Chapter 8 Manual Transmission Construction and Operation 163 Copyright by Goodheart-Willcox Co., Inc. Tapered roller bearing Front bearing cup Rubber O-ring (install on cup) Countershaft gear Select size spacer Rear bearing cup Rear bearing cone shown in Figure 8-9. The mainshaft gears include one gear for each available reduction ratio and the overdrive ratio. In addition, while not the case in this particular design, there is typically a gear for reverse. Each mainshaft gear (and the main drive gear) com- prises two sets of teeth. Next to the set of large, helical teeth is a set of small, straight-cut teeth. The straight-cut teeth are called clutch teeth. Each gear has a tapered, or cone, surface next to the clutch teeth. Typically, each mainshaft gear has a bushing pressed into its inside diameter. The bushing is a close fit with the matching machined area, or race, on the shaft. The bushing turns on the race, and the transmission lubricant reduces friction. In modern transmissions, the gears can turn freely (and independently) on the output shaft. Each gear turns at a different speed, depending on the gear ratio between it and its mating countershaft gear. The output shaft itself will not begin to turn until one of the mainshaft gears becomes locked to the shaft. Synchronizers, which are splined to the output shaft, are the devices that lock the gears to the shaft. Normally, only one gear will be engaged, or locked to the shaft, at a time. Reverse idler gear The reverse idler gear is used to reverse the rotation of the output shaft for backing up the vehicle. This gear is turned by the countershaft reverse gear. It mounts on the reverse idler shaft, which is normally stationary. Refer to the reverse idler gear assembly shown in Figure 8-10. Needle bearings are used between the gear and shaft to reduce wear. Thrust washers, which are placed between the ends of the gear assembly and the transmission case, control endplay. The interior of the transmission case often contains a special projection to hold one end of the idler shaft. Synchronizers Constant-mesh manual transmissions, used on mod- ern vehicles, make use of synchronizers. These assemblies are required for shifting from one gear to another. They permit gears to be selected without clashing, or grinding. Synchronizers do this by synchronizing (matching) the speed of mating mainshaft gears or the main drive gear to the output shaft speed before the gears engage. Gear clash makes shifting difficult and causes damage to gears and synchronizers. Through the use of synchronizers, main- shaft gears can be locked to the output shaft, or in the case of direct drive, input and output shafts can be locked together without damage. Modern transmissions are synchronized in forward gears. Such transmissions are referred to as synchromesh transmissions. Reverse gear is not usually synchronized, since the vehicle should be stopped before being shifted into reverse. When the synchronizer is away from a mainshaft gear, the gear mainshaft freewheels, or spins, on the output shaft. No power is transmitted to the output shaft. When shifting gears, the clutch is disengaged, which means the input shaft is disengaged from the engine. Shift forks slide the synchro- nizer against a gear, locking the gear to the synchronizer and to the output shaft. The clutch is then re-engaged, and power is sent out the transmission to the drive shaft. Synchronizer operation is described in greater detail in upcoming paragraphs. Synchronizer construction The most popular type of synchronizer for an automo- tive manual transmission is the block synchronizer. This assembly consists primarily of the following components: inner hub, outer sleeve, inserts, insert springs, and blocking rings. See Figure 8-11. The inner hub is splined to and rotates with the output shaft. It is held in a stationary position between the trans- mission gears. The hub also has external splines that mate with the outer sleeve. The outer sleeve has internal splines that mate with the external splines of the inner hub. The sleeve can slide back and forth along the splines of the hub. The internal splines of the outer sleeve also mate with the straight-cut teeth of the gears next to which they are mounted. The inserts serve, in part, as detents. They fit into slots along the inner hub. The insert springs fit around the back- side of the inner hub, in the recessed area. They push against the inserts, forcing the notches on the inserts into engage- ment with the notches in the internal splines of the sleeve. This holds and centers the sleeve on its hub. The inserts slide back and forth with the outer sleeve. The blocking rings are toothed brass or copper rings. One is located between each sleeve-and-hub assembly and mainshaft gear it is not attached to either, Figure 8-12. The blocking ring acts as a clutch to speed up or slow down the mating gears to match the speed of the output shaft. Figure 8-8. The countershaft gear sometimes gets its sup- port from tapered roller bearings on each end. The bearings also control endplay. This design does not use a countershaft. (Chrysler)