Which of the following statements are correct for mating gears with involute profiles?

• A. The pressure angle, from the start of the engagement to the end of the engagement, remains constant.
• B. The pressure angle is maximum at the beginning of the engagement, reduces to zero at pitch point, starts decreasing and again becomes maximum at the end of the engagement.
• C. The face and flank of the teeth are generated by a single curve and the normal to this curve at any point is tangent to the base circle of the gear.
• D. The centre distance for a pair of mating gears can be varied within limits without altering the velocity ratio.