Thrust Bearings: Ball, Cylindrical & Spherical Roller Types (29xxx Series)

Thrust bearings are a category of rolling bearing engineered specifically to carry axial loads (thrust loads), transmitting force parallel to the shaft centerline — fundamentally different from ball bearings or angular contact bearings that prioritize radial or combined loads.

A thrust bearing consists of two flat washers clamping rolling elements — balls, cylindrical rollers, spherical rollers, or needles — arranged in a plane perpendicular to the shaft axis. This configuration handles axial thrust from a few hundred Newtons to over 2,000 kN depending on the series. This article analyzes four main families: thrust ball (51xxx, 52xxx), thrust cylindrical roller (81xxx), thrust spherical roller (29xxx), and thrust needle (AXK, NTA) — based on data from SKF, FAG/Schaeffler, ZVL Slovakia, NTN, NSK, and ISO 104:2015.

Axial Load Principle

An axial load (thrust load) is a force acting parallel to the shaft centerline, pushing the shaft along its longitudinal axis. In practice, axial loads originate from multiple sources: worm shaft thrust, hydraulic cylinder pressure in presses, self-weight of vertical shafts, or reaction forces from bevel and helical gears.

Standard radial bearings — deep groove ball bearings 62xx, 63xx — handle only incidental axial loads, typically below 25% of the rated radial capacity. When axial load is the primary or dominant load component, dedicated thrust bearings are required.

Basic Construction

Every thrust bearing comprises three components:

• Shaft washer: a flat ring mounted on and rotating with the shaft
• Housing washer: a flat ring seated in the housing, remaining stationary
• Rolling element assembly + cage: balls, cylindrical rollers, spherical rollers, or needles — held in evenly spaced pockets by a cage

The axial force path runs: shaft → shaft washer → rolling elements → housing washer → housing. Rolling elements convert sliding friction into rolling friction, reducing the friction coefficient from 0.15–0.30 (plain bearings) to 0.002–0.005 (rolling bearings).

Single-Direction and Double-Direction

Single-direction thrust bearings carry axial load in one direction only — series 511xx, 512xx, 811xx, 292xx. This is the most common configuration.

Double-direction thrust bearings carry axial load in both directions — series 522xx, 523xx. Construction includes three washers: one center washer (mounted on the shaft) and two outer washers (seated in the housing), sandwiching two sets of rolling elements.

Thrust Ball Bearings: Series 51100, 51200, 52200

Thrust ball bearings use balls as rolling elements and represent the simplest, most common thrust bearing type. Point contact between balls and raceways permits high rotational speeds but limits load capacity compared to roller types.

Series 51100 — Light Section

Series 51100 features a thin cross-section with small height B, suitable for tight axial space constraints. Dynamic load rating C ranges from 6 kN (51100, d=10 mm) to 90 kN (51130, d=150 mm).

Practical example: 51105 (d=25, D=42, B=11 mm) is a common designation in small CNC rotary table spindles and valve adjusters. Dynamic load rating C ≈ 13.2 kN, grease limiting speed 7,500 rpm — adequate for most light thrust applications.

Series 51200 — Medium Section

Series 51200 has a larger D than 51100 at the same bore d, accommodating more balls with larger ball diameter. Result: C rating 40–60% higher than 51100 at the same bore.

Example: 51210 (d=50, D=78, B=22 mm) has C ≈ 43 kN, C₀ ≈ 72 kN. This designation is widely used in industrial gearboxes, ball screw shafts, and medium-size machine tool rotary tables. Grease limiting speed approximately 4,000 rpm.

Series 52200 — Double-Direction

Series 52200 is the double-direction version of 51200. Three washers instead of two, two ball sets. Height B is nearly twice that of 51200 at the same bore.

Used where the shaft experiences alternating axial load — for example CNC table ball screws, bidirectional rotary tables, or double-acting press main shafts.

Table 1: Common Thrust Ball Bearing Specifications

Note: Specifications per SKF 2023 catalog. ZVL values deviate less than 2% at the same designation — for example, ZVL 51210 has C = 42.5 kN versus SKF's 43.0 kN. The difference falls within manufacturing tolerance and does not reflect a quality gap.

Raceway and Contact Angle

The raceways on shaft and housing washers are concave circular arcs conforming to the balls, with a raceway conformity ratio of approximately 52–54%. Contact angle α = 90° — meaning balls transmit force purely in the axial direction, carrying zero radial load. This is the fundamental difference from angular contact bearings (α = 15–40°), which handle both radial and axial loads simultaneously.

Key limitation of thrust ball bearings: they carry NO radial load. Radial force pushes balls out of the raceway grooves, causing rapid failure. Therefore, thrust ball bearings must always be paired with separate radial bearings (typically deep groove ball bearings or cylindrical roller bearings) to support the radial component.

Thrust Cylindrical Roller Bearings: Series 81100, 81200

Thrust cylindrical roller bearings use cylindrical rollers instead of balls, creating line contact with the raceways. The contact area is many times larger than ball point contact — the foundation for substantially higher axial load capacity.

Construction Details

Cylindrical rollers are guided by a cage — typically machined brass or pressed steel. The flat raceways on shaft and housing washers are ground to Ra ≤ 0.2 μm. Rollers have crowned ends to reduce edge stress concentration.

Unlike thrust ball bearings, thrust cylindrical roller bearings can support a small radial load component — but this capability is very limited (below 5% of axial load). A separate radial bearing is still required in practice.

Series 81100 — Light Section

Series 81100 has a thin cross-section, suitable for tight installation envelopes. Load rating C is 50–80% higher than thrust ball bearing 51100 at the same bore, thanks to line contact.

Series 81200 — Medium Section

Series 81200 has larger D and B, accommodating more cylindrical rollers. This is the most widely used series for industrial applications requiring medium to heavy thrust loads.

Example: 81212 (d=60, D=95, B=26 mm) has C ≈ 132 kN, C₀ ≈ 355 kN. Comparison: thrust ball bearing 51212 at the same bore has C ≈ 47 kN — the 81212 carries 2.8 times more load. Grease limiting speed is approximately 1,800 rpm — significantly lower than the thrust ball equivalent, a trade-off for the superior load capacity.

Speed Limitations

Thrust cylindrical roller bearings have limiting speeds 40–60% lower than thrust ball bearings. The reason: cylindrical rollers slide on the flat raceway near the center because the tangential velocity differs between the inner and outer edges of the roller. This sliding generates heat and wear, capping the maximum rotational speed.

Solution: use circulating oil lubrication instead of grease when operating above 60% of the grease limiting speed. Some manufacturers, including SKF and ZVL, supply rollers with a logarithmic profile to reduce edge sliding, improving the limiting speed by 10–15%.

Thrust Spherical Roller Bearings: Series 29200, 29300, 29400

Thrust spherical roller bearings are the most capable thrust bearing type — combining extremely high axial load capacity, simultaneous radial load capacity (up to 55% of the axial load), and 2–3° misalignment compensation. No other thrust bearing type offers all three characteristics in a single unit.

Unique Construction

Fundamentally different from thrust ball and thrust cylindrical roller bearings, thrust spherical roller bearings feature:

• Shaft washer: a conical (not flat) raceway, inclined relative to the shaft axis
• Housing washer: a concave spherical raceway — similar to the outer ring of radial spherical roller bearings
• Asymmetric barrel rollers: barrel-shaped but with one end larger than the other, matching the conical raceway geometry on the shaft washer

The concave spherical housing washer allows the barrel rollers to self-align when the shaft tilts — the same principle as radial SRBs. The contact angle is approximately 45–60°, meaning the majority of force is transmitted axially while a significant radial component is also supported.

Series 29200 — Light

Series 29200 has D/d ≈ 1.5–1.7, with moderate height B. Dynamic load rating C ranges from 80 kN (29212) to 450 kN (29240). Suitable for plastic extruder screws, small hydraulic presses, and vertical centrifugal pump shafts.

Series 29300 — Medium

Series 29300 has larger D and B than 29200 at the same bore, accommodating more rollers. This is the most popular series in the thrust spherical roller family.

Example: 29320 (d=100, D=170, B=42 mm) has C = 280 kN, C₀ = 670 kN. This designation is commonly specified for 200–500-ton hydraulic presses, large-diameter plastic extruder screws, and slewing cranes. Misalignment tolerance up to 2° allows stable operation even when machine foundations are not perfectly level.

Series 29400 — Heavy

Series 29400 has the largest D/d ratio and thickest B — containing the maximum number of barrel rollers. Dynamic load rating C can exceed 1,500 kN at larger sizes (29464, d=320 mm). Used in heavy hydraulic presses (1,000+ tons), tower crane slewing rings, oil drilling rig main shafts, and vertical hydroelectric turbine shafts.

Table 2: Thrust Spherical Roller Bearing Series Comparison

Note: Specifications per SKF and ZVL catalogs. The 29xxx series is a strategic product line for ZVL — their Slovakia factory produces the full size range from 29200 through 29400, with quality equivalent to SKF/FAG at competitive European pricing.

Three Advantages of 29xxx Over Other Thrust Types

Three advantages that make the 29xxx series irreplaceable in heavy-duty applications:

1. Combined load capacity: Axial + radial load simultaneously (radial ≤ 55% of axial). Thrust ball and thrust cylindrical roller bearings CANNOT do this.
2. 2–3° misalignment compensation: Enabled by the concave spherical housing washer. Thrust ball and thrust cylindrical roller bearings require near-perfect alignment (misalignment < 0.05°).
3. Extremely high load ratings: Line contact on barrel rollers yields C values far exceeding thrust ball bearings at the same bore — 29320 (C=280 kN) versus 51220 (C≈70 kN), a factor of 4.

Limitations: lowest limiting speed among all thrust types (50–70% of thrust ball bearings at the same bore), and mandatory continuous lubrication — circulating oil is recommended for applications above 50% of the grease limiting speed.

Thrust Needle Bearings: AXK and NTA Series

Thrust needle roller bearings use needle rollers — cylindrical rollers with a length-to-diameter ratio ≥ 3:1 — delivering an extremely thin cross-section (B of just 2–5 mm) while still carrying meaningful axial loads.

AXK Series — Standard Thin Section

The AXK series (or AXK...AS when supplied with washers) is the most widely used thrust needle bearing. Key feature: extremely small height B — for example, AXK 2035 has d=20, D=35, B=2 mm — only 2 mm tall yet carrying C ≈ 17 kN.

Comparison: AXK 2035 versus thrust ball bearing 51104 (d=20, D=35, B=10 mm, C≈13.4 kN) — the AXK is 5 times thinner while delivering comparable load capacity. The reason: needle rollers create line contact, yielding far greater load-carrying area per unit width than balls.

AXK vs 51xxx — Selection Guide

NTA Series — Inch Dimensions

The NTA series is the Imperial (inch-dimensioned) version of thrust needle bearings, common in equipment imported from the United States. Bore d and OD are in inches, but load ratings are comparable to metric AXK equivalents at corresponding sizes. When replacing NTA with metric AXK, verify d, D, and B dimensions carefully — some designations are not exact equivalents.

Load Capacity Comparison Across Thrust Types

The table below compares all four thrust bearing types at a common bore diameter of d = 60 mm, illustrating the clear differences in load capacity, speed, and special capabilities.

Table 3: Four Thrust Bearing Types Compared at d = 60 mm

Analysis: at d = 60 mm, the thrust spherical roller 29312 delivers C = 190 kN — 4 times the thrust ball 51212 and 1.4 times the thrust cylindrical roller 81212. However, the 29312 requires more space (D=150 mm) and has a lower limiting speed. The thrust needle AXK 6085 achieves C=72 kN within just 4 mm of height — outstanding load-to-height efficiency.

Selecting the right thrust bearing type depends on three factors: required load capacity, rotational speed, and available installation space. No single type is optimal for every application — only the right type for each specific set of operating conditions.

Speed Limits and Lubrication Requirements

Thrust bearings present greater lubrication challenges than radial bearings. The reason: rolling elements run on flat (or near-flat) surfaces — they do not self-retain lubricant the way deep-groove raceways do. Grease or oil is easily flung from the contact zone by centrifugal force.

Grease Lubrication — Most Common

EP (Extreme Pressure) grease with a lithium complex base is the standard choice for thrust bearings in most industrial applications. EP grease contains extreme-pressure additives — essential because Hertzian contact pressures at the roller-raceway interface reach 1,500–3,000 MPa.

Specific requirements:

• Thrust ball 51xxx: lithium complex grease, NLGI grade 2, operating temperature up to 120°C
• Thrust cylindrical roller 81xxx: EP lithium complex or calcium sulfonate grease, re-lubrication every 2,000–4,000 hours
• Thrust spherical roller 29xxx: EP grease or circulating oil — see bearing lubrication for details — circulating oil is mandatory above 50% of grease limiting speed
• Thrust needle AXK: light grease or oil, initial fill is often sufficient for light-load applications

Circulating Oil — Mandatory for High-Speed 29xxx

Thrust spherical roller bearings 29xxx at moderate speeds and above REQUIRE circulating oil lubrication. The reason: large barrel rollers generate significant heat, and grease cannot dissipate it fast enough. Circulating oil both lubricates and cools — oil flows through the bearing, carrying heat to an external cooler.

SKF and FAG catalogs specify clearly: 29xxx bearings with d > 120 mm and speeds above 200 rpm should use circulating oil. For d < 80 mm and speeds below 100 rpm (e.g., hydraulic presses), EP grease is adequate — but re-lubrication intervals must be strictly followed.

Table 4: Lubrication Summary by Thrust Bearing Type

Limiting Speed — the Decisive Factor

Thrust bearing limiting speeds are significantly lower than radial bearing limiting speeds at the same bore. At d = 50 mm: thrust ball bearing 51210 reaches 4,000 rpm, while radial deep groove 6210 reaches 11,000 rpm — a 2.75x difference.

The reason: in thrust bearings, rolling elements travel on flat surfaces where tangential velocity differs between the inner and outer edges. Sliding at the non-uniform velocity zone generates heat, capping the maximum rpm.

Quick selection rule by speed:

• > 3,000 rpm at d=50 mm: Thrust ball 51xxx
• 1,000–3,000 rpm at d=50 mm: Thrust cylindrical roller 81xxx or thrust needle AXK
• < 1,000 rpm at d=50 mm: Thrust spherical roller 29xxx (when heavy load is needed)

Applications: Presses, Screw Shafts, Cranes, Rotary Tables

Thrust bearings appear in every machine with significant axial loading. Four application groups dominate:

Hydraulic Presses

Hydraulic presses from 100 to 2,000 tons generate axial force on the main shaft via hydraulic cylinders. This force transmits through thrust bearings at the main shaft support.

At an automotive parts factory in Binh Duong province, a 500-ton press uses a pair of 29324 bearings (d=120, D=210, B=54, C=425 kN) on the main shaft. Operating at 15–20 rpm with 350 kN axial load and EP grease lubrication, the bearings achieved over 5 years of service (approximately 8,000 load hours) before replacement. For lighter loads (< 200 kN), thrust cylindrical roller 81xxx can substitute, saving space.

Screw Shafts and Ball Screws

Plastic extruder screws, CNC ball screws, and screw jacks all generate axial force as the shaft rotates. Thrust bearings support this force, maintaining the shaft's axial position.

Typical application: a plastic extruder screw d=80 mm uses 29316 (C=245 kN) or 81216 + 51216 (combined thrust cylindrical roller for primary load and thrust ball for secondary load). Small CNC ball screws use 51105 or equivalent AXK — light load but higher speed.

Cranes and Slewing Rings

Slewing rings on tower cranes, crawler cranes, and drilling equipment carry axial load from self-weight plus lifted load. Thrust spherical roller bearings 29xxx are the preferred solution thanks to combined load capacity and misalignment compensation.

At a construction site in Ho Chi Minh City, a Potain MC 85 tower crane uses a 29430 thrust spherical roller bearing (d=150, D=300, B=90, C=1,020 kN) at the slewing ring. Axial load is approximately 400–600 kN (boom weight plus lifted load), rotation speed 0.2–0.5 rpm, with automatic EP grease lubrication. The 2–3° misalignment tolerance of the 29xxx is especially critical because tower cranes experience continuous wind loading and oscillation that causes ongoing shaft deflection.

Machine Tool Rotary Tables

CNC rotary tables on 4-axis or 5-axis machines need precision thrust bearings to position the axis with minimal runout. Thrust ball bearings 51xxx in P5 or P4 precision class are the standard choice. Thrust needle AXK is used when height clearance is severely constrained.

At a mold-making facility in Dong Nai province, a Haas HRT-160 rotary table uses a pair of 51110 thrust ball bearings (d=50, D=70, B=14), achieving over 20,000 operating hours thanks to light loading (< 5 kN axial) and sealed grease lubrication requiring no re-greasing.

Housing Design and Washer Requirements

Thrust bearings — especially the 29xxx series — demand housing design considerations that differ substantially from radial bearings. Misunderstanding these requirements is the most common cause of premature thrust bearing failure.

Flat Seating Surface Requirements

The housing washer of a thrust bearing must seat on a precision-flat surface within the housing. The seating surface must be:

• Perpendicular to the shaft centerline within 0.01 mm per 100 mm diameter
• Surface finish Ra ≤ 0.8 μm (precision ground)
• Sufficiently hard — HB ≥ 200 for cast iron, HRC ≥ 55 for steel — to prevent plastic deformation under load

If the seating surface is not perpendicular, load distributes unevenly across the rolling elements — some elements carry 2–3 times the nominal load, causing early spalling. This is a frequent failure mode when thrust bearings are installed in custom housings that were not precision-machined.

Washers — Sold Separately or Included?

Thrust ball bearings 51xxx and 52xxx are typically sold as complete sets (shaft washer + housing washer + cage-roller assembly). However, thrust needle bearings AXK are usually sold as the needle-cage assembly only — AS (axial washer) must be purchased separately. Forgetting to order AS washers is a common purchasing error with AXK bearings.

Thrust spherical roller bearings 29xxx are always sold as complete assemblies — the shaft washer and housing washer cannot be separated because the concave spherical raceway is precision-ground as a matched set.

Minimum Axial Load — Why Thrust Bearings Must Never Run Unloaded

Thrust bearings require a continuous minimum axial load (Fa_min). Unlike radial bearings — which can run unloaded — thrust bearings running without axial load cause rolling elements to slide rather than roll, producing rapid wear and overheating.

Minimum axial load is specified in catalogs: Fa_min ≈ 0.005 × C₀ for thrust ball, 0.01 × C₀ for thrust cylindrical roller, and A × (n/1000)² for thrust spherical roller 29xxx (where A is a size-dependent coefficient, n is speed in rpm). If the actual axial load does not reach Fa_min, preload springs or mechanical design features must maintain the minimum load.

Brand Landscape and ZVL

Tier 1 — Leading Manufacturers

• SKF (Sweden): The most complete thrust bearing catalog — from 51100 through 29400, with online calculation tools. Explorer class optimizes service life. Highest market pricing.
• FAG/Schaeffler (Germany): Thrust spherical roller series 292xx/293xx/294xx excel in steel and cement applications. X-life technology extends fatigue life 15–20%.
• ZVL (Slovakia): Manufactures all four thrust bearing families at their European factory — 51xxx, 81xxx, 29xxx, and thrust needle. Technical specifications equivalent to SKF/FAG (C deviation < 3%). Competitive European pricing driven by Slovakia's manufacturing cost structure — European quality, ISO 9001 certified, not a budget alternative. Multiple cement and mining plants in Vietnam have transitioned to ZVL 29xxx with results matching SKF/FAG performance.
• Timken (USA): Primarily known for tapered roller bearings, but their 29xxx thrust spherical roller line is also high quality. Common in US/Australian mining equipment.
• NTN (Japan): ULTAGE line for thrust spherical rollers. Consistent quality, common in Japanese equipment.
• NSK (Japan): Strong in high-precision thrust ball bearings for machine tools.

Counterfeit Bearings — A Real Risk

Counterfeit SKF and FAG thrust bearings flood the Vietnamese market, particularly the high-value 29xxx series. Warning signs:

• Blurred laser markings, wrong fonts, mispositioned designations — see how to identify genuine vs counterfeit bearings
• Weight 5–15% below catalog specification
• No Certificate of Conformance (CoC) with traceable batch code
• Packaging print misalignment, poor paper quality

Purchasing from authorized distributors is the only way to guarantee authenticity — especially for 29xxx thrust spherical roller bearings used in critical equipment.