What is overhung load?

Overhung load is the radial force from a pulley, sprocket, pinion, gear or similar drive member mounted outside the nearest shaft bearing. It is not just the force on the drive member. Because that force acts through a lever arm, it bends the shaft and can make the nearest bearing carry more radial load than the force that was applied at the pulley or gear.

That is why overhung load belongs in the same workflow as shaft diameter, bearing load and bearing life. First estimate the radial shaft load from torque or from the known belt, chain or gear force. Then distribute that load to the two bearing reaction centers. Finally use the peak bending moment for shaft sizing and the bearing reactions for catalog load and life checks.

Overhung load from torque

A common first-pass estimate for the radial load from a shaft-mounted sprocket, pulley, pinion or gear is:

F = 2 T K / D

where F is the radial shaft load, T is shaft torque, D is the pitch diameter of the mounted member, and K is a drive connection factor. In SI units, use T in N*m and D in metres to get newtons. In inch units, use torque and pitch diameter in consistent force-length units.

The pitch diameter matters. Use the pitch diameter of a sprocket, timing pulley, sheave or gear, not the outside diameter. A larger pitch diameter reduces the radial load for the same torque because the same torque is carried at a larger radius. This is why replacing a very small sprocket or sheave with a larger one can reduce shaft and bearing load even though the transmitted torque is unchanged.

If you already know the radial force from a belt tension calculation, chain pull calculation or gear mesh force calculation, use that known force directly instead of recalculating from torque. The overhung load calculator supports both approaches.

Why the nearest bearing can carry more than the load

Model the shaft as two bearing reaction centers: bearing A at x = 0 and bearing B at x = L. Put the radial load F at position x, measured from bearing A. Static equilibrium gives:

RB = F x / L   RA = F - RB

When the load lies between the bearings, both reactions are usually positive and the bearings share the force. When the load is outside the span, one reaction becomes negative. That negative sign is not an error. It means the far bearing is reacting in the opposite radial direction to balance the moment from the overhung load.

For the common right-side overhung case, put the load e past bearing B, so x = L + e. The equations become:

RB = F(1 + e/L)   RA = -F e/L

If the overhang is 30% of the bearing span, bearing B carries about 1.30F and bearing A carries 0.30F in the opposite direction. If the overhang equals the whole bearing span, the near bearing carries 2F. This is the leverage problem people often miss when they compare only the applied belt, chain or gear force to a bearing rating.

Peak bending moment

Bearing reactions tell you the bearing load, but the shaft also needs a bending check. For a right-side overhung load, the peak bending moment is at bearing B:

Mmax = F e

For a left-side overhung load, the peak is at bearing A. For a load between bearings, the peak is at the load point and equals either reaction times its lever arm. That bending moment is the value to carry into the shaft diameter calculator or a detailed shaft stress model. Torque alone is not enough when a pulley, sprocket or gear is hanging outboard of the bearing.

Worked example

A shaft transmits 200 N*m through a 120 mm pitch-diameter pinion. Use K = 1.25 for a pinion or gear drive:

F = 2 x 200 x 1.25 / 0.120 = 4167 N

The bearings are 250 mm apart, and the pinion load center is 320 mm from bearing A. That is 70 mm past bearing B, so the reactions are:

RB = 4167 x 320 / 250 = 5333 N

RA = 4167 - 5333 = -1167 N

The applied radial load is 4167 N, but bearing B carries 5333 N. The load amplification is 5333 / 4167 = 1.28. The peak bending moment at bearing B is:

Mmax = 4167 x 0.070 = 292 N*m

That example shows the important design point: the bearing and shaft do not feel only the drive force. They feel the drive force multiplied by the geometry of the shaft layout.

What to check after the reactions

Once you have RA and RB, use the absolute value of each signed reaction as the radial load at that bearing. Then continue with the checks that match the hardware:

• Shaft strength. Use the peak bending moment and transmitted torque in the shaft diameter calculator.
• Shaft stiffness. If deflection at the gear, seal or coupling matters, check it with the shaft deflection calculator or a full shaft model.
• Bearing equivalent load. If the bearing also sees axial thrust, combine radial and axial components with catalog X and Y factors in the bearing load calculator.
• Bearing life. Use the equivalent dynamic load and speed in the bearing life calculator.

Common mistakes

• Using outside diameter instead of pitch diameter. The force is based on the pitch radius where torque is transmitted, not the outer edge of the part.
• Comparing the applied load directly to a bearing rating. First resolve the shaft reactions. The nearest bearing can see substantially more than the applied radial load.
• Ignoring the negative reaction. A negative reaction means opposite radial direction, not a negative bearing load. Use the absolute value for radial bearing load.
• Mixing load planes. Horizontal and vertical loads should be solved in their own planes, then combined by vector sum at each bearing.
• Treating the first-pass model as a catalog approval. Real bearing selection still depends on axial thrust, preload, fit, lubrication, temperature, shock, speed and duty cycle.

References

• Boston Gear / Penn State Gearology for the overhung-load definition, F = 2TK/D form and connection factors.
• NTN Bearing Load Calculation for shaft load factors, belt/chain/gear load handling and bearing load distribution.
• CalcResource support reactions article for the equilibrium basis behind two-support reaction calculations.

Frequently asked questions

What is an overhung load?

An overhung load is a radial force applied outside the nearest bearing support, usually from a pulley, sprocket, pinion or gear mounted on a shaft extension. It bends the shaft and changes the radial load carried by each bearing.

Why can a bearing see more load than the applied overhung force?

The overhung force has leverage. For a load past bearing B by distance e, static equilibrium gives RB = F(1 + e/L) and RA = -F e/L, where L is the bearing span. The near bearing can carry more than the applied force while the far bearing reacts in the opposite direction.

How do I reduce overhung load?

Use a larger pulley, sprocket or pinion pitch diameter, move the load closer to the nearest bearing, increase bearing span where practical, reduce service shock, or use a flexible coupling so the shaft is not carrying a belt, chain or gear radial load.

Last reviewed: 2026-05-31.