There are two load capacity specifications for recirculating linear guides and ball screws—static load capacity and dynamic load capacity. Most engineers are familiar with dynamic load capacity, since this is the value used to calculate bearing life. But it’s also important to consider the component’s static load capacity in order to avoid premature bearing failure. Here’s a quick look at each specification and their use for sizing and selection.
Dynamic Load Capacity
The stress of loading on linear bearing balls and raceways causes these surfaces to wear over time. This wear, or fatigue, causes flaking of the load-bearing surfaces, which becomes progressively worse and induces vibrations and rough motion. A bearing’s dynamic load capacity, C, is the load, constant in magnitude and normal to the load-bearing surfaces, at which 90 percent of a sample will operate for a defined travel distance (linear bearings) or number of rotations (ball screws) without fatigue.
Dynamic load capacity can be based on a travel life of 50 km or 100 km, and it’s important to note which distance is used as a basis. If comparing linear guides between manufacturers who use different ratings, a conversion factor should be applied in order to make an equivalent comparison.
Static Load Capacity
Static load capacity, C0, is defined as the load or moment at which permanent deformation of the ball and raceway equals 0.0001 times (0.01 percent) the ball diameter. This deformation can be caused by excessive load or by a single impact. Deformation of the load-bearing surfaces has adverse effects on the smoothness of operation and can reduce bearing life.
A safety factor for calculating the allowable static load is typically recommended, depending on the type of application, or the likelihood and severity of vibration or impact.
S0 = static load safety factor
C0 = static load capacity
F0max = maximum load (shock, vibration)
It is rare for an application to produce a load in just one direction for the entire travel distance. When different types of loads are encountered, or when the load varies with time, life expectancy calculations are based on an equivalent bearing load. Equivalent load is a single value that would cause a life expectancy equal to the life resulting from all the application conditions combined. Simply put, all the various loads, directions, and durations are converted into one vertical load on the bearing.
The equivalent static load takes into account static loads from all directions, as well as static moments. Since dynamic loads are applied over time, the equivalent dynamic load takes into account not only varying loads, but also the duration and distance (or number of rotations, in the case of ball screw drives) of each load.
Some linear bearings maintain the same load ratings regardless of the direction of the load (upward, downward, or lateral), while some have different load ratings for upward or lateral loads than for downward loads. In addition, most manufacturers recommend de-rating linear bearing life when certain conditions are present, such as extreme temperatures or contamination.
While the actual life of a recirculating linear guide or ball screw is dependent on many factors, understanding the two types of load capacity, how they are determined, and how to apply them correctly is important for sizing and selecting a bearing that can achieve the desired life.