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KC#1: In order for screwdriver to function properly, Bit Holder rotation axis must be concentric with Power Generator axis. Thus concentricity is a key characteristic.
KC#2: The distance between PG3 Internal Gear & PG1 Internal Gear is important because this subassembly has to fit within the space provided by Transmission/Bit Holder housing. So Stack-up length is a key characteristic.
KC#3-#5: Hi/Low Speed Configuration and Their Key Characteristics
The gear configurations for the high/low speeds and the transition are shown in the following figure.
Figure 2. Hi/Lo speed gear configurations.
There are distinct key characteristics for each configuration as follows.
High Speed: The Coupling Gear must successfully engage to lock PG2 Carrier and PG2 Pinion Gears together so that they become an integral unit. Therefore, the engagement of PG2 Carrier and Pinion Gears is the KC (KC#3).