ISO 7902-3-2020 pdf free download – Hydrodynamic plain journal bearings under steady-state conditions — Circular cylindrical bearings — Part 3: Permissible operational parameters.
6 Operational parameters to avoid excessive mechanical loading The maximum permissible specific bearing load, p lim , is the result of the requirement that a deformation of the sliding surfaces may not result in impairment of correct functioning and the formation of cracks. In addition to the composition of the bearing material, there are many other decisive influencing factors, such as the method of manufacture, the material structure, the thickness of bearing material and the geometry and type of bearing liner backing. Independently of these, a check shall be made as to whether the bearing is subjected to the full load when starting. If the specific bearing load during starting, p, is greater than 2,5 N/mm 2 to 3 N/mm 2 , it may be necessary to provide relief by pressurized oil (auxiliary hydrostatic device). Otherwise wear can occur on the sliding surfaces. Table 3 gives empirical values for p lim.
7 Operational parameters to avoid excessive thermal loading The maximum permissible bearing temperature, T lim , is dependent on the bearing material and the lubricant. With increasing temperature, hardness and strength of the bearing material decrease. This is particularly noticeable for lead and tin alloys because of their low melting points. In addition, lubricant viscosity decreases as temperature increases. The load-carrying capacity of the plain bearing is therefore reduced, resulting, under certain circumstances, in mixed friction with wear. Furthermore, at temperatures greater than 80 °C, ageing of lubricants based on mineral oils increases. During steady-state operation of the plain bearing, the temperature field is constant. When carrying out the plain bearing calculation in accordance with ISO 7902-1, the thermal bearing load may be described by the bearing temperature, T B , or by the lubricant outlet temperature, T ex , provided they do not exceed, T lim . Table 4 gives general empirical values for T lim in which it is taken into account that the maximum value of the temperature field is greater than the calculated bearing temperature, T B , or than the calculated lubricant outlet temperature, T ex.
From the total quantity of lubricant available for bearing lubrication, there is always only a small proportion for a limited period within the clearance gap and hence at an increased temperature level. This means that T B or T ex is important for the service life of the lubricant flow rate. This ratio is, in general, more favourable for circumferentially-lubricated bearings than for self-lubricated bearings.
8 Operational parameters for the bearing clearance The bearing clearance greatly influences the operational behaviour of the plain bearing. Although its magnitude would actually be determined taking into account all other operational data, in practice, the following approximate formula has proven of value. According to this formula, the mean relative bearing clearance, ψ , in thousandths, is calculated as a function of the peripheral speed, U J , in metres per second, only: ψ = = 0 8 4 , U U J J 0,409 6 Experience has shown that it is sometimes difficult to realize an appropriate clearance fit using the tolerances specified in ISO 286-2. Sometimes deviations greater than the mean value calculated using this formula occur. For this reason, the bearing clearance should preferably be 0,56; 0,8; 1,12; 1,32; 1,6; 1,9; 2,24; 3,15 (‰) in which the main relative bearing clearances are in steps based on the preferred number series. Table 5 gives general empirical values for ψ , based not only on the peripheral speed but also on the diameter. The reason for this is that for the same Sommerfeld numbers and the same B/D ratios, the highest temperature measured in the bearing increases as the diameter decreases. Choosing a bearing clearance from Table 5 reduces this tendency.ISO 7902-3 pdf download.