Question 34 DDE04 - Designated Duty Engineer - 1000-4000 HP

The Correct Answer is D **Explanation for why Option D is Correct:** The primary function of the timing gears (also called drive gears or synchronizing gears) on a Roots-blower is not to time the blower to the engine's crankshaft (the blower is typically driven by a separate drive mechanism, like a belt or gears, which handles the speed relationship). Instead, the crucial role of these gears is to maintain the precise synchronization and spacing between the two rotor lobes within the blower housing. The rotors must spin extremely close to each other and the housing walls without actually touching (maintaining a "close tolerance"). If the timing gears were to slip or wear significantly, the lobes would collide, causing catastrophic failure. Furthermore, modern, high-quality Roots-blowers (especially those used in marine applications like auxiliary engines on tugs) utilize **helically cut gears**. Helical gears offer several advantages over straight-cut (spur) gears, including smoother, quieter operation, and increased strength due to the gradual engagement of the teeth, making them ideal for high-speed blower applications. **Explanation for why the other options are incorrect:** * **A) The timing gears are straight cut and ensure that the blower is properly timed to the engine's crankshaft.** This is incorrect for two reasons: 1) The gears are typically helically cut for performance, and 2) the gears synchronize the rotors, they do not time the blower's operation relative to the engine's cycles (crankshaft position). * **B) The timing gears are straight cut and ensure that the blower rotor lobes are properly spaced apart with a close tolerance.** This is partially correct in describing the function (spacing the lobes), but it is incorrect in stating that the gears are straight cut. They are typically helically cut. * **C) The timing gears are helically cut and ensure that the blower is properly timed to the engine's crankshaft.** This is incorrect because, while the gears are helically cut, their function is to synchronize the two internal rotors, not to time the blower's intake/exhaust phase relative to the engine's crankshaft position.