depends size. It is generally of the order of 2 to 4. It should be noted on the material and its that above of sliding between the material, trough and during progress of material, over and screw, the material slide against each other which results in internal friction. There- particles fore, is naturally expected than normal coefficient of friction for the material. to be more Drive power of the screw at no load, PN is comparatively low. It is proportional to the screw diameter and total length of the screw. The recommended formula is