Combined Loads: Radial and Axial Together

Every rolling bearing exists to carry a load across a rotating joint, and the first question any experienced selector asks is not which brand or which size but which direction the load comes from. Radial load pushes across the shaft; axial (thrust) load pushes along it; combined load does both at once. Bearing families differ enormously in how they treat these directions, and most gross selection mistakes - a ball bearing where a roller belonged, a thrust washer asked to carry radial load, a deep groove ball bearing preloaded by thermal growth - trace back to skipping this step.

This article treats the load direction as the ranking decision it is. It defines the three cases, explains how each family turns a thrust force into contact stress, shows why orientation and duty change the answer, and works two short cases - a conveyor and a spindle - that reach opposite conclusions from the same discipline.

Load direction is the first decision

A radial load acts perpendicular to the shaft axis: the weight of a rotor, the belt pull on a pulley, the reaction to a gear's tangential force. A pure axial load acts along the axis: the screw force in a hoist, the weight of a vertical rotor, the helical component of gear mesh. Combined load is the normal case in gearboxes, wheels and machine tool spindles, where both act at once. The ratio of axial to radial load is what separates the families, because the internal contact geometry decides how gracefully each design converts axial force into rolling contact stress.

It is worth pausing on why the direction is so decisive. A rolling contact can only push along its own normal; a load that tries to push the rolling element sideways along the raceway demands a different internal geometry to keep the contact intact. Families that are excellent at one direction are often deliberately barred from the other, and that is a design intent, not a defect. The full comparison of how the families weigh up is set out in bearing type selection.

Radial-only, thrust-only and combined geometries

Three structural ideas cover the whole catalogue. A radial-only bearing, such as a plain cylindrical roller, has no way to react axial force and either slides or separates when thrust is applied. A thrust-only bearing, such as a cylindrical roller thrust bearing or a thrust ball bearing, is built to take axial load but offers nothing useful across the axis and must be given a separate radial partner. Between them sit the combined-load families - angular contact, tapered roller, spherical roller and, within limits, deep groove ball - whose geometry lets a single set of rolling elements feel both directions at once.

Point contact versus line contact under thrust

Under a thrust load a ball bearing's contact patch stays small, so the local stress climbs quickly and the practical thrust ceiling stays modest. A roller made for thrust spreads the load along a line, carries more axial load for its size, and is more sensitive to even slight misalignment because the contact no longer tolerates a cocked position.

How a thrust force becomes contact stress

When axial load is applied to a combined-duty bearing, the ball or roller slides up the raceway groove until the contact normal can balance the force. The bearing carries thrust by moving along that load line, not by any separate mechanism - which is exactly why the contact angle matters. The larger the angle between the load line and the radial plane, the more of the contact normal points along the axis and the more thrust the row can carry before contact stress becomes the limit.

This is the mechanical reason a steeper design trades radial reserve for axial capacity. It is also why a bearing can suddenly lose its thrust capability: apply load in the reverse direction and the rolling element is asked to move off the end of the groove, where there is no surface to support it.

Where the contact normal points

The load line is the direction along which the rolling element transmits force, and the useful part of it under thrust is the axial component. A family with a steeper load line converts a larger share of the applied force into a supported contact. A radial-only roller has a load line lying entirely in the radial plane, so an axial force finds no component to push against and the roller simply slides along the raceway.

Families that carry combined load

The last row is the one most often misapplied: a thrust-only bearing must never be asked to hold the shaft across the axis.

The axial-to-radial ratio is a fast first screen; only after that does the catalogue and the ISO 281 life check come into play.

Read the ratio as a threshold, not a precise figure. A traction motor turning 1,500 rpm with 8 kN radial and 0.5 kN axial sits at 0.06, where radial geometry is enough. A pump at 3,600 rpm with 5 kN radial and 2.5 kN axial sits at 0.5, right at the edge where the choice changes. A bevel pinion at 2,500 rpm with 6 kN radial and 4 kN axial sits at 0.67, where the geometry must take real thrust.

Why orientation and duty flip the answer

Two practical points close the loop. First, orientation matters: a horizontal motor and a vertical pump with the same rotor impose completely different axial shares, so copying a bearing arrangement between machines without checking orientation is a known error. Second, duty cycles matter: a crane wheel carries a huge radial load when lifting and almost none when idle, while a fan sees its worst radial load at start-up with a dirty rotor. Sizing for the steady case and then checking the peaks against the static rating covers both without overbuilding the machine.

When the axial share is small

If the thrust is a small fraction of the radial load and does not reverse harshly, a general-purpose radial bearing may absorb it with no geometry change. The moment the axial share becomes a real fraction of the load - or the machine relies on the bearing to locate the shaft - the family question reopens and a combined-duty design is the honest answer.

Locating and floating: the second decision

Once the family is chosen, decide which position locates the shaft and which is free to slide. A common and reliable pattern pairs a locating bearing that takes the axial load with a floating bearing that carries only radial load and lets the shaft grow with heat. Mixing this up - two bearings both fighting to locate, or a thrust-bearing position with nothing to hold it centred - produces the classic thermal preload failure described in the note on a thrust bearing carrying load the wrong way.

Which position is free to slide

The floating position is not a decoration; it is what stops thermal growth from becoming an axial load. A plain NU cylindrical roller or an N-type design floats by construction, and it should carry radial load only. If the machine instead uses two rigidly located bearings, the shaft's growth has nowhere to go and the resulting axial force is absorbed by whatever bearing is weakest in thrust - usually not the one the designer intended.

Two worked cases: conveyor and spindle

Consider a belt conveyor head pulley. Belt tension sums to a heavy radial load, the drive adds a small torque reaction, and the pulley carries its own weight. Axial load is nearly zero, alignment across a long frame is imperfect, and dust is present. The family answer is a self-aligning or spherical roller position in a housed unit: radial capacity, tilt tolerance and sealing in one package.

Now take a machine tool spindle. Cutting gives a light radial load, machining forces give significant bidirectional thrust, the shaft must be extremely stiff, and alignment is near-perfect. There the answer is a paired angular contact set with controlled preload. Same discipline, opposite conclusions - which is exactly why the load-direction step comes first.

Reading the load direction first narrows the family; locating, speed and environment then finish the selection.

A load-first selection checklist

• Direction. Is the dominant load radial, axial or genuinely combined?
• Magnitude ratio. What is the axial-to-radial ratio, and does it reverse?
• Orientation. Does the machine's mounting turn a radial load into an axial one?
• Duty. What is the peak load at start-up or a jam, and the static rating it needs?
• Locating. Which position holds the shaft, and which is free to float?
• Partner. If the family is thrust-only, what carries the radial load?
• Reversals. Does the load change direction in service, and does the chosen set carry both ways?

Load ratings for every family in this catalogue can be filtered in the bearing selector, and the life arithmetic behind the peak checks is set out in the note on L10 life and load rating. The NSK and equivalent catalogues tabulate the same ratios, so a quick comparison across makers is straightforward once the direction is settled. Match the family to the load direction before opening any data page, and most of the remaining decisions become small ones.