Radial vs axial loads: what is the difference?

What is a radial load?

A radial load is a force acting at right angles to the shaft axis, along a radius. Gravity on a horizontal shaft is the simplest example: the shaft presses down onto the bottom of the bearing bore. Belt tension, chain pull, the separating force between spur gears, out-of-balance forces and the reaction at a pivot pin are all radial.

In a plain bearing the radial load is carried by the cylindrical running surface. The shaft does not touch the whole bore; it rests on a loaded arc that, for design purposes, is treated as the projected area bore diameter × length. That is why the specific load on a sleeve bushing is written p = F / (d × L): the same force on a longer or larger-bore sleeve produces a lower pressure.

What is an axial load?

An axial load, also called thrust, acts parallel to the shaft axis. It tries to push the shaft through the bearing rather than across it. Sources include the reaction of a pump impeller or a propeller, the thrust component of helical or worm gears, the clamping force along a screw or spindle, and the weight of a vertical shaft.

A cylindrical bore cannot carry an axial load; the shaft would simply slide through it. Thrust is carried by a flat face at right angles to the axis: a thrust washer between the shaft shoulder and the housing, the flange of a flanged bushing, or the end face of a sleeve with a shoulder on the shaft. The specific load is force divided by the bearing face area, p = F / A, where A is the annular area of the washer or flange minus grooves.

Examples of radial and axial loads

The same machine usually contains both. An excavator boom pin carries the boom weight and the digging reaction radially, while side loads on the bucket produce axial thrust on the pin ends. A centrifugal pump shaft sees radial load from the impeller’s hydraulic imbalance and axial load from the pressure difference across the impeller. A vehicle wheel hub carries the vehicle weight radially and the cornering force axially.

Combined loads and how they are carried

Most joints see both loads at once. Three arrangements carry them. A plain sleeve with a separate thrust washer keeps each load on its own surface and lets each be sized and replaced independently; it is the usual choice for heavy machinery. A flanged bushing combines the two in one part: the sleeve carries the radial load and the flange carries a light-to-moderate axial load while also locating the bushing in the housing. A pair of bushings with shoulders on the shaft is the third arrangement, common in small mechanisms.

The flange of a flanged bushing is not a full thrust bearing. It is thin, it is usually unlubricated on its back face and its area is small, so it suits locating loads and occasional thrust rather than continuous axial load. When the axial component is significant, specify a thrust washer.

What each load does to the bearing

Radial load determines the wall pressure on the loaded arc, the hydrodynamic film in an oil-lubricated bearing and, through the running clearance, how far the shaft moves off centre. Too much radial load for the material produces plastic flow of the bearing layer (bronze and bimetal), cold flow of the polymer (PTFE, POM) or fatigue cracking under cyclic load. Misalignment turns a distributed radial load into an edge load at one end of the bore, which is the most common cause of early bushing failure in pivots.

Axial load determines the face pressure on the thrust surface. Because thrust faces are small and their sliding speed rises with radius, they run hotter than sleeves at the same pressure; the PV limit, not the load limit, is usually what is exceeded first. Lubricant must be led onto the face by radial grooves, since it cannot enter from the side as it does in a sleeve.

Specific load p and sliding speed v are checked against the material’s p max, v max and (p·v) max separately. Reaching one limit does not leave room for the others.

Factors that change how the load is distributed

The nominal calculation assumes the load spreads evenly over the projected area. Several things make it worse in practice.

• Misalignment between shaft and housing concentrates the radial load at one end of the bore (edge loading). Self-aligning constructions, shorter bushings or a spherical seat reduce it.
• Length-to-diameter ratio: a long, slender sleeve (L/d above about 1.5) is more sensitive to misalignment; a short one (L/d below 0.5) carries less load and lets more lubricant escape.
• Running clearance: too little clearance starves the film and seizes the bearing; too much lets the shaft move off centre and hammer under reversing load.
• Housing stiffness: a thin or slotted housing lets the bushing deform under load, so the bore does not stay round.
• Shock and reversal: cyclic and reversing loads fatigue the bearing layer at a lower pressure than a steady load; the dynamic load limit applies, not the static one.
• Thermal expansion: at temperature, the shaft, bushing and housing grow at different rates and the clearance changes; the clearance is set for the operating temperature, not the assembly temperature.

How to calculate the specific load

For a sleeve bushing of bore d and length L carrying a radial force F, the specific load is p = F / (d × L). For a thrust washer with inside diameter d₁ and outside diameter d₂ carrying an axial force F, the face area is A = π (d₂² − d₁²) / 4 minus any grooves, and p = F / A. Sliding speed for a rotating sleeve is v = π d n / 60 (d in metres, n in rev/min); for a washer, use the mean diameter (d₁ + d₂) / 2.

Worked example: a 50 mm bore × 60 mm long bronze sleeve carrying 60 kN radially runs at p = 60 000 / (50 × 60) = 20 N/mm². At 30 rev/min the sliding speed is π × 0.050 × 30 / 60 = 0.079 m/s, so p·v = 1.6 N/mm²·m/s. Both values are then checked against the limits published for the chosen material.

Our bearing load calculator runs these numbers for sleeves and washers. Where the duty is close to a limit, send it with the enquiry and we will confirm the material against it.

Choosing the bearing element for the load

For a mainly radial load, specify a sleeve bushing and size it by p and p·v; lubricated bronze, bimetal or hardened steel for heavy, slow duty; PTFE composite, graphite bronze or sintered bronze where the joint cannot be greased. For a mainly axial load, specify a thrust washer in the same materials and lead lubricant onto the face with radial grooves. For a combined load, use a sleeve plus washer where the thrust is continuous, or a flanged bushing where it is a locating or occasional load.

Whatever the arrangement, put the load direction, magnitude and duty cycle on the enquiry. A drawing shows the geometry; only the loads tell us which material will survive in it.