPedia lists it as a specialty bearing for special applications only, typically large fans where removing the shaft is a major job. A split pillow block splits only the housing; its insert still slides on the shaft. Temperature (B) is handled by fixed and floating bearings, cost (C) points the other way, and axial load (D) is not the deciding factor.

Concept Tested: Totally Split Roller Bearing

See: Pillow Block Housings

4. A competitor quotes a bearing with a "200,000-hour average life," and the Twin City Fan quote says "L-10 40,000 hours." What is the best interpretation?

  1. The competitor's bearing will last five times longer than the TCF bearing
  2. The TCF bearing is better, because L-10 is a stricter test than average life
  3. The two numbers cannot be compared, because one is in hours and one is in years
  4. The two selections may be identical, because L-50 (average life) is five times L-10
Show Answer

The correct answer is D. L-50, also called average life, is the hours at which 50% of a bearing population is expected to fail, and FanPedia gives L-50 as five times L-10. An L-10 of 40,000 hours therefore corresponds to an L-50 of 200,000 hours. Both numbers describe the same bearing at different points on its failure curve, so option A mistakes a different statistic for a longer-lived bearing. Option B overstates the difference, and both figures are in hours (C).

Concept Tested: L-50 Bearing Life

See: Bearing Life: L-10 and L-50

5. A plenum fan in Arrangement 3VA is mounted vertically and uses split roller bearings at 200°F. Which bearing configuration do the fixed-floating rules call for?

  1. Two fixed bearings
  2. Fixed bearing at the drive end, floating bearing at the non-drive end
  3. Floating bearing at the drive end, fixed bearing at the non-drive end
  4. Two floating bearings
Show Answer

The correct answer is C. Split roller bearings use one fixed and one floating bearing at every temperature, so two fixed bearings (A) are out. Placement then depends on orientation: horizontal fans put the fixed bearing on the drive side, but vertically mounted fans, including plenum Arrangements 3VA and 3VS, put the floating bearing on the drive side. Option B is the horizontal-fan layout. Two floating bearings (D) would leave nothing to locate the shaft axially.

Concept Tested: Fixed-Floating Bearing Rules

See: The Fixed-Floating Bearing Rules

6. A customer asks for the L-50 life, in years, of a bearing rated L-10 20,000 hours on a fan that runs 24 hours a day, 7 days a week. Which answer is closest?

  1. 11.4 years
  2. 2.3 years
  3. 22.8 years
  4. 4.6 years
Show Answer

The correct answer is A. First convert the rating: L-50 = 5 × 20,000 = 100,000 hours. Continuous running is 8,760 hours a year, and 100,000 ÷ 8,760 ≈ 11.4 years, which matches FanPedia's table. Option B is the L-10 answer, from forgetting the five-times conversion. Option C is the L-50 of a 40,000-hour bearing, and option D is the L-10 of a 40,000-hour bearing.

Concept Tested: L-50 Bearing Life

See: Bearing Life: L-10 and L-50

7. A horizontally mounted Arrangement 3SI fan uses ball bearings at 250°F. How should the bearing configuration be handled?

  1. Order two fixed bearings, as for any ball bearing fan at 300°F and below
  2. Order a fixed bearing at the drive end and a floating bearing at the non-drive end without review
  3. Order a floating bearing at the drive end, because the impeller sits between the bearings
  4. Flag it for engineering review, because ball bearings on 3SI may still need one fixed and one floating
Show Answer

The correct answer is D. The general rule allows two fixed ball bearings at 300°F and below, but FanPedia adds a footnote for Arrangements 3SI, 7SI, 3DI and 7DI, where the bearings straddle the impeller and the shaft between them is long: ball bearings there may still require one fixed and one floating bearing. The chapter treats that case as an engineering review item rather than a lookup. Option A ignores the footnote, B skips the review, and C applies the vertical-fan placement rule to a horizontal fan.

Concept Tested: Fixed-Floating Bearing Rules

See: The Fixed-Floating Bearing Rules

8. A fan must run fast in a dusty plant on split pillow block spherical roller bearings, and the fan speed is above the standard taconite seal's limit. Which bearing seal fits best?

  1. A lip seal, because contact seals have the highest speed limits
  2. A Canadian type (non-contact) taconite seal
  3. A standard type taconite seal, since the seal does not limit bearing speed
  4. A gas purge shaft seal on the fan housing
Show Answer

The correct answer is B. Taconite seals are designed for dirty or abrasive service on split pillow block spherical roller bearings, but the standard type has a lower speed limit than a labyrinth seal. FanPedia notes a non-contact Canadian type taconite seal for higher speeds. The seal type does set the bearing's speed limit, so C is wrong. Lip seals (A) rub on the shaft and suit only low to moderate speeds. A gas purge seal (D) is a shaft seal for the airstream, not a bearing seal.

Concept Tested: Bearing Seal Speed Limit

See: Bearing Seals and Seal Speed Limits

9. In a gas purge shaft seal, why must the purge gas be supplied at a pressure above the gas pressure inside the housing at the shaft?

  1. So the purge gas cools the shaft enough to replace a shaft cooler
  2. So the higher pressure lubricates the carbon rings and packing
  3. So any leakage past the inner ring is clean purge gas flowing into the fan, and process gas never reaches the outside
  4. So the lantern ring is pushed tight against the shaft and stops all leakage
Show Answer

The correct answer is C. No shaft seal is perfectly tight, so a purge seal decides what leaks. Purge gas fed to the lantern ring at a pressure above the housing gas means leakage past the inner ring is clean gas entering the fan, and leakage past the outer ring is clean gas leaving it. The lantern ring is an open spacer and does not clamp the shaft or stop all leakage (D). Cooling (A) and lubrication (B) are not the purpose of the purge.

Concept Tested: Gas Purge Seal

See: Gas Purge Seals

10. An Arrangement 1 fan has a stainless steel housing handling 550°F exhaust, and the specification forbids welding stainless steel to carbon steel. Which pedestal approach addresses both the heat and the dissimilar-metal requirement?

  1. A separated pedestal with a heat shield, joined to the housing with buffer strips or built as a non-rotatable bolt-on pedestal
  2. A rotatable bolt-on pedestal, because every fan above 500°F must have a rotatable housing
  3. A standard mild steel pedestal welded to the housing, with a grease pan under the bearings
  4. A mild steel pedestal welded to the housing, with taconite seals to keep heat out of the bearings
Show Answer

The correct answer is A. A separated pedestal leaves a gap that reduces housing heat conducted into the pedestal and bearings, and a heat shield adds protection. Because the mild steel pedestal cannot be welded to the stainless housing, FanPedia offers buffer strips or a non-rotatable bolt-on pedestal for dissimilar metals. A rotatable bolt-on pedestal (B) is for models whose housing rotates to several discharge positions. Options C and D weld the pedestal, which the specification forbids, and taconite seals keep out dirt, not heat.

Concept Tested: Fan Pedestal

See: Fan Pedestals