Figure 13-11.-Steering colunn transmission control lever and linkage.synchromesh transmission that uses constant-meshtruck, the auxiliary transmission doubles the mechanicalhelical gears for the three forward speeds and a slidingspur gear for reverse.Some transmissions are controlled by a steeringcolumn control lever (fig. 13-11). The positions for thevarious speeds are the same as those for the verticalcontrol lever except that the lever is horizontal. Theshifter fork is pivoted on bell cranks that are turned bya steering column control lever through the linkageshown. The poppets shown in figure 13-10 engagenotches at the inner end of each bell crank. Other typesof synchromesh transmissions controlled by steeringcolumn levers have shifter shafts and forks moved by alinkage similar to those used with a vertical controllever.AUXILIARY TRANSMISSIONThe auxiliary transmission allows a rather smalltruck engine to move heavy loads by increasing theengine-to-axle gear ratios. The auxiliary transmissionprovides a link in the power trains of constructionvehicles. This link diverts engine power to drive fourand six wheels and to operate accessory equipmentthrough transfer cases and power takeoff units. (Seefig. 13-12).Trucks require a greater engine-to-axle gear ratiothan passenger cars, particularly when manufacturersput the same engine in both types of equipment. In aadvantage. It connects to the rear of the maintransmission by a short propeller shaft and universaljoint. Its weight is supported on a frame crossmemberas shown in figure 13-12. The illustration also showshow the shifting lever would extend into the driver’scompartment near the lever operating the maintransmission.In appearance and in operation, auxiliarytransmissions are similar to main transmissions, exceptthat some may have two and some three speeds (low,direct, and overdrive).TRANSFER CASESTransfer cases are put in the power trains of vehiclesdriven by all wheels. Their purpose is to provide thenecessary offsets for additional propeller shaftconnections to drive the wheels.Transfer cases in heavier vehicles have two speedpositions and a declutching device for disconnecting thefront driving wheels. Two speed transfer cases, such asthe one shown in figure 13-13, serve also as auxiliarytransmissions.Some transfer cases are complicated. When theyhave speed-changing gears, declutching devices, andattachments for three or more propeller shafts, they areeven larger than the main transmission. A cross section13-10
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