Bearing Knowledge Encyclopedia (5)

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For example, if a bearing needs to support a load and has a power loss of 4%, and the expected operating life is over 1000 hours, the percentage of life given in the table is 53% of the normal L bearing life. To meet these requirements, divide the required lifespan by the correction percentage: 1000 / 0.53 = 1887 (hours) This result shows that to achieve a 96% reliability level, the bearing must be designed for a life of 1887 hours. It's important to use the dynamic bearing capacity equation to calculate the bearing size for a 90% reliability rating. When calculating loads and equivalent loads, it's essential to determine the additional load the bearing can handle while meeting performance requirements. This involves considering all forces acting on the equipment, as well as the moments generated by each force during operation. Various working conditions should be listed in a table. Note that the load unit is in Newtons (N), speed in revolutions per minute (r/min), and the useful time for each condition is expressed in terms of bearing life. These conditions can be simplified into an equivalent load P, and each condition uses the appropriate formula to determine this value. If both speed and load remain constant throughout the operating cycle, the equivalent load is simply the actual load: P = P (Equation A) If the speed is constant but the load varies from a minimum P_min to a maximum P_max over a long period, then the equivalent load is calculated using a different approach. In cases where speed is constant, but the load changes nonlinearly (e.g., step function, power function, sine wave, or a combination), and the pattern repeats randomly during the bearing’s life, the equivalent load is calculated accordingly. Here, P₁, P₂, ..., Pₙ represent the effective load at selected times t₁, t₂, ..., tₙ. Some of these P values may be used with Equation A or B. If both load and speed vary, and each load change corresponds to a speed change, then the equivalent load depends on the speed and load combinations. In this case, P₁, P₂, ..., Pₙ represents the effective load at speeds n₁, n₂, ..., nₙ, and q₁, q₂, ..., qₙ indicate the percentage of time spent at each speed. The modified lifespan formula should be used to account for varying speeds. The coefficient "500" in Equation E represents a time value of 500 hours, based on the international standard speed of 33 r/min, which equates to a bearing life of 500 hours. In many applications, oscillating loads require adjustments. The bearing stress can be significantly improved under such conditions. Similarly, situations involving initial or permanent axial misalignment often require higher load capacity. To compensate for oscillating loads, the maximum oscillating force is considered (worst-case scenario) and compared to the stable radial load. A correction factor K is added to the equivalent load equation. Table 1-6 provides coefficients for two scenarios: one where the oscillating force P exceeds the stable load P, and another where P is based on daily fluctuations.

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