Bearings, Shafts and Shaft Seals¶
Summary¶
This chapter introduces the bearings and shafts that carry the impeller. It covers bearing life, ball and spherical roller bearings, pillow block and flange mount bearings, two-bearing housings, set screw mounting, fixed bearings, lubrication, bearing seals, positioners, restrained and insulated bearings, and vibration and temperature sensors. It also covers shaft features (keyways, turndowns, sleeves, cladding, locking collars) and the shaft seals that keep air in the housing: lip, friction, carbon ring, and stuffing box seals. After completing it, readers can describe how a fan's rotating assembly is supported and sealed.
Concepts Covered¶
This chapter covers the following 25 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Shaft Turndown | 2 |
| Shaft Sleeve | 1 |
| Clad Shaft | 1 |
| Shaft Seal | 162 |
| Bearing Life | 3 |
| Ball Bearing | 2 |
| Spherical Roller Bearing | 3 |
| Pillow Block Bearing | 5 |
| Flange Mount Bearing | 1 |
| Two-Bearing Housing | 1 |
| Set Screw Mount | 3 |
| Fixed Bearing | 6 |
| Bearing Lubrication | 7 |
| Bearing Seal | 6 |
| Bearing Positioner | 4 |
| Bearing Vibration Sensor | 16 |
| Restrained Bearing | 1 |
| Shaft Keyway | 2 |
| Shaft Locking Collar | 1 |
| Lip Shaft Seal | 1 |
| Friction Shaft Seal | 1 |
| Carbon Ring Seal | 49 |
| Stuffing Box Seal | 48 |
| Bearing RTD and Thermocouple | 1 |
| Insulated Bearing | 1 |
Prerequisites¶
This chapter builds on concepts from:
- Chapter 2: Airstreams, Total Pressure and Standard Air
- Chapter 3: Basic Fan Anatomy
- Chapter 8: Centrifugal and Axial Impeller Types
- Chapter 14: Fan Motors and Enclosures
- Chapter 15: Motor Ratings, Speed Control and Positioners
The Rotating Assembly¶
Every fan you quote, ship or service has a rotating assembly at its heart: the impeller and the shaft it is fastened to. FanPedia calls the shaft "the core piece of the rotor assembly." The shaft is carried by two bearings, and wherever it passes through the housing wall it opens a path for air to leak in or out. That one sentence sets the agenda for this chapter. First we look at the shaft and the features machined into it, then at the bearings that hold it, and finally at the shaft seals that close the hole where it enters the housing.
The Parts That Spin
The shaft, its bearings and its seal decide whether a fan runs smoothly for decades or comes back on a service call. When a customer asks "what bearings are on that?" or "will it leak?", this chapter is where your answer comes from. Let's follow the airflow, right up to the point where it tries to sneak out along the shaft.
Recall from Chapter 3 that the two bearings support the shaft in one of two basic layouts. In an overhung layout the impeller sits on the end of the shaft, outboard of both bearings (Arrangement 8, and also Arrangements 1, 9 and 10). In a center hung layout the impeller sits between the two bearings (Arrangement 3, and also Arrangement 7). In Arrangement 4 the impeller is mounted on the motor shaft, so the two bearings are the motor's own and there is no separate fan shaft. Shaft diameter is chosen to meet the structural, vibration and balance needs of the assembly, and the shaft material is chosen for the environment the fan works in. Mild steel (Grade 1045 or 4140, turned, ground and polished) is typical; stainless steel, Monel, nickel-aluminum bronze and titanium are used when the job calls for them, as Chapter 20 explains.
Shaft Features¶
A plain cylinder of steel is not yet a fan shaft. The shop machines features into it so that it can drive the impeller and the sheave, sit correctly in the bearings, and survive the airstream.
A shaft keyway is a groove cut lengthwise into the shaft to accept a piece of keystock. The keystock (the key) fits half in the shaft keyway and half in a matching groove in the hub, bushing, coupling or sheave, so torque passes from shaft to part through the key rather than through friction alone. A square key is the normal supply; a stepped key, wider on one side than the other, is used only when a square key is not possible. If you see a keyway called out at both ends of a shaft, one usually carries the impeller hub and the other the drive sheave or coupling. FanPedia also lists a tachometer hole, a small dimple in the shaft end that lets a technician read speed with a hand-held mechanical tachometer.
A shaft turndown is a section of shaft machined ("turned") to a smaller diameter than the rest, on one or both ends. Turndowns let one shaft present different diameters to different parts: a larger major diameter for stiffness under the impeller, and smaller diameters where a standard bearing bore, seal or coupling must fit. When the design needs them, FanPedia lists the typical turndown locations as the hub, the shaft seal(s), the bearings and the coupling. Center hung fans such as Arrangement 3SI/DI and 7SI/DI may carry several turndowns along one shaft. On very large Arrangement 3SI and 3DI shafts with multiple turndowns, a forged shaft, usually supplied at customer request, reduces the machining time and cost.
The list below collects the shaft features and their reasons in one place before we add the accessories that slide onto the shaft.
- Keyway: transmits torque to the hub, sheave or coupling through a key.
- Turndown: steps the diameter down to fit a bearing, seal, hub or coupling.
- Tachometer hole: gives a mechanical tachometer a center point for reading speed.
A shaft locking collar (FanPedia lists it as "Shaft (Locking) Collar") is a ring clamped to the shaft that holds the shaft in place axially, so it cannot slide and strike another object such as a bearing or the impeller. You will meet it on Type A and Type B spark-resistant construction, where rubbing metal is a hazard, and on vertical fans, where gravity is always pulling the shaft along its own axis. Do not confuse it with the locking collar that is part of some bearings; that collar fastens the bearing's inner ring to the shaft and is covered with bearing mounts later in this chapter.
A shaft sleeve is a tube fitted over the shaft where the shaft would otherwise be exposed to the airstream. Its job is to shield the shaft, usually because the shaft and the airstream components are made of dissimilar materials, for example a steel shaft on a fan whose wetted parts are stainless or aluminum. Shaft sleeves are typically used on Arrangements 1, 8, 9 and 10. Arrangement 4 fans can use a protruding hub instead of a sleeve, and a hub cap does the same shielding job at the shaft end. One exception matters on orders: FanPedia's nominally leak-tight construction says not to use a shaft sleeve and cap, because the joint between sleeve and shaft is one more leak path.
A clad shaft solves the same dissimilar-material problem on big fans in a cheaper way. When the shaft is large (9 inches in diameter or more) and the airstream calls for a special material such as stainless steel, a solid stainless shaft is expensive. Instead, a mild steel shaft and hub (common grades 1018 and 8620) are covered with cladding of the airstream material. The cladding is either thin-gauge sheet fabricated in two halves that wrap around the shaft, or thin-wall pipe machined to a good surface where it runs through the shaft seal.
Diagram: Rotating Assembly Callout Map¶
Run Rotating Assembly Callout Map Fullscreen
Rotating Assembly Callout Map
Type: infographic
sim-id: ch16-rotating-assembly-callout-map
Library: html
Status: Specified
Bloom Level: Remember
Bloom Verb: identify
Learning Objective: The learner will identify ten parts of an overhung fan's rotating assembly, given a one-sentence description of each part's job.
Prerequisites: rotating assembly, overhung impeller, shaft keyway, shaft turndown, shaft sleeve, shaft locking collar, pillow block bearing, drive end, shaft seal, bearing positioner, vibration sensor tap (all defined in the chapter prose; bearing and seal terms are defined in the sections that follow, and the learner reaches this map from those sections as well).
Evidence of Mastery: In quiz mode, the learner is shown one description at a time and clicks the callout it describes. A click is correct when it lands on the callout named in the Content table. Mastery is 8 of 10 correct on the first attempt. Hovering and clicking callouts in explore mode is exploration, not evidence.
Misconceptions: (1) The shaft sleeve is a bearing part (it shields the shaft from the airstream). (2) The shaft locking collar and a bearing's locking collar are the same part. (3) The shaft seal keeps grease in the bearing (that is the bearing seal's job; the shaft seal keeps air in the housing).
Instructional Rationale: Remember-level identification needs repeated retrieval with immediate correction, so the learner first explores labeled callouts on a schematic cross-section of an overhung fan (housing wall, impeller, shaft, two pillow block bearings on a pedestal) and then must retrieve each part from its job description alone.
Content: The picture is a schematic side cross-section of an overhung (Arrangement 1 style) centrifugal fan: impeller inside the housing at the left, shaft passing through the housing drive-side wall, two pillow block bearings on a pedestal, and a drive sheave on the right-hand shaft end. Ten callouts, each with a label and an infobox:
| # | Callout label | Infobox text (also the quiz description) |
|---|---|---|
| 1 | Shaft keyway | A groove cut into the shaft that accepts keystock so torque passes to the hub or sheave. |
| 2 | Shaft turndown | A length of shaft machined to a smaller diameter to fit a bearing, seal, hub or coupling. |
| 3 | Shaft sleeve | A tube over the shaft inside the housing that shields the shaft from the airstream when materials differ. |
| 4 | Shaft locking collar | A ring clamped to the shaft that keeps the shaft from sliding axially into another part. |
| 5 | Shaft seal | A seal mounted on the housing wall where the shaft passes through, limiting air leakage in or out. |
| 6 | Non-drive end bearing (inboard) | The bearing nearer the impeller, farther from the sheave. |
| 7 | Drive end bearing (outboard) | The bearing nearer the sheave or coupling. |
| 8 | Bearing seal | A seal built into the bearing that keeps dirt out of the bearing. |
| 9 | Bearing positioner | A threaded bolt on a bracket beside the bearing, used for fine adjustment of bearing location. |
| 10 | Vibration sensor tap | A drilled and tapped hole in the bearing housing where a vibration sensor is mounted. |
Provenance: FanPedia (Twin City Fan, 2021): shaft overview p. 79; keyways and turndowns p. 80; turndown locations p. 81; shaft collar and shaft sleeve p. 129; drive end and inboard bearing names p. 91; bearing seals p. 100; vibration sensor tap p. 101; bearing positioner p. 102 and Technical Descriptions p. 176. The cross-section is schematic and must be labeled "schematic, not to scale".
Rules: Each callout has one correct description. Quiz items are drawn without repetition. A click on empty space is ignored and does not use an attempt.
Learner Activity:
- In explore mode the learner hovers or clicks each callout; its infobox opens with the label and text from Content.
- The learner switches to quiz mode. All labels are hidden; callout markers remain.
- The sim shows one description; the learner clicks the matching callout. The sim marks it right or wrong and shows the feedback.
- After ten items the sim shows the score and returns all labels.
Feedback: Ten items, shuffled order, two attempts per item. Correct: "Correct:
Starting State: Explore mode with all ten labels visible and the prompt "Click any part to see what it does, then start the quiz."
Chapter Anchors: The chapter names ten callouts in this map. The chapter states that sleeves are typical on Arrangements 1, 8, 9 and 10, and that FanPedia's leak-tight construction does not use a shaft sleeve and cap.
Bearings¶
A bearing lets the shaft turn while holding it in position. Fan bearings are rolling element bearings: an inner ring fixed to the shaft, an outer ring held in a housing, and a row of balls or rollers between them that roll instead of slide. The bearing housing is then bolted to the pedestal or frame of the fan. Two questions follow for every fan: how long will the bearings last, and which kind of bearing and housing does this fan need?
Bearing Life¶
Bearing life is a statistical estimate of how long bearings will run before they fail from metal fatigue. FanPedia's starting point is blunt: under laboratory conditions with controlled loads and proper lubrication, bearings fail by fatigue, and all bearings have a finite life. The estimate depends on bearing geometry, load and speed. The common rating is L-10 life: the hours by which 10% of a large population of identical bearings, at a given speed and load, are expected to have failed. Most Twin City Fan models offer L-10 life of 40,000 hours, and some models are offered at 20,000 to 100,000 hours; the product catalog for the model is the authority. Turning hours into calendar years depends on the operating schedule.
Bearing Life in Calendar Years¶
where:
- \( Y \) is the expected life in years
- \( L_{10} \) is the rated L-10 life in hours
- \( h \) is the operating hours per day
- \( d \) is the operating days per week
A 40,000-hour bearing on a fan that runs 16 hours a day, 5 days a week, gives \( Y = 40{,}000 \div (16 \times 5 \times 52) = 40{,}000 \div 4{,}160 \approx 9.6 \) years, the same figure FanPedia tabulates; the same bearing running around the clock lasts about 4.6 years. Chapter 17 adds L-50 life and the full tables.
Ball and Spherical Roller Bearings¶
The rolling element is the first choice to make. A ball bearing uses steel balls as its rolling elements. FanPedia describes ball-type pillow blocks as providing smooth, low-friction motion in rotary applications, and ball bearings are the usual choice on smaller, lighter-duty fans.
A spherical roller bearing uses barrel-shaped rollers whose surfaces are crowned (spherical), running in a matching spherical outer raceway. FanPedia states the trade-off directly: spherical roller bearings are superior for high loads and for loads that require tolerance to shock, but they have limited speed capability. The spherical raceway also lets the bearing accept small angular misalignment between shaft and housing, which is why it is a common choice for heavy industrial fans with large shafts.
Bearing Housings: Pillow Block, Flange and Two-Bearing¶
The rolling element sits inside a housing, and the housing shape depends on which way the mounting surface faces.
A pillow block bearing has a housing that bolts to a mounting surface parallel to the shaft axis, such as the flat top of a fan pedestal. Its bolt holes are usually slotted so the bearing can be adjusted along the pedestal during mounting. Pillow blocks come with ball or spherical roller elements, and in two housing styles: solid, where the housing is one piece, and split, where the housing separates into a cap and base for easier inspection and bearing replacement. On Twin City Fan pedestals, shafts of 2-15/16 inch diameter and below normally use a two-bolt pillow block and shafts of 3-7/16 inch and above a four-bolt one.
A flange mount bearing sits in a flanged housing that bolts to a surface perpendicular to the shaft axis, such as a plate or wall the shaft passes through. On the fan shaft, flange bearings come with ball or spherical roller elements and are used mostly on some axial fans and special applications. Small flange bearings also turn up on dampers and on the control linkage of inlet vanes.
A two-bearing housing, also called a monoblock bearing, puts both pillow block bearings inside one common housing. Because both bearing seats are machined into one casting, the housing preserves precise alignment between the two bearings. It requires a special shaft for each application and is a specialty bearing, used on special applications only.
The table below compares the three housings described above.
| Housing | Mounting surface relative to shaft | Typical use in Twin City Fan products |
|---|---|---|
| Pillow block (solid or split) | Parallel to the shaft axis | Standard fan bearings on pedestals and frames |
| Flange mount | Perpendicular to the shaft axis | Some axial fans, special applications, dampers and inlet vane linkage |
| Two-bearing housing (monoblock) | Parallel, one housing for both bearings | Special applications needing precise bearing alignment |
Mounting the Bearing to the Shaft¶
The bearing's inner ring must be locked to the shaft or it will creep and wear the shaft. The simplest method is the set screw mount: the bearing's inner ring carries two set screws that are tightened down onto the shaft. Two set screws are always required, and the angle between them varies by bearing manufacturer, from 65° to 120° around the ring. The set screws clamp at two points, so the ring sits slightly off center on the shaft; FanPedia contrasts this with "concentric" mounts, in which a partially segmented inner ring is drawn evenly around the shaft by a locking collar or an adapter sleeve. Those concentric mounts are detailed in Chapter 17. In the field, a set screw mount is easy to spot: two socket screws in the inner ring next to the bearing face.
Read the Bearing Side That Locks
Before you take a bearing question over the phone, ask the caller to look for the screws or collar on the bearing's inner ring and say which side of the bearing they are on. That one observation tells you the mounting method and, with FanPedia's locking-collar orientation drawings, which end of the shaft you are talking about.
Fixed Bearings¶
A shaft needs to be located in two directions: radially (up, down and sideways) and axially (along its own length). Both bearings carry radial load, but only one should control axial position. A fixed bearing, also called a non-expansion, thrust or locating bearing, is a bearing that holds the shaft axially in place relative to the stationary housing. Its partner, the floating (expansion) bearing, lets the shaft slide slightly within it, so the shaft can grow and shrink with temperature without forcing the two bearings against each other. A shaft that warms from 70°F to several hundred degrees gets measurably longer; if both bearings are fixed, that growth has nowhere to go but into the bearings as thrust load.
FanPedia's guidelines decide when both bearings may be fixed:
- At 300°F and below, ball bearings use two fixed bearings; roller and split roller bearings use one fixed and one floating.
- At 301°F and above, all types use one fixed and one floating bearing.
- On horizontally mounted fans the fixed bearing is located on the drive side; on vertically mounted fans the floating bearing is on the drive side.
Worked example. An inside salesperson is checking two orders.
- Order A is an Arrangement 1 single-width fan handling 450°F air on split spherical roller pillow blocks. The temperature is above 300°F, so one bearing is fixed and one floating. The fan is horizontal, so the fixed bearing goes on the drive end (outboard) and the floating bearing on the inboard end near the impeller.
- Order B is the same fan size handling 120°F air on ball bearings. At 300°F and below, ball bearings may both be fixed, so both bearings are ordered as fixed. FanPedia's exception, that ball bearings at 300°F and below may still need one fixed and one floating, applies to center hung Arrangements 3SI, 7SI, 3DI and 7DI, not to Arrangement 1.
The full placement rules by arrangement, and the floating bearing itself, come in Chapter 17.
Restrained Bearings and Bearing Positioners¶
A restrained bearing is a pillow block bearing whose housing is mechanically prevented from shifting on its pedestal, beyond what the hold-down bolts alone provide. FanPedia names bearing dowel pins (the first choice) and bearing stop blocks as the restraining methods, with slotted steel spring pins as another option. Restrained bearings are part of Type A and Type B spark-resistant construction, where a bearing that walked along the pedestal could let the impeller rub. Both methods are detailed in Chapter 17.
A bearing positioner is a threaded bolt mounted in a bracket on each side of a fan bearing, also known as a bearing alignment jacking screw. Turning the bolts pushes the bearing housing along its slotted holes in small, controlled steps, so an installer can fine-tune the bearing's location and shaft alignment before torquing the hold-down bolts. It works on the same principle as the motor positioners of Chapter 15, applied to the bearing instead of the motor. Positioners adjust; dowel pins and stop blocks then hold.
Adjust First, Then Restrain
A common field mistake is tightening the jacking screws hard and treating them as the restraint, then later finding the bearing has moved anyway. Use positioners to set the bearing where it belongs, tighten the hold-down bolts, and rely on dowel pins or stop blocks when the construction calls for a restrained bearing.
Bearing Lubrication¶
Bearing lubrication is the film of grease or oil that separates the rolling elements from the raceways. Without it, metal touches metal, heat builds, and the bearing fails long before its rated life; recall that FanPedia's bearing life figures assume proper lubrication. Most fan bearings are grease lubricated: grease is added through a grease fitting (a "zerk") on the bearing housing, on a schedule set by speed, temperature and bearing size in the bearing manufacturer's instructions. A grease pan, offered as a fan base accessory, collects the excess grease the bearings emit.
Temperature changes the grease. FanPedia's high temperature construction requires high temperature grease (stated as over 275°F on one product page), together with fixed and floating bearings. For special applications only, Twin City Fan offers two oil systems:
- Static oil (Trico oiler): a separate oiler for each bearing keeps a set oil level in the bearing sump and tops it up automatically; the oiler reservoir sits on the left for clockwise impeller rotation and on the right for counterclockwise.
- Oil mist: one pump unit serves both bearings, feeding an oil mist into the top of each bearing and returning liquid oil from the bottom. The slight overpressure in the bearing housing also keeps dirt out.
Worked example. A dryer exhaust fan runs at 350°F, clockwise, on spherical roller pillow blocks, and the customer's plant prefers oil.
- Temperature: 350°F is above 275°F, so if the bearings are greased, the order must call for high temperature grease. It is also above 300°F, so one bearing is fixed and one floating.
- Oil preference: oil systems are used on special applications only, so confirm with engineering that the option fits this model before quoting it. If a static oil system is approved, the order needs two oilers (one per bearing), and for clockwise rotation each oiler reservoir is on the left.
- Record the choice on the order as a lubrication line item so that whoever services the fan years later knows which lubricant system is fitted.
Chapter 17 describes the static oil and oil mist systems component by component.
Bearing Seals¶
A bearing seal is a seal built into the bearing housing, around the shaft, that keeps dust, moisture and other foreign material out of the bearing (and the lubricant in). It is separate from the shaft seal on the fan housing. Exact construction and material vary by bearing manufacturer, but FanPedia names three common types, and the seal type sets the bearing's maximum allowable speed.
- Lip seal (contact seal): a flexible lip rides on the shaft. Used on ball bearings at low to moderate shaft surface speeds.
- Labyrinth seal (non-contact seal): a winding path of close-clearance grooves with no rubbing contact. Used for higher speeds, on spherical roller bearings in solid and split pillow blocks.
- Taconite seal: designed for dirty or abrasive environments, used on split pillow block spherical roller bearings. The standard taconite seal has lower speed limits than a standard labyrinth; a non-contact Canadian type is available for higher speeds.
Worked example. A fan for an aggregate drying plant will run in heavy dust on split spherical roller pillow blocks. The dust points to a taconite seal. Two follow-up checks come straight from FanPedia. First, compare fan speed with the seal's speed limit; if the standard taconite seal's limit is too low, ask for the non-contact (Canadian) type. Second, taconite seals make the bearing wider, which can require a longer shaft and repositioning the bearing on the pedestal or a larger bearing support. That is a design change, not a line item, so flag it before the quote is final. FanPedia points to Twin City Fan's Fan Engineering Letter FE-1200 for the details.
Insulated Bearings¶
An insulated bearing is a bearing whose outer race is coated with an electrically insulating material, for example SKF's INSOCOAT, so that electrical current cannot pass from the shaft through the bearing to the frame. The problem it solves comes from variable frequency drives: as Chapter 15 describes, a VFD can induce voltage on the motor shaft, and when that voltage discharges through a bearing it pits the raceways. FanPedia lists insulated bearings as a motor modification, alongside the shaft grounding ring, which gives the current a path to ground that bypasses the bearings. NEMA MG-1 Part 31 is the standard FanPedia cites for these precautions.
Monitoring the Bearings¶
Bearings rarely fail without warning. They run hotter and they vibrate more first, so two kinds of sensors are offered to catch those signs.
A bearing RTD (resistance temperature detector) or bearing thermocouple is a temperature sensor whose tip sits in the bearing housing and reports bearing temperature to the plant's monitoring system. FanPedia shows a 3-wire RTD and a 2-wire Type K thermocouple, installed either through a T-fitting on the existing grease fitting (so the bearing can still be greased) or in a hole drilled and tapped into the bearing housing. On motors, bearing RTDs sense the heat rise of the motor bearings.
A bearing vibration sensor is a sensor, usually an accelerometer, mounted on the bearing housing to monitor vibration levels at the fan bearings. Rising vibration is the classic early sign of imbalance, misalignment, looseness or a bearing defect. Twin City Fan's standard provision is a hole drilled and tapped into the bearing housing, 1/4"-28 UNF with a 1-inch spot face (a machined flat so the sensor seats squarely); a 1/4"-18 NPT tap is optional. The sensors are typically shipped loose for field mounting because they can be damaged in transit, and the sensor cord is supplied by others. Other mounting methods are a bracket held by a bearing bolt or epoxy mounting to the housing.
Worked example (illustrative specification). A specifying engineer's fan specification says: "Provide provisions for vibration monitoring in the horizontal and vertical directions at each fan bearing. Sensors by others." Reading it line by line:
| Spec phrase | What it means for the order |
|---|---|
| "Provisions for vibration monitoring" | Holes drilled and tapped in the bearing housings, not the sensors themselves |
| "Horizontal and vertical at each fan bearing" | Two bearings × two directions = 4 tapped holes |
| "Sensors by others" | Contractor supplies and wires sensors and cords; nothing to ship loose |
| No thread stated | Standard 1/4"-28 UNF with 1-inch spot face; ask if the plant standard is 1/4"-18 NPT |
If the specification had said "provide sensors", the order would include sensors shipped loose, and the cords would still be by others. The specification wording here is illustrative; the tap sizes and shipping practice are from FanPedia. Vibration measurement itself is covered in Chapter 23.
Sensors Turn Bearing Life Into a Forecast
Notice that L-10 life is a statement about a population of bearings, not about the one in front of you. Temperature and vibration sensors close that gap: they let the plant watch its own bearing and replace it on evidence, before fatigue becomes a failure.
Shaft Seals¶
A fan housing is a pressure vessel with a hole in it, and the shaft runs through that hole. A shaft seal is a seal mounted on the fan housing around the shaft where it passes through the housing wall, to reduce leakage of air or gas along the shaft. An overhung fan needs one, on the drive side; a center hung fan whose shaft passes through both sides of the housing needs two. It does not touch the bearings and has nothing to do with lubrication; its job is the airstream. Which way the leak runs depends on pressure: where the housing is above atmospheric pressure, gas pushes out along the shaft, and where it is below, outside air is drawn in. FanPedia lists what excessive leakage can do: it can be dangerous when the gas carries hazardous contaminants, waste energy, create an environmental or safety hazard, damage fan bearings, and create excessive noise. A hot, dusty or corrosive gas blowing out along the shaft lands directly on the drive-end bearing a few inches away.
Shaft seals fall into three families, defined by what does the sealing:
- Contact seals rub a soft material against the shaft: the friction seal, the V-ring (axial) seal and the lip seal. They are simple and inexpensive.
- Carbon ring seals surround the shaft with split carbon rings held in a housing, with very small clearance; a purge connection can be added between two rings.
- Stuffing box (packing) seals compress rings of packing into a box around the shaft; a purge connection can be added here too.
As you move down that list, seals become more effective at limiting leakage, more expensive, and more likely to be vendor supplied. Whatever the type, the seal assembly bolts to the housing on mounting studs welded inside the housing and is sealed to the housing with PTFE joint sealant.
Every Seal Is a Leak Allowance
FanPedia says a fan generally cannot be built totally leak-tight, which is why the term is "nominally leak-tight." So don't ask "does this seal leak?"; ask "how much leakage can this job accept, and in which direction?" That question picks the seal family for you.
Lip and Friction Shaft Seals¶
The two contact seals you will see most often are the lip seal and the friction seal.
A lip shaft seal uses an elastomer lip, held in a seal housing on the fan housing wall, that presses against the turning shaft. Buna-N (Buna rubber) is standard and Viton is the special, higher-performance option, and a double lip version is available. The seal housing carries a grease fitting, and grease pumped into it lubricates the lip and fills the gap, which is why order lines and field staff also call it a grease seal or grease purge seal. Lip seals appear on FanPedia's list of shaft seals for nominally leak-tight construction.
A friction shaft seal is FanPedia's standard type: a ring of soft seal material clamped by a cover plate (typically aluminum) against the housing so that it rubs lightly on the shaft. It is also called a tacky cloth seal, after its most common material. Typical materials are tacky cloth, Teflon, Viton, Nomex mineral wool, silicone sheet and Fiber Frax (ceramic felt), chosen for the temperature and gas. The V-ring or axial shaft seal, a Teflon plate design, is a close relative.
Carbon Ring Seals¶
A carbon ring seal, which FanPedia calls a mechanical shaft seal, surrounds the shaft with one or two rings of carbon held in a seal housing behind a cover plate. Each ring is split into segments so it can be installed around the shaft without removing the impeller, and the rings "float" with the shaft, which gives the seal its other name, floating circumferential carbon ring seal. Carbon runs against steel with low friction at very small clearance, so leakage is lower than through a soft contact seal. These seals are vendor supplied by specialist seal manufacturers, and FanPedia also lists "labyrinth shaft seal" as another name for them.
They come in two forms:
- Single ring (single carbon ring): one split carbon ring in the seal housing.
- Double ring (double carbon ring): two split carbon rings with a lantern ring between them at a purge hole. A lantern ring is a spacer ring with openings that lets purge gas, fed in from outside, fill the space between the two rings.
The purge is what makes the double ring seal special. When clean air or an inert gas is supplied to the lantern ring at a pressure slightly above the gas in the fan, any leakage across the inner ring is purge gas flowing into the fan, not process gas flowing out. The purge supply is the customer's.
Worked example. Compare two jobs that both need carbon ring seals.
- A regenerative thermal oxidizer (RTO) fan in an automotive paint plant. FanPedia's RTO construction lists friction, single carbon ring, and commercially available carbon ring seals. A single carbon ring meets the requirement without a purge supply.
- A nominally leak-tight Arrangement 8 fan handling a gas the customer must not release. FanPedia's leak-tight construction lists lip, stuffing box (Graphoil) and double ring mechanical (double carbon ring) seals, so the single ring is not on the list. Order the double carbon ring and confirm the customer will pipe a purge line to the purge connection.
The difference between the two is not the ring material; it is whether the job's tolerance for leakage requires a purge barrier.
Diagram: Contact and Carbon Ring Shaft Seals¶
Run Contact and Carbon Ring Shaft Seals Fullscreen
Contact and Carbon Ring Shaft Seals
sim-id: ch16-shaft-seal-types-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the friction, V-ring and lip contact seals and the single and double carbon ring seals on FanPedia's exploded drawings, given a description of each.
Content: Five numbered zones, one per exploded view: friction shaft seal, V-ring shaft seal, lip shaft seal, single carbon ring seal, and double carbon ring seal.
Provenance: FanPedia pages 149-153, "Shaft Seals: exploded views of the friction (p. 149), V-ring (p. 150), lip (p. 151), single ring mechanical (p. 152) and double ring mechanical (p. 153) shaft seals", with the printed labels removed. Drawing: FanPedia, Twin City Fan, pp. 149-153.
Stuffing Box Seals¶
A stuffing box seal, also called a packing gland seal or Graphoil seal, works like the packing on a valve stem. An inside plate carrying the seal housing is mounted on the fan housing wall; rings of packing are stacked in it around the shaft; and an outside plate with a retaining ring presses the stack, drawn up on threaded mounting rods whose nuts are welded inside the housing. In Twin City Fan's design the inner rings are Graphoil (flexible graphite) split rings and the outer rings are carbon yarn packing. A variant replaces the two inner rings with hard seals. Because packing is compressed against the shaft, it can be retightened to take up wear.
The stuffing box with gas purge adds a Teflon lantern ring at a purge hole in the ring stack, between the Graphoil split ring and the carbon yarn packing, on the same principle as the double carbon ring. The fan ships with a 1/8-inch NPT pipe plug in the purge hole, which is replaced in the field with the customer-supplied purge line.
Diagram: Stuffing Box Seal With Gas Purge¶
Run Stuffing Box Seal With Gas Purge Fullscreen
Stuffing Box Seal With Gas Purge
sim-id: ch16-stuffing-box-seal-overlay
Library: html + FP.Overlay
Status: built
Bloom Level: Remember
Learning Objective: The learner will identify the ten labeled parts of a stuffing box shaft seal with gas purge on an exploded view, given each part's job or material.
Content: Ten numbered markers on the exploded view: shaft, fan housing wall, threaded mounting rods, 1/8-inch NPT pipe plug, PTFE joint sealant, inside plate with seal housing, carbon yarn packing, Graphoil split ring, Teflon lantern ring, and outside plate with retaining ring.
Provenance: FanPedia page 155, "Shaft Seals: Stuffing Box Type Shaft Seal With Gas Purge" (exploded view with the assembled inset), with the printed labels removed. Drawing: FanPedia, Twin City Fan, p. 155.
Worked example. An order line reads "Shaft Seal: Graphoil Stuffing Box With Purge" on a nominally leak-tight fan. Decode it as follows.
| Order line element | What ships and what it means |
|---|---|
| "Graphoil Stuffing Box" | Inside plate with seal housing, Graphoil split ring(s), carbon yarn packing rings, outside plate with retaining ring |
| "With Purge" | Teflon lantern ring at the purge hole, plugged with a 1/8-inch NPT pipe plug for shipment |
| Field work | Contractor removes the plug and connects the customer's purge line |
| Leak-tight check | Stuffing box (Graphoil) type is on FanPedia's leak-tight seal list; no shaft sleeve and cap |
The same line without "With Purge" ships without the lantern ring and purge hole.
The Seal Changes the Shaft
It's tempting to treat a seal upgrade as a bolt-on accessory added after the order. The shaft may need a turndown or a machined surface at the seal location, and on high temperature fans FanPedia shows them on a high temperature pedestal with a shaft cooler, so decide the seal before the shaft is released to the shop.
Choosing a Shaft Seal: Worked Example¶
The table below gathers the construction rules mentioned above into one place. It lists only the seal types FanPedia names for each construction; it is a starting point, and the current catalog and engineering review have the final word.
| Construction | Shaft seal types FanPedia lists | Other seal notes |
|---|---|---|
| Standard | Friction (standard type) | Standard high temperature fans use the standard seal with a shaft cooler |
| High temperature, upgraded seal | Stuffing box or mechanical (carbon ring) | Shown on a high temperature style pedestal with shaft cooler |
| Nominally leak-tight (Arr. 1, 8, 9) | Lip, stuffing box (Graphoil), double ring mechanical | No shaft sleeve and cap; not recommended over 600°F |
| Regenerative thermal oxidizer | Friction, single carbon ring, commercially available carbon ring | Dampers, if required, must have stuffing boxes |
| AMCA A spark-resistant with specialty seal | Specialty shaft seal | Monel shaft used |
Worked example. A specifying engineer sends three fans on one project and asks which shaft seal to quote on each.
- General exhaust, 90°F, Arrangement 9. No leakage requirement is stated. The standard friction seal is the default. If the engineer later adds "minimize leakage," the lip seal is the next step up.
- Process gas, nominally leak-tight, 400°F, Arrangement 8. Leak-tight construction allows lip, Graphoil stuffing box or double carbon ring. Because the gas must not escape, a purged seal is the stronger choice: a stuffing box with gas purge or a double carbon ring. Note on the quote that the customer supplies the purge line, and that no shaft sleeve and cap are used. At 400°F the fan is also in high temperature territory, so expect fixed and floating bearings, high temperature grease and a shaft cooler.
- RTO process fan. Quote a friction or single carbon ring seal per FanPedia's RTO list, and remember the RTO note that any dampers on that fan need stuffing boxes too.
Finally, before the classification activity below, make sure you can name the parts that identify each seal type: the aluminum cover plate and soft seal material of the friction seal; the Teflon plate of the V-ring seal; the elastomer lip and greased seal housing of the lip seal; the split carbon ring or rings, with or without a lantern ring at a purge hole; and the Graphoil rings and carbon yarn packing of the stuffing box.
Diagram: Shaft Seal Identifier¶
Run Shaft Seal Identifier Fullscreen
Shaft Seal Identifier
Type: microsim
sim-id: ch16-shaft-seal-identifier
Library: p5.js
Status: Specified
Bloom Level: Understand
Bloom Verb: classify
Learning Objective: The learner will classify a fan shaft seal into one of seven FanPedia seal types from a description of its parts, materials or order-line name, with at least 10 of 12 correct on the first attempt.
Prerequisites: shaft seal, friction shaft seal, V-ring (axial) shaft seal, lip shaft seal, carbon ring seal (single and double), lantern ring, purge, stuffing box seal, stuffing box with gas purge, Graphoil, carbon yarn packing (all defined in the "Shaft Seals" section above).
Evidence of Mastery: For each of 12 cards, the learner commits a card to one of seven seal-type bins. A placement is correct when it matches the "Correct bin" column in Content. Mastery is 10 of 12 correct on the first attempt. Opening the reference cross-sections in explore mode is exploration, not evidence.
Misconceptions: (1) Any seal with a purge is a carbon ring seal (stuffing boxes can be purged too). (2) A lip shaft seal and a bearing lip seal are the same part (the shaft seal is on the fan housing and sealing air; the bearing seal keeps dirt out of the bearing). (3) "Labyrinth" always means a bearing seal (FanPedia also lists "labyrinth shaft seal" as another name for carbon ring shaft seals). (4) Single and double carbon ring seals differ only in size (the double ring adds a lantern ring and purge).
Instructional Rationale: Classifying unfamiliar descriptions into categories requires the learner to recognize each seal's defining parts rather than recall a label. Each card asks for a prediction committed before feedback, and the feedback states the identifying part so the learner builds the part-to-type mapping.
Content:
Seven bins, each with a reference infobox (shown in explore mode and after each answer):
| Bin | Reference infobox |
|---|---|
| Friction shaft seal | Standard type; soft seal material (tacky cloth, Teflon, Viton, Nomex mineral wool, silicone sheet or Fiber Frax ceramic felt) held by a cover plate, typically aluminum. Also called tacky cloth seal. |
| V-ring shaft seal | Teflon plate design with Chekseal and PTFE joint sealant. Also called axial shaft seal or Teflon shaft seal. |
| Lip shaft seal | Elastomer lip (Buna-N standard, Viton special) in a seal housing with a grease fitting. Also called grease seal, grease purge, double lip seal. |
| Single carbon ring seal | One split carbon ring in a seal housing behind a cover plate; vendor supplied. Also called floating circumferential carbon ring seal, labyrinth shaft seal. |
| Double carbon ring seal | Two split carbon rings with a lantern ring at a purge hole; vendor supplied. |
| Stuffing box seal | Inside plate with seal housing, Graphoil split inner rings, carbon yarn packing outer rings, outside plate with retaining ring. Also called Graphoil seal, packing gland seal. |
| Stuffing box seal with gas purge | Stuffing box with a Teflon lantern ring at a purge hole; ships with a 1/8-inch NPT pipe plug replaced in the field by the customer's purge line. |
Twelve cards:
| # | Card text shown to the learner | Correct bin | Why (shown as feedback) |
|---|---|---|---|
| 1 | An aluminum cover plate clamps a strip of tacky cloth against the shaft. | Friction shaft seal | Soft material held by a cover plate is the friction (standard) seal. |
| 2 | Order line: "Shaft Seal: Buna Rubber (lip type), Standard". | Lip shaft seal | Buna rubber lip is the standard lip seal. |
| 3 | A seal housing on the fan housing wall carries a grease fitting. | Lip shaft seal | Only the lip seal is greased through its own housing. |
| 4 | One split carbon ring sits in a seal housing behind a cover plate; there is no purge hole. | Single carbon ring seal | One carbon ring without a purge is the single ring mechanical seal. |
| 5 | Two split carbon rings with a lantern ring between them at a purge hole. | Double carbon ring seal | Two carbon rings plus a lantern ring is the double ring mechanical seal. |
| 6 | Graphoil split rings inside, carbon yarn packing outside, and an outside plate with a retaining ring; no purge hole. | Stuffing box seal | Graphoil and carbon yarn packing compressed in a box is a stuffing box. |
| 7 | Carbon yarn packing and a Teflon lantern ring at a hole closed by a 1/8-inch NPT pipe plug. | Stuffing box seal with gas purge | Packing plus a lantern ring and purge plug is the purged stuffing box. |
| 8 | A Teflon plate with Chekseal, called an axial shaft seal. | V-ring shaft seal | "Axial shaft seal" is another name for the V-ring type. |
| 9 | Seal material is Fiber Frax ceramic felt under a cover plate. | Friction shaft seal | Fiber Frax is one of the friction seal's materials. |
| 10 | The order calls it a "packing gland seal" and lists no purge. | Stuffing box seal | Packing gland seal is another name for the stuffing box. |
| 11 | Order line: "Shaft Seal: Viton (lip type), Special". | Lip shaft seal | Viton is the special lip material; the part is still a lip seal. |
| 12 | Order line: "Shaft Seal: Graphoil Stuffing Box With Purge". | Stuffing box seal with gas purge | "With Purge" adds the lantern ring and purge connection. |
Provenance: FanPedia (Twin City Fan, 2021), Shaft Seals pages 149 (friction), 150 (V-ring), 151 (lip), 152 (single ring mechanical), 153 (double ring mechanical), 154 (stuffing box) and 155 (stuffing box with gas purge). Card wording is written for this chapter from those pages.
Rules: Each card has exactly one correct bin. A card is answered by committing it to a bin; once committed it cannot be moved except on the second attempt after a wrong first attempt. Score counts first-attempt correct answers only.
Learner Activity:
- In explore mode the learner opens any bin's reference infobox to review the seven seal types.
- The learner starts the quiz. One card is shown at a time.
- The learner commits the card to a bin. The sim marks it right or wrong, shows the feedback, and opens the correct bin's infobox.
- After 12 cards the sim shows the first-attempt score and lists any missed cards with their correct bins.
- The learner can restart; cards reshuffle.
Feedback: 12 cards, shuffled order, two attempts per card. Correct: "Correct:
Starting State: Explore mode showing the seven bins and the prompt "Every shaft seal is identified by what touches the shaft. Open the bins, then start the quiz."
Chapter Anchors: The chapter describes seven seal types (friction, V-ring, lip, single carbon ring, double carbon ring, stuffing box, stuffing box with gas purge) in three families (contact, carbon ring, stuffing box). The purge plug is 1/8-inch NPT. Mastery is 10 of 12.
Supported and Sealed
You can now trace a rotating assembly from the keyway at the sheave to the seal at the housing wall, and say why each bearing, collar, sensor and seal is there. Next time an order line reads "Graphoil Stuffing Box With Purge," you will know exactly what ships and what the contractor must connect.
Key Takeaways¶
- The shaft and impeller form the rotating assembly, carried on two bearings in an overhung or center hung layout.
- Keyways carry torque through keystock; turndowns step the shaft diameter down at hubs, seals, bearings and couplings; locking collars keep the shaft from moving axially.
- Shaft sleeves shield a shaft from an airstream of a different material (typical on Arrangements 1, 8, 9 and 10, but not used on nominally leak-tight fans); clad shafts do the same job on shafts of 9 inches and larger at lower cost.
- Bearing life is a fatigue statistic; most Twin City Fan models offer L-10 life of 40,000 hours, which is about 9.6 years at 16 hours a day, 5 days a week.
- Ball bearings suit lighter, smoother duty; spherical roller bearings carry high and shock loads at more limited speed.
- Pillow blocks mount parallel to the shaft, flange bearings perpendicular to it, and two-bearing housings hold both bearings in precise alignment for special jobs.
- Set screw mounts lock the inner ring with two screws; fixed bearings locate the shaft axially, and above 300°F every fan uses one fixed and one floating bearing.
- Positioners adjust bearing location; dowel pins and stop blocks restrain it, as spark-resistant Types A and B require.
- Grease is standard, high temperature grease is needed on hot fans, and static oil and oil mist systems are special-application options.
- Bearing seals (lip, labyrinth, taconite) protect the bearing and set its speed limit; insulated bearings block VFD shaft currents.
- RTDs, thermocouples and vibration sensors mount on tapped bearing housings and give early warning of bearing trouble.
- Shaft seals limit leakage where the shaft enters the housing. Friction and lip seals are simple contact seals; carbon ring and stuffing box seals limit leakage further, and their purged versions use a lantern ring and a customer-supplied purge line.