Gear Tooth Angle Rules and Requirements for Spur and Helical Gears

1. Spur Gears

1.1 Pressure Angle:

• Common values: 20° (standard), also 14.5° or 25°.
• Definition: The pressure angle is the angle between the line of action (the force direction during gear contact) and the tangent to the pitch circle.
• Rules and Requirements:
  • A 20° pressure angle offers better load capacity than 14.5°, and is widely used in modern machinery.
  • Meshing gears must have the same pressure angle and module (tooth size).
  • A larger pressure angle increases root thickness and strength but also increases noise.
  • A smaller pressure angle results in smoother engagement but weaker tooth strength.

1.2 Tooth Profile:

• Typically an involute curve, which ensures constant speed ratio and ease of manufacturing.

2. Helical Gears

2.1 Helix Angle:

• Definition: The angle between the gear tooth line and the gear axis.
• Common range: 15°–30°, often 20° or 25°.
• Purpose and Requirements:
  • Increases the length of contact between teeth for smoother and quieter operation.
  • Allows greater load capacity.
  • Larger helix angles create more axial force, requiring additional axial support (e.g., thrust bearings).
  • Mating helical gears must have the same helix angle in magnitude but opposite directions (one right-hand, one left-hand).

2.2 Pressure Angle:

• Two types: Normal pressure angle and transverse pressure angle.
  • The normal pressure angle (usually 20°) refers to the pressure angle in the plane perpendicular to the tooth.
  • Due to the helix, the transverse pressure angle is slightly larger than the normal pressure angle.

Comparison Table: