ASNU20 One Way Clutch One Clutch ASNU20 = NFS20 ASNU20 is a one-way clutch. One way Sprag/Clutch Freewheel and also called as Backstops. ASNU20 is used in any applications that requires one direction rotation, such as mining, hydraulic, textile, printing and washing machines, conveyor systems that carry products to a higher level should not be reversed because of gravity during any power shortage.