To 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 countershaft gears are constantly in mesh with the main shaft gears. The main shaft meshing gears are arranged so that they cannot move endwise. They are supported by roller bearings that allow them to rotate independently of the main shaft (figs. 13-7 and 13-8).

(fig. 13-8, A) toward the third-speed gear (fig. 13-8, D). This action engages the external teeth of the clutch gear with the internal teeth of the third-speed gear. Since the third-speed gear is rotating with the rotating counter-shaft gear, the clutch gear also must rotate. The clutch gear is splined to the main shaft, and therefore, the main shaft rotates with the clutch gear. This principle is carried out when the shift lever moves from one speed to the next.

Constant-mesh gears are seldom used for all speeds. Common practice is to use such gears for the higher gears, with sliding gears for first and reverse speeds, or for reverse only. When the shift is made to first or reverse, the first and reverse sliding gear is moved to the left on the main shaft. The inner teeth of the sliding gear mesh with the main shaft first gear.

The synchromesh transmission is a type of constant-mesh transmission. It synchronizes the speeds of mating parts before they engage to allow the selection of gears without their clashing. It employs a combination metal-to-metal friction cone clutch and a dog or gear positive clutch. These clutches allow the main drive gear and second-speed main shaft gear to engage with the transmission main shaft. The friction cone clutch engages first, bringing the driving and driven members to the same speed, after which the dog clutch engages easily without clashing. This process is accomplished in one continuous operation when the driver declutches and moves the control lever in the usual manner. The construction of synchromesh transmissions varies somewhat with different manufacturers, but the principle is the same in all.

The construction of a popular synchromesh clutch is shown in figure 13-9. The driving member consists of a sliding gear splined to the transmission main shaft with bronze internal cones on each side. It is surrounded by a sliding sleeve having internal teeth that are meshed with the external teeth of the sliding gear. The sliding sleeve has grooves around the outside to receive the shift fork. Six spring-loaded balls in radially drilled holes in the gear fit into an internal groove in the sliding sleeve. That prevents the sliding sleeve from moving endwise relative to the gear until the latter has reached the end of its travel. The driven members are the main drive gear and second-speed main shaft gear. Each has external cones and external teeth machined on its sides to engage the internal cones of the sliding gear and the internal teeth of the sliding sleeve.

Some transmissions are controlled by a steering column control lever (fig. 13-11). The positions for the various speeds are the same as those for the vertical control lever except that the lever is horizontal. The shifter fork is pivoted on bell cranks that are turned by a steering column control lever through the linkage shown. The poppets shown in figure 13-10 engage notches at the inner end of each bell crank. Other types of synchromesh transmissions controlled by steering column levers have shifter shafts and forks moved by a linkage similar to those used with a vertical control lever.