Load Rating & Rating Life Linear Ball Bearings
Static Safety Factor (fs) – This factor is used based on the application condition.
Load Rating Basic Dynamic Load Rating (C) –
This term is arrived at based on an evaluation of a number of identical linear systems individually run in the same conditions, if 90% of them can run with the load (with a constant value in a constant direction) for a distance of 50 km without damage caused by rolling fatigue.
STATIC SAFETY FACTORS Table 1
Condition of use
Low Limit of fs
This is the basis of the rating. Allowable Static Moment (M) –
When the shaft has less deflection and shock
1 to 2
When elastic deformation should be considered with respect to cantilever load When the equipment is subject to vibration and impacts
2 to 4
This term defines the allowable limit value of static moment load, with reference to the amount of permanent deformation similar to that used for evaluation of basic rated load (Co).
3 to 5
Basic Static Load Rating (Co) – This term defines a static load such that, at the contacting position where the maximum stress is exercised, the sum of the permanent deformation of the rolling elements and that of the rolling plain is 0.0001 times of the diameter of the rolling elements.
Rating Life
Rating Life of the Linear System As long as the linear system reciprocates while being loaded, continuous stress can cause flaking on the rolling bodies and planes due to material fatigue. The system’s distance of travel until the first flaking occurs is called the life of the system. The life of the system varies even when similar conditions are used – dimensions, structure, material, heat treatment, and processing method. The material fatigue causes the essential variations. The rating life defined bellow is used as an index for the life expectancy of the linear system. Rating Life (L) Rating life is the total travelling distance that 90% of a group of systems of the same size can reach without causing any flaking when they operate under the same conditions. The rating life can be obtained from the following equation with the basic dynamic load rating and the load on the linear system: For ball type:
Consideration and influence of vibration impact loads and distribution of load should be taken into account when designing a linear motion system. It is difficult to calculate the actual load. The rating life is also affected by the operating temperature. In these conditions, the expression (1) is arranged as follows:
For ball type: L: Rating life (km)
fh: Hardness factor (See Fig.1) C: Basic dynamic load rating (N) fr: Temperature coefficient (See Fig.2) p: Load (N) fc: Contact coefficient (See Table 2) fw: Load coefficient (See Table 3)
1
The rating life in hours can be calculated by obtaining the travelling distance per unit time. The rating life in hours can be obtained from the following expression when the stroke length and the number of strokes are constant:
L: Rating life (km) C: Basic dynamic load rating (N) p: Load (N)
Lh : Rating life in hours (hr) l s : Stroke length (m) L : Rating life (km) n 1 : No. of strokes per minute (cpm)
pbclinear.com • Engineering Your Linear Motion Solutions 153
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