The wall fan is used at a different incline angle. During rotation of motor, oil flows out of the bearing by the way of the shaft. Shaft rings are located before the bearing and rotate simultaneously with the shaft. Oil which has flowed out is shaken away by centrifugal force and received back by the oil stopper to be absorbed in the felt ring, then moves again to the bearing. But if the incline angle is at minimum position (as shown) , oil will drip out instead of being collected back by the felt.
Wall fan is used normally at commercial and industrial places which are exposed to high temperature, humidity, dust and chemical atmosphere. In this condition, oil condensation is at maximum rate and unwanted particles are trapped in between the shaft and bearing.