Figure 13-8.—Dissembled main shaft assembly.(fig. 13-8, A) toward the third-speed gear (fig. 13-8,D). This action engages the external teeth of theclutch gear with the internal teeth of the third-speedgear. Since the third-speed gear is rotating with therotating counter-shaft gear, the clutch gear also mustrotate. The clutch gear is splined to the main shaft,and therefore, the main shaft rotates with the clutchgear. This principle is carried out when the shiftlever moves from one speed to the next.Constant-mesh gears are seldom used for allspeeds. Common practice is to use such gears for thehigher gears, with sliding gears for first and reversespeeds, or for reverse only. When the shift is made tofirst or reverse, the first and reverse sliding gear ismoved to the left on the main shaft. The inner teethof the sliding gear mesh with the main shaft firstgear.SYNCHROMESH TRANSMISSIONThe synchromesh transmission is a type ofconstant-mesh transmission. It synchronizes thespeeds of mating parts before they engage to allowthe selection of gears without their clashing. Itemploys a combination metal-to-metal friction coneclutch and a dog or gear positive clutch. Theseclutches allow the main drive gear and second-speedmain shaft gear to engage with the transmissionmain shaft. The friction cone clutch engages first,bringing the driving and driven members to the samespeed, after which the dog clutch engages easilywithout clashing. This process is accomplished in onecontinuous operation when the driver declutches andmoves the control lever in the usual manner. Theconstruction of synchromesh transmissions variessomewhat with different manufacturers, but theprinciple is the same in all.The construction of a popular synchromesh clutchis shown in figure 13-9. The driving member consistsof a sliding gear splined to the transmission mainshaft with bronze internal cones on each side. It issurrounded by a sliding sleeve having internal teeththat are meshed with the external teeth of the slidinggear. The sliding sleeve has grooves around theoutside to receive the shift fork. Six spring-loadedballs in radially drilled holes in the gear fit into aninternal groove in the sliding sleeve. That preventsthe sliding sleeve from moving endwise relative to thegear until the latter has reached the end of its travel.The driven members are the main drive gear andsecond-speed main shaft gear. Each has externalcones and external teeth machined on its sides toengage the internal cones of the sliding gear and theinternal teeth of the sliding sleeve.The synchromesh clutch operates as follows: whenthe driver moves the transmission control lever to thethird-speed, or direct-drive, position the shift forkmoves the sliding gear and sliding sleeve forward as aunit until the internal cone on the sliding gearengages the external cone on the main drive gear.This action brings the two gears to the same speedand stops endwise travel of the sliding gear. Thesliding sleeve slides over the balls and silentlyengages the external teeth on the main drive gear.This action locks the main drive gear andtransmission main shaft together as shown in13-8
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