From the start of the engagement to the end of the engagement, the pressure angle of gear, having an involute profile is:

Explanation:-

Involute profile

It is defined as the locus of the point on the line which rolls without slipping on the fixed circle.

Pressure angle - The angle between the pressure line and common tangent to the pitch circle is called Pressure angle or angle of obliquity.

here,

R = radius of the pitch circle of the gear, r = radius of the pitch circle of pinion

R_A = radius of addendum in gear, r_A  = radius of addendum in pinion

Start of engagement  point = K, End of engagement point  = L

Now,

Line of action

• Pass through the pitch point
• Tangent to both the base circle.

Here the point of contact is changing but the line of action is not changing.

Thus, ϕ  is constant

Locus of ϕ  =  Straight line