Understanding Gear Tooth Dimensions

Gears are fundamental components in many mechanical systems, used to transmit power and motion. Understanding the various parts and dimensions of a gear tooth is crucial for designing and analyzing gear trains. A key reference point for gear tooth dimensions is the pitch circle. The pitch circle is an imaginary circle on which the gear's teeth are based; it's where the gear theoretically meshes with another gear.

The question asks about the specific distance from the pitch circle down to the very bottom of the tooth space. Let's look at the options provided to identify this distance.

• Hub: The hub is the central part of the gear body, often where it connects to a shaft. This is not related to the dimensions of the gear teeth themselves from the pitch circle.
• Addendum: The addendum is the radial distance from the pitch circle to the top surface of the gear tooth (the addendum circle). This is the distance measured outwards from the pitch circle.
• Dedendum: The dedendum is the radial distance from the pitch circle to the bottom land of the tooth space (the root circle). This is the distance measured inwards from the pitch circle. The bottom of the tooth space is also known as the root.
• Bore: The bore is the hole in the center of the gear through which a shaft passes for mounting. This is unrelated to the gear tooth dimensions relative to the pitch circle.

Based on the definitions, the distance from the pitch circle to the bottom of the tooth space (or root circle) is specifically defined as the Dedendum. The root circle is the circle that defines the bottom of the tooth spaces.

Key Gear Tooth Terminology

Here's a summary of some important terms related to gear tooth geometry:

• Pitch Circle: The imaginary circle on which the gear's teeth are based. Used as a reference for other dimensions.
• Addendum: The radial distance from the pitch circle to the addendum circle (tooth top).
• Dedendum: The radial distance from the pitch circle to the root circle (tooth bottom).
• Addendum Circle: The circle passing through the tips of the gear teeth.
• Root Circle: The circle passing through the bottoms of the tooth spaces.
• Tooth Depth: The total radial distance from the addendum circle to the root circle. It is the sum of the addendum and the dedendum.

Therefore, the term that describes the distance from the pitch circle to the bottom of the tooth space in a gear is the Dedendum.

Revision Table: Gear Dimensions from Pitch Circle

Additional Information on Gear Geometry

Understanding the Dedendum is part of grasping the overall geometry of a gear tooth. Other dimensions that relate to the gear shape and how teeth mesh include:

• Circular Pitch: The distance measured along the pitch circle from a point on one tooth to the corresponding point on the next tooth.
• Tooth Thickness: The thickness of the tooth measured along the pitch circle.
• Tooth Space: The gap between adjacent teeth, measured along the pitch circle. Ideally, tooth thickness and tooth space are equal along the pitch circle.
• Root Clearance: The small radial distance between the top of a tooth on one gear and the bottom of the tooth space on the mating gear when they are meshed. This prevents interference. The dedendum is typically slightly larger than the addendum to provide this clearance.
• Whole Depth: The total depth of the tooth space, equal to the sum of the addendum and the dedendum.

All these dimensions are crucial for ensuring proper gear meshing, load carrying capacity, and overall performance of a gear system.