| 摘要 |
Speed-change shaft (12) of a key-shift transmission mechanism (15) is divided axially into hollow first and second shaft halves (21, 23) through which one (14) of co-axially disposed wheel axles (13, 14) extends. A plurality of speed-change gears (17F1-17F5) are rotatably mounted on the first shaft half (21), whereas a shifter sleeve (22) for shifting a gear-coupling shift key (19) is slidably mounted on the second shaft half (23). A brake drum (25) including therein an aperture (25a) for passing the shift key is sandwiched between and engaged to the shaft halves for providing a vehicle-braking brake (24) within the casing (10). The second shaft half (23) may preferably be made of a polymer material for cost- and weight-reducing purposes. A key-receiving axial groove (18) may be formed in each of the shaft halves (21, 23) with ease because of a shortness of such each shaft half. In a preferred embodiment, a speed-reduction shaft (29) is disposed co-axially with and in close proximity to a drive shaft (11) of the transmission mechanism (15) for interposing two sets of reduction gear trains between the change shaft (12) and wheel axles (13, 14) in a compact fashion.
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