Page 7 of catalogue Miniature profile rail guides of SKF Linearsysteme GmbH

Load carrying capacity
Static load rating C0 The static load rating C0 is the load which corresponds to an arithmetical Hertzian Pressure of 4 200 MPa between raceway and balls. This pressure produces a permanent deformation of approximately 0,0001 of the ball diameter. Static moments “MA, MB, MC” The permissible static moments correspond to a moment load that produces the same permanent deformation as in the static load rating C0. Load direction SKF miniature Profile rail guides are designed to accommodate loads in all directions. Dynamic load rating C The dynamic load rating C is the constant load which gives a theoretical system life of 100 000 m of travel with a certainty of 90 %.

Life calculation
The life of a profile rail guide is defined as the total linear distance travelled before the appearance of the first Signs of material fatigue on the raceways or rolling elements. Both in laboratory trials and in practice it is found that the life of apparently similar rail guides under completely identical operating conditions can differ. Calculation of the requisite bearing size therefore requires a clear statistic definition of the term bearing life. All references to dynamic load rating of profile rail guides apply to the basic rating life as covered by the ISO definition, in which life is understood as that operating period reached or exceeded by 90 % of a large group of identical bearings. The SKF life calculation is based on 100 000 metres of travel. Other calculation models assume merely 50 000 metres of travel. In such cases, the dynamic load ratings must be divided by a factor of 1,26 to ensure comparability with the SKF values. Where the stroke length and frequency are constant it is often easier to calculate the basic rating life in hours of operation using the equation:

Permissible operating conditions
Permissible maximum load DIN 636, Part 2, stipulates that the calculation of bearing life is valid only when the equivalent dynamic loading of a profile rail guide does not exceed 0,5 C. Any higher loading leads to an imbalance of stress distribution which can have a negative effect on bearing life. Where such conditions prevail, the user should turn to SKF for recommendations and advice on bearing life calculation. Requisite minimum load In order to assure slip-free running of profile rail guides, they must be subjected to a certain minimum load. The general guideline is a minimum value of P = 0,001 C. The minimum load is of special importance in profile rail guides which operate at high speed or with high acceleration. In such cases, the inertia forces of the balls as well as the rolling friction in the lubricant can have an adverse effect on the rolling conditions in the guide and can lead to damaging slip conditions between the balls and raceways.

L10 =

1C2 P

3

L10 = basic rating life, 105 m C = dynamic load rating, N P = equivalent dynamic bearing load, N

L10 h = 50 000 000 C s · n · 60 P

12

3

L10 h = basic rating life, hours of operation s = stroke, mm n = frequency of stroke, min-1 (number of movements from one end position to the other end and back again)

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