BS ISO 7902-1:2020 pdf download – Hydrodynamic plain journal bearings under steady-state conditions — Circular cylindrical bearings — Part 1: Calculation procedure
5.2 Assumptions and preconditions
The following idealizing assumptions and preconditions are made, the permissibility of which has been sufficiently confirmed both experimentally and in practice.
a) The lubricant corresponds to a Newtonian fluid.
b) All lubricant flows are laminar.
c) The lubricant adheres completely to the sliding surfaces.
d) The lubricant is incompressible.
e) The lubricant clearance gap in the loaded area is completely filled with lubricant. Filling up of the unloaded area depends on the way the lubricant is supplied to the bearing.
f) Inertia effects, gravitational and magnetic forces of the lubricant are negligible.
g) The components forming the lubrication clearance gap are rigid or their deformation is negligible;
their surfaces are ideal circular cylinders.
h) The radii of curvature of the surfaces in relative motion are large in comparison with the lubricant film thicknesses.
i) The lubricant film thickness in the axial direction (z-coordinate) is constant.
j) Fluctuations in pressure within the lubricant film normal to the bearing surfaces (y-coordinate) are negligible.
k) There is no motion normal to the bearing surfaces (y-coordinate).
l) The lubricant is isoviscous over the entire lubrication clearance gap.
m) The lubricant is fed in at the start of the bearing liner or where the lubrication clearance gap is widest; the magnitude of the lubricant feed pressure is negligible in comparison with the lubricant film pressures.
5.3 Boundary conditions
The boundary conditions for the generation of lubricant film pressure fulfil the following continuity
— at the leading edge of the pressure profile: p z ϕ 1 0 , ( ) = ;
— at the bearing rim: p z B ϕ , / =± ( ) = 2 0 ;
5.4 Basis of calculation
The numerical integration of the Reynolds equation is carried out (possibly by applying transformation of pressure as suggested in References [5], [13] and [14]) by a transformation to a differential formula which is applied to a grid system of supporting points, and which results in a system of linear formulae. The number of supporting points is significant to the accuracy of the numerical integration; the use of a non-equidistant grid as given in References [8] and [15] is advantageous.
After substituting the boundary conditions at the trailing edge of the pressure profile, integration yields the pressure distribution in the circumferential and axial directions. The application of the similarity principle to hydrodynamic plain bearing theory results in dimensionless magnitudes of similarity for parameters of interest, such as load-carrying capacity, frictional behaviour, lubricant flow rate and relative bearing length. The application of magnitudes of similarity reduces the number of numerical solutions required of Reynolds equation specified in ISO 7902-2. Other solutions may also be applied, provided they fulfil the conditions laid down in ISO 7902-2 and are of a similar numerical accuracy.
5.5 Permissible operational parameters ISO 7902-3 includes permissible operational parameters towards which the result of the calculation shall be oriented in order to ensure correct functioning of the plain bearings. In special cases, operational parameters deviating from ISO 7902-3 may be agreed upon for specific applications.
6 Calculation procedure
6.1 General Calculation is understood to mean determination of correct operation by computation using actual operating parameters (see Figure 2), which can be compared with operational parameters. The operating parameters determined under varying operating conditions shall therefore lie within the range of permissibility as compared with the operational parameters. To this end, all operating conditions during continuous operation shall be investigated.
6.2 Freedom from wear Freedom from wear is guaranteed only if complete separation of the mating bearing parts is achieved by the lubricant. Continuous operation in the mixed friction range results in failure. Short-time operation in the mixed friction range, for example, starting up and running down machines with plain bearings is unavoidable and does not generally result in bearing damage. When a bearing is subjected to heavy load, an auxiliary hydrostatic arrangement may be necessary for starting up and running down at a slow speed. Running-in and adaptive wear to compensate for deviations of the surface geometry from the ideal are permissible as long as they are limited in area and time and occur without overloading effects. In certain cases, a specific running-in procedure may be beneficial, depending on the choice of materials.BS ISO 7902-1 pdf download.