Involute gears, widely used for their constant velocity ratio and ease of manufacture, primarily suffer from one significant disadvantage: the occurrence of interference with the pinion.
The Primary Disadvantage: Gear Interference
The main drawback of involute gears is a phenomenon known as interference. This occurs when the mating of two non-conjugate teeth takes place, meaning parts of the gear teeth are trying to mesh in a way that doesn't follow the smooth involute curve. During interference, the tips of the teeth on one gear (often the driving pinion) overlap or cut into the non-involute flank of the mating gear's teeth (typically the driven gear) below the base circle.
What is Gear Interference?
Interference can be visualized as one gear's tooth tip gouging the root of the mating gear's tooth. Specifically:
- Approach Interference: Occurs when the tip of the driven gear's tooth contacts the non-involute portion of the driving gear's flank.
- Recess Interference: Occurs when the tip of the driving gear's tooth contacts the non-involute portion of the driven gear's flank.
This destructive contact can happen when the path of contact extends beyond the tangent points of the base circles. The involute profile for a gear tooth only exists outside its base circle. Any contact occurring inside the base circle will not be smooth involute action and will lead to interference.
Why Is Interference a Problem?
Interference is a critical issue in gear design and operation for several reasons:
- Excessive Wear: The non-conjugate contact results in high localized stresses and sliding, leading to rapid wear of the tooth surfaces.
- Noise and Vibration: The sudden impact and scraping action generate significant noise and undesirable vibrations during operation.
- Reduced Efficiency: Interference increases friction, which in turn reduces the overall mechanical efficiency of the gear train.
- Increased Stress and Breakage: The teeth are subjected to stress concentrations beyond their design limits, potentially leading to tooth fatigue and eventual breakage.
- Manufacturing Difficulties: If not properly accounted for, interference can make it impossible for the gears to mesh or rotate freely.
Causes of Interference
Interference is typically more pronounced under specific design conditions:
- Low Number of Teeth on the Pinion: When a pinion has too few teeth, the curvature of its involute profile can become very sharp, increasing the likelihood of interference with a larger gear.
- Small Pressure Angles: Gears with smaller pressure angles are more prone to interference because the base circle is closer to the addendum circle, leaving less true involute profile length.
- Large Addendum Heights: If the addendum (the height of the tooth above the pitch circle) is too large, the tip of the tooth can extend too far, increasing the chance of it digging into the root of the mating tooth.
Solutions to Avoid or Minimize Interference
Fortunately, engineers employ several strategies to prevent or mitigate gear interference:
- Addendum Modification (Tooth Topping): This involves adjusting the addendum and dedendum heights of the mating gears.
- Shortening the addendum: The most common method, especially for the pinion, to prevent its tip from extending into the interference zone.
- Lengthening the dedendum: To compensate for the shortened addendum and maintain the full tooth depth.
- Using a Larger Pressure Angle: Increasing the pressure angle effectively moves the base circle further away from the pitch circle, providing a longer involute profile and reducing the chances of interference. However, larger pressure angles can also lead to increased radial forces on bearings.
- Increasing the Number of Teeth: Ensuring that both the pinion and gear have a sufficient minimum number of teeth helps to avoid interference. This minimum number depends on the pressure angle and addendum. For standard 20° pressure angle full-depth involute gears, the minimum number of teeth on a pinion to avoid interference with a rack (or a very large gear) is often cited as 18 teeth.
- Stub Teeth: These are gear teeth with shorter addendum and dedendum compared to standard full-depth teeth. They are stronger and less prone to interference but have a shorter contact ratio.
- Undercutting: While undercutting can remove the interfering material from the root of a gear tooth, it also weakens the tooth at its base, making it less desirable as a primary solution. It's often an unavoidable consequence when attempting to cut a gear with too few teeth.
By carefully designing the gear geometry, engineers can effectively eliminate or manage interference, ensuring smooth, efficient, and durable operation of involute gear systems.
