Sleeve bearings of porous, sintered iron or bronze alloys are used in fans. The liquid lubricant is stored in the sintered pores and is discharged when the shaft rotates. Due to the hydrodynamic effect, a lubricant cycle is created that only functions freely in a relatively tight temperature range (approx. 0-60°C). Due to surface errors, micro-contamination and natural wear during running the fan in and running down, reliability is considerably lower than with ball bearings. It Sleeve bearings however, are frequently used thanks to their extremely favourable price.