Constant Pressure Angle in Gear Pair with Involute Tooth Profile:
The pressure angle is an important parameter in gear design and affects the performance and efficiency of gear pairs. In the case of a gear pair with an involute tooth profile, the pressure angle remains constant throughout the contact between the gear teeth.

Explanation:
To understand why the pressure angle is constant in a gear pair with an involute tooth profile, let's first define the pressure angle. The pressure angle is the angle between the line of action and the common tangent to the pitch circles of the mating gears.

In an involute tooth profile, the shape of the tooth is determined by an involute curve. This curve is generated by unwrapping a taut string from the base circle of the gear. The involute tooth profile has several advantages, including smooth and gradual engagement, good load distribution, and constant pressure angle.

Key Points:
- The pressure angle is the angle between the line of action and the common tangent to the pitch circles.
- In an involute tooth profile, the shape of the tooth is determined by an involute curve.
- The involute tooth profile offers advantages such as smooth engagement, good load distribution, and constant pressure angle.

Constant Pressure Angle:
The pressure angle remains constant throughout the contact between the gear teeth. This means that regardless of the position of the contact point along the tooth profile, the angle between the line of action and the common tangent remains the same.

When the gears are in contact, the line of action is the imaginary line along which the force is transmitted between the meshing teeth. The common tangent is the line that touches both pitch circles at the point of contact.

Since the involute tooth profile is generated by unwrapping a string from the base circle, the tangent to the involute curve at any point is always perpendicular to the line of action. This property ensures that the pressure angle remains constant throughout the contact.

Conclusion:
In conclusion, the pressure angle remains constant in a gear pair with an involute tooth profile. This is due to the nature of the involute curve, which ensures that the tangent to the curve at any point is always perpendicular to the line of action. The constant pressure angle is one of the advantages of the involute tooth profile, as it allows for smooth and efficient power transmission between gear pairs.