Differences Between Static & Dynamic Load Ratings

Static Load vs Dynamic Load

Data sheets and catalog pages for rolling element linear bearings or actuators provide two load capacity specifications — static load capacity and dynamic load capacity. These ratings can have a significant impact on your choice of linear bearing. It’s important to understand the differences between these two specifications so that you can properly evaluate and select an ideal bearing for your application. Note that some actuators may list a maximum force instead of a static capacity.

Understanding Static Load Capacity

Definition of Static Load Capacity

Static load capacity refers to the amount of a load the bearing can support in a given non-moving instance. This specification is typically designated as the symbol C0 and is measured in units of force, such as Newtons (N).

Importance of Static Load Capacity in Applications

The static load capacity is an important factor to consider when evaluating whether a system can withstand shock loads, repeated loading and unloading, and emergency stop conditions. If a bearing is subjected to a load that exceeds its static load capacity, the raceways may develop brinelling, resulting in noisy operation and premature failure. It is therefore crucial to choose a bearing with a static load capacity that is appropriate for your application’s needs.

How Static Load Capacity Was Defined

Historical Definition of Static Load Capacity

For many years, the basic static load capacity in a rolling element bearing was defined as the amount of load that would result in a permanent deformation of the rolling element equal to 0.0001 times its diameter.

Updated Standard

However, the standard has been updated to specify a load amount that can cause 480,000 pounds per square inch (psi) [3,310 megapascals (Mpa)] of Hertzian contact stress in the raceway. Regardless of how it is defined, be sure to check your static capacity safety factor using the following formula and guidelines:

• Under normal load and operating conditions: f.s = 1-2
• Under impact load or vibration operating conditions: f.s = 2-3
• For best dynamic performance and service life, aim for a f.s >5.

Understanding Dynamic Load Capacity

Definition of Dynamic Load vs Static Load

Dynamic load capacity is not a stand-alone load rating, but rather a parameter used in an equation to calculate the theoretical service life of a bearing system. It is typically defined as C and specified in force units of N. At Rollon, our dynamic load capacity corresponds to a nominal service life of 100 kilometers of linear travel in accordance with DIN ISO 14728-1 in the United States and Europe. In some regions, the nominal service life is based on 50 kilometers.

Comparing Static and Dynamic Load Values

When comparing static and dynamic load values, consider the lowest value of the two as a starting point when assessing the bearing’s ability to carry the expected load. When specifying a bearing, it’s important to already know the weight of your static load and possibly your lifetime requirement, too. This information, along with the dynamic load rating, will help determine the lifetime capacity of the linear bearing system.

In most situations, multiple forces will act on your bearing. For example, if you anticipate a load that is 50 percent of the static load and also expect to see twisting, moment or other types of load, the lifetime calculation equation will incorporate these values as part of a single effective equivalent load value — P.eq, or just P.