fig1 shows a bearing 1 for a wind turbine in a perspective view , fig2 shows the bearing 1 in a sectional view . the bearing 1 comprises a standing inner ring 2 , an outer ring 3 , rotatable around the inner ring 2 and lubricated rollers 4 , contained in cage 5 , as can be seen in fig2 . lubrication of the rollers 4 is effected by passive oil circulation which means that the oil is distributed in the bearing 1 by the bearing rotation only . referring now to fig3 , it can be seen that bearing 1 comprises a tank 6 , disposed at the bottom of the standing inner ring 2 . at the lower side of tank 6 an opening is provided so that oil from tank 6 can flow directly into the standing inner ring to rollers 4 and cage 5 so that lubrication is effected . oil flows from the tank 6 to the outer ring 3 under the influence of gravity , therefore no separate pump for distributing oil is necessary . as can be seen best in fig2 a means for draining oil from the top region 8 is provided . this means comprises a pipe 9 connecting the top region 8 of the bearing 1 to the tank 6 . fig4 is an enlarged sectional view of the outer and the inner ring in the top region 8 of bearing 1 . the pipe 9 comprises a branching 10 with two ends 11 , 12 , which are connected to channels 7 in the standing inner ring 2 . during rotation of the wind turbine a small flow of oil is constantly drained from the top region 8 of the bearing 1 . this oil is fed to the tank 6 and through an opening in the bottom of the tank 6 the oil flows through the inner ring 2 to rollers 4 and cage 5 of bearing 1 . during rotation of bearing 1 oil is constantly conveyed from the lower region to the top region . fig5 shows a detail of the outer ring 3 which comprises recesses 13 , 14 which are provided in the rotatable outer ring 3 in order to ensure that oil is conveyed to the top region 8 of the bearing 1 when the wind turbine is turning slowly . the recesses 13 , 14 are formed as “ buckets ” in the outer ring 3 . these recesses 13 , 14 are automatically filled up with oil when they pass the bottom under the tank 6 . when the wind turbine is turning slowly most of the oil will be in the tank 6 . when the outer ring 3 and the recesses 13 , 14 elevate to the top region 8 of the bearing 1 the oil will gradually be drained out as the recesses 13 , 14 , which are formed as buckets , are turned upside down . fig6 is a schematic diagram of the oil distribution at stand still . when the wind turbine is not turning the oil is collected in the bottom of bearing 1 and in the oil tank 6 . the oil level 15 is indicated by the hatching . fig7 shows the oil distribution when the bearing is turning . in this state rollers , cage and the rotating outer ring 3 carry oil around so that oil from tank 6 flows down through the channels to the inside of bearing 1 . with a limited amount of oil in the bearing 1 shortly all oil will be distributed all around in the bearing 1 , as is indicated by the hatching . the oil will almost entirely be moved around by rollers 4 and cage 5 as long as the wind turbine turns . fig8 is a schematic diagram of the bearing 1 which shows that the tank 6 comprises a separate compartment 16 inside the tank 6 which is used as a pump sump . a pump 17 is connected to the compartment 16 and pumps the oil through a filter 18 from where the oil flows to the top region 8 . pump 17 and filter 18 are integrated in a single unit which further comprises pressure transducers and a temperature sensor . the combined unit is placed close to the bearing 1 inside the hollow shaft of the wind turbine , in other embodiments it may be placed in the back end of the nacelle . when the wind turbine is stopped all oil will be in the bottom of bearing 1 and in oil tank 6 and compartment 16 and the quantity of the oil can be checked on the basis of the oil level . when the wind turbine is rotating all oil will be spread around in the bearing 1 and there will be almost no oil in tank 6 . in order to monitor that there is oil enough in the bearing 1 the flow in the drain from the top region 8 of bearing 1 is used as an indicator . as long as oil flows through filter 18 , which is supplied from drain through pipe 9 and compartment 16 this indicates that there is oil in the top region 8 of bearing 1 . to monitor this the pressure drop through filter 18 is monitored and as long as the pressure drop through filter 18 is in a certain interval this indicates that a sufficient amount of oil flows through filter 18 . if the pressure drop is outside a predetermined interval an alarm is triggered and eventually the wind turbine is stopped in order to prevent damage due to insufficient lubrication . compartment 16 comprises a level switch measuring the oil level so that the pump 17 is stopped when the oil level is too low . consequently the pressure which is measured in filter 18 drops and after a certain delay the wind turbine is stopped .