Transmission design — advantages of helical gears Compared with spur gears used in gearboxes, helical gears offer which two key advantages that justify their widespread use in automotive transmissions?

Introduction / Context:
Gear selection in transmissions balances noise, strength, efficiency, and manufacturability. Helical gears dominate modern automotive gearboxes due to their meshing characteristics and load capacity.

Given Data / Assumptions:

• Comparison between spur and helical gear sets under similar module and face width.
• Automotive use where noise and smoothness are critical.
• Proper bearings are provided to react axial (end) thrust.

Concept / Approach:
Helical gears have angled teeth causing gradual engagement across the tooth face. This increases the contact ratio and distributes load over multiple teeth, improving strength and smoothness while reducing noise and vibration. The trade-off is axial thrust that must be supported by bearings and slightly higher sliding losses.

Step-by-Step Solution:
Identify attributes: helical → higher contact ratio → greater load capacity.Gradual tooth engagement → quieter operation than spur gears.Therefore, the principal benefits are reduced noise and increased strength.

Verification / Alternative check:
NVH measurements confirm lower tonal noise with helical gears; gearbox design books cite higher permissible torque for the same size.

Why Other Options Are Wrong:
Helical gears create more end thrust, not less; so (b) is incorrect.

Cost is typically higher than spur due to complex manufacture; so (a) and (d) are not primary advantages.

Option (e) is wrong because helical gears have axial thrust and not necessarily lower friction.

Common Pitfalls:
Ignoring axial load in bearing selection; excessive helix angle increases sliding losses and thrust beyond desired limits.

Final Answer:
noise level and strength