Figure 13-7.-Constant-mesh transmission assembly—sectional view.All transmissions do not have four speeds forward,and all do not have the same gear reductions at thevarious speeds. Passenger cars, for example, usuallyhave only three forward speeds and one reverse speed.Their gear ratios are about 3 to 1 in both low and reversegear combinations. You must remember, the gearreduction in the transmission is only between the engineand the propeller shaft. Another reduction gear ratio isprovided in the rear axle assembly. If you have acommon rear axle ratio of about 4 to 1, the gearreduction from the engine of a passenger car to the rearwheels in low gear would be approximately 12 to 1. Inhigh gear the ratio would be 4 to 1 since the transmissionwould have no reduction of speed.CONSTANT MESH TRANSMISSIONTo eliminate the noise developed in the old spur-tooth type of gears used in the sliding gear transmission,the automotive manufacturers developed the constant-mesh transmission that contains helical gears.In this type of transmission, certain countershaftgears are constantly in mesh with the main shaft gears.The main shaft meshing gears are arranged so that theycannot move endwise. They are supported by rollerbearings that allow them to rotate independently of themain shaft (figs. 13-7 and 13-8).In operation, when you move the shift lever to third,the third and fourth shifter fork moves the clutch gear13-7
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