Why are 14.5°, 20° and 25° pressure angles used commonly in gears? | Q & A

The pressure angle in gears is an important design parameter that affects the performance and characteristics of the gear pair. The commonly used pressure angles of 14.5°, 20°, and 25° are chosen due to specific advantages and trade-offs they offer in gear design and operation. Here are the reasons for the common usage of these angles:

1. Smoother Operation: Gears with a 14.5° pressure angle tend to have smoother and quieter operation due to the lower angle of contact between the teeth.
2. Higher Contact Ratio: This angle typically results in a higher contact ratio, meaning more teeth are in contact at any given time, which distributes the load more evenly and reduces stress on individual teeth.
3. Lower Load Capacity: However, the lower pressure angle results in weaker teeth that cannot handle as much load compared to higher pressure angles.
4. Historical Use: The 14.5° pressure angle was commonly used in older gear designs but has been largely replaced by the 20° pressure angle in modern applications.

20° Pressure Angle

1. Standardization: The 20° pressure angle has become the standard for most gear designs, offering a good balance between strength and smooth operation.
2. Higher Load Capacity: Gears with a 20° pressure angle have stronger teeth compared to those with a 14.5° pressure angle, allowing them to handle higher loads.
3. Adequate Smoothness: While not as smooth as 14.5° pressure angle gears, 20° pressure angle gears still provide relatively smooth and quiet operation.
4. Manufacturing and Interchangeability: The 20° pressure angle is widely used, making it easier to find compatible gears and simplifying manufacturing and maintenance.

25° Pressure Angle

1. Increased Strength: Gears with a 25° pressure angle have even stronger teeth, making them suitable for high-load applications.
2. Lower Contact Ratio: The higher pressure angle results in a lower contact ratio, meaning fewer teeth are in contact at any given time. This can lead to increased stress on individual teeth and potentially noisier operation.
3. Greater Interference: Higher pressure angles can also lead to increased interference, which might require more precise manufacturing and alignment.
4. Specific Applications: The 25° pressure angle is used in applications where the primary concern is load capacity rather than smoothness of operation.
5. 17.5° Pressure Angle
6. Intermediate Choice: Offers a compromise between the 14.5° and 20° pressure angles.
7. Smooth Operation: Provides smoother operation than 20° but slightly less strength.
8. Special Applications: Used in applications where a specific balance between smoothness and strength is needed.

22.5° Pressure Angle

• Higher Strength: Provides higher strength than 20° but not as strong as 25°.
• Moderate Contact Ratio: Offers a balance between strength and contact ratio.
• Less Common: Not as widely used as the standard angles but can be found in specific industrial applications.

Custom Angles

• Tailored Design: In some specialized gear systems, custom pressure angles may be designed to meet specific performance criteria.
• Application-Specific: Custom pressure angles can be optimized for particular loads, speeds, and operating conditions.

Considerations for Choosing Non-Standard Angles

• Load Requirements: Higher pressure angles for higher loads and lower pressure angles for smoother operation.
• Noise and Vibration: Lower pressure angles generally result in quieter operation.
• Manufacturing Capabilities: Precision in manufacturing to maintain proper tooth profiles and avoid interference.
• Compatibility: Ensuring that the chosen pressure angle is compatible with other gears in the system.

Although 14.5°, 20°, and 25° are the most standardized and widely used pressure angles due to their well-understood characteristics and broad applicability, other angles like 17.5° and 22.5° can also be used based on specific design needs and engineering considerations.

Summary

The choice of pressure angle involves trade-offs between strength, smoothness, and load capacity:

• 14.5°: Smoother, quieter, higher contact ratio, but weaker teeth.
• 20°: Standard balance of strength and smoothness, widely used and standardized.
• 25°: Stronger teeth for higher loads, but lower contact ratio and potentially noisier operation.

The specific application and requirements of the gear system dictate the appropriate pressure angle to be used.