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Bevel Gear: Characteristic and Classification

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Bevel gears are cone-shaped gears primarily used to transfer power between intersecting shafts at right angles. Their teeth can be configured as straight or spiral. Straight teeth are preferred for peripheral speeds of up to 1000 feet per minute; beyond this, bevel gears tend to become noisy.

Characteristics and Classification

  • Design Purpose: Bevel gears are designed for efficient power and motion transmission between intersecting shafts. A common example is the primary mechanism in a hand drill, where the vertical rotation of the drill handle is converted into horizontal rotation of the chuck by the bevel gear.
  • Assembly Flexibility: They allow slight adjustments during assembly and accommodate some misalignment caused by deflection under operational loads without concentrating the load at the gear tooth ends.
  • Types of Bevel Gears:
    1. Straight Bevel Gears: These are typically used for pitch-line speeds up to 1000 ft/min (5 m/s) where noise levels are not a concern. They are widely available in standard sizes and are cheaper to produce than other bevel gears, especially in small quantities.
    2. Spiral Bevel Gears: Recommended for higher speeds and applications where noise reduction is critical. These gears have helical teeth, providing smoother operation compared to straight bevel gears, with similar pitch surface and contact properties.
    3. Zerol Bevel Gears: These gears have curved teeth but with a zero-degree spiral angle. They generate less axial thrust compared to spiral bevel gears, making them a suitable alternative to straight bevel gears. Zerol bevel gears are produced using the same tools as spiral bevel gears. For design purposes, follow the same procedures as for straight bevel gears, then substitute with Zerol bevel gears.
    4. Hypoid Gears: Hypoid gears are a variation of spiral bevel gears with an offset between the axes. The distinguishing feature of hypoid gears is that the pinion and ring gear shafts do not intersect.
    5. Spiroid Gears: Hypoid gears typically have a small offset, but for larger offsets, the pinion begins to resemble a worm gear, classified as spiroid gearing.

Bevel Gear Force Analysis

To determine the shaft and bearing loads in bevel gear applications, a common approach is to calculate the tangential load or transmitted load, assuming all forces are concentrated at the gear’s mid-tooth point. While the actual resultant force may be slightly off-center, this assumption introduces minimal error.

The transmitted load is formulated as:

Wt=T/rav

Where:

  • T= torque
  • rav = pitch radius at the mid-point of the gear tooth under consideration.

The forces acting at the gear tooth center are depicted in the diagram below. The resultant force Whas three components:

  1. Tangential force (Wt)
  2. Radial force (Wr)
  3. Axial force (Wa)

Using trigonometry:

Wr=Wt tan⁡ϕcos⁡γ

Wa=Wt tan⁡ϕsin⁡γ

The three forces (Wt, Wr, Wa) are mutually perpendicular and can be used to calculate bearing loads using static physics principles.

By Caesar Wiratama

Source:

Budynas, Richard G dan J. Keith Nisbett. 2011. Shigley’s Mechanical Engineering Design: Ninth Edition. Amerika Serikat: The McGraw-Hill Companies, Inc.

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