A primary disadvantage of helical gears is their higher manufacturing cost compared to spur gears and the generation of significant thrust forces that necessitate additional components like thrust bearings.
While helical gears are valued for their smooth, quiet operation and higher load-carrying capacity, these benefits come with specific drawbacks that impact their overall system cost and complexity.
Higher Manufacturing Cost
The intricate design of helical gears directly contributes to their increased production expense. Unlike spur gears, which have straight teeth parallel to the axis of rotation, helical gears feature teeth cut at an angle. This complex geometry requires more sophisticated machining processes and specialized tooling, making them more time-consuming and expensive to manufacture. This increased cost is a critical factor in the initial investment for systems utilizing helical gears.Generation of Axial Thrust Force
Perhaps the most significant operational disadvantage of helical gears is the axial thrust force they produce: * **Cause:** Due to the helix angle of their teeth, when helical gears mesh and transmit power, they generate a component of force along the axis of rotation. This force, known as axial thrust, can be substantial, especially under heavy loads. * **Consequence:** This axial thrust must be managed to prevent damage to the gear system and connected components. It necessitates the inclusion of specialized **thrust bearings** in the assembly. * Thrust bearings are designed specifically to absorb and support these axial loads, protecting the gear shafts and housing from excessive stress and wear. * The requirement for thrust bearings adds to the overall system complexity, increases the number of components needed, and further drives up the total assembly cost beyond just the gears themselves. * Proper lubrication, sealing, and maintenance of these thrust bearings are also crucial for the longevity and reliability of the entire gear train.Comparison of Disadvantages: Helical vs. Spur Gears
To illustrate these disadvantages, here's a brief comparison with spur gears:
| Feature | Helical Gears | Spur Gears |
|---|---|---|
| Manufacturing Cost | Higher (complex tooth geometry) | Lower (simpler, straight-cut teeth) |
| Thrust Force | Generates significant axial thrust | No axial thrust generated |
| Bearing Requirement | Requires thrust bearings to absorb axial load | Standard radial bearings typically sufficient |
| System Complexity | Higher (due to thrust bearings and mounting) | Lower |
Practical Considerations and Solutions
Engineers must carefully weigh the advantages of helical gears (such as reduced noise and vibration, and higher strength) against these disadvantages. For applications where these drawbacks are critical, alternative solutions or mitigation strategies include:- Double Helical Gears (Herringbone Gears): These gears feature two sets of helical teeth cut in opposite directions on the same gear. This design effectively cancels out the axial thrust forces internally, eliminating the need for external thrust bearings. However, they are even more complex and costly to manufacture than single helical gears.
- Careful System Design: Proper shaft and housing design is paramount to effectively accommodate the thrust bearings and manage the loads. This includes selecting appropriate bearing types and ensuring adequate space and lubrication.
Ultimately, while helical gears offer superior performance in many aspects, their increased manufacturing cost and the need to manage axial thrust forces are significant considerations in their selection and implementation.
