A thrust bearing exists for one reason - to carry a load that runs along the shaft axis instead of across it. Every radial bearing depends on a contact angle between the rolling element and the raceway. When the load pushes straight down the axis, the balls or rollers are forced against a shoulder, friction climbs, and the cage is usually the first component to fail.
This guide explains what changes when the load turns vertical: how the thrust families differ, how the designation encodes the series and the bore, why roller thrust bearings need a minimum axial load, and how washer seating, flatness and lubrication decide whether the bearing ever reaches its rating life. It is written for engineers sizing a vertical pump, a crane slew drive or a gearbox reaction point.
- Thrust bearings are separate from the radial system. They are dimensioned by shaft washer bore and housing washer outside diameter under ISO 104 and ISO 199, not by the radial ISO 15 envelope.
- Balls for speed, rollers for load. A cylindrical roller thrust bearing of the same envelope carries several times the axial load of a thrust ball bearing, at a lower speed ceiling.
- Roller thrust bearings need a minimum load. Below the load floor the rollers skid, the film breaks down and the raceway smears; a light spring preload is the usual fix.
- Two-way types take thrust in both directions. The 522, 523 and 524 series accept axial load wherever it points, which matters on reversible and vertical shafts.
Why a radial bearing cannot take a pure axial load
A deep groove ball bearing will carry some thrust while it is also carrying radial load, because the contact angle rises from roughly 0° at no load to a useful value under combined load. The permissible axial force is set by a factor Y that depends on the ratio of axial to radial load, so the capacity exists only while a radial component is present. A pure axial load with no radial support sits outside that design envelope.
A thrust bearing solves the problem by turning the geometry through 90°. The washers sit square to the shaft, the rolling elements run on flat or grooved raceways, and the whole contact area is loaded in the direction of the applied force. The design consequence is simple: thrust bearings are not interchangeable with radial bearings of the same bore. A 51105 and a 6205 both take a 25 mm shaft and share nothing else - not the outside diameter, not the height, not the mounting shoulders.
The thrust families and what each one is for
Four families cover almost every industrial axial position, and the thrust bearing range groups them by exactly these differences: the shape of the rolling element, the number of directions they accept and the misalignment they tolerate.
Thrust ball bearings: 511 to 514 and the two-way 522 to 524
Single-direction thrust ball bearings use a shaft washer, a housing washer and a ball-and-cage assembly. The first digit group sets the height and diameter series: 511 is the flattest and lowest-capacity series, and 512, 513 and 514 step up in height and diameter for the same bore, carrying progressively more axial load at progressively lower speed. The two-way 522, 523 and 524 designs add a central shaft washer and a second housing washer so the bearing takes axial load in both directions - the usual choice on a reversing or vertically floating shaft.
Cylindrical roller thrust: 811, 812, 893 and 894
Replacing balls with short cylindrical rollers raises axial capacity several times for the same envelope, because the contact becomes a line rather than a point. The 811 and 812 series are the standard single-row designs; 893 and 894 denote the heavier and the double-row arrangements. The trade for that capacity is speed and a hard minimum-load requirement, both covered below.
Spherical roller thrust: 292, 293 and 294
Spherical roller thrust bearings use barrel-shaped rollers running on a spherical housing washer raceway. That geometry lets the bearing accept shaft deflection and misalignment of a degree or two while still carrying very high axial load plus a modest radial component, which is why it is the default for large vertical machines and heavily loaded gearbox reaction points. The design is covered inside the self-aligning roller family as well as the thrust group.
Tapered thrust arrangements
Tapered roller bearings can be arranged in opposed and face-to-face pairs to take thrust alongside radial load, and a matched two-row tapered set is often used where the axial load is moderate but the radial load is high and the housing length is constrained. Our tapered roller range lists the pairs and the assembled widths, because a thrust arrangement is only valid when the cone, cup and spacing are specified together.
| Family | Series | Direction | Misalignment | Axial capacity | Speed |
|---|---|---|---|---|---|
| Thrust ball | 511 / 512 / 513 / 514 | One way | Very low | Low | High |
| Thrust ball | 522 / 523 / 524 | Two way | Very low | Low to medium | High |
| Cylindrical roller thrust | 811 / 812 / 893 / 894 | One way | Very low | High | Low |
| Spherical roller thrust | 292 / 293 / 294 | One way, small radial | 1° to 2° | Very high | Low to medium |
| Tapered thrust pair | matched sets | One or two way | Low | Medium to high | Medium |
Reading a thrust bearing designation
The designation carries the type and the size in the same way a radial bearing does, but the numbers mean something different. In a 51208, the 512 is the type and series - single-direction thrust ball in the 2 height series - and the 08 is the bore code, so the bore is 8 × 5 = 40 mm. In an 81210, the 812 is a single-row cylindrical roller thrust bearing and the 10 gives a 50 mm bore.
Because the shaft washer is the locating element, the bore is the first dimension to confirm. The outside diameter and the height that follow are set by ISO 104 for thrust ball bearings and ISO 199 for thrust roller bearings, and they do not follow the radial diameter series. An 81207 does not share a housing with a 6207 no matter how similar the trailing digits look, and the height difference between a 51108 and a 51408 with the same 40 mm bore is large enough to change the whole enclosure. The thrust dimension guide works through the tables bore by bore.
Minimum axial load, skidding and the load floor
Roller thrust bearings rely on a lubricant film built by rolling motion. When the axial load is too small, the rollers do not roll - they slide, the film collapses locally and the raceway smears. This is the failure mode known as skidding, and it looks like a polished or bruised track rather than a spalled one, which is why it is often misread as a lubrication problem. The required minimum load is quoted as a fraction of the dynamic rating, and for cylindrical and spherical roller thrust bearings it is far higher than for thrust ball bearings.
The practical fix on a lightly loaded vertical shaft is to add a spring preload that holds the rollers against the raceway even when the working load falls away. The preload guide covers how to size that spring so it stays below the working load but above the skidding floor.
| Series group | Rolling element | Minimum load guide | Typical fix |
|---|---|---|---|
| 511 / 512 / 513 / 514 | Ball | Very low, often below 0.01 C | No preload normally required |
| 811 / 812 | Cylindrical roller | Approx. 0.01 to 0.02 C | Spring preload on the housing washer |
| 893 / 894 | Cylindrical roller, heavy | Approx. 0.02 C | Spring preload plus axial load check |
| 292 / 293 / 294 | Spherical roller | Approx. 0.01 C plus radial component | Preload spring and radial load maintained |
Washers, seat flatness and perpendicularity
A thrust bearing has almost no capacity to absorb a badly machined seat. The load path is short and stiff, so any error in the housing or the shaft shoulder is transferred straight into the raceway as an edge load.
Shaft and housing washer pairing
The shaft washer has a ground bore that is a close fit on the shaft, and the housing washer has a ground outside diameter that is a close fit in the housing. They are not interchangeable without consequence: fitting a plain washer on the rotating side turns the shaft into a raceway that is too soft and insufficiently accurate, and the bearing fails early. Always confirm which washer is the tight, rotating one before assembly - the SKF and ANDGE listings both record the washer types separately for this reason.
Seat flatness and perpendicularity
The supporting face must be flat and square to the axis. For a thrust ball bearing a perpendicularity error of 0.05 mm across the seat is already significant, and for high-speed positions the tolerance is tighter still.
Speed limits and oil lubrication
Thrust bearings have a lower speed ceiling than radial bearings of comparable size, and the ceiling falls sharply as the element changes from ball to roller to spherical roller. The reason is centrifugal force: at speed, the rolling elements are thrown outward against the housing washer, and the resulting friction cannot be carried away by grease alone.
For sustained high-speed thrust duty, oil is the answer. Oil circulation or an oil bath keeps the contact supplied and removes heat, and the oil lubrication guide covers the flow rates and the viscosity selection from ISO 3448.
Applications: cranes, extruders, vertical pumps and gearbox reaction
The family choice follows the application. A crane slewing system, a vertical pump and an extruder shaft each load the bearing differently, and the axial load path usually sits opposite a radial support bearing that does the locating.
| Application | Typical family | Why this family |
|---|---|---|
| Crane slew and hook block | Spherical roller thrust 292 / 293 / 294 | Very high axial load with structural deflection |
| Plastic and rubber extruder thrust point | Cylindrical roller thrust 811 / 812 / 893 / 894 | Steady high axial load at moderate speed |
| Vertical pump and mixer | Thrust ball 511 to 514, or two-way 522 to 524 | Moderate axial load, higher speed, compact height |
| Gearbox reaction bearing | Tapered pair or thrust ball | Combined load with axial reaction from the gear mesh |
| Reversible vertical shaft | Two-way thrust ball 522 / 523 / 524 | Load changes direction without a second bearing |
In every case the thrust bearing takes the axial load and a separate radial bearing - often a cylindrical roller bearing - takes the radial load.
How to choose a thrust bearing
Work through the sequence in order; each step removes families that cannot do the job.
- Confirm the load is truly axial. Separate the axial and radial components and check whether a radial support bearing already exists. If not, a combined design may be needed.
- Fix the direction. One-way or two-way, based on whether the axial load changes sign during operation.
- Size for load first, then speed. Choose ball for speed, cylindrical roller for pure axial capacity and spherical roller thrust where deflection or misalignment is present.
- Check the minimum load. Compare the lightest working load against the skidding floor and add a preload spring if the margin is thin.
- Set the seat geometry. Specify washer fit, seat flatness and perpendicularity before the housing is machined, using the shaft and housing fits guide for the tolerance bands.
- Choose the lubrication. Grease for low speed, oil circulation for sustained speed, with the viscosity taken from the operating temperature.
If you already know the bore and the axial load, the bearing selector filters the whole catalogue by dimension and type, and the glossary defines the thrust terms used above. For the wider picture of how thrust and radial load share a shaft, combined load arrangements and the bearing type selection guide both pick up where this one stops.
Common mistakes
- Using a thrust ball bearing where misalignment exists. A thrust ball bearing has almost no tilting capacity; putting one under a deflecting shaft edge-loads the raceway and destroys the cage. Use a spherical roller thrust bearing instead.
- Ignoring the minimum load. A lightly loaded roller thrust bearing skids rather than rolls, and the damage is often mistaken for contamination.
- Fitting the wrong washer on the rotating side. Swapping the shaft and housing washers removes the hardened, ground raceway from the load path and shortens life dramatically.
- Machining the seat without a squareness callout. A seat that is out of square by a few hundredths of a millimetre is enough to wipe the lubricant from one side of the track.
- Run the bearing on grease above its speed limit. Beyond the grease speed ceiling the heat cannot leave the contact, and failure follows quickly.
Frequently asked questions
Can a thrust bearing carry radial load?
Thrust ball bearings should not take radial load at all. A spherical roller thrust bearing can accept a limited radial component, typically a small fraction of the axial load, which is why it is used where some deflection is unavoidable. For any real radial duty, add a separate radial bearing.
What does the minimum load rule mean for a roller thrust bearing?
A cylindrical or spherical roller thrust bearing needs enough axial load to keep the rollers rolling rather than sliding. If the load falls below roughly one to two percent of the dynamic rating, the lubricant film breaks down and the raceway smears. A spring preload is the normal way to keep the rollers loaded.
What is the difference between a 511 and a 514 thrust ball bearing?
Both are single-direction thrust ball bearings and the trailing digits give the bore. The 511 series is the flattest and lightest, and 512, 513 and 514 increase the height and diameter for the same bore, raising axial capacity while lowering the speed ceiling. A wider series needs a taller and larger seat.
When should I use a two-way thrust bearing?
When the axial load changes direction during operation, or when a vertical shaft can float both ways. The 522, 523 and 524 series place a shaft washer between two housing washers so each direction is carried by its own raceway. They are taller than the one-way type and need a seat on both sides.
Why does a thrust bearing run hotter than a radial bearing?
The contact geometry concentrates load and the elements are continuously thrown against the housing washer by centrifugal force, so frictional heat is higher and harder to remove. That is why thrust bearings have lower speed limits than radial bearings of the same size and why sustained duty usually needs oil rather than grease.
How do I confirm which thrust washer rotates?
The shaft washer has the close bore fit and always rotates with the shaft; the housing washer has the close outside diameter fit and stays with the housing. Check the catalogue listing, because the two are supplied as a matched set and fitting the plain washer on the rotating side removes the hardened raceway from the load path.