🔍 Key Concepts

• double helical (herringbone) gear design and how the two opposite helix angles work together • effects of gear tooth arrangement on axial (end) thrust, load distribution, and noise • what happens to gear tooth contact and vibration at high rotational speeds

💭 Think About

• Think about what forces a single helical gear produces along the shaft, and how adding a second, opposite helix changes those forces. • Consider whether a double helical arrangement would tend to make the gear set smoother and quieter, or rougher and noisier, in operation. • Ask yourself: would an engineer choose a more complex double helical gear in a main reduction gear if it increased stress, deflection, or reduced tooth contact?

✅ Before You Answer

• Verify which choice correctly describes the main mechanical advantage of using a double helical gear in heavy marine reduction gears. • Eliminate any options that would obviously make the gear set weaker, less stable, or noisier under load. • Check your basic machine design knowledge: how do opposite helix angles affect axial thrust and gear smoothness of operation?