Build reliable slewing bearing load cases by combining axial force, radial force, tilting moment, duty and dynamic effects before model selection.
A slewing bearing should be selected from complete operating load cases, not from one maximum number. The bearing simultaneously transmits axial force, radial force and tilting moment between two structures. Those components change as the boom, payload, counterweight, wind direction or machine attitude changes, so the condition with the largest lifted load is not automatically the condition that governs selection.
This guide explains how to turn machine geometry and operating duty into a traceable load-case table. It complements the broader slewing bearing structure selection guide and provides the input needed before a candidate model is checked on a static limiting-load diagram.
Start with a free-body diagram at the slewing axis
Draw the rotating upper structure and place the reference point at the bearing center. Show each mass, payload, cylinder reaction, wind force and external process force with its distance from that point. Resolve forces into the bearing axial and radial directions. The sum of vertical components becomes the resulting axial load, while horizontal components form the radial load. Each force multiplied by its perpendicular lever arm contributes to the tilting moment.
Keep sign conventions consistent. A counterweight may reduce the moment in one orientation and increase the reverse moment in another. Record whether axial load acts in compression or uplift and whether the moment reverses. Reversing duty affects roller or ball contact zones, bolts and fatigue even when the numerical peaks appear similar.
- Fa: resulting axial force through the bearing axis.
- Fr: resulting radial force in the bearing plane.
- Mk: resulting tilting moment about the bearing center.
- T: drive torque transmitted by the gear or other drive interface.
Create simultaneous load combinations
Do not place the maximum Fa, maximum Fr and maximum Mk in one row unless they genuinely occur together. Instead, calculate combinations for rated load, maximum radius, minimum radius, acceleration, braking, emergency stop, wind, transport and parked survival conditions. For cranes and platforms, a light payload at the longest outreach can produce a higher tilting moment than the rated payload at a shorter radius.
Add the movement state and time share to every row. A static survival case may govern permanent-deformation safety, while a lower repeated working case governs rolling-contact fatigue. If the machine operates on slopes, outriggers, a vessel or a flexible chassis, include the attitude and support condition that changes the reaction at the slewing interface.
Apply service factors without losing the original data
Manufacturer selection methods usually apply an application or service factor to account for shock, operating severity and uncertainty. Keep both calculated and factored values in the worksheet. The factor must match the equipment type and duty definition used by the selected bearing supplier; a value copied from another catalogue is not automatically transferable.
Plot every factored combination on the candidate bearing static limiting-load diagram. The point must remain inside both the raceway curve and the bolt curve under the stated mounting and preload assumptions. A point that passes the raceway curve but crosses the bolt curve is not an acceptable selection. The joint, support and bearing must be reviewed as one system.
Separate static safety from service life
Static verification limits permanent raceway deformation and joint separation during the most severe cases. Fatigue verification evaluates repeated load over time. Build the life spectrum from representative load levels, rotation speed, oscillation angle, number of cycles and duration. Continuous full rotation and repeated small-angle oscillation do not lubricate or load the raceway in the same way.
If stiffness or positioning is critical, add an allowable deflection requirement at a stated load. The same nominal size can behave differently with normal clearance, reduced clearance or preload. The crossed roller selection guide explains why clearance and mounting deformation must be included in that decision.
What the engineering worksheet should deliver
A reviewable worksheet shows geometry, units, sign convention, assumptions, calculated combinations, applied factors and the source of each factor. It identifies which point governs the raceway, which governs the bolts and which governs life. It also keeps drive torque separate from tilting moment so gear-tooth loads are not confused with bearing overturning capacity.
- Approve the free-body diagram and machine coordinate system.
- Calculate all simultaneous operating and survival combinations.
- Apply documented service factors and preserve unfactored values.
- Check raceway and bolt limiting curves for every point.
- Complete fatigue, stiffness, gear and mounting-interface checks before release.
FAQ
Can I select a slewing bearing from axial load alone?
No. Axial force, radial force and tilting moment act together and must be checked as simultaneous combinations.
Which crane load case usually governs?
There is no universal answer. Maximum outreach, wind, braking, reverse moment or parked survival may govern instead of maximum payload.
Does passing the static diagram confirm service life?
No. Static safety and rolling-contact fatigue are separate checks based on different parts of the duty.
Engineering references
For a drawing-based review, send MERYDOM the application, load cases, dimensions and required documentation. Final selection and service instructions must follow the approved drawing and equipment manufacturer requirements.
