You are here: Home » News and Events » Fundamental Knowledge of Dynamic And Static Loads for Bearings in Gearboxes

Fundamental Knowledge of Dynamic And Static Loads for Bearings in Gearboxes

Views: 0 Author: Site Editor Publish Time: 2025-10-29 Origin: Site

Bearings are critical components in mechanical systems, and their load-carrying capacity is directly related to the service life and reliability of equipment. The loads on bearings can be divided into dynamic loads and static loads, which correspond to different working conditions and failure modes respectively. This article systematically introduces their definitions, calculation principles, influencing factors, and engineering applications, providing references for engineering practitioners.

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)

According to the ISO 281 standard, it refers to the maximum radial or axial load that a bearing can withstand under a service life of 1 million revolutions (L₁₀).Formula:C = P × (1,000,000 / L₁₀)^(1/p)

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

Equivalent Dynamic Load (P)

Converts combined loads (radial + axial) into an equivalent pure radial load:Formula:P = X·Fᵣ + Y·Fₐ

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

The corrected service life considering factors such as material, lubrication, and contamination (extended formula of ISO 281):Formula:Lₙₘ = a₁·aISO·L₁₀

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)

The maximum load allowed for a bearing in a stationary state, corresponding to the contact stress as follows:

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

Equivalent Static Load (P₀)

The equivalent static load under combined loads:Formula:P₀ = X₀·Fᵣ + Y₀·Fₐ

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

Static Safety Factor (S₀)

Used to verify whether the bearing meets the static strength requirements:Formula:S₀ = C₀ / P₀ > 1.5 (general requirement)

3. Comparison of Influencing Factors

Parameters	Impact on Dynamic Load	Impact on Static Load
Rotational Speed	Fatigue risk increases significantly at high speeds	Almost irrelevant
Lubrication	Oil film thickness affects fatigue life	Only affects rust prevention
Temperature	High temperature reduces the fatigue strength of materials	High temperature may cause creep deformation
Impact	Accelerates the propagation of fatigue cracks	Directly causes plastic deformation
Installation Error	Aggravates eccentric load and shortens service life	Causes local stress concentration

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:

Select tapered roller bearings (with high C value).
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:

Select self-aligning roller bearings (with high C₀ value).
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:

Assemble angular contact ball bearings (back-to-back installation).
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.