as we know that most of the gears in use today are designed on the basis of two curves involute (path traced by a point on a wire wound on a cirle when it is unwound along its circumference) and cycloidal (locus of any point on a circle rolling over another circle). lets take the latter case. as we say “rolling” it means friction has to be present (a condition for rolling to occur). the gear tooth faces transmit power by applying preesure along the normal to their faces (the direction is called line of action)…but if pressure only is required, can it not be done without friction….even if the above is possible, how is it going to affect the power transimission by a pair of gear teeth?