In the belt type oil skimmer, one drum is down in water. A belt rotates and takes up floating oil or floating oil is drawn by a belt when a belt moves up from water. Then this oil on belt surface is removed by a scraper.
The illustration below is an example of steel belt type oil skimmer. One pulley down in water has flanges. Flanges are for belt tracking for one thing. At the same time they increase weight of a pulley so that it stays in water.
A black steel belt is covered with red floating oil. Oil is removed by a scraper.
Pulleys are flange type and holds magnets inside so that even in water metal belt rotates along pulleys.
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