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Static vs Dynamic load capacity
Linear motion components with rolling element linear bearings, such as profiled linear guides, linear shafts and bearings, crossed roller bearings and ball screws have two different load capacity specifications. These are static and dynamic load capacity. When looking at static vs dynamic load capacity, these should be evaluated independent of each other as they are based on different operating parameters and performance criteria. Understanding the difference between them, as well as understanding ball bearing arrangement, is essential to suitably size and select the correct linear motion product.
Static load capacity
Simply put, when looking at static vs dynamic loads, a static load refers to stationary loads, when the linear component is fixed or stationary. A dynamic load will involve movement of the carried load.
Static load capacity, C0, is the amount of load a linear rolling bearing can withstand before the sum of the raceway deformation and of the ball equals 0.01 percent of the ball diameter. This is defined by ISO 14728-2:2017.
Static load capacity will nearly always be higher than dynamic load capacity. As the load is being applied to the bearing in a static, stationary non-moving state (or very slow moving) the failure point is limited to deformation of the raceway material and the ball bearing.
The application should always be considered when considering static loads. Linear product manufacturers recommend applying a static safety factor depending on the operating conditions. This factor is the ratio between the maximum static load and the basic static load rating. If the linear component is to operate under smooth conditions with low risk of vibrations this can be as low as 2, with a factor as high as 5 or 6 if the application conditions have severe shock loads and, or vibration.
Dynamic load capacity
Dynamic load capacity, C, is based on observational testing in which a load that is constant in size and ‘normal’ to the load carrying ball bearing surfaces allows the bearing to achieve a defined travel distance for linear guides or number of revolutions for ball screws without any signs of fatigue. Any flaking on the surface of the raceway material or the linear rolling elements is defined as fatigue.
The dynamic load capacity is used to determine the rated life of a linear rolling element bearing. This is commonly referred to as the L10 life. So called L10 life as it is the life that 90 percent of a group of identical bearings is expected to achieve under set conditions of speed and load.
The L10 life calculation and dynamic load capacity are again defined by the ISO 14728-1:2017 standard for linear bearings, and by the ISO 3408-5:2006 standard for ball screws. The ball screw standard specifies that dynamic load capacity is based on an L10 life of 1 million revolutions. The linear bearing standard for dynamic load capacity specifies an L10 life of 50,000m or 100,000m. CPC (Chieftek Precision Company) have market leading static and dynamic load capacities due to their innovative linear guide ball arrangement allowing in many cases for the designer to choose a lighter linear product with smaller dimensions than other manufacturers. All calculations and important specifying information are available through Kiwi Motion.
The importance of ball bearing arrangement
The design of ball bearing raceway arrangement is key when manufacturers define static and dynamic load capacity. All linear motion manufacturers will list their static and dynamic load capacity ratings, but not all linear guides were created equal. Let’s look at the difference between raceway arrangements. Linear rails and guides should always offer good travel accuracy and high stiffness. They can withstand downward, upward and side loads. They can also withstand overhung, or moment loads. The larger the rail, the more moment capacity it has of course, but crucially, depending on the raceway arrangement, ‘O’ or ‘X’, this directly impacts the moment loads the linear guide can withstand.
The ‘X’ type arrangement does provide equal load capacity in all directions as per the ‘O’ type design; however, it results in a shorter moment arm where overhung loads are applied thus reducing moment load capacity.
The ‘O’ type arrangement provides a larger moment arm and gives higher moment load capacities as well as providing equal load capacity in all directions.
CPCs ARC/HRC/ERC/WRC linear guide series’ use four rows of re-circulating steel balls arranged in the O-shape design and at 45º contact angle with the raceway. This achieves superior load capacity and track rail rigidity compared to other linear rail manufacturers.
The O-shape design allows the use of larger steel balls in greater quantities to increase the load capacity, moment capacity and rigidity. This allows the machine designer to use a smaller product, so vitally important when trying to reduce overall machine weight and to save on purchasing costs.
The whole CPC range can be purchased in the UK & Ireland through UK & Ireland partner Kiwi Motion.
Static and dynamic capacity conclusion
In conclusion, when assessing linear motion components such as profiled linear guides, linear shafts, and ball screws, it’s essential to differentiate between static and dynamic load capacity. CPC’s ARC/HRC/ERC/WRC linear guide series, featuring an O-shape ball bearing arrangement, demonstrates superior load capacity and rigidity. Understanding the nuances of static vs. dynamic load capacity and the importance of ball bearing arrangement is crucial for precise sizing and selection of linear motion products.
