FOUR-SPEED TRUCK TRANSMISSIONThe gear shift lever positions shown in the smallinset in figure 13-6 are typical of most four-speed trucktransmissions. The gear shifting lever, shown in A, B,C, D, and E of the figure, moves the position of the twoshifting forks that slide on separate shafts secured in thetransmission case cover. Follow the separate diagramsto learn what takes place in shifting from one speed toanother. For example, as you move the top of the gearshift toward the forward left position, the lower arm ofthe lever moves in the opposite direction to shift thegears. The fulcrum of this lever is in the transmissioncover.Shifting transmission gears requires the use of theclutch to disengage the engine. Improper use of theclutch will cause the gears to clash and may damagethem by breaking the gear teeth. A broken tooth or pieceof metal can wedge itself between two moving gears andruin the entire transmission assembly.When you shift from neutral to first, or low, speed(fig. 13-6, A), the smallest countershaft gear engageswith the large sliding gear. Low gear moves the truck atits lowest speed and maximum power. The arrows showthe flow of power from the clutch shaft to the propellershaft.The second-speed position is obtained by movingthe gear shift lever straight back from the low-speedposition. You will, of course, use the clutch whenshifting. In figure 13-6, B, you will see that the next tothe smallest countershaft gear is in mesh with the secondlargest sliding gear. The largest sliding gear (shift gear)has been disengaged, The flow of power has beenchanged as shown by the arrow. The power transmittedto the wheels in second gear (speed) is less, but the truckwill move at a greater speed than it will in low gear ifthe engine speed is kept the same.In shifting from the second-speed to the third-speedposition, you move the gear shift lever through theneutral position. You must do that in all selective geartransmissions. From the neutral position the driver canselect the speed position required to get the powerneeded. In figure 13-6, C, notice that the gear shift leveris in contact with the other shifting fork and that theforward sliding gear meshes with the secondcountershaft gear. The power flow through thetransmission has again been changed, as indicated bythe arrow, and the truck will move at an intermediatespeed between second and high.You shift into the fourth, or high-speed, position bymoving the top of the shift lever back and to the rightfrom the neutral position. In the high-speed position, theforward shift or sliding gear is engaged with the constantspeed gear as shown in figure 13-6, D. The clutch shaftand the transmission shaft are now locked together, andthe power flow is in a straight line. In high, the truckpropeller shaft revolves at the same speed as the enginecrankshaft, or at a 1 to 1 ratio.You shift to reverse by moving the top of the gearshift lever to the far right and then to the rear. Mosttrucks have a trigger arrangement at the gear shift ballto unlock the lever so that it can be moved from neutralto the far right. The lock prevents unintentional shiftsinto reverse. Never try to shift into reverse until theforward motion of the vehicle has been completelystopped.In figure 13-6, F, you can see how the idler gear fitsinto the transmission gear train. In figure 13-6, E, youcan see what happens when you shift into reverse. Anadditional shifting fork is contacted by the shift lever inthe far right position. When you shift to reverse, this forkmoves the idling gear into mesh with the smallcountershaft gear and the large sliding gear at the sametime. The small arrows in the inset show how the enginepower flows through the transmission to move thepropeller shaft and the wheels in a reverse direction.The different combination of gears in thetransmission case makes it possible to change thevehicle speed while the engine speed remains the same.It is all a matter of gear ratios. That is, having large gearsdrive small gears, and having small gears drive largegears. If a gear with 100 teeth drives a gear with 25 teeth,the small gear will travel four times as fast as the largeone. You have stepped up the speed. Now, let the smallgear drive the large gear, and the large gear will makeone revolution for every four of the small gear. You havereduced speed, and the ratio of gear reduction is 4 to 1.In the truck transmission just described, the gearreduction in low gear is 7 to 1 from the engine to thepropeller shaft. In high gear the ratio is 1 to 1, and thepropeller shaft turns at the same speed as the engine.This principle holds true for most transmissions. Thesecond- and third-speed positions provide intermediategear reductions between low and high. The gear ratio insecond speed is 3.48 to 1, and in third is 1.71 to 1. Thegear reduction or gear ratio in reverse is about the sameas it is in low gear, and the propeller shaft makes onerevolution for every seven revolutions of the engine.13-6
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business