Short Answer:

Addendum is the part of a gear tooth that lies above the pitch circle. It is the radial distance between the pitch circle and the top of the gear tooth. The addendum determines how deeply one gear tooth can engage with another. In simple terms, it represents the height of the gear tooth above the pitch circle.

The value of the addendum is usually equal to one module (for standard gears). Proper addendum ensures smooth meshing between gears and avoids interference during motion. It plays an important role in defining the strength and performance of the gear system.

Detailed Explanation :

Addendum

Addendum is an important term used in the study of gears. It refers to the portion of a gear tooth that extends beyond or above the pitch circle toward the outermost point of the gear tooth, known as the addendum circle. The pitch circle is an imaginary circle that represents the size of the gear and is used to determine the gear ratio and motion between two meshing gears. The addendum defines how far the teeth project beyond this pitch circle, helping the gears to interlock properly with their mating gears.

In simpler terms, addendum is the height of the gear tooth measured from the pitch circle up to the tip of the tooth. It ensures the proper engagement and smooth transfer of motion between two gears. If the addendum is too large, it can cause interference, while too small an addendum can lead to incomplete meshing and reduced contact.

Definition and Meaning

Addendum can be defined as:
“The radial distance between the pitch circle and the top of the tooth is called the addendum.”

It represents the upper working portion of the gear tooth. This part is in contact with the dedendum of the mating gear during meshing. The dedendum is the distance between the pitch circle and the bottom of the tooth space. Together, addendum and dedendum define the overall tooth height of a gear.

In standard gear design, the addendum is equal to one module (m). The module is a basic unit that relates the size of the teeth to the pitch diameter. Hence,

This ensures that gears of the same module can mesh correctly regardless of the number of teeth.

Purpose of Addendum

The main purpose of the addendum is to allow proper engagement between two meshing gears. It ensures that the teeth of one gear fit correctly into the spaces of the mating gear without slipping or jamming. Addendum helps in maintaining the correct contact ratio, which means there should always be at least one pair of teeth in contact during gear rotation.

A proper addendum value also ensures:

• Smooth and continuous power transmission
• Reduced wear and tear of gear teeth
• Uniform motion between gears
• Prevention of interference during operation

Addendum Circle

The outermost boundary of the gear tooth is known as the addendum circle. This circle is concentric with the pitch circle and passes through the tips of the gear teeth. The diameter of this circle is called the addendum circle diameter, which can be calculated as:

This circle defines the outer size of the gear and helps in determining the clearance between mating gears.

Importance in Gear Design

Addendum plays a vital role in the design and performance of gears. It affects several parameters such as:

1. Contact Ratio:
   The addendum determines how many teeth are in contact during rotation. A higher addendum increases the contact ratio, improving smoothness but may cause interference.
2. Strength of Teeth:
   A larger addendum leads to thinner tooth tips, which may reduce strength. Hence, standard values are preferred to maintain durability.
3. Backlash and Clearance:
   Proper addendum ensures sufficient clearance between gears to prevent binding or excessive friction.
4. Interchangeability:
   Since standard addendum values are used, gears with the same module can easily mesh together even if they are from different sources.
5. Efficiency:
   A correct addendum reduces vibration, noise, and energy loss during transmission.

Standard Values

In standard gear systems, the addendum depends on the gear type and standard followed.

• For involute spur gears, the addendum is usually equal to one module (m).
• For stub teeth, the addendum is slightly less than one module (commonly 0.8m).
  These standard proportions ensure that gears of different sizes but the same module can mesh without modification.

Example

Suppose a gear has a module of 5 mm.
Then,

If the pitch circle diameter is 100 mm, then the addendum circle diameter will be:

This means the gear’s outer circle extends 5 mm beyond the pitch circle on all sides.

Conclusion

Addendum is a key element in gear geometry that defines the height of the gear tooth above the pitch circle. It directly influences gear meshing, strength, and performance. Maintaining a correct addendum value ensures smooth motion, prevents interference, and promotes longer gear life. In standard gears, the addendum is usually equal to one module, ensuring compatibility and reliable power transmission. Without proper addendum, gears may not mesh correctly, leading to mechanical inefficiency or damage.