Spherical Roller Bearings: Construction, Series 222–240, W33 & SNL Guide

Spherical roller bearings (SRB) are a type of rolling bearing that uses two rows of barrel-shaped rollers running on a continuous concave spherical outer raceway, accommodating 1–2° of shaft misalignment while carrying very heavy radial loads from 100 kN at the smallest sizes to over 2,000 kN at extra-heavy series.

SRB is the standard selection for heavy industrial rotating equipment: cement mills, mining crushers, steel rolling mills, vibrating screens, and large conveyor pulleys. Line contact between barrel rollers and raceways creates a load-carrying area 5–8 times larger than the point contact of self-aligning ball bearings at the same bore. No other bearing type combines this load capacity with misalignment tolerance at the same external envelope.

Construction and operating principle

An SRB consists of four components: inner ring, outer ring, two rows of barrel-shaped rollers, and a cage.

The outer ring has a continuous concave spherical inner surface. The center of curvature coincides with the geometric center of the bearing. When the inner ring tilts with the shaft, the rollers continue rolling on the spherical surface without sliding or jamming. Maximum misalignment is 1–2° depending on series — sufficient to compensate for shaft deflection and frame distortion in most industrial applications [SKF Rolling Bearings Catalog, PUB BU/P1 10000/2 EN, 2023].

Barrel rollers create line contact with both inner and outer ring raceways. This is why SRBs carry far higher loads than self-aligning ball bearings at the same bore. Point contact in ball bearings limits dynamic load rating to 15–55 kN; line contact in barrel rollers pushes that to 100–2,000+ kN.

Roller profile is crowned using a logarithmic curve that distributes stress evenly across the full roller length, eliminating edge stress concentration when the shaft deflects.

Cage variants — three main types

For shock loading or continuous temperatures above 100°C, brass cages (CA or MA) are preferred. Pressed steel CC cages tend to crack after a few thousand hours on high-intensity vibrating screens.

E-design — 15–20% load capacity increase

E-design is the most significant SRB improvement in the past 30 years. Earlier designs (CC or no suffix) have a center guide rib on the inner ring. This rib occupies space, limiting roller size.

E-design eliminates the center rib, replacing its function with an inner ring guided cage. The freed space accommodates longer, larger rollers with greater contact area. Result: dynamic load rating C increases 15–20% compared to the older design at the same external dimensions D and width B [Schaeffler Technical Pocket Guide, TPI 110, 2022].

Concrete example: SKF 22220 E has C = 365 kN. The older 22220 CC reaches approximately 310 kN. Using the E-design on the same equipment increases L10 life by approximately 75% (since L10 scales with C^(10/3)/P^(10/3)).

ZVL, SKF, FAG, NSK, and NTN all apply equivalent designs. ZVL designates its enhanced SRB line per ISO conventions, allowing direct interchange with SKF E and FAG E at matching part numbers.

When sourcing replacements: always confirm the E suffix or manufacturer-equivalent. Installing the older design instead of E can significantly reduce calculated bearing life even though the external dimensions are identical.

Series classification and part number decoding

ISO 15:2017 classifies SRBs by the ratio of outer diameter D to bore d and width B. Each series serves a different application range.

SRB series overview

Part number decoding example — 22220 EK/C3:

• 222 = series 222, light
• 20 = bore d = 100 mm (multiply by 5)
• E = Enhanced design (larger rollers, higher C)
• K = tapered bore 1:12
• C3 = greater than normal radial clearance

Real specifications for common SRB part numbers

Source: SKF Rolling Bearings Catalog, 2023

At d = 100 mm, the 22220 EK/C3 delivers D = 180 mm, B = 46 mm, C = 365 kN. A self-aligning ball bearing 2220 at the same bore achieves only C ≈ 73 kN — the SRB carries five times the load. See also bearing construction and how to read bearing codes.

W33 suffix and SNL plummer block housings

What W33 means

W33 designates an outer ring machined with an annular lubrication groove running around the inner circumference, plus three drilled holes spaced 120° apart from the outer surface through to the groove. Grease pumped through the housing grease fitting travels through the housing body, into the W33 holes, around the annular groove, and distributes evenly to both roller rows. Periodic relubrication requires no housing disassembly.

When W33 is mandatory

W33 is mandatory when the bearing is installed in a plummer block housing (SNL, SN, SD). Installing a bearing without W33 in an SNL housing means grease pumped from the external fitting cannot reach the rolling elements. The bearing runs dry and typically fails within 2,000–4,000 operating hours instead of the designed 30,000–50,000 hours [SKF Bearing Housings Catalog, PUB BU/P2 07200 EN, 2022].

At a cement plant in northern Vietnam, a technician replaced a 23024 with an imported bearing missing the W33 suffix. The bearing fit the SNL 524 housing perfectly dimensionally. After approximately 2,200 hours, housing temperature climbed from 55°C to 95°C. On disassembly: internal grease had dried completely, rollers showed blue discoloration from lubrication starvation. Root cause: grease pumped through the housing nipple could not enter the bearing because the W33 holes were absent.

Complete SNL unit ordering

A correct plummer block unit for a 100 mm shaft consists of four matched components:

• 22220 EK/C3 W33 — bearing
• H 320 — adapter sleeve with lock nut and lock washer
• SNL 520-617 — housing
• 2× TSNG 520 — labyrinth seals (or TACONITE seals for heavy dust)

SNL — SRB — Adapter sleeve compatibility

Source: SKF Bearing Housings Catalog, 2022

Tapered bore installation — adapter sleeve method

Bearings with the K suffix have a tapered bore (1:12 taper). The adapter sleeve method is the most common industrial installation approach — it works on plain shafts without precision machining and permits rapid removal.

Drive-up procedure

Step 1 — Measure initial clearance: Use a feeler gauge to measure radial clearance between rollers and outer ring at the 12 o'clock position. Record the value.

Step 2 — Mount the adapter sleeve: Position the bearing on the shaft. Insert the adapter sleeve into the tapered bore from the lock nut end.

Step 3 — Controlled tightening: Use a hook wrench to tighten in quarter-turn increments. Re-measure clearance after each step.

Step 4 — Stop at the specified reduction: Stop when the clearance reduction matches the values below. Over-tightening reduces clearance to zero, causing overheating and seizure.

Clearance reduction table during mounting (C3 group)

Source: SKF Rolling Bearings Catalog, 2023

At a cement plant in Hai Phong, a maintenance team installed SRB 22320 E/C3 W33 on a clinker conveyor drive shaft without measuring clearance. After 800 hours, bearing temperature climbed from 65°C to 95°C. Inspection revealed zero radial clearance from over-tightening. Replacement cost and production stoppage exceeded 80 million VND.

Lubrication

SRBs perform best with lithium complex or polyurea grease, base oil viscosity 100–150 cSt at 40°C. Low-speed applications (n × dm < 75,000 mm/rpm) use NLGI 2–3. Higher speeds use NLGI 1 or oil lubrication.

Initial grease fill quantity (grams): G = 0.005 × D × B

Example: 22220 E (D = 180, B = 46): G = 0.005 × 180 × 46 = 41.4 grams.

Over-greasing is also damaging. Excess grease creates churning resistance, generating heat that pushes grease past the seals. Temperature rises 10–15°C, grease life halves [Hamrock, Schmid, Jacobson — Fundamentals of Fluid Film Lubrication, 2004].

Relubrication intervals in SNL housings

SRB vs CARB toroidal — selection guide

SKF's CARB bearing (C series) combines angular misalignment compensation like an SRB with axial displacement accommodation up to ±2 mm — a capability SRBs lack entirely.

Direct comparison at d = 100 mm

Choose SRB when: radial load dominates and maximum C is needed, or cost is a factor. SRBs are significantly more cost-competitive than CARB at the same bore [NTN Spherical Roller Bearings CAT. No. 2203/E, 2021].

Choose CARB when: thermal shaft expansion exceeds 0.5 mm axially (dryers, hot rolling), or a combined arrangement is desired — tapered roller bearing at the locating position, CARB at the non-locating position, replacing the traditional SRB + cylindrical roller bearing pair.

Industrial applications

Cement

Ball mills use SRBs from series 230xx and 232xx on main trunnion shafts (Ø200–400 mm), with C up to 1,500–2,000 kN. Operating conditions: fine dust, temperatures to 80°C, significant vibration. Specification: K tapered bore + adapter sleeve, SNL housing with TACONITE seals for dust exclusion.

At a cement plant in Binh Duong province, upgrading from CC-design to E-design SRBs on the mill shaft extended L10 from 18,000 hours to 28,000 hours — eliminating one bearing replacement within a 3-year major overhaul cycle.

Mining

Jaw crushers, cone crushers, and vibrating screens all use SRBs. Vibrating screens impose short-cycle impact loading. Brass cages (CA) are mandatory — pressed steel CC cages crack under sustained high-intensity vibration. C4 clearance is required for high-intensity screens because friction from vibration generates rapid temperature rise.

Steel rolling

Roll neck shafts use series 231xx and 240xx, with loads reaching 2,000+ kN. Continuous temperatures to 120°C require high-temperature grease and water-resistant seals against cooling water ingress. High-speed hot rolling applications switch to circulating oil lubrication.

Paper and pulp

Dryer roll shafts extend up to 10 m at 500–1,500 rpm. Long shafts deflect 0.3–0.8° between supports — well within SRB capability. Series 222xx and 231xx are common. Above 1,000 rpm, oil lubrication is preferred to manage heat.

ZVL spherical roller bearings

ZVL (ZVL-ZKL Group, Slovakia) manufactures SRBs per ISO standards at European facilities certified to ISO 9001. The ZVL SRB range covers all series from 222xx to 240xx, including E-design, K suffix, and W33 [ZVL Spherical Roller Bearings Catalog, ZVL-ZKL Group, 2023].

ZVL 22220 EK/C3 W33 carries equivalent specifications to SKF 22220 EK/C3/W33 (C = 365 kN, D = 180 mm, B = 46 mm). External dimensions comply with ISO 15:2017, enabling direct interchange with SKF E, FAG E, NSK EA, and NTN E at matching part numbers. Load rating deviation versus SKF is under 2–3%, within calculation method variation, not a quality difference.

ZVL offers significantly more competitive pricing than SKF and FAG, making it a practical choice for high-volume periodic replacement applications where maintaining technical specification is non-negotiable. Multiple cement plants and mining operations in Vietnam have transitioned to ZVL with equivalent service life results.

View the full catalog and request pricing at the spherical roller bearing product page. For sizing calculations, see bearing life calculation.

Selecting an SRB for a new application

A five-step selection procedure per ISO 15:2017:

Step 1 — Determine loads: Calculate equivalent load P = X × Fr + Y × Fa. If Fa/Fr > 0.3, consider tapered roller bearings instead.

Step 2 — Select series: If outer diameter D is constrained, consider series 230xx (smaller D/d ratio with wider B). If axial width B is constrained, series 222xx.

Step 3 — Check dynamic load rating: C ≥ P × (L10h × n × 60/10⁶)^(1/3)

Where L10h = target life in hours, n = speed in rpm.

Step 4 — Check static load rating: C₀ ≥ s₀ × P₀ (s₀ = 3–4 for shock loads, 2 for smooth loading).

Step 5 — Select clearance and suffixes:

• SNL housing installation: K + W33 + C3
• Direct shaft press-fit: cylindrical bore, C3 or CN
• Temperatures > 100°C or vibrating screens: C4

See how to select bearings for detailed calculation worked examples.

Failure diagnosis

High temperature (> 90°C)

Probable causes in order of frequency: (1) over-greasing, (2) wrong grease type, (3) insufficient radial clearance from over-tightening during installation, (4) actual load exceeding design load, (5) seal failure causing contamination.

Premature failure (< 20% of calculated life)

At a paper mill in Dong Nai province, SRB 22316 E/C3 on the paper machine shaft failed after 4,000 hours against a design life of 25,000 hours. Surface analysis showed flaking concentrated in one roller row — diagnostic pattern for misalignment exceeding 2°. Root cause: uneven foundation settlement over three years. Corrective action: reinforce foundation, switch to K-suffix SRB with SNL housing to simplify future realignment.

Condition monitoring targets

Three parameters to track continuously:

• Temperature: Normal 40–70°C. Warning above 80°C. Shutdown above 100°C.
• Vibration: Velocity > 7 mm/s (ISO 10816) at the housing indicates need for inspection.
• Noise: A steady low hum is normal. A periodic click or thump synchronised with shaft rotation indicates raceway spalling.