When learning about bearings, you will often come across two terms: static load and dynamic load. If you are new to bearings, you may wonder what these terms mean, how they differ, how they are used in calculations, and how to select a suitable bearing for your application.
In this guide, I will explain the key information you need to know about static and dynamic loads, including their definitions, differences, calculation methods, and roles in bearing selection.
After understanding these concepts, if you are a purchaser looking for a bearing manufacturer, you can contact BKZ Industry. With 15 years of experience in bearing manufacturing, BKZ Industry can recommend and manufacture bearings with suitable static and dynamic load ratings based on your operating conditions and application requirements.
Part 1. Static Load vs Dynamic Load: What Are They?
To fully understand static and dynamic loads, you also need to understand four related terms: basic static load rating, basic dynamic load rating, equivalent static bearing load and equivalent dynamic bearing load. Here, I will explain what these terms mean and show you their differences and relationships.
What Are Static Loads and Dynamic Loads?
Static load is the load acting on a bearing when the bearing is stationary, oscillating slowly or rotating at an extremely low speed. Momentary heavy loads and shock loads that occur during equipment startup, shutdown or operation must also be considered when evaluating static load. This is because they may cause permanent deformation of the rolling elements or raceways.
Dynamic load is the load acting on a bearing when continuous or repeated relative rolling motion occurs between the rolling elements and raceways. Even if the magnitude and direction of the applied load remain constant, it is still considered a dynamic load because the rolling elements repeatedly pass through the loaded areas of the raceways.
In simple terms, static load is mainly associated with stationary or extremely slow-moving bearings, while dynamic load is associated with bearings undergoing continuous or repeated rotation.
What Are Equivalent Static and Dynamic Bearing Loads?
The equivalent static bearing load, represented by P0, is a hypothetical radial load for a radial bearing or axial load for a thrust bearing. It produces the same maximum contact stress between the rolling elements and raceways as the actual combination of static radial and axial loads. It is compared with the basic static load rating C0 to evaluate static safety and the risk of permanent deformation.
The equivalent dynamic bearing load, represented by P, is a hypothetical radial load for a radial bearing or axial load for a thrust bearing. It has the same effect on the bearing’s basic rating life as the actual load or combination of static radial and axial loads. It is used together with the basic dynamic load rating C to calculate the bearing’s basic rating life.
What Are the Basic Static and Dynamic Load Ratings?
The basic static load rating, represented by C0, is a standardized load rating corresponding to a specified level of permanent deformation at the most heavily loaded contact point between the rolling element and raceway. It is used to evaluate the bearing’s static load-carrying capacity. However, it does not represent the maximum static load that the bearing can carry.
The basic dynamic load rating, represented by C, is the constant load under which a sufficiently large group of apparently identical bearings can achieve a basic rating life of one million revolutions under specified conditions. The corresponding basic rating life is based on 90% reliability. The basic dynamic load rating is used for bearing life calculations and does not represent the maximum dynamic load that the bearing can carry.
What Are Their Relationships and Differences?
These terms describe different but directly related concepts. Static and dynamic loads are the actual loads acting on a bearing under specific operating conditions. Basic static and dynamic load ratings are standardized performance values of the bearing. Equivalent static and dynamic bearing loads are calculated values used to represent the effects of the actual loads.
Actual static loads are converted into the equivalent static bearing load P0, which is compared with the basic static load rating C0 to evaluate static safety. Actual dynamic loads are converted into the equivalent dynamic bearing load P, which is used together with the basic dynamic load rating C to calculate the bearing’s basic rating life.
Part 2. Static Load vs Dynamic Load: Key Differences
Now that you understand static and dynamic loads and their related terms, you may already recognize their key differences from the definitions. The following table provides a clearer comparison.
| Item | Static Load | Dynamic Load |
| Main Evaluation | Static safety and permanent deformation risk | Rolling-contact fatigue and basic rating life |
| Working condition | Stationary, slow oscillation, extremely low speed or momentary heavy loading | Continuous or repeated rotation under load |
From this table, you can see that the key differences lie in the operating conditions and the related evaluation. Static load is mainly associated with stationary or extremely slow-moving bearings and is considered when evaluating static safety and the risk of permanent deformation. Dynamic load is associated with continuous or repeated relative rolling motion and is considered when evaluating rolling-contact fatigue and calculating the bearing’s basic rating life.
Part 3. Why Are Bearing Static and Dynamic Loads So Important?
If the static load acting on a bearing is too high for the bearing under the required safety conditions, the following problems may occur:
- Permanent deformation and indentations at the contact points between the raceways and rolling elements, known as true brinelling
- Reduced running accuracy
- Increased noise and vibration
- Uneven load distribution and possible damage to other bearing components
Dynamic load is important because it affects the bearing’s rolling-contact fatigue life. In general, a higher dynamic load results in greater repeated contact stress and a shorter basic rating life. Excessive dynamic load may cause premature fatigue damage, such as pitting and flaking.
Moreover, the actual static and dynamic loads are used to calculate the equivalent static and dynamic bearing loads. These calculated loads are then used with the basic static and dynamic load ratings provided by the manufacturer to evaluate static safety and calculate the bearing’s basic rating life. Therefore, accurately determining the static and dynamic loads is essential for bearing selection.
Part 4. Static Load and Dynamic Load Calculation
To calculate the static and dynamic loads acting on a bearing, first identify all external forces and then determine the radial and axial loads carried by each bearing.
Step 1. Calculate the External Forces
Common external forces include component weight, transmitted torque, belt or gear forces, axial thrust and inertial forces.
Component weight:
W = m × g
Inertial force during acceleration or deceleration:
F_i = m × a
Tangential force produced by torque:
F_t = M / r = 2M / d
where mm is mass, gg is gravitational acceleration, aa is acceleration, MM is torque, rr is radius and dd is diameter.
Step 2. Calculate the Load on Each Bearing
For a shaft supported by bearings A and B, with a force FF acting between them:
R_A = (F × b) / L
R_B = (F × a) / L
where:
- R_A and R_B are the reaction loads at bearings A and B;
- a and b are the distances from the applied force to bearings A and B;
- L = a + b is the distance between the bearings.
For multiple forces, calculate the reaction caused by each force and combine the reactions according to their directions.
Step 3. Determine the Radial and Axial Loads
If the bearing is subjected to radial force components in two perpendicular directions, calculate the total radial load as follows:
F_r = √(F_x² + F_y²)
The axial load F_a is the total load acting parallel to the shaft axis.
Loads acting when the bearing is stationary, oscillating slowly or rotating at an extremely low speed are treated as static loads. Loads acting during continuous or repeated rotation are treated as dynamic loads.
Step 4. Calculate the Equivalent Bearing Loads
Equivalent static bearing load:
P_0 = X_0 × F_r + Y_0 × F_a
Equivalent dynamic bearing load:
P = X × F_r + Y × F_a
The load factors X_0, Y_0, X and Y depend on the bearing type and must be obtained from the bearing manufacturer’s catalogue.
Finally, use P_0 with the basic static load rating C_0 to evaluate static safety, and use P with the basic dynamic load rating C to calculate the bearing’s basic rating life.
Part 5. How to Identify Bearing Static and Dynamic Load in Size Chart?
Bearing size charts generally do not list the actual static and dynamic loads or the equivalent static and dynamic bearing loads because these values depend on the specific operating conditions. Instead, they usually list the basic static and dynamic load ratings using the following symbols:
Basic Static Load Rating:
- C₀: Basic static load rating
- C₀r: Basic static radial load rating
- C₀a: Basic static axial load rating
Basic Dynamic Load Rating:
- C: Basic dynamic load rating
- Cᵣ: Basic dynamic radial load rating
- Cₐ: Basic dynamic axial load rating
Radial bearing charts commonly show Cᵣ and C₀r, while thrust bearing charts commonly show Cₐ and C₀a. However, many manufacturers simplify these symbols to C and C₀, so you should always check the column headings, definitions and units in the specific catalogue.
These values are load ratings provided by the manufacturer. They are usually expressed in N or kN.
Part 6. How to Select a Bearing Based on Static and Dynamic Loads
A bearing size chart provides basic static and dynamic load ratings. These values are used to evaluate static safety, the risk of permanent deformation and the bearing’s basic rating life. Does this mean that higher load ratings are always better?
Not necessarily. Higher C₀ and C values generally indicate greater load-carrying capacity, but they may also come with a larger bearing size, higher cost, greater weight and potentially higher friction. The correct bearing should meet the application requirements without being unnecessarily oversized.
So the correct step to select a suitable bearing, first determine the actual radial and axial loads under static and dynamic operating conditions. Then use the calculations introduced in Part 4 to obtain the equivalent static bearing load P₀ and equivalent dynamic bearing load P.
Next, select a candidate bearing and obtain its C₀ and C values from the manufacturer’s catalogue. Calculate its static safety factor and basic rating life, and check whether the results meet the required safety factor and service-life target of the application.
For static load selection, calculate the static safety factor:
s₀ = C₀ / P₀
For dynamic load selection, calculate the bearing’s basic rating life:
L₁₀ = (C / P)ᵖ
Please note that static load capacity and dynamic load capacity are only two factors to consider when selecting a suitable bearing. You should also consider speed, dimensions, bearing type, lubrication, temperature, accuracy, alignment, installation and other operating conditions.
Ending Note
From this article, you now know that static and dynamic loads are the actual loads acting on a bearing. These actual loads can be converted into equivalent static and dynamic bearing loads. The calculated values are then used with the basic static and dynamic load ratings to evaluate the bearing’s static safety and calculate its basic rating life. This information can help you select a bearing that meets your application requirements.
If you are looking for bearings with suitable static and dynamic load ratings, contact BKZ Industry. We can recommend and manufacture suitable bearings based on your loads, operating conditions and application requirements.
