Slide bearings - Wolf Kunststoff-Gleitlager GmbH

CCCCCClubricant dissipation and therefore deficient lubrication.7.2.3. EvaporationAll lubricants have a steam pressure depending on theirchemical composition. That means, every lubricant emitsin certain limits and to a certain extent molecules to thesurrounding atmosphere. This process is influenced bythe ambient pressure and the ambient temperature. Alow ambient pressure leads to a quicker evaporation. Insome cases the boiling range of lubricants with a lowviscosity can be exceeded. But modern lubricants canachieve considerable evaporation rates also withoutboiling the fluid. These evaporation rates can causelubricant dissipations of several percent per day, if thesystems are not closed and sealed. Apart from thetemperature and the atmospheric pressure, this processdepends on the free surface. Here are especially sensitiveoil films developped by means of migration. In thiscase a very big evaporated area faces a very low quantityof lubricants. This leads to very quick evaporationprocesses in these ranges. The reinforcements into theevaporated surfaces ensues by means of capillary effectsreleased inside the roughnesses of the surfacesthemselves.7.2.4. AgeingWith increasing temperature lubricants becomechemically more reactive in the presence of oxygen. Thechemical processes which run very slowly at roomtemperature are drastically speeded up with increasingtemperature. Here applies the van t´Hoff rule, accordingto which the reaction rate of a chemical reaction isincreased two to four times by means of a raising oftemperature by 10 K. Thus the reaction rate is 2 18 (260000)times quicker at 200 °C than at 20 °C. In practice thismeans that when operating at room temperature weachieve a working life of 20 years, whereas at atemperature of 200 °C we achieve a working life of almost1 hour. The lubricants change their chemical composition,the viscosity and the neutralising or acid value increaseand additives are decomposed in every case. Especiallythe viscosity increase (oxidative polymerisation)drastically changes the characteristics of the lubricant.Some lubricants which are long-term exposed to highambient temperatures are transformed into stickymaterial, rubber-like substances or tar- and asphalt-likeproducts. These lubricants are consequently no longererational for bearing systems at low temperatures.7.2.5. Yield point of greasesWhen operating with greases at high temperatures, thethickener which is in most cases a metal soap starts tomelt. The lubricant loses its internal structure andbecomes completely liquid. In general this process isreversible, however, operations at temperaturesexceeding the melting point of the thickener may createspontaneous flat-running.LubricantsHigh temperature applications thus require theconstructive guarantee that sufficient quantities oflubricants are provided in the area of tribological stressedelements.We recommend the constructive installation of oil poolsand oil barriers or the epilamisation of the componentsurfaces in order to reduce the migration of the oil out ofthe bearing seat.7.3. Lubricants7.3.1.Lubricating oilsAccording to their origin lubricants can be subdividedinto:- mineral oils- animal and vegetable oils- synthetic oils- other, e.g. waterMineral oils which can be extracted from petroleum andpartly from coal have the biggest importance. Theyconsist of paraffine, naphthene and aromatic solvents.Animal and vegetable oils such as castor oil, fish oil,olive oil a.o. are used for special applications, e.g. inthe field of precision engineering.The lubricants must have a whole lot of physical andchemical properties in order to be able to fulfil theircomplex tasks.The viscositiy is of decisive importance when achievinga hydrodynamic or elastohydrodynamic lubricatingcondition; it is a measure of the internal friction of thelubricating oil. Often so-called additives are added tolubricating oils in order to make them able to fulfil theirfunction when operating subject to high, complex stressover a long-term period. The most important lubricantadditives are quoted with their tasks, active substancesand modes of action in table 8.Wolf Kunststoff-Gleitlager GmbH • Heisenbergstr. 63 - 65 • 50169 Kerpen-Türnich • Tel. 02237/9749-0 • Fax. 02237/9749-20PrefaceContentsMaterialoverviewIndex Slide bearings Slide bearings Conditions ofTribologyMathematical Design(with collar) (cylindrical) installationexample guidelines21