SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Let us investigate whether this condition is satisfied b the involute. It is clear that since the string is taut at all times the path of the point of contact between the two involutes is a straight line. This line intersects the line of centers(AB)at P. Also, the involute is by definition normal to its generating line (i.e, the string) at all times, since the involute is a circular arc with everincreasing radius, and a radius is al ways perpendicular to its circular arc. Therefore, if we can prove that point P is the pitch point, we have satisfied all the above mentioned requirements Triangles ACP and BPD are similar since their corresponding angles are equal. Like the friction drive at the beginning of this chapter, A and b have the same circumferential velocity at points C and D. We may therefore state thatSCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Let us investigate whether this condition is satisfied by the involute. It is clear that since the string is taut at all times, the path of the point of contact between the two involutes is a straight line. This line intersects the line of centers (AB) at P. Also, the involute is by definition normal to its generating line (i.e., the string) at all times, since the involute is a circular arc with everincreasing radius, and a radius is always perpendicular to its circular arc. Therefore, if we can prove that point P is the pitch point, we have satisfied all the above mentioned requirements. Triangles ACP and BPD are similar since their corresponding angles are equal. Like the friction drive at the beginning of this chapter, A and B have the same circumferential velocity at points C and D. We may therefore state that