Question 22

The Correct Answer is D **Explanation for Option D (Correct Answer):** Option D is correct because it accurately describes the function and design of the timing gears in a typical Roots-type blower used on auxiliary engines (like those found on fishing trawlers). 1. **Helically Cut Gears:** Roots blowers, especially those used in engine applications where noise reduction is important, utilize **helically cut (or helical) gears**. Straight-cut gears (spur gears) are much noisier and produce vibration, making helical gears the preferred design for smoother operation and reduced noise level. 2. **Function of Timing Gears:** The primary purpose of the timing gears in a Roots blower is *not* to time the blower to the engine crankshaft (that is handled by the drive mechanism). Instead, the timing gears link the two separate rotor shafts. They ensure the precise synchronization of the two rotors, maintaining the extremely small, critical clearance between the rotor lobes and between the rotors and the housing. If this clearance is lost (if the rotors touch), the blower will fail instantly. Therefore, the gears **ensure that the blower rotor lobes are properly spaced apart with a close tolerance**. **Explanation for Incorrect Options:** * **A) The timing gears are straight cut and ensure that the blower rotor lobes are properly spaced apart with a close tolerance.** This is incorrect because modern, high-speed Roots blowers typically use **helically cut** gears for quieter operation, not straight-cut gears. The function described (spacing the lobes) is correct, but the gear type is wrong. * **B) 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: they are usually helical (not straight cut), and their function is to time the *rotors relative to each other*, not to time the blower to the crankshaft. * **C) The timing gears are helically cut and ensure that the blower is properly timed to the engine's crankshaft.** This is incorrect because the timing gears' role is internal to the blower (synchronizing the rotors), not external (timing the entire blower assembly relative to the crankshaft).