1. Basic Concepts of Dynamic and Static Loads

1.1 Dynamic Load

• Definition: The load borne by a bearing when it is in a rotating or moving state, usually associated with rotational speed, vibration, and impact.
• Characteristics:
  • Periodically changing (e.g., gear meshing forces, unbalanced centrifugal forces).
  • Causes fatigue failure (e.g., pitting, spalling).
• Typical Application Scenarios: Motor spindles, automobile wheel hubs, industrial gearboxes.

1.2 Static Load

• Definition: The load borne by a bearing when it is stationary or operating at an extremely low speed (< 10 rpm).
• Characteristics:
  • Constant or slowly changing (e.g., equipment self-weight, preload).
  • Causes permanent deformation (e.g., indentation, plastic deformation).
• Typical Application Scenarios: Crane support seats, large structural components, transmission systems in shutdown state.

2. Calculation of Dynamic and Static Loads

2.1 Calculation of Dynamic Load

Basic Dynamic Load (C)

• P: Equivalent dynamic load (unit: N).
• p: Life exponent (p = 3 for ball bearings, p = 10/3 for roller bearings).

Equivalent Dynamic Load (P)

• For radial bearings (e.g., deep groove ball bearings):
  • Fᵣ: Radial force.
  • Fₐ: Axial force.
  • X, Y: Coefficients (to be checked in bearing manuals).

Corrected Service Life Calculation

• a₁: Reliability coefficient (e.g., a₁ = 0.21 for 99% reliability).
• aISO: Working condition correction coefficient.

2.2 Calculation of Static Load

Basic Static Load (C₀)

• Ball bearings: 4,200 MPa (per ISO 76).
• Roller bearings: 4,000 MPa.

Equivalent Static Load (P₀)

• X₀, Y₀: Static load coefficients (to be checked in tables).

Static Safety Factor (S₀)

3. Comparison of Influencing Factors

4. Engineering Applications

4.1 Dynamic Load-Dominated Scenario: Wind Turbine Spindle Bearings

• Problem: Under variable-speed working conditions, alternating loads lead to fatigue spalling of the raceway.
• Solutions:
  1. Select tapered roller bearings (with high C value).
  2. Calculate the equivalent dynamic load through KISSsoft and optimize the preload.

4.2 Static Load-Dominated Scenario: Bridge Support Bearings

• Problem: Bear the self-weight of the bridge for a long time, requiring prevention of plastic deformation.
• Solutions:
  1. Select self-aligning roller bearings (with high C₀ value).
  2. Verify that S₀ ≥ 2.0 to ensure a safety margin.

4.3 Combined Load Scenario: Machine Tool Spindles

• Problem: High-speed rotation + cutting force (combined dynamic + static loads).
• Solutions:
  1. Assemble angular contact ball bearings (back-to-back installation).
  2. Calculate P and P₀ respectively to ensure L₁₀ > 20,000 h and S₀ > 1.8.

5. Load Considerations for Bearings

• Dynamic Load Priority:High-speed, continuous operation equipment (e.g., motors, pumps).Selection basis: Basic dynamic load (C) and service life (L₁₀).
• Static Load Priority:Low-speed, heavy-load or intermittent working conditions (e.g., cranes, punch presses).Selection basis: Basic static load (C₀) and safety factor (S₀).
• Combined Load:Need to meet both L₁₀ and S₀ requirements (e.g., slewing bearings for construction machinery).

6. Common Misunderstandings and Corrections

• Misunderstanding 1: Ignore the impact of axial load on radial bearings.Correction: Even if the axial force is small, the equivalent load must be calculated (e.g., when Fₐ/Fᵣ > e, the Y coefficient increases significantly).
• Misunderstanding 2: Do not verify the static load condition.Correction: Impact loads may cause the instantaneous P₀ to far exceed the design value (e.g., wheel hub bearings when a vehicle bumps).
• Misunderstanding 3: Only focus on dynamic load under high-speed and light-load conditions.Correction: Centrifugal force at high speed may increase the load on the balls, requiring dynamic correction (refer to SKF formulas).

7. Summary

• Dynamic load determines the fatigue life of the bearing, and attention should be paid to rotational speed, lubrication, and load spectrum.
• Static load determines the instantaneous load-carrying capacity of the bearing, and precautions should be taken against plastic deformation and failure.
• In engineering, both loads should be comprehensively evaluated, and optimal design should be carried out in combination with standards (e.g., ISO, DIN) and simulation tools.