Bearing Types: Ball, Roller and Plain Compared

Every bearing selection starts with the same four questions, and only one of them is about the catalogue: which direction does the load act, how fast does the shaft turn, how much misalignment must be tolerated, and how much space is there. Get those four right and the family almost chooses itself. Get them wrong and no amount of precision, brand or price will rescue the application.

This guide is the map we use at the quotation desk. It compares the ball, roller, thrust and plain families against the four questions, sets out the numeric limits that separate them, and finishes with a worked example on a 25 mm shaft where four different families are defensible answers with four different consequences. Along the way it links the fourteen product families in our catalogue so you can move from principle to an orderable part number without leaving the page.

The load-direction test comes first

Classify the load before looking at any catalogue. Three cases matter, and most real applications are the third one.

• Purely radial - the force is perpendicular to the shaft axis. Deep groove ball, cylindrical roller and needle roller bearings are all built for this.
• Purely axial - the force acts along the shaft axis. Thrust ball and thrust roller bearings exist specifically for this, and no radial bearing should be asked to carry it alone.
• Combined radial and axial - by far the most common. The decision then turns on the ratio of axial to radial load, because that ratio decides whether a radial bearing can absorb the thrust itself or whether you need a design with a real contact angle.

Write the ratio down as a number. A bearing that carries an axial load equal to 10 % of its radial load is doing something completely different from one carrying 80 %, even if the radial load and the bore are identical.

The families at a glance

The table below is the compressed version of this article. Every column is a constraint you cannot trade away, and the notes column links to the family page in our catalogue.

Ball or roller: the contact-geometry decision

Between two bearings of the same boundary dimensions, the ball version and the roller version differ by roughly a factor of two to three in load rating. That difference comes entirely from the shape of the contact.

Point contact versus line contact

A ball touches the raceway over a small elliptical area. Contact stress is high for a given load, but friction is low and the rolling motion stays clean at high speed. A roller touches the raceway along a line, which spreads the same load over far more material and lowers the stress. The price is that a line contact is sensitive to any skewing, so it needs accurate alignment and generally cannot match the speed ceiling of a ball bearing.

When roller capacity is wasted

A common mistake is to specify a roller bearing "for safety" on a lightly loaded shaft. The result is a heavier, more expensive, slower bearing whose extra capacity is never used, and whose higher friction torque shows up as heat. Roller bearings earn their place when the radial load is genuinely large, when the shaft must be stiff, or when the envelope is radially tight - which is exactly the case for needle roller bearings, where the roller replaces the inner ring entirely.

Radial or thrust: matching the load direction

Radial bearings can carry some axial load, but the amount depends on the contact angle. A deep groove ball bearing can absorb an axial load of roughly a quarter to a third of its radial rating on a sustained basis. A cylindrical roller bearing in the standard NU form cannot carry axial load at all, while the NJ and NUP forms introduce ribs that take thrust in one or both directions - a distinction that has caused more than one returned shipment.

Combined loads and the equivalent load P

When both components act together, they are not added directly. The standard method converts them to a single equivalent dynamic load using factors that depend on the contact angle and the load ratio, and that equivalent load is what goes into the life equation. This is why the axial component cannot be ignored on the grounds that "the radial load is the bigger one" - a modest thrust component can raise the equivalent load substantially in a bearing with a small contact angle.

Speed: the DN factor and its limits

Speed limits are usually expressed as a product of bore diameter in millimetres and rotational speed in revolutions per minute. A 25 mm bore turning at 12,000 rpm is running at DN 300,000, and that number is directly comparable across sizes, which is why it is the practical currency of speed selection.

Reference speed versus limiting speed

Catalogues publish two numbers. The thermal reference speed is the speed at which a defined reference condition produces an equilibrium temperature rise - it is a comparison figure for normal lubrication and load. The limiting speed is the mechanical or thermal ceiling for a specific configuration, and it moves with closure type, cage material, lubrication method and load. A shielded bearing and a sealed bearing of the same size can differ by a factor of two in permissible speed.

• Open bearings with oil lubrication reach the highest speeds and suit jet or mist systems.
• Shielded 2Z bearings run almost as fast as open bearings because the shield does not touch the rings.
• Contact-sealed 2RS bearings lose the most speed but need no relubrication and resist contamination.
• Hybrid designs with ceramic balls run cooler and permit higher speed, and they also break the electrical circuit path that damages motor bearings.

Misalignment: when the shaft is not straight

Misalignment between shaft and housing is unavoidable in fabricated frames, long shafts and anything that deflects under load. Two families solve it by geometry: self-aligning ball bearings tolerate roughly two to three degrees, and spherical roller bearings carry heavy loads while tolerating one to two and a half degrees. Everything else tolerates very little, and the failure mode when the limit is exceeded is edge loading - the roller or ball contacts only part of the raceway, stress concentrates and the surface fatigues early.

The practical misalignment budget

Add up the contributions before choosing a rigid family: housing bore out-of-roundness and squareness, shaft deflection under load, thermal growth differences, and settling of a fabricated frame. If the total exceeds the published permissible misalignment, either choose a self-aligning family or correct the geometry. No amount of extra load capacity compensates for misalignment.

Rigidity, preload and running accuracy

Some applications are limited by deflection rather than by fatigue life. Machine tool spindles, printing cylinders and metrology stages care about how far the shaft moves under load, not how many hours the bearing lasts. There, the relevant properties are stiffness and running accuracy, and the selection looks different: a roller family for stiffness, a paired angular contact set with controlled preload for accuracy, and precision tolerance classes defined in ISO 492.

Why preload is a system property

Preload is not a bearing attribute you order; it is a condition you create during assembly, and it depends on the housing, the spacers, the locknut torque and the thermal environment. Specifying a preloaded pair without controlling those variables produces a preload you cannot reproduce. Our guide to preload in bearing sets covers the mechanics.

Space and weight: the envelope trade

Boundary dimensions are fixed by ISO 15 for radial bearings and ISO 199 for thrust bearings, so a given bore comes with a limited menu of outside diameters and widths. The three diameter series - extra-light, light and medium - are the practical lever. For a 25 mm bore the light series gives 52 mm outside diameter and the medium series 62 mm, with a corresponding jump in load rating and a drop in speed ceiling. When the radial space is tighter than any standard series allows, the answer is usually a needle roller bearing or a drawn cup unit, both of which pack the rolling elements directly against a hardened shaft.

Environment decides closure and material

Once the geometry is settled, the environment decides the closure, the lubricant and sometimes the material. This is where an application that is technically perfect on paper fails in service.

Mounting and maintenance reality

The best bearing on paper is the one that can be fitted correctly with the tools and skills available. Two families dominate because they are forgiving: deep groove ball bearings, which tolerate reasonable handling, and ball bearing units, which are self-aligning, pre-lubricated, sealed and simply bolted down. A split spherical roller housing costs more but can be serviced without dismantling the shaft line, which in a conveyor or a pump station is often the decisive factor.

Interference fits change the clearance

A rotating inner ring is normally fitted with interference, and that fit stretches the ring and consumes internal clearance. A bearing that measures correctly on the bench can be running with almost no clearance at temperature. This is why the clearance class and the fit have to be chosen together, following the ISO 286 fit tables rather than habit.

A step-by-step selection sequence

1. Draw the load case. Direction, magnitude, whether the load rotates with the inner or the outer ring, and the duty cycle including shock and stall.
2. Compute the axial-to-radial ratio. It decides the family more often than the total load does.
3. Fix the envelope. Available bore, outside diameter and width, plus the diameter and width series you can accept.
4. Check the speed. Compare the required speed against the reference and limiting speeds for the closure you intend to use.
5. Quantify misalignment. Sum the geometric and deflection contributions and compare with the family limit.
6. Select the family from the ratio, envelope, speed and misalignment results - then confirm the capacity using the equivalent load and the required life.
7. Choose clearance and tolerance class together with the fit, using the temperature differential as the governing input.
8. Choose closure and lubrication from the environment table above, and decide whether relubrication is possible in service.
9. Confirm availability and cost against real stock rather than a catalogue promise.

Worked example: a 25 mm shaft, 4 kN radial and 1 kN axial

Take a gearbox input shaft: 25 mm bore, 4 kN radial and 1 kN axial, 3,000 rpm, moderate misalignment from a fabricated frame, space for a 52 mm outside diameter, expected life 20,000 hours. The axial-to-radial ratio is 0.25, which sits right on the boundary between a deep groove solution and a pair with a real contact angle. Four families could be argued, and the table shows what each choice commits you to.

On this duty, the deciding factor is not the load - all four options carry it - but the misalignment and the continuity of the load. If the 4 kN is a steady running load in a gearbox with a machined housing, the tapered roller pair is the conventional and correct answer. If misalignment is real and cannot be corrected, the spherical roller family is the one that will survive, even though it is not the cheapest or the fastest.

Cost of ownership: why the cheapest bearing is rarely the cheapest

Purchase price is the smallest number in the calculation for most industrial duty. The costs that matter are the ones that appear later.

• Premature failure caused by a clearance class chosen to match the last order rather than the temperature.
• Unplanned downtime, which in a continuous process line is measured in production hours, not parts.
• Repeat labour when a rigid family is fitted into a misaligned frame for the second time.
• Energy from friction torque in an oversized or over-specified bearing running below its design load.
• Inventory complexity when every position uses a different non-interchangeable variant.

This is why we publish boundary dimensions alongside every listing: a cross-reference that shares the envelope is a genuine alternative, and one that only shares the bore is not. The interchange lookup shows equivalents across brands, the selector filters the whole catalogue by bore, outside diameter, width and brand, and the bearing glossary defines the terminology used throughout this guide. Our brand directory covers the eleven brands we stock, from SKF, FAG and TIMKEN through to our own house brand, and the deep groove selection guide applies this method in full detail to the most common family in the world.