BS ISO 7902-3:1998 download.Hydrodynamic plain journal bearings under steadystate conditions Circular cylindrical bearings Part 3: Permissible operational parameters.
1 Scope
This part of ISO 7902 specifies empirical permissible values for hbm, T and ,.
The empirical values stated can be modified for certain applications, for example if informat,on supplied by the manufacturer is to be taken into account. The descriptions of the symbols and calculation examples are given in
ISO 7902.1.
2 NormatIve reference
The following standard contains provisions which, through reference in this text, constitute provisions of this part of ISO 7902. At the time of publication, the edition indicated was valid. All standards are subject to revision, and parties to agreements based on this part of ISO 7902 are encouraged to investigate the possibility of applying the most recent edition of the standard indicated below. Members of lEG and ISO maintain registers of currently valid International Standards.
ISO 7902-1:1998. Hydrodynamc plain journal bearings under steady-state conditions — Circular cylindrical bearings — Pal? 1: Calculation procedure.
3 Operational parameters to avoid wear
3.1 The aim of keeping above the minimum permissible lubrication film thickness h, is to retain complete lubrication of the plain bearing in order to attain least possible wear and low susceptibility to faults. The lubricant should be free of contaminating particles, otherwise increased wear, scoring and local overheating can result, thus impairing correct functioning of the plain bearing. If necessary, appropriate filtering of the lubricant should be provided for.
3.2 The minimum permissible lubrication film thickness hhm, as a characteristic parameter for the transition to mixed friction (see ISO 7902-1:1998, 6.6), can be determined from the following equation:
This takes into account
— the sum o the man pe.k-lo.vaO.y he.ghts 04 beanng and shaft at the ide& location (line X-X) (R:B • Rj)
— the misalignment (line V-Y) within the beaflng length (1/2 Ni)
— the mean deflection (line Z-Z) (1,2 )
3.3 If wavy geomeincal deviations occur in the sliding surfaces (bearing or shaft) in the circumferentiai direction. they at. taken into account during the determination of by the effective waviness tot the most unfavourable shaft position. In this case. Is the effective waviness of the bearing under static loading or the effective waviness 04 the shaft under rotating loading, respectively.
The effective waviness and die maximum permissible effective waviness at a given operating point (r or kb,.J can be determined using figure 2 it roughnesses. deformations and tat positions are known.
3.4 In accordance with eation (1), the following applies:
4 Operational parameters to avoid excessive mechanical loading
The maximum pefrrssible specific bearing load. p. is the result of the requirement that a delormahon of the shdlng surfaces may riot result in impairment of correct lunctionng and the formation of cracks. in addition to the composition of the bearwig rnatertal, there are many other decisive influencing factors, such as the method of manufacture, the malarial structure, the thickness of bearing material and the geometry and type of beaflng liner backing. Independemntty of thess, a check shalt be made as to whether the bearing Is subecled to the lull load when starbng II the specific bearng load during starting, p. ii greeter than 2.5 N/mm2 to 3 Nrnrn2. it may be necessary to provide relief by pressunzed oil (auxiliary hydrostatic device). Otherwise wear can occur on the along surfaces. Table 2 gives empirical values for hm
5 Operational parameters to avoid excessive thermal loading
5.1 The rnaimum pem,lssle beanng temperature. Tm, is dependent On the beaflng matenal and the lubricant.
With incroaalng temperature, har*iess and strength 01 the bearing material decrease. Ths particularly noticeable br lead and tin alloys because 01 their low melting points.
In addition, Iubncant viscosity decreases as temperature increases. The load-carrying capacity 01 the plain bearing Is therefore reduced, resulting, under cerialn circumstances, in mixed Inchon with wear Furthermore, at temperatures greater than 80 C, ageing of lubricants based on mineral ods increases.
5.2 During steady-state operation of the plain beanng, the temperature PlaId Is constant. When carrying out the plain bearing calculation in accordance with ISO 7902. the thermal bearing load may be descrtied by the beanng temperature. T, Dr by the lubricant outlet temperature, T., provided they do not exceed 7j,,.
5.3 Table 3 gives general empirical values for T In etich it is taken Into account that the maximum value 01 the temperature field is greater than the calculated bearing temperature. re or than the calculated biricanI outlet temperature T.