9.11 TRANSMISSIONS
COMPOUND REVERTED GEAR TRAINS are commonly used in manual (nonautomatic) automotive transmissions to provide user-selectable ratios between the engine and the drive wheels for torque multiplication (mechanical advantage). These gearboxes usually have from three to six forward speeds and one reverse. Most modem transmissions of this type use helical gears for quiet operation. These gears are not moved into and out of engagement when shifting from one speed to another except for reverse. Rather, the de-sired ratio gears are selectively locked to the output shaft by synchromesh mechanisms as in Figure 9-43 which shows a four-speed, manually shifted, synchromesh automotive transmission.
The input shaft is at top left. The input gear is always in mesh with the left-most gear on the countershaft at the bottom. This countershaft has several gears integral with it, each of which meshes with a different output gear that is freewheeling on the output shaft. The output shaft is concentric with the input shaft, making this a reverted train, but the input and output shafts only connect through the gears on the countershaft except in "top gear" (fourth speed), for which the input and output shafts are directly coupled together with a synchromesh clutch for a I: I ratio.
The synchromesh clutches are beside each gear on the output shaft and are par-tially hidden by the shifting collars which move them left and right in response to the driver's hand on the shift lever. These clutches act to lock one gear to the output shaft at a time to provide a power path from input to output of a particular ratio. The arrows on the figure show the power path for third-speed forward, which is engaged. Reverse gear, on the lower right, engages an idler gear which is physically shifted into and out of mesh at standstill.