How to Calculate Specific Load, Sliding Speed

The factors, MPa and U, are used to determine the suitability of a given bearing design and choice of bearing material to the various application requirements.  For example, by first selecting a bearing material, the designer can specify the proper bearing dimensions that will meet the application requirements.  Alternatively, by first determining the bearing dimensions the designer may then select a bearing material that will meet the various application requirements. 

An important factor in designing for a dry bearing is the product of Specific Load (MPa) and Sliding Speed, known as the U factor.  The U factor in combination with the coefficient of friction determines the rate of heat generated by thermal friction for a given dry bearing design which relates the bearing material’s ability to resist heat.  GGB brochures list the , U and U limits for its various bearing materials. 

In a lubricated application, the ability of a sleeve bearing to develop a hydrodynamic lubrication film between the shaft and the sleeve bearing surface is determined by the relationship between Specific Load () and Sliding Speed (U), the dynamic viscosity (centiPoise) of the lubricant and the bearing length-to diameter ratio (B/D).  The relationship between these factors is: where the 7.5 value is a proportionality factor based on ISO units.

To determine the bearings capability of resisting permanent deformation under worst case scenarios we must first determine the maximum applied force, F_max,.  To determine the maximum force, which is critical to a robust bearing design, we must consider: anticipated design loads; load history based on other similar designs; measured loads; power source information like torque versus speed; shock loads.  Maximum specific load, p _max, is used to determine if the bearing material has sufficient load capacity to support the maximum load.  

When determining bearing life for select GGB products, an average or weighted average bearing load, F, is used to determine if the bearing material will provide sufficient life when considering the sliding speed.  The average bearing load is calculated when load data is limited to minimum and maximum values.  If the load range is relatively small (less than 25%) between the min/max loads, then simply take the average of the two values.  If the load range is relatively large then take 2/3 of the difference and add it to the minimum load for a “conservative” average.  If a load versus time history is available, assuming a steady speed, then a weighted average is possible: 

where t_n  and F_n are the times and loads respectively for each time/load increment and St. 

When the speed varies, substitute the number of revolutions, n₁, n₂ ... n_n and Sn for the time increments t₁, t₂ ... t_n and St. 

Now that the maximum and average forces have been determined, the specific load  is very easy to calculate: 

• For sleeve bearings the projected area, A = D_i ´ B, based on the sleeve bearing inside diameter, Di, multiplied by bearing length, B:    
• For thrust washers, A = 0.25 ´ p ´ (D_o² – D_i²), where D_o and D_i are the washers outside and inside diameters respectively. 
• For flanged bearings thrust surfaces, A = 0.04 ´ p ´ (D_o² – D_i²), where D_o is the flange outside diameter and D_i is the flanged bearing inside diameter. 
• For linear slideways, A = L ´ W, where L = bearing material length; W = bearing material width. 

Sliding Speed, U, also called velocity, is usually not that difficult to determine especially in applications that are driven by motors or engines.  Speed generates heat at the bearing/mating surface interface which, over time, will affect bearing performance.  The greater the speed, the greater the amount of heat generated.  Very slow or very occasional periods of relative motion may not develop sufficient heat to degrade the bearing material’s properties. 

Speed, U, in meters per second, is calculated based on the basic type of application: 

Continuous rotation: 

• For sleeve bearings, D_i = sleeve bearing ID in mm, N = shaft speed, rpm: 
• For thrust washers, D_o = washer OD in mm, D_i = washer ID in mm, N = speed in rpm:

Oscillating motion: 

• For sleeve bearings, D_i = ID in mm, N_osc = shaft oscillating speed, cpm: 
• For thrust washers, D_o = OD in mm, D_i = ID, N_osc in mm = oscillating speed in cpm: 

Linear motion: 

• For sleeve bearings and linear slideways, L_s = linear stroke length in mm, c = cycling rate in cpm: