Bearing housings are mechanical assemblies comprising a housing body and an integrated bearing inside, designed to position and secure a rotating shaft within a machine structure.
Without the correct housing type, bearings cannot distribute loads along the intended axis — resulting in shaft misalignment, uneven load distribution, and significantly reduced bearing life. The three most common housing lines in the Vietnamese market are SNL (self-aligning spherical roller bearing housings), SN (standard cast-iron body), and UCP (cylindrical block with Y-type insert bearing). Selecting the correct housing line can double equipment lifespan compared to improper installation.
Definition and Function of Bearing Housings
Bearing housings perform three simultaneous functions: (1) position the shaft on the correct centerline per design, (2) transmit loads from the shaft to the machine structure, and (3) protect the bearing from dust, moisture, and environmental factors. According to SKF Bearing Housings Catalogue, housing design directly influences load-carrying capacity — cast-iron bodies have higher stiffness than steel bodies and suit heavy-load, high-vibration applications.
Classification by housing shape:
- Pillow block (plummer block) — split housing body, bearing installed from top, mounted on horizontal surfaces. Easy to disassemble, most common in industry
- Flanged housing — body with mounting flange for vertical or angled bolt fastening. Saves installation space
- Cartridge housing — cylindrical form, inserted through a support plate. Common for conveyor belts and roller applications
Standard materials: gray cast-iron GJL-200 for general duty, nodular cast-iron GJS-400 for impact loads, cast steel for elevated temperatures. ISO 15243:2017 classifies bearing failures by root cause — many defects originate from incorrect housing selection or poor installation.
Housings are classified by the bearing type inside: SNL line uses spherical roller bearings (SRB), SN line uses SRB and tapered roller bearings, UCP includes Y-type insert bearings from the manufacturer. Each line has its own catalog codes — mismatched codes mean misaligned installation and complete assembly failure.
Beyond these three main types, the market offers specialized housings: take-up units for belt conveyors with tension-adjustment mechanisms, vertical housings for vertical shafts, and sensor-integrated housings with built-in temperature or vibration transducers. The last category grows increasingly common in predictive maintenance systems, enabling continuous monitoring without external sensors.
SNL Housing — Plummer Block for Self-Aligning Spherical Roller Bearings
The SNL line (SKF) is a heavy-duty pillow block designed exclusively for spherical roller bearings (SRB) in the 22xxx and 23xxx series. Cast-iron body GJL-200, split horizontally, allowing direct bearing installation on the shaft and placement into the housing.
Key technical features of SNL:
- Tapered adapter sleeve: Bearing mounts on shaft via tapered sleeve H3xx/H2xx, requiring no shaft shoulder machining. Suitable when the shaft is standard-sized and not custom-machined
- Self-aligning angle: SRB tolerates angular misalignment ±1.5–2°, requiring no absolute centering during installation
- Integrated seals: Oil seal ring TSN and labyrinth seal TSNG prevent dust entry and oil leakage. Sealing grade IP55–IP65 selectable
- Lubrication path / grease port: Standard Zerk grease nipple compatible with common grease guns
Typical complete assembly example: Shaft d = 100 mm → bearing 22220 EK/C3 (d=100, D=180, B=46, C=365 kN) + adapter sleeve H320 + housing SNL 520-617. This assembly sustains radial load to 365 kN, maximum speed 3,000 rpm with lithium EP2 grease. Per ZVL-ZKL Industrial Bearings 2022, SRB series 222xx bearings mount directly into SNL housings of the same series.
| Housing Code | Compatible Bearing | Shaft d (mm) | Static Load C₀ (kN) | Typical Application |
|---|---|---|---|---|
| SNL 509-607 | 22209 EK | 45 | 98 | Small conveyor belt, agitator shaft |
| SNL 513-611 | 22213 EK | 65 | 215 | Industrial fan, pump |
| SNL 517-615 | 22217 EK | 85 | 330 | Small mill, gearbox |
| SNL 520-617 | 22220 EK | 100 | 530 | Heavy-duty conveyor belt |
| SNL 524-620 | 22224 EK | 120 | 780 | Mill, mining equipment |
| SNL 528-626 | 22228 EK | 140 | 1,060 | Crusher shaft, heavy mixer |
| SNL 532-630 | 22232 EK | 160 | 1,430 | Large-diameter roller press, rolling mill |
SNL suits heavy-duty conveyor belts, mills, large fans, mixers, and mining equipment. Dusty environments, heavy vibration, quick change requirements — SNL is the first choice.
SN Housing — Standard Cast-Iron Body
The SN line is a standard pillow block in cast iron, used for spherical roller bearings (SRB) and tapered roller bearings (TRB). Simpler design and lower cost than SNL, suitable for medium to heavy loads in less severe environments.
Key differences from SNL:
- No separate bearing cover: SN uses simpler integrated seals, fewer sealing options than SNL
- Lower cost: 20–35% cheaper than SNL for the same shaft size — suitable when environment does not demand high sealing
- Adapter sleeve compatibility: Uses H3xx sleeve but narrower selection
- No advanced seal options: No labyrinth seal TSNG as found in SNL
Specifications for common SN sizes:
| Housing Code | Compatible Bearing | Shaft d (mm) | Static Load C₀ (kN) | Max Speed (rpm) |
|---|---|---|---|---|
| SN 505 | 22205 EK | 25 | 36 | 9,000 |
| SN 508 | 22208 EK | 40 | 80 | 5,500 |
| SN 511 | 22211 EK | 55 | 146 | 4,300 |
| SN 515 | 22215 EK | 75 | 250 | 3,600 |
| SN 519 | 22219 EK | 95 | 440 | 3,000 |
Example: Shaft d = 40 mm → bearing 22208 EK/C3 + adapter sleeve H2308 + housing SN 508. Maximum radial load 80 kN. Speed limit 5,500 rpm with standard grease. Per SKF Rolling Bearings Catalogue PUB BU/P1 10000/2 EN, C3 clearance is mandatory when using adapter sleeves — the sleeve narrows internal clearance, and C3 compensates for that tightening.
SN is typically selected for indoor, medium-load, non-critical environments: factory drive shafts, light centrifugal pumps, warehouse conveyor belts. When dust is heavy or water spray occurs, upgrade to SNL with labyrinth seal. Cost increase from SN to SNL for the same shaft size usually runs only 25–40% — trivial compared to unplanned downtime from early housing failure.
UCP Housing — Cylindrical Block with Y-Type Insert Bearing
UCP (Unit Cast Pillow block) differs fundamentally from SNL and SN. Instead of installing a loose bearing into the housing, UCP is sold complete with a Y-type insert bearing from the factory — bearing and housing form a single unit.
The Y-type insert bearing in UCP has a spherical outer raceway, allowing minor self-alignment (±2–3°) within the housing bore. The bearing is locked to the shaft via set screw or eccentric locking collar — no adapter sleeve needed, no precision shaft shoulder required.
Typical UCP unit specifications:
| Housing Code | Shaft d (mm) | C (kN) | C₀ (kN) | Max Speed (rpm) | Seal Type |
|---|---|---|---|---|---|
| UCP 204 | 20 | 12.8 | 6.95 | 1,600 | Rubber seal 2RS |
| UCP 205 | 25 | 14.8 | 7.80 | 1,400 | Rubber seal 2RS |
| UCP 208 | 40 | 22.4 | 13.2 | 1,100 | Rubber seal 2RS |
| UCP 210 | 50 | 27.5 | 18.0 | 950 | Rubber seal 2RS |
| UCP 212 | 60 | 33.8 | 23.2 | 800 | Rubber seal 2RS |
UCP speed limits are markedly lower than SNL/SN. SKF Y-bearing units catalogue recommends UCP for n ≤ 1,600 rpm. Above this threshold, the insert bearing overheats due to poor housing heat dissipation.
The primary advantage of UCP is installation speed: no centering, no adapter sleeve, no specialized tools. A general mechanic can install it in 15–20 minutes. Ideal for: agricultural conveyor belts, small food processing machinery, packaging equipment, small ventilation fans. Not suitable for heavy loads, high speeds, or environments above 100°C.
One technical note: the UCP set screw creates stress concentration on the shaft surface at contact. Small shafts (d < 30 mm) risk shaft indentation after repeated assembly/disassembly cycles. For frequent removal, prefer UCP models with eccentric locking collar instead of set screw — load distributes more evenly, no shaft marking.
Comparison: SNL vs SN vs UCP vs Flanged Housing
Selecting the correct housing line depends on four factors: load, speed, environment, and installation requirements. The table below summarizes the primary distinctions:
| Criterion | SNL | SN | UCP | Flanged Housing (FY/FYTB) |
|---|---|---|---|---|
| Bearing Included | No (buy SRB separately) | No (buy SRB/TRB separately) | Yes (Y-insert included) | Yes (Y-insert included) |
| Radial Load Capacity | Very high (>365 kN) | High (>200 kN) | Medium (<35 kN) | Medium (<35 kN) |
| Maximum Speed | High (>5,000 rpm) | High (>5,000 rpm) | Low (<1,600 rpm) | Low (<1,600 rpm) |
| Sealing Ability | IP55–IP65 (labyrinth) | IP44–IP55 | IP54 (2RS) | IP54 (2RS) |
| Centering Requirement | Low (SRB self-aligns) | Low (SRB self-aligns) | Very low (set screw lock) | Very low |
| Installation | Requires adapter, expertise | Requires adapter | No adapter needed | No adapter needed |
| Cost (same size) | Highest | Medium | Lowest | Low |
| Typical Application | Heavy industry, mining | Manufacturing, factories | Agriculture, packaging | Vertical mounting, tight space |
A common misconception: UCP is cheaper than SNL but not a direct replacement. They serve entirely different load classes. Using UCP instead of SNL for a cement conveyor belt is wrong — UCP will fail within hundreds of hours.
Flanged housings (FY, FYTB, FYTBK) use the same Y-type insert bearing as UCP but add a flange for vertical bolt mounting. Used when the shaft is horizontal but no flat surface exists for a pillow block — for example, roller shafts in printing machinery or small textile looms.
Criteria for Selecting Bearing Housings
The standard housing selection process comprises 5 steps, in order:
Step 1 — Determine Load Type and Magnitude
Pure radial load → use SRB or deep-groove ball bearing (DGBB). Large axial load → use tapered roller bearing (TRB) or angular-contact ball bearing (ACBB). Combined load → use SRB (handles both directions) or symmetric TRB. Calculate equivalent load P = X·Fr + Y·Fa per ISO 281:2007 before consulting catalogs.
Step 2 — Determine Shaft Diameter and Housing Size
Shaft d defines the bearing size range. Example: d = 65 mm → SRB 22213 EK/C3 + housing SNL 513-611 or SN 513. Never undersizing the housing — insufficient body stiffness fails to transmit load to the base.
Step 3 — Evaluate Environment
| Environment | Sealing Requirement | Suitable Housing Line |
|---|---|---|
| Indoor, low dust | IP44 | SN standard |
| Medium dust, no water | IP55 | SNL with TSN seal |
| Heavy dust, water spray | IP65 | SNL with labyrinth TSNG |
| Food, chemicals | IP66 + stainless steel | Specialized INOX housings |
| Temperature > 150°C | Heat-resistant | VL0241 bearing (insulated) or C4 clearance |
Step 4 — Verify Speed
Compare actual operating speed against catalog limit. If n > 70% n_max → switch from grease to oil lubrication for better cooling. Bearing 22220 EK in SNL 520 has n_max = 3,000 rpm (grease); running at 2,500 rpm requires closer temperature monitoring and shorter grease intervals.
Step 5 — Confirm Design Life L₁₀
Calculate L₁₀ = (C/P)^p × 10^6 / (60 × n) hours. For SRB, p = 10/3. If L₁₀ < design requirement, increase bearing size or reduce load (revisit mechanical design). ISO 281:2007 provides complete formulas with SKF adjustment factors.
ZVL Slovakia manufactures SRB series 222xx and 23xx to ISO standards, equivalent to SKF/FAG — compatible directly with SNL and SN housings. Many Vietnamese factories use ZVL spherical roller bearings with competitive pricing versus other European brands.
Real-World Case — Upgrading a Conveyor Housing
At an animal feed processing plant in Dong Nai, heavy conveyor belts (1,200 kg/m) suffered repeated roller shaft failures. Replacement cycles averaged only 3–4 months — three times the design target. The engineering team identified the root cause via vibration analysis per ISO 10816-3:2009: amplitude at 2× shaft frequency surged, a signature of housing misalignment.
Root cause: UCP 208 housings (d=40 mm) were sustaining 28 kN radial load — 26% over the static rating C₀ = 13.2 kN for UCP 208. Impact loads from raw material worsened the situation. The 2RS seal on UCP was insufficient against feed dust and wood fiber in the plant air.
Upgrade solution:
- Replace UCP 208 with SNL 508-606 assembly + bearing 22208 EK/C3 + adapter sleeve H2308. Dynamic load capacity C = 80 kN — 2.8× actual load margin
- Add labyrinth seal TSNG 508 — IP65 protection versus IP54 of the old UCP
- Establish grease schedule: SKF LGMT 2 every 500 hours (heavy-dust environment)
- Install SKF CMSS temperature sensor on the lead housing — alert if T > 80°C
Results after 14 months: zero roller shaft failures. Maintenance interval extended from 3 months to 14 months. Initial cost rose 4.2×, but 18-month total maintenance cost fell 61% — chiefly from eliminated unplanned downtime.