ation ▁prevents ▁wear ▁in ▁moving ▁parts , ▁and ▁metal ▁to ▁metal ▁contact ▁is ▁prevent ed . ▁ ▁H yd rod ynamic ▁lub ric ation ▁requires ▁thin , ▁conver ging ▁fluid ▁films . ▁ ▁These ▁flu ids ▁can ▁be ▁liquid ▁or ▁gas , ▁so ▁long ▁as ▁they ▁exhib it ▁vis cos ity . ▁ ▁In ▁computer ▁fan ▁and ▁spin ning ▁device , ▁like ▁a ▁hard ▁disk ▁drive , ▁heads ▁are ▁supported ▁by ▁hyd rod ynamic ▁lub ric ation ▁in ▁which ▁the ▁fluid ▁film ▁is ▁the ▁atmosphere . ▁ ▁The ▁scale ▁of ▁these ▁films ▁is ▁on ▁the ▁order ▁of ▁mic rom eters . ▁ ▁Their ▁convergence ▁creates ▁press ures ▁normal ▁to ▁the ▁surfaces ▁they ▁contact , ▁forcing ▁them ▁apart . ▁ ▁Three ▁types ▁of ▁bear ings ▁include : ▁ ▁Self - act ing : ▁Film ▁exists ▁due ▁to ▁relative ▁motion . ▁e . g . ▁spir al ▁gro ove ▁bear ings . ▁S que e ze ▁film : ▁Film ▁exists ▁due ▁to ▁relative ▁normal ▁motion . ▁Extern ally - press ur ized : ▁Film ▁exists ▁due ▁to ▁external ▁press ur ization . ▁ ▁Con cept ually ▁the ▁bear ings ▁can ▁be ▁thought ▁of ▁as ▁two ▁major ▁geometric ▁classes : ▁bearing - j ournal ▁( anti - fr iction ), ▁and ▁plane - slider ▁( fr iction ). ▁ ▁The ▁Reyn olds ▁equations ▁can ▁be ▁used ▁to ▁derive ▁the ▁gover ning ▁principles ▁for ▁the ▁flu ids . ▁ ▁Note ▁that ▁when ▁g ases ▁are ▁used , ▁their ▁deriv ation ▁is ▁much ▁more ▁involved . ▁ ▁The