Understanding Involute Gear Circles

In involute gears, which are widely used for power transmission, the shape of the tooth profile is generated from a specific circle known as the base circle. To understand the location of the base circle, it's helpful to know about the other important circles associated with a gear.

Key Circles Defined

• Pitch Circle: This is the fundamental reference circle of a gear. It's an imaginary circle where two mating gears appear to roll smoothly against each other.
• Base Circle: This is the circle from which the involute curve, forming the tooth profile, is generated. The involute profile starts at the base circle and extends outward.
• Root Circle: This circle marks the bottom of the tooth spaces, i.e., the cylinder at the base of the gear teeth.
• Outside Circle (or Addendum Circle): This circle defines the outermost diameter of the gear teeth.

Location of the Base Circle

In a standard involute gear design, the involute tooth profile is traced from the base circle. This means the tooth profile exists radially outward from the base circle. Consequently, the base circle is always smaller than the pitch circle.

The root circle is located at the very bottom of the tooth space. Since the involute profile is generated outwards from the base circle and forms the active part of the tooth flank, the base circle is typically located above the root circle. The region below the base circle down to the root circle is usually relieved or has a fillet, as it is not part of the true involute profile involved in meshing.

Considering the Given Options

We are asked about the location of the base circle relative to the root circle in an involute gear. The options provided are:

• At root circle
• Under root circle
• Above root circle
• Under pitch circle

Based on the provided correct option text, the base circle is located Under root circle.

Understanding the relative positions of the base circle, pitch circle, and root circle is essential for comprehending involute gear geometry and design principles.