Question 52 AEL01 - Assistant Engineer - Limited

The Correct Answer is B **Explanation for Option B (Helical):** Helical gears (often configured as double-helical or herringbone gears in marine applications) are overwhelmingly preferred for medium-speed main propulsion reduction gearing. This is due to several key advantages essential for high-power, continuous-operation marine environments: 1. **Smooth, Quiet Operation:** The angled tooth engagement (helix) allows for gradual contact between teeth, leading to significantly smoother and quieter operation compared to spur gears. This reduces vibration and wear, which is critical for ship longevity and comfort. 2. **High Load Carrying Capacity:** The larger contact ratio (more teeth in contact simultaneously) inherent in helical designs allows them to transmit very high loads and torque efficiently, making them ideal for powerful main propulsion systems. 3. **High Efficiency:** Helical gearing offers very high mechanical efficiency (typically over 98%), minimizing power loss between the engine and the propeller shaft. **Explanation of Incorrect Options:** **A) Hypoid:** Hypoid gears are a type of spiral bevel gear primarily used when the input and output shafts are offset and perpendicular (e.g., in automotive differentials). They are complex, less efficient than helical gears, and not suited for the high-power, parallel-shaft arrangements typical of medium-speed marine reduction gears. **C) Cycloidal/Cyclical:** While cycloidal gear profiles are sometimes used in specific, low-speed industrial gearboxes (like cycloidal drive speed reducers), they are not suitable for the high-speed, high-power transmission requirements of main ship propulsion reduction gearing. The term "cyclical" is also not a standard marine reduction gear type. **D) Spur:** Spur gears have teeth parallel to the axis of rotation. While simple and robust, they are characterized by instantaneous, line-contact engagement, which results in high impact stress, significant noise, and increased vibration, especially at medium and high speeds. They are generally limited to lower speed or auxiliary applications and are not the optimal choice for transmitting the continuous, high power of main propulsion units.