Understanding Motion Between Gear Teeth

When two gears are in mesh, power and motion are transmitted from one gear to the other through the contact of their teeth. The type of motion that occurs at the contact point between the mating teeth is a fundamental concept in gear design and mechanics.

Analyzing Gear Tooth Contact Motion

Consider the profiles of two gear teeth that are in contact. As the gears rotate, the point of contact moves along the tooth profiles. At different points during the engagement, the relative motion between the two tooth surfaces at the contact point is a combination of two fundamental types of motion:

• Rolling motion: This occurs when the point of contact on one surface moves relative to the point of contact on the other surface without slipping. Imagine two circles rolling against each other.
• Sliding motion: This occurs when the point of contact on one surface moves relative to the point of contact on the other surface with slipping. Imagine rubbing two surfaces against each other.

For standard gear tooth profiles, such as the involute profile, the motion between the teeth is not purely rolling or purely sliding. It is a combination of both.

• At the pitch point, which is the point where the pitch circles of the two gears touch, the relative velocity between the tooth profiles is theoretically zero. This means that instantaneous pure rolling occurs at the pitch point.
• However, away from the pitch point, as the contact moves along the tooth profiles during the approach and recess phases of engagement, there is always some relative velocity component parallel to the tooth surfaces. This parallel component causes sliding.

The amount of sliding varies along the path of contact. It is zero at the pitch point and increases as the point of contact moves towards the beginning or end of the engagement.

Therefore, the motion transmitted between the teeth of gears in mesh is generally not pure rolling or pure sliding, but a mixture of both. It is partly sliding and partly rolling.

Conclusion on Gear Tooth Motion

Based on the analysis of how gear teeth interact during meshing, the motion at the contact point involves both rolling and sliding. While instantaneous pure rolling happens at the pitch point, the overall motion throughout the engagement is a combination.

Thus, the motion transmitted between the teeth of gears in mesh is partly sliding and partly rolling.