SKF
Principles of Selection (Spherical Plain Bearings)
Selecting the correct spherical plain bearing or rod end requires a systematic analysis of load type, magnitude, and duration (static vs. dynamic), sliding velocity, operating temperature, required misalignment angle, and environmental factors. SKF’s selection principles guide engineers through bearing terminology (sphere diameter, housing bore, shank thread), material and sliding surface selection, load rating and pV-value calculations, friction torque estimation, and design of the surrounding housing and shaft interfaces. Proper sizing ensures the bearing meets its service life without overheating or excessive wear, especially under dynamic oscillating loads. Lubrication needs are driven by the sliding material choice. Mounting and dismounting procedures must protect the spherical surface from brinelling. By following these principles, designers arrive at a robust bearing specification that minimises linkage weight and cost while maximising reliability.
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Structured Selection Workflow
Step-by-step from load definition to final part number.
pV Value Calculation
Validates sliding speed and pressure combination to prevent liner overheat.
Material Selection Matrix
Matches steel, PTFE, or composite to load, speed, and environment.
Clearance and Preload Guidance
Proper internal fit for rigidity or misalignment compensation.
Corrosion and Temperature Mapping
Environmental stress factored into material and seal choice.
Design Integration Support
SKF provides housing bore tolerances and shaft pin recommendations.
pV Value and Its Importance in Plain Bearing Selection
The product of contact pressure (p) and sliding velocity (V) defines the thermal load on the sliding surface. Exceeding the permissible pV for a given material leads to rapid wear or liner extrusion. SKF publishes pV limits for each material combination, making it a critical check in dynamic applications like hydraulic grabs and excavator linkages.
How to Define the Load Spectrum for Spherical Bearings
Unlike rolling bearings, spherical plain bearings often operate under variable-direction, pulsating loads. The selection process uses the maximum dynamic and static loads with appropriate safety factors. For oscillation, the load angle and frequency define the sliding wear path, which SKF factors into the calculated bearing life.