Question 66 OSE01 - Chief Engineer - OSV

The Correct Answer is B. **Explanation for Option B (Correct):** Option B is correct because it accurately describes the primary function and characteristics of the timing gears within a Roots-blower unit (like the one commonly used on auxiliary engines). 1. **Rotor Lobe Spacing (Synchronization):** The most critical function of these gears is to synchronize the two rotors (lobes) so that they rotate in opposite directions without touching each other. Roots-blowers rely on extremely close tolerances (typically thousandths of an inch) between the rotor lobes and the housing for efficient air displacement. If the rotors touch, the blower will fail catastrophically. The timing gears ensure this crucial, non-contacting rotation is maintained. 2. **Helical Cut:** Timing gears in Roots-blowers are generally **helically cut**. Helical gears offer quieter operation and smoother transfer of torque compared to straight-cut gears, which is beneficial for high-speed machinery like engine blowers. **Explanation for Incorrect Options:** **A) The timing gears are helically cut and ensure that the blower is properly timed to the engine's crankshaft.** This option is incorrect because the timing gears *inside* the blower only ensure the synchronization of the two blower rotors (lobe spacing). While the blower assembly as a whole is driven by the engine, the timing relationship between the rotors and the engine crankshaft is established by the drive mechanism (belts, gears, or coupling) connecting the engine to the blower input shaft, not by the internal rotor timing gears. **C) The timing gears are straight cut and ensure that the blower is properly timed to the engine's crankshaft.** This option is incorrect for two reasons: 1) The gears are typically helical (not straight cut) for smoother operation, and 2) the internal timing gears do not time the blower to the engine crankshaft; they time the rotors to each other. **D) The timing gears are straight cut and ensure that the blower rotor lobes are properly spaced apart with a close tolerance.** This option correctly identifies the primary function (spacing/synchronizing the rotor lobes) but incorrectly identifies the gear type as "straight cut." They are generally designed as **helically cut** gears for quieter, more efficient operation.