SKF
Selecting Bearing Size (Spherical Plain)
Sizing a spherical plain bearing involves checking the static load rating (C₀) against the maximum applied static load with a safety factor, and the dynamic load rating (C) against the equivalent dynamic load and required total sliding distance or oscillation cycles. For steel-on-steel bearings, the permissible surface pressure must not be exceeded. For PTFE-lined bearings, the specific dynamic load rating (based on pV) is used. SKF provides formulas and selection tables for each bearing type. Correct sizing prevents both catastrophic failure under peak loads and premature wear under cyclic duty, ensuring the bearing achieves its intended service life.
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Static Load Safety Factor
s₀ ≥ 2 for normal conditions, higher for shock and vibration.
Dynamic Load and Life Formula
Relates bearing load C, equivalent load P, and sliding distance.
pV Verification
Ensures sliding speed × pressure does not exceed material limit.
Contact Pressure Checks
Surface pressure under maximum static load must stay below yield.
Catalogue Load Ratings
C₀ and C tables for each bearing size and material.
Recalculation for Angular Misalignment
Load capacity derating when tilt exceeds a certain angle.
Step‑by‑Step Sizing Example for a Cylinder Pin
Define maximum hydraulic force. 2) Calculate equivalent dynamic load considering oscillation. 3) Select trial bearing size from catalogue, check C₀. 4) Verify pV at maximum sliding speed. 5) Confirm safety factor ≥ 2. This structured approach, following SKF’s charts, eliminates guesswork and prevents under‑sizing the critical pivot.
When to Increase the Safety Factor
On linkages where impact loads are frequent (e.g., rock breaker attachments), SKF recommends a static safety factor of 3 or higher. The additional margin compensates for the force peaks that occur when the tool hits an obstruction, preventing brinelling of the spherical surfaces and extending the bearing’s life in heavy‑demolition cycles.