▁and ▁p ump ing ▁power , ▁but ▁may ▁increase ▁fr iction . ▁ ▁H yd rod ynamic ▁bear ings ▁rely ▁on ▁bearing ▁motion ▁to ▁su ck ▁fluid ▁into ▁the ▁bearing , ▁and ▁may ▁have ▁high ▁fr iction ▁and ▁short ▁life ▁at ▁spe eds ▁lower ▁than ▁design , ▁or ▁during ▁starts ▁and ▁stops . ▁An ▁external ▁p ump ▁or ▁secondary ▁bearing ▁may ▁be ▁used ▁for ▁startup ▁and ▁shut down ▁to ▁prevent ▁damage ▁to ▁the ▁hyd rod ynamic ▁bearing . ▁A ▁secondary ▁bearing ▁may ▁have ▁high ▁fr iction ▁and ▁short ▁operating ▁life , ▁but ▁good ▁overall ▁service ▁life ▁if ▁bearing ▁starts ▁and ▁stops ▁are ▁inf re quent . ▁ ▁H yd rod ynamic ▁lub ric ation ▁H yd rod ynamic ▁( HD ) ▁lub ric ation , ▁also ▁known ▁as ▁fluid - film ▁lub ric ation ▁has ▁essential ▁elements : ▁ ▁A ▁lub ric ant , ▁which ▁must ▁be ▁a ▁vis c ous ▁fluid . ▁ ▁H yd rod ynamic ▁flow ▁behavior ▁of ▁fluid ▁between ▁bearing ▁and ▁journal . ▁The ▁surfaces ▁between ▁which ▁the ▁fluid ▁films ▁move ▁must ▁be ▁conver gent . ▁ ▁H yd rod ynamic ▁( full ▁film ) ▁lub ric ation ▁is ▁obtained ▁when ▁two ▁m ating ▁surfaces ▁are ▁completely ▁separated ▁by ▁a ▁coh es ive ▁film ▁of ▁lub ric ant . ▁ ▁The ▁thick ness ▁of ▁the ▁film ▁thus ▁exceed s ▁the ▁combined ▁rough ness ▁of ▁the ▁surfaces . ▁The ▁coefficient ▁of ▁fr iction ▁is ▁lower ▁than ▁with ▁boundary - layer ▁lub ric ation . ▁H yd rod ynamic ▁lub ric