Quiz: Drives and Standard Fan Arrangements¶
Test your understanding of direct and belt drives, overhung and center hung impellers, and the standard numbered fan arrangements with these review questions.
1. Which statement best describes an overhung impeller?¶
- The impeller is mounted between two bearings, one on each side of it
- The impeller is mounted on the end of the shaft, beyond both bearings
- The impeller is mounted on a shaft that is carried by bearings inside the inlet
- The impeller is mounted on a vertical shaft above the motor
Show Answer
The correct answer is B. An overhung impeller sits at the end of the shaft, past both bearings, like the load on a diving board. The two bearings sit together on a bearing pedestal outside the housing. An impeller mounted between the bearings (A) is a center hung impeller. A bearing in the inlet (C) belongs to a center hung single width fan, and a vertical shaft (D) describes a vertical arrangement, not the impeller's position on the shaft.
Concept Tested: Overhung Impeller
See: Overhung Impeller
2. Which arrangement has the impeller mounted directly on the motor shaft, with no fan shaft and no fan bearings?¶
- Arrangement 1
- Arrangement 7
- Arrangement 3
- Arrangement 4
Show Answer
The correct answer is D. In Arrangement 4 the impeller is mounted directly to the motor shaft, so the motor's own bearings carry it. FanPedia notes that it offers low maintenance because there are no fan bearings, fan shaft or drive parts to maintain. Arrangement 1 has two fan bearings on a pedestal, Arrangement 3 carries its bearings on the housing, and Arrangement 7 couples the motor to a separate fan shaft.
Concept Tested: Arrangement 4
See: Arrangement 4
3. Why does FanPedia describe Arrangement 1 as ideal for high temperature or contaminated air?¶
- Its two bearings sit on a pedestal outside the housing, out of the airstream
- Its impeller is center hung, so the bearing loads are shared evenly
- Its motor is mounted inside the housing, where the airflow cools it
- It is always direct driven, so there are no belts to degrade in the heat
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The correct answer is A. Arrangement 1 overhangs the impeller, so both bearings sit together on a pedestal beside the housing, out of the airstream. Hot air would thin and break down bearing grease, and dirty air would attack the seals, so keeping the bearings out of the air is the advantage. Arrangement 1 is overhung, not center hung (B); its motor is mounted separately on the floor or fan base (C); and it is usually belt driven (D).
Concept Tested: Bearings Out of Airstream
4. How does the drive service factor differ from the motor service factor on the nameplate?¶
- They are the same number, written once on the nameplate and once on the drive order
- The drive service factor is always lower than 1.0, while the motor service factor is above 1.0
- The drive service factor multiplies motor horsepower to size belts and sheaves; the motor service factor says how far above rated horsepower the motor may run
- The drive service factor applies only to direct drive fans, and the motor service factor only to belt drives
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The correct answer is C. The drive service factor comes from the belt drive manufacturer's table and multiplies motor horsepower to give the design horsepower used to select belts and sheaves. The motor service factor is a nameplate rating: FanPedia's example is 1.15 on a 100 HP motor, allowing 115 HP. Confusing the two (A) is the trap the chapter warns about. The drive service factor is at least 1.0 (B) and applies to belt drives (D).
Concept Tested: Drive Service Factor
See: Drive Service Factor
5. An arrangement drawing shows a flexible coupling joining the motor shaft to the fan shaft. What can you conclude?¶
- The fan is belt driven, and the coupling replaces the fan sheave
- The fan is direct driven, has its own shaft and bearings, and is therefore not Arrangement 4
- The fan must be Arrangement 4, because every direct drive fan is Arrangement 4
- The impeller must be center hung, because couplings are used only on center hung fans
Show Answer
The correct answer is B. A coupling joins two shafts end to end, so fan speed equals motor speed and the fan has its own shaft and bearings. Arrangement 4 has no fan shaft to couple to, so a coupled fan is a different arrangement, such as 7 (center hung) or 8 (overhung). A coupling is not part of a belt drive (A). Arrangement 8 shows that a coupled fan can be overhung, so D is wrong.
Concept Tested: Shaft Coupling
See: Shaft Coupling
6. A belt drive has a motor running at 1,760 rpm, a motor sheave with a 5.0 in pitch diameter and a fan sheave with an 8.0 in pitch diameter. How fast does the fan turn?¶
- 1,100 rpm
- 2,816 rpm
- 1,760 rpm
- 1,320 rpm
Show Answer
The correct answer is A. Fan speed equals motor speed times the motor sheave diameter divided by the fan sheave diameter: 1,760 × 5.0 / 8.0 = 1,100 rpm. The larger fan sheave slows the fan, so the answer must be below motor speed. Option B, 2,816 rpm, comes from inverting the ratio. Option C ignores the sheaves, as if the fan were direct driven.
Concept Tested: Sheave
See: Sheave
7. A 20 hp belt-driven fan uses a drive service factor of 1.3, and the chosen V-belt section can carry 7.0 hp per belt at this belt speed. How many belts does the drive need?¶
- 2 belts
- 3 belts
- 4 belts
- 5 belts
Show Answer
The correct answer is C. Design horsepower is 20 × 1.3 = 26 hp. Dividing by the per-belt rating gives 26 / 7.0 = 3.71, which is rounded up to 4 belts because a fraction of a belt cannot share the load. Rounding down to 3 (B) would overload every belt. Three belts also results from forgetting the service factor (20 / 7.0 = 2.86, rounded up).
Concept Tested: V-Belt
See: V-Belt
8. An outline drawing shows a belt-driven single inlet centrifugal fan. Both bearings sit together on a pedestal, nothing is on the inlet side of the impeller, and the motor is on an adjustable base inside the bearing pedestal. Which arrangement is it?¶
- Arrangement 1
- Arrangement 3
- Arrangement 9
- Arrangement 10
Show Answer
The correct answer is D. Belt drive with both bearings on a pedestal means an overhung fan: Arrangement 1, 9 or 10. These three differ only in where the motor goes. A motor on its own base on the floor is Arrangement 1, a motor on the side of the pedestal is Arrangement 9, and a motor inside the pedestal is Arrangement 10. Arrangement 3 is center hung, with a bearing on each side of the impeller.
Concept Tested: Fan Arrangement
See: The Fan Arrangement
9. An oven builder needs a large belt-driven plug fan to recirculate hot air. The oven wall is a light-gauge insulated panel, and the line is sensitive to vibration. Which plug fan arrangement fits best, and why?¶
- Arrangement 9, because it is the most common plug fan arrangement
- Arrangement 4, because a direct drive fan has no belts to wear in the heat
- Arrangement 9P, because the pedestal carries the fan at grade instead of the wall and helps with lower vibration
- Arrangement 8, because the coupled motor shortens the assembly
Show Answer
The correct answer is C. Arrangement 9 is the most common plug fan arrangement (A), but it hangs the whole fan on the customer's wall. FanPedia says the pedestal versions suit larger fans where the wall may not be enough, and that mounting to the foundation helps meet lower vibration requirements. Arrangement 4 is direct drive and limited by hot air (B). Arrangement 8 is the least common plug fan arrangement because of its long assembly (D).
Concept Tested: Plug Fan Arrangements
10. A large DWDI fan works well in a standard Arrangement 3 when it moves ambient outdoor air, but the same fan must now handle 300°F process air. What explains the problem, and what change addresses it?¶
- Both center hung bearings sit in the airstream, so moving to Arrangement 3DI, whose inlet boxes put the bearings outside the air, addresses it
- The overhung impeller puts too much leverage on the bearing nearest the housing, so switching to a center hung fan addresses it
- The V-belts lose grip in hot air, so switching to an Arrangement 3 with a larger fan sheave addresses it
- The housing cannot withstand 300°F, so changing the discharge position addresses it
Show Answer
The correct answer is A. A DWDI fan has inlets on both sides, so both center hung bearings are in the airstream, and FanPedia limits standard DWDI fans to clean air at 130°F. Arrangement 3DI uses integrated inlet boxes to locate both bearings outside the airstream on independent pedestals. Option B describes an overhung fan, not a DWDI Arrangement 3. Belt grip (C) and discharge position (D) have nothing to do with the bearing temperature limit.
Concept Tested: Arrangement 3 SI and DI