Reduction Gears: Helical vs. Spur Teeth & Single vs. Double Helical
(a) Helical Teeth vs. Spur Teeth:
Helical Teeth (Advantages):
- Smoother Operation: The angled teeth of helical gears engage gradually, resulting in smoother meshing and quieter operation compared to spur gears with straight teeth.
- Reduced Load: The helical teeth transmit force along an oblique path, leading to a more even distribution of load across the tooth width compared to spur gears, which concentrate load at a single point of contact. This reduces stress on individual teeth and bearings.
- Higher Load Capacity: Due to the more even load distribution, helical gears can typically handle higher loads than spur gears of similar size.
- Reduced Axial Thrust: Helical teeth generate an axial thrust force along the shaft in addition to the rotational force. However, the opposing helix angle on the mating gear cancels out most of this thrust in properly designed helical gear sets.
Helical Teeth (Disadvantages):
- Higher Manufacturing Cost: The machining process for helical gears is more complex than for spur gears, making them slightly more expensive to manufacture.
- Thrust Bearing Requirements: The axial thrust force generated by helical gears necessitates the use of thrust bearings to accommodate this additional load.
Spur Teeth (Advantages):
- Lower Manufacturing Cost: Spur gears are simpler to manufacture compared to helical gears, making them a more cost-effective option.
- No Axial Thrust: Spur gears do not generate any axial thrust force, eliminating the need for additional thrust bearings.
Spur Teeth (Disadvantages):
- Noisier Operation: The abrupt engagement of straight teeth in spur gears results in higher noise levels compared to helical gears.
- Higher Stress on Teeth: The concentrated load at the point of contact in spur gears can lead to higher stress on individual teeth, potentially limiting load capacity.
- Higher Vibration: The abrupt engagement of spur teeth can also generate higher vibration levels compared to helical gears.
(b) Single Helical vs. Double Helical Gears:
Single Helical Gears (Advantages):
- Simpler Design: Single helical gears are simpler to design and manufacture compared to double helical gears.
- Lower Cost: Due to their simpler design, single helical gears are generally less expensive than double helical gears.
Single Helical Gears (Disadvantages):
- Axial Thrust: As mentioned earlier, single helical gears generate an axial thrust force that needs to be accommodated by thrust bearings. This can be a disadvantage in applications where space is limited for additional bearings.
- Shaft Deflection: The axial thrust force can cause some deflection in the shaft, which can be undesirable in high-precision applications.
Double Helical Gears (Advantages):
- Balanced Axial Thrust: Double helical gears have opposing helix angles on each gear, resulting in cancellation of the axial thrust forces within the gear set. This eliminates the need for separate thrust bearings and reduces shaft deflection.
- Smoother Operation: Double helical gears offer even smoother operation and quieter noise levels compared to single helical gears due to the balanced opposing forces.
Double Helical Gears (Disadvantages):
- Higher Complexity: The design and manufacturing process for double helical gears is more complex compared to single helical gears, leading to higher cost.
- Increased Weight: Double helical gears typically require slightly more material due to the additional gear teeth, resulting in increased weight compared to single helical gears.
In conclusion, the choice between helical and spur teeth, and single or double helical gears, depends on the specific application requirements. Helical gears offer smoother operation, higher load capacity, and quieter operation at the expense of higher cost and potential thrust bearing requirements. Double helical gears provide the advantage of balanced axial thrust but are more complex and expensive than single helical gears.