Bearing Details, Lubrication and Pedestals¶
Summary¶
This chapter adds the details needed to specify bearings and their supports. It covers L-10 and L-50 bearing life, split, solid, and totally split pillow blocks, adapter sleeve and concentric mounts, fixed and floating bearings and the rules for placing them, drive-end and non-drive-end bearings, grease fittings, static oil and oil mist lubrication, labyrinth, taconite, and gas purge seals, seal speed limits, and bearing stop blocks and dowel pins. It closes with fan pedestals, separated and bolt-on pedestals, and shaft and coupling guards. After completing it, readers can choose bearing and lubrication options for a given duty.
Concepts Covered¶
This chapter covers the following 26 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| L-10 Bearing Life | 2 |
| Split Pillow Block Bearing | 2 |
| Adapter Sleeve Mount | 1 |
| Floating Bearing | 5 |
| Grease Fitting | 1 |
| Static Oil Lubrication | 1 |
| Oil Mist Lubrication | 1 |
| Labyrinth Seal | 2 |
| Taconite Seal | 2 |
| Bearing Stop Blocks | 1 |
| Bearing Dowel Pins | 1 |
| Fan Pedestal | 24 |
| Gas Purge Seal | 48 |
| Solid Pillow Block Bearing | 1 |
| Concentric Bearing Mount | 1 |
| Keystock | 1 |
| L-50 Bearing Life | 1 |
| Totally Split Roller Bearing | 1 |
| Separated Pedestal | 1 |
| Drive End Bearing | 3 |
| Two and Four Hole Mount | 1 |
| Bearing Seal Speed Limit | 1 |
| Shaft and Coupling Guards | 1 |
| Bolt-On Pedestal | 1 |
| Non-Drive End Bearing | 1 |
| Fixed-Floating Bearing Rules | 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 7: Housing Construction and Exploded Views
- Chapter 11: Drives and Standard Fan Arrangements
- Chapter 12: Discharge Positions and Rotation
- Chapter 16: Bearings, Shafts and Shaft Seals
Two Small Blocks, One Long Fan Life
Most fan service calls start at the bearings, and most bearing questions on a quote come down to a short list of choices: which life rating, which housing, which seal, which lubricant, and what holds it all up. By the end of this chapter you will be able to read a bearing and pedestal line on an order and say why each choice is there. Let's follow the airflow, and then follow the shaft out to where it meets the steel.
Chapter 16 introduced bearings as parts: ball and spherical roller elements, pillow block and flange housings, the fixed bearing, lubrication in general, and the shaft seals that keep air inside the housing. This chapter adds the details that show up when a fan is actually specified and built. You will learn how bearing life is rated and converted to calendar time, how the bearing housing and its mount are chosen, which bearing on the shaft is fixed and which floats, how oil replaces grease on special jobs, how bearing seals and gas purge shaft seals differ, and how the pedestal, its options and the guards around the drive complete the assembly.
Bearing Life: L-10 and L-50¶
Even a bearing that is perfectly installed, loaded and lubricated eventually fails from metal fatigue in its rolling elements and races. Because fatigue varies from one bearing to the next, bearing life is a statistical estimate: it predicts when a given percentage of a large population of identical bearings, running at a stated speed and load, will have failed. FanPedia (p. 90) puts it plainly: all bearings have a finite life.
L-10 bearing life is the number of operating hours at which 10% of a population of bearings at a given speed and loading condition is expected to fail. Equivalently, 90% of the bearings are expected to survive past that number of hours. This is the rating Twin City Fan catalogs quote. Most TCF fan models offer L-10 life of 40,000 hours, and some models are offered at 20,000, 40,000, 60,000, 80,000 or 100,000 hours; the product catalog for each model states which.
L-50 bearing life, also called average life, is the number of hours at which 50% of the population is expected to fail. FanPedia gives a simple conversion: L-50 is five times L-10. A bearing selected for L-10 of 40,000 hours therefore has an L-50 of 200,000 hours. The two numbers describe the same bearing; L-50 only sounds longer because it describes a later point in the failure curve. When a competitor quotes "200,000-hour average life" and a TCF quote says "L-10 40,000 hours," the two may be identical selections.
Hours become meaningful to a customer only when they are converted to years of that customer's operating schedule. The conversion divides the rated hours by the hours the fan runs in a year:
The factor \( 365/7 \approx 52.14 \) is the number of weeks in a year. The table below reproduces FanPedia's two examples (p. 90).
| Rating | Hours | 24 h/day, 7 days/wk | 24 h/day, 5 days/wk | 16 h/day, 5 days/wk | 8 h/day, 5 days/wk | 2 h/day, 5 days/wk |
|---|---|---|---|---|---|---|
| L-10 | 40,000 | 4.6 years | 6.4 years | 9.6 years | 19 years | 77 years |
| L-50 | 200,000 | 22.8 years | 32 years | 48 years | 96 years | 385 years |
| L-10 | 20,000 | 2.3 years | 3.2 years | 4.8 years | 9.6 years | 39 years |
| L-50 | 100,000 | 11.5 years | 16 years | 24 years | 48 years | 193 years |
The calculation lands within a few tenths of a year of every cell in the table; the small differences come from rounding and from whether a year is counted as exactly 52 weeks or as 365 days.
The 8,760 Shortcut
A year of continuous running is 8,760 hours, so for a 24/7 process fan just divide the L-10 hours by 8,760: 40,000 hours is about 4.6 years. For anything less than 24/7, multiply that answer by 168 and divide by the fan's actual hours per week.
The MicroSim below lets you practice the conversion for both ratings and several duty schedules before you have to do it on a customer call.
Diagram: Bearing Life to Calendar Years¶
Run Bearing Life to Calendar Years Fullscreen
Bearing Life to Calendar Years
Type: microsim
sim-id: ch17-bearing-life-calendar-years
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the expected calendar life in years of an L-10 or L-50 bearing rating for a stated daily and weekly duty schedule, to within 2% of the model value.
Prerequisites: bearing life as a statistical estimate, L-10 bearing life, L-50 bearing life, the L-50 = 5 × L-10 conversion, and the years formula (all defined in the section "Bearing Life: L-10 and L-50" above).
Evidence of Mastery: For five challenge problems, the learner types the expected life in years before the answer is revealed. An answer is correct when it is within ±2% of the model value computed by the Rules. Mastery is 4 of 5 correct on the first attempt. Changing the inputs in free-exploration mode is exploration, not evidence.
Misconceptions: (1) L-50 is a better or longer-lived bearing than L-10, rather than a different statistical point on the same bearing. (2) L-10 hours equal calendar hours regardless of how many hours a day the fan runs. (3) L-10 is the time at which every bearing fails.
Instructional Rationale: Converting a rating to years is a calculation applied to new duty schedules, so the learner commits a number before seeing the model value. Free exploration afterwards lets the learner see how strongly the duty schedule, not just the rating, sets the calendar life.
Content:
The learner can set four inputs:
| Input | Min | Max | Step | Default | Unit |
|---|---|---|---|---|---|
| L-10 rating | 20,000 | 100,000 | 20,000 | 40,000 | hours |
| Hours per day | 1 | 24 | 1 | 24 | h/day |
| Days per week | 1 | 7 | 1 | 7 | days/week |
| Rating shown | L-10 or L-50 | L-10 |
Challenge problems, in this order:
| # | Rating given to learner | Hours per day | Days per week | Model answer (years) | Feedback when wrong |
|---|---|---|---|---|---|
| 1 | L-10 40,000 h | 24 | 7 | 4.6 | 24 × 7 × 52.14 = 8,760 h per year; 40,000 ÷ 8,760 = 4.6 years. |
| 2 | L-10 40,000 h | 8 | 5 | 19.2 | Only 2,086 operating hours a year: 40,000 ÷ 2,086 = 19.2 years. Calendar life depends on the schedule. |
| 3 | L-50 for a bearing rated L-10 20,000 h | 24 | 7 | 11.4 | First convert: L-50 = 5 × 20,000 = 100,000 h. Then 100,000 ÷ 8,760 = 11.4 years. |
| 4 | L-10 60,000 h | 16 | 5 | 14.4 | 16 × 5 × 52.14 = 4,171 h per year; 60,000 ÷ 4,171 = 14.4 years. |
| 5 | L-10 100,000 h | 12 | 6 | 26.6 | 12 × 6 × 52.14 = 3,754 h per year; 100,000 ÷ 3,754 = 26.6 years. |
A comparison table of FanPedia's published values (the four rows of the chapter table above) is shown after challenge 5 so the learner can check the model against the source.
Provenance: L-10 and L-50 definitions, the L-50 = 5 × L-10 rule, the available L-10 ratings (20,000, 40,000, 60,000, 80,000 and 100,000 hours) and the published year tables: FanPedia p. 90 (Bearings: Bearing Life Overview). Challenge values 4 and 5 and all free-exploration values are computed from the Rules; the 12 h/day, 6 days/week schedule is illustrative and is labeled "illustrative schedule".
Rules: Annual hours = hours per day × days per week × 365/7. Years = rated hours ÷ annual hours, rounded to 0.1 year. L-50 hours = 5 × L-10 hours. An answer is correct when |answer − model| ≤ 0.02 × model. Annual hours can never be zero because both minimums are 1. When the learner switches between L-10 and L-50, the hours and years shown both update and the rating label changes; the underlying L-10 rating is unchanged.
Learner Activity:
- The learner reads challenge 1 (rating and schedule) and types an answer in years.
- The learner presses Check. The sim shows the annual operating hours, the model answer and the learner's answer side by side.
- The learner completes challenges 2 to 5 the same way. Challenge 3 asks for an L-50 answer from an L-10 rating, so the learner must apply the five-times rule first.
- After challenge 5, free exploration unlocks. The learner changes any input and the annual hours and years update immediately, with a bar comparing L-10 and L-50 years for the same bearing. The learner should notice that switching from L-10 to L-50 changes the years by exactly five times while the bearing itself is unchanged.
Feedback: Five challenges, fixed order, two attempts each. Correct: "Correct:
Starting State: Challenge 1 is shown with an empty answer box. The question on screen is "A process fan runs 24 hours a day, 7 days a week on bearings rated L-10 40,000 hours. How many years until 10% of such bearings are expected to fail?"
Chapter Anchors: L-10 40,000 h at 24/7 is 4.6 years; L-50 is five times L-10; L-10 40,000 h has L-50 of 200,000 h; 8,760 hours in a continuous year; the chapter table's 20 year values (4 rows by 5 schedules); TCF offers L-10 20,000, 40,000, 60,000, 80,000 and 100,000 hours on some models.
Pillow Block Housings: Solid, Split and Totally Split¶
A pillow block bearing is a bearing insert carried in a housing that bolts to a surface parallel to the shaft, which on most fans is the top of the pedestal. FanPedia describes three housing styles (pp. 92, 178-179), and the difference among them is how much of the bearing must come off the shaft for inspection or replacement.
A solid pillow block bearing has a one-piece cast housing. It is offered with ball or spherical roller elements, its bolt holes are usually slotted for adjustment during mounting, and it is the common choice on standard fans. To replace one, the mechanic generally has to slide it off the end of the shaft, which may mean pulling the sheave or coupling first.
A split pillow block bearing has a housing split horizontally through the shaft centerline, so the top cap lifts off. FanPedia notes that the split exists "for easy bearing replacement and inspection." Split pillow blocks are offered with ball or spherical roller elements and are common on larger, heavy-duty fans.
A totally split roller bearing goes one step further: the housing is split and every internal part (inner ring, roller cage, outer ring) is split into two halves. The internal parts can be removed without removing the shaft at all. FanPedia lists it as a specialty bearing for special applications only, which typically means a large fan where pulling the shaft is a major outage.
| Housing style | What splits | What a bearing change involves | FanPedia note |
|---|---|---|---|
| Solid pillow block | Nothing | Slide the bearing off the shaft end | Standard on most fans |
| Split pillow block | Housing only | Lift the cap; the insert still slides on the shaft | Easy inspection and replacement |
| Totally split roller | Housing and all internal parts | Remove halves with the shaft in place | Special applications only |
Locking the Bearing to the Shaft¶
A bearing insert has to grip the shaft so the inner ring turns with it. Chapter 16 described the set screw mount, in which two set screws bear directly on the shaft. The alternative family is the concentric bearing mount, in which the inner ring is clamped evenly around the whole circumference of the shaft instead of being pushed to one side by screws. FanPedia shows two concentric styles (p. 92), and both tighten to the shaft using a partially segmented inner ring.
- D-Lok / Skwezloc style: a split locking collar is tightened with a cap screw, squeezing the segmented inner ring onto the shaft. FanPedia shows it on solid pillow blocks.
- Adapter mount: the locking collar is tightened with a spanner wrench. FanPedia notes this style is used for both solid and split pillow block bearings.
The adapter sleeve mount is the common industrial form of the adapter mount: a thin, slit, tapered sleeve sits between the shaft and a bearing with a tapered bore. Tightening the locknut with a spanner wrench draws the sleeve into the taper, which closes it onto the shaft and seats the bearing. Because the clamp is uniform around the shaft, a concentric mount runs truer than a set screw mount and is less likely to mark or fret the shaft, which is why it is the usual partner for spherical roller bearings on heavier fans.
Diagram: Pillow Block Bearings and Shaft Mounts¶
Run Pillow Block Bearings and Shaft Mounts Fullscreen
Pillow Block Bearings and Shaft Mounts
sim-id: ch17-pillow-block-bearing-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the solid (ball and spherical roller) and split pillow block housings and the set screw, D-Lok / Skwezloc and adapter shaft mounts on FanPedia's drawings, given a description of each.
Content: Six numbered zones: solid pillow block bearing, ball type; solid pillow block bearing, spherical roller type; split pillow block bearing; set screw mount; D-Lok / Skwezloc style (concentric); adapter mount (concentric).
Provenance: FanPedia page 92, "Bearings: Pillow Block Bearings" (top row) and "How Bearings Connect to Shaft" (bottom row), with the captions removed. Drawing: FanPedia, Twin City Fan, p. 92.
Bearings are not the only parts that must turn with the shaft. Keystock is the method FanPedia (p. 80) gives for connecting the shaft to a rotating element such as an impeller hub, bushing, coupling or sheave. A piece of keystock sits half in the shaft keyway and half in the matching keyway of the hub, so torque passes through the key rather than through friction alone. A square key is typically supplied; a stepped key, which is wider on one side than the other, is used only when a square key is not possible. Keystock also reappears later in this chapter as a material for bearing stop blocks.
Bolting the Bearing Down: Two and Four Hole Mounts¶
The pillow block housing is held to the pedestal by bolts through its base. A two hole mount has one bolt on each side of the shaft; a four hole mount has two on each side. More bolts spread the load and resist the housing rocking under larger shaft loads. FanPedia (p. 91) gives the TCF standard by shaft diameter:
| Fan shaft diameter | TCF standard mount |
|---|---|
| 2-15/16 in. and below | 2 hole mount |
| 3-7/16 in. and above | 4 hole mount |
FanPedia adds that some bearing manufacturers offer only a two hole or only a four hole mount beyond these ranges, so a replacement bearing from a different manufacturer may not drop onto the existing pedestal holes. That is worth checking before a field replacement is ordered.
Drive End and Non-Drive End Bearings¶
On a fan with two shaft bearings, the bearings are named by which end of the shaft they serve. The drive end bearing is the one nearest the sheave or coupling that brings power into the shaft. FanPedia (p. 91) also calls it the outboard bearing, and notes that on a pedestal it is set 3/4 in. in from the end of the pedestal top plate. The non-drive end bearing is the one nearer the impeller, also called the inboard bearing.
Which side of the fan the drive end falls on depends on the arrangement. For SWSI Arrangements 1, 8, 9, 9F and 10, the drive end is on the side away from the housing. For SWSI Arrangements 3, 3SI, 7 and 7SI, and for DWDI Arrangements 3, 3DI, 7 and 7DI, the bearings sit on both sides of the impeller; FanPedia notes that on Arrangement 3 inlet-driven fans the drive end bearing is on the inlet side. The same drawing shows which face of each bearing carries the locking collar, so the assembler knows how to orient it.
Fixed and Floating Bearings¶
A steel shaft grows longer as it warms. On a fan handling hot air, the shaft between the two bearings may grow by a noticeable amount compared with the pedestal that holds the bearings. If both bearings clamp the shaft axially, that growth has nowhere to go, and it turns into an axial load that shortens bearing life.
FanPedia's answer (pp. 97-99) is to let only one bearing locate the shaft. The fixed bearing (also called the non-expansion, thrust or locating bearing, covered in Chapter 16) controls the shaft's axial position. The floating bearing (also called the expansion or non-locating bearing) supports the shaft radially but lets the outer ring slide axially inside its housing, so the shaft can grow or shrink without loading the bearings.
One Bearing Decides, One Bearing Follows
Think of the fixed bearing as the shaft's anchor and the floating bearing as its slide. The shaft always grows away from the anchor, so where you put the anchor decides where all the thermal movement goes.
The Fixed-Floating Bearing Rules¶
FanPedia states the placement and quantity rules as guidelines. These fixed-floating bearing rules have two parts: where the fixed bearing goes, and whether a floating bearing is needed at all.
Placement depends on how the fan is mounted:
- Horizontal mounted fans: the fixed bearing is on the drive side, and the floating bearing is on the non-drive side.
- Vertical mounted fans (for example, Arrangement 9 axial or plug fans mounted vertically, or plenum Arrangements 3VA and 3VS): the floating bearing is on the drive side.
Whether a floating bearing is needed depends on bearing type and operating temperature:
| Operating temperature | Ball type | Roller type | Split roller type |
|---|---|---|---|
| 300°F and below | 2 fixed | 1 fixed, 1 floating | 1 fixed, 1 floating |
| 301°F and above | 1 fixed, 1 floating | 1 fixed, 1 floating | 1 fixed, 1 floating |
There is one footnote to watch. On Arrangements 3SI, 7SI, 3DI and 7DI, where the bearings straddle the impeller and the shaft between them is long, FanPedia notes that ball type bearings at 300°F and below may still require one fixed and one floating bearing. Treat that case as an engineering review item rather than a lookup. FanPedia's high-temperature construction pages (Chapter 21) repeat the rule: hot fans use fixed and floating bearings and high-temperature grease.
Four Questions, in Order
The rules look like a lot at first because they mix orientation, bearing type, temperature and arrangement. Ask them in a fixed order (horizontal or vertical, ball or roller, above or below 300°F, straddle arrangement or not) and every case comes out in under a minute; the sim below lets you practice exactly that.
Diagram: Fixed or Floating Bearing Assignment¶
Run Fixed or Floating Bearing Assignment Fullscreen
Fixed or Floating Bearing Assignment
Type: microsim
sim-id: ch17-fixed-floating-bearing-assignment
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: apply
Learning Objective: The learner will apply the fixed-floating bearing rules to assign the correct bearing configuration to ten fan scenarios defined by mounting orientation, bearing type, operating temperature and arrangement, with at least 8 of 10 correct.
Prerequisites: drive end bearing, non-drive end bearing, fixed bearing, floating bearing, ball, roller and split roller bearing types, horizontal and vertical mounting, and the fixed-floating bearing rules (all defined in the sections "Drive End and Non-Drive End Bearings" and "Fixed and Floating Bearings" above).
Evidence of Mastery: For each of 10 scenario cards, the learner selects one of four configurations and commits it. An answer is correct when it matches the Correct configuration column. Mastery is 8 of 10 correct on the first attempt. Viewing the shaft diagram before committing is exploration, not evidence.
Misconceptions: (1) Every fan needs a floating bearing. (2) The fixed bearing is always on the drive end, regardless of orientation. (3) Temperature does not matter for ball bearings. (4) The 300°F cutoff applies the same way to straddle arrangements with ball bearings.
Instructional Rationale: The rules are a decision procedure, so the learner applies them to new combinations and commits an answer per card. Showing the shaft with the bearings labeled after each answer connects the rule to the physical layout.
Content:
The four configurations offered for every card:
- A. Two fixed bearings
- B. Fixed bearing at the drive end, floating bearing at the non-drive end
- C. Floating bearing at the drive end, fixed bearing at the non-drive end
- D. Flag for engineering review: may need one fixed and one floating
Scenario cards, in this order:
| # | Scenario shown to the learner | Correct configuration | Why (shown as feedback) |
|---|---|---|---|
| 1 | SWSI centrifugal, Arrangement 1, horizontal mount, ball bearings, 150°F | A | Ball bearings at 300°F and below use two fixed bearings. |
| 2 | SWSI centrifugal, Arrangement 1, horizontal mount, spherical roller solid pillow blocks, 200°F | B | Roller bearings always use one fixed and one floating; on a horizontal fan the fixed bearing is on the drive side. |
| 3 | SWSI centrifugal, Arrangement 8, horizontal mount, split roller bearings, 650°F | B | Above 301°F every type uses one fixed and one floating; horizontal puts the fixed bearing at the drive end. |
| 4 | Axial fan, Arrangement 9, vertical floor mount, roller bearings, 120°F | C | On vertical mounted fans the floating bearing is on the drive side. |
| 5 | Plug fan, Arrangement 9, vertical up mount, ball bearings, 100°F | A | Ball bearings at 300°F and below use two fixed bearings, whatever the orientation. |
| 6 | SWSI centrifugal, Arrangement 9, horizontal mount, ball bearings, exactly 300°F | A | The two-fixed rule covers 300°F and below, and 300°F is included. |
| 7 | DWDI centrifugal, Arrangement 3, horizontal mount, roller bearings, 180°F | B | Roller bearings use one fixed and one floating; horizontal puts the fixed bearing at the drive end. |
| 8 | SWSI centrifugal, Arrangement 3SI, horizontal mount, ball bearings, 250°F | D | FanPedia's footnote: on 3SI, 7SI, 3DI and 7DI, ball bearings at 300°F and below may require one fixed and one floating. |
| 9 | Plenum fan, Arrangement 3VA, vertical down mount, split roller bearings, 200°F | C | Split roller bearings use one fixed and one floating, and a vertical fan puts the floating bearing on the drive side. |
| 10 | SWSI centrifugal, Arrangement 1, horizontal mount, ball bearings, 301°F | B | At 301°F and above every bearing type uses one fixed and one floating; horizontal puts the fixed bearing at the drive end. |
Provenance: All rules and arrangement lists: FanPedia pp. 97-99 (Bearings: Fixed & Floating Bearings), including the footnote on Arrangements 3SI, 7SI, 3DI and 7DI. Drive end and non-drive end definitions: FanPedia p. 91. The scenario temperatures are illustrative and are labeled "illustrative scenario".
Rules: Step 1: if the operating temperature is >= 301°F, the configuration has one fixed and one floating bearing; go to step 3. Step 2: if the temperature is <= 300°F and the bearings are ball type, the answer is A, except for Arrangements 3SI, 7SI, 3DI and 7DI, where it is D. Roller and split roller types at <= 300°F have one fixed and one floating. Step 3: for a configuration with a floating bearing, a horizontal mount gives B and a vertical mount gives C.
Learner Activity:
- The learner reads scenario card 1 and selects one of the four configurations.
- The learner presses Commit. The sim draws the shaft with its two bearings labeled drive end and non-drive end, marks each as fixed or floating according to the correct answer, and shows the feedback.
- The learner moves to the next card. Committed cards cannot be changed.
- After card 10, the sim shows the score and lists any missed cards with the rule step that decided them. The learner should notice that orientation only matters once a floating bearing is needed.
Feedback: Ten cards, fixed order, one attempt each. Correct: "Correct" followed by the Why text. Incorrect: "Not quite" followed by the Why text and the correct configuration letter. The correct answer is revealed immediately after each commit. A running score "n of 10" is shown, and the final screen states whether the 8 of 10 mastery level was reached.
Starting State: Card 1 is shown with no configuration selected and an unlabeled shaft with two bearings. The question on screen is "Which bearings on this fan should be fixed, and which should float?"
Chapter Anchors: horizontal fans: fixed bearing on the drive side; vertical fans: floating bearing on the drive side; 300°F and below: ball type 2 fixed, roller and split roller 1 fixed and 1 floating; 301°F and above: all types 1 fixed and 1 floating; ball bearings on 3SI, 7SI, 3DI and 7DI at 300°F and below may require one fixed and one floating.
Lubrication: Grease Fittings and Oil Systems¶
Nearly all fan bearings are grease lubricated. A grease fitting, often called a zerk, is a small threaded nipple with a spring-loaded check ball, screwed into the bearing housing so that a grease gun can push fresh grease into the bearing while the ball keeps dirt out. FanPedia (p. 101) shows the standard zerk on a pillow block, and the same port is used for a bearing temperature sensor added through a T-fitting (Chapter 16). When a bearing is hard to reach, lube lines carry the fitting to a convenient location; FanPedia's high-temperature pages call for metal lube lines and high-temperature grease. A grease pan on the pedestal collects excess grease purged from the bearings. Do not confuse two different temperatures here. The 300°F and 301°F limits in this chapter are airstream temperatures, but Twin City Fan engineering letter FE-1300 recommends special high-temperature grease when the bearing temperature exceeds 160°F, which a bearing can reach well below a 300°F airstream.
For special applications, FanPedia (p. 95) shows two oil systems instead of grease. Static oil lubrication keeps a constant oil level in a sump in the bearing housing. Each bearing has its own constant-level oiler (FanPedia names the Trico oiler) whose reservoir automatically tops up the sump as oil is used. The supply line enters at the bottom of the bearing, and the bearing needs either a pressure relief line routed back to the oiler or a breather tube or vent on top. The oiler reservoir is placed on the side set by impeller rotation: left for clockwise, right for counterclockwise.
Oil mist lubrication uses one pump unit for both bearings. A metered quantity of oil is broken into fine droplets by a continuous flow of compressed air and carried to the bearings, entering through a line on top of each bearing; liquid oil drains from a line on the bottom and returns to the pump unit. Because the mist keeps the bearing housing at a slight overpressure, dirt is kept away from the bearing, which is a useful side benefit in dusty plants.
| Method | Supply | Hardware per fan | Typical use |
|---|---|---|---|
| Grease | Grease gun through a fitting, on a schedule | One fitting per bearing, optional lube lines and grease pan | Standard on nearly all fans |
| Static oil | Constant-level oiler tops up a sump | One oiler per bearing, relief line or vent | Special applications |
| Oil mist | Pump unit sends oil mist, oil returns | One pump unit for both bearings, supply and return lines | Special applications; slight overpressure keeps dirt out |
Bearing Seals and Seal Speed Limits¶
A bearing seal is part of the bearing itself: it closes the gap between the housing and the shaft so grease stays in and dirt stays out. It is different from the shaft seal on the fan housing, which keeps the airstream in. FanPedia (p. 100) describes three common bearing seals and makes one rule that drives selection: the seal type dictates the speed limit of the bearing.
A labyrinth seal, also called a non-contact seal, uses a winding path of close-clearance grooves between rotating and stationary rings. Nothing rubs, so it adds no friction or heat, and FanPedia lists it for higher-speed applications on spherical roller bearings, both solid and split pillow block. A lip seal, by contrast, is a contact seal that rubs on the shaft and is used for low to moderate shaft surface speeds, typically on ball bearings.
A taconite seal is designed for dirty or abrasive environments, taking its name from the iron-ore pellet plants where it is common. It is used on split pillow block spherical roller bearings and combines a labyrinth with a grease-purged cavity that pushes contaminants back out. The standard type taconite seal has a lower speed limit than a standard labyrinth seal; a Canadian type taconite seal, which is non-contact, is available for higher speeds.
The bearing seal speed limit is the maximum shaft speed (rpm) that the bearing manufacturer allows for a given bearing and seal combination. It is separate from the bearing's own speed rating, and it can be the lower of the two. A fan that must run fast and dirty is therefore a real selection problem: the seal that protects best from dust may not be allowed at the fan's speed. FanPedia refers to Fan Engineering Letter FE-1200 for detail.
A Taconite Seal Makes the Bearing Wider
Adding taconite seals to a quote looks like a simple seal swap, but FanPedia notes they widen the bearing, which can require a longer shaft, moving the bearing on the pedestal, or a larger bearing support such as a bearing bar. Flag the change to engineering before the order is released, not after the shaft is cut.
Gas Purge Seals¶
The bearing seals above protect the bearing. The shaft seals of Chapter 16 protect the surroundings by limiting how much gas leaks through the hole where the shaft passes through the housing. When even a small leak is unacceptable, because the gas is toxic, flammable, odorous or would condense on the bearings, a gas purge seal is used. It is a shaft seal with a connection between its sealing rings through which a clean purge gas is supplied. FanPedia shows two versions (pp. 153 and 155):
- Double ring mechanical shaft seal (double carbon ring): two split carbon rings with a lantern ring between them, located at the purge hole.
- Stuffing box type shaft seal with gas purge (also called a graphoil seal or packing gland seal with purge): carbon yarn packing rings and a graphoil split ring, with a Teflon lantern ring at the purge hole.
The lantern ring is a spacer with an open channel around it. Purge gas fed into the purge hole fills that channel and surrounds the shaft. If the purge gas is supplied at a pressure above the gas pressure inside the housing at the shaft, any leakage past the inner ring is clean purge gas flowing into the fan, and any leakage past the outer ring is clean purge gas flowing out. Process gas never reaches the outside. The purge gas is usually clean, dry air or, when the process gas is flammable, an inert gas such as nitrogen; the choice belongs to the customer's process engineer.
The Seal Leaks on Purpose
No shaft seal is perfectly tight, so a purge seal does not try to stop leakage. It decides what leaks, by making sure the gas at the lantern ring is always the cleanest and highest-pressure gas in the neighborhood.
FanPedia notes that the purge hole ships with a 1/8 in. NPT pipe plug, which is replaced in the field with a customer-supplied purge line. That one sentence has a real consequence for the order: the purge gas supply, its piping, its pressure regulation and any flow monitoring are by others.
Worked example (illustrative). A customer needs an Arrangement 1 fan to move solvent-laden air from a coating line to a thermal oxidizer. The fan is downstream of the process blower, so the housing is under positive pressure at the shaft. The specification says "nominally leak-tight construction; no process gas release at the shaft." Working through it:
- Find the direction of leakage. Positive pressure at the shaft means process gas will try to leak out. A plain lip seal limits that leakage but does not stop it, so a purged seal is justified.
- Check the allowed seal list. FanPedia's nominally leak-tight construction page (Chapter 21) permits lip type, stuffing box (graphoil) and double ring mechanical shaft seals on the fan shaft, with no shaft sleeve or cap. Of these, the stuffing box type and the double ring mechanical type are offered with a purge connection.
- Choose the purge gas. The process gas is flammable, so the customer's process engineer specifies nitrogen rather than plant air.
- Set the purge pressure. The purge supply must be above the housing pressure at the shaft at every operating point, including start-up and damper changes. The required margin and flow come from the seal vendor's data and the customer's process engineer; the quote should not invent them.
- Write the scope split. The quote states that TCF supplies the seal with its purge hole plugged with a 1/8 in. NPT plug, and that the purge line, nitrogen supply and regulation are supplied and installed by others.
Note that the leak-tight requirement also affects the bearings: a sealed, hot housing usually means fixed and floating bearings, so the example connects to the rules earlier in this chapter.
Holding the Bearing in Place: Stop Blocks and Dowel Pins¶
Once a bearing is aligned on its pedestal, something should prevent it from creeping out of position during shipment, start-up or years of vibration. FanPedia calls bearings held this way restrained bearings and offers two methods (pp. 102, 176-177).
Bearing stop blocks are welded brackets, or short pieces of keystock, welded to the pedestal against each side of the bearing base. They confirm and hold the bearing's location along the shaft. Stop blocks are also listed among the pedestal accessories for use next to motors.
Bearing dowel pins are rods fixed to the pedestal that pass through a hole drilled through the bearing base. A doweled bearing can be removed and reinstalled in exactly the same position, which preserves alignment after service.
Restrained bearings matter most in spark resistant construction. FanPedia's Type A spark resistant page lists restrained bearings (bearing dowel pins or bearing stop blocks) as a fan modification, with dowel pins as the first choice, because a bearing that walks along its pedestal can let the impeller rub (see Chapter 20).
Fan Pedestals¶
A fan pedestal is the steel structure that supports the bearings, shaft and often the motor, and through them carries the impeller. FanPedia (pp. 83-87) adds a second job: the pedestal provides the structural stability the assembly needs to be balanced and to meet vibration requirements. A typical pedestal has a top, sides, feet, a front and sometimes a back, built from angled, straight or formed sheet metal. Not every fan has one: an Arrangement 3 fan carries its bearings on the housing, and air kits ship without a pedestal, but every fan needs some support, supplied by TCF or by others in the field.
Several pedestal features follow from choices made earlier in this chapter:
- Bearing or motor pads, or a machined top, give a flat, stiff surface for bearing alignment.
- Hand holes give access to bearing and motor fasteners.
- Reinforcements (angles, flat bar, channels or gussets) stiffen the pedestal; rebar can be welded inside for a concrete fill.
- Heat shields protect the bearings on high-temperature fans, and buffer strips are used when the customer specifies that a stainless steel housing may not be welded to a mild steel pedestal.
- Positioners, stop blocks and a grease pan are pedestal accessories.
Diagram: Fan Pedestal Features¶
Run Fan Pedestal Features Fullscreen
Fan Pedestal Features
sim-id: ch17-fan-pedestal-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the top, pillow block bearing, hand holes, sides, front, feet and bearing or motor pads of a fan pedestal on FanPedia's drawing, given each feature's job or look.
Content: Seven numbered markers: top, pillow block bearing, hand holes, sides, front, feet, bearing or motor pads.
Provenance: FanPedia page 85, "Pedestals: Features (pedestal on a SWSI fan) with the 'Bearing or Motor Pads' inset", with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 85.
Two pedestal variants deserve their own names. A separated pedestal is used on high-temperature applications. It leaves a gap between the fan housing and the pedestal, which reduces the housing heat conducted into the pedestal and bearings and leaves room for insulation. A bolt-on pedestal bolts to the housing instead of being welded to it. FanPedia lists two kinds: a non-rotatable bolt-on pedestal, used when the pedestal must come off the housing to fit the fan through a smaller opening, to send the pedestal out for special machining, or to join dissimilar metals such as a stainless steel or aluminum housing on a mild steel pedestal; and a rotatable bolt-on pedestal, used on specific models whose housing can be rotated to several discharge positions.
Worked example (illustrative). A quote calls for an Arrangement 1 SWSI fan with a stainless steel housing, handling corrosive exhaust at 550°F. The customer specification says stainless steel may not be welded to carbon steel, and the selection has a 3-7/16 in. shaft. Reading the job against this chapter:
- Pedestal material and attachment. The pedestal is mild steel, so it cannot be welded to the stainless housing. Two FanPedia features answer that requirement: buffer strips, or a non-rotatable bolt-on pedestal. The bolt-on pedestal also makes the fan easier to rig into the building if access is tight.
- Heat path. At 550°F, a separated pedestal and a heat shield reduce the heat reaching the bearings; the high-temperature construction packages in Chapter 21 set the rest, such as a shaft cooler.
- Bearing mount. A 3-7/16 in. shaft is at the four hole mount threshold, so the pillow blocks bolt down with four holes each.
- Bearing configuration. Above 301°F, every bearing type uses one fixed and one floating bearing; the fan is horizontal, so the fixed bearing is the drive end bearing, set 3/4 in. in from the end of the pedestal top plate. High-temperature grease is specified.
- Retention and lubrication. Stop blocks or dowel pins are added if the customer wants restrained bearings, and a grease pan keeps purged grease off the floor.
Each line on the resulting order traces to a source reason, which is exactly what a reviewer at the factory or the customer's engineer will ask for.
Shaft and Coupling Guards¶
The last parts on the drive side protect people rather than bearings. FanPedia's common accessories page (p. 126) shows a shaft/bearing guard, which covers the exposed shaft and bearings, and a coupling guard, which encloses the flexible coupling on direct-drive arrangements where the motor shaft is coupled to the fan shaft. Together these shaft and coupling guards complete the guarding that the belt guard provides on belt-driven fans (see Chapter 7), and a cooler guard covers the shaft cooler on high-temperature fans. Guards are usually bolted to the pedestal or base so they can be removed for bearing service, then must be reinstalled before the fan is restarted. Whether guards are included, and to what standard, should be stated on every quote rather than assumed, because local safety rules require exposed rotating parts to be guarded.
Key Takeaways¶
- L-10 bearing life is the hours at which 10% of a bearing population is expected to fail; L-50 (average life) is five times L-10. Convert either to years by dividing by the fan's annual operating hours; 40,000 hours of continuous duty is about 4.6 years.
- Solid pillow blocks are standard; split pillow blocks split the housing for easy inspection; totally split roller bearings split every part so the shaft can stay in place.
- Concentric mounts, including the adapter sleeve mount, clamp the whole shaft circumference; keystock carries torque between the shaft and hubs, sheaves and couplings.
- The TCF standard is a two hole mount for shafts 2-15/16 in. and below and a four hole mount for 3-7/16 in. and above.
- The drive end bearing (outboard) is nearest the drive; the non-drive end bearing (inboard) is nearest the impeller.
- The fixed-floating bearing rules: horizontal fans put the fixed bearing on the drive side, vertical fans put the floating bearing there; ball bearings at 300°F and below use two fixed bearings, and every other case uses one fixed and one floating bearing.
- Bearings are greased through a grease fitting; static oil and oil mist lubrication are for special applications.
- Labyrinth seals suit higher speeds, taconite seals suit dirty service but widen the bearing, and the bearing seal speed limit can cap fan speed.
- A gas purge seal feeds clean gas to a lantern ring at higher pressure than the process gas, so only clean gas leaks; the purge line is customer-supplied.
- Bearing stop blocks and bearing dowel pins restrain bearings; dowel pins are the first choice for Type A spark resistance.
- The fan pedestal supports and stiffens the rotating assembly; separated pedestals keep heat away from bearings and bolt-on pedestals solve dissimilar-metal, access and rotatable-discharge problems. Shaft and coupling guards finish the drive side.
You Can Specify the Whole Shaft Line
You can now take a bearing life, housing, mount, fixed-floating layout, lubricant, seal and pedestal from a specification and explain each one in shop terms. That shaft line is where most field problems start, and now it is where your quotes are strongest.