160 Manual Drive Trains and Axles Copyright by Goodheart-Willcox Co., Inc. Countershaft The countershaft holds several gears of different sizes. It is the central shaft of the countershaft gear assembly. It is located below the input shaft. Normally, the countershaft does not turn in the transmission case. Instead, it is locked in place. Manual transmission output shaft The manual transmission output shaft, also called the mainshaft, is attached to the vehicle drive shaft assembly. This rotating shaft delivers power to the drive shaft assembly. This shaft holds the output gears and synchronizers. The front end of the output shaft fits into the end of the input shaft, where it rides on needle bearings, sometimes called the output shaft pilot needle bearings. The output shaft and the input shaft are aligned, so they look like one shaft. The two shafts, however, are free to rotate separately, except when in direct drive, where they are locked together and rotate at the same speed. Sometimes, a thrust bearing is used to control endplay between the input and output shafts. The output shaft extends to the back of the extension housing. The rear end is splined and connects to the drive shaft assembly. The shaft is supported by one or two anti- friction bearings. Typically, the transmission rear bearing provides the central support for the shaft. The rear bearing is commonly held in place either by the transmission case or the front part of the extension housing however, in some designs the rear bearing is held in the rear of the extension housing. Reverse idler shaft The reverse idler shaft is a short shaft that supports the reverse idler gear. It is normally mounted in the case about midway between the countershaft and output shaft. A key or other retainer is used to hold the reverse idler shaft so it does not turn. Transmission Gears Transmission gears transmit power. These are gears of different sizes that are used to obtain different gear ratios— either reducing output speed to multiply torque or increasing output speed to reduce engine rpm. In addition, certain other transmission gears are used to move a vehicle in reverse. Transmission gears are made of high-strength steel. They are machined and then hardened by a heat-treating process. Two types of gears are commonly used in manual transmissions. Spur gears, which have straight-cut teeth, are used for gears that are only used occasionally. An example is reverse gear. Most other transmission gears are helical gears. Helical gears run more quietly and are stronger because of increased contact of the teeth, which are cut at an angle. Helical gears must be mounted with a minimum of endplay, since they tend to slide apart due to thrust forces caused by the angular tooth contact. Transmission gears are held on transmission shafts. Transmission gears may be an integral part of the shaft or they may be splined to the shaft. Some gears rotate on the shaft. Transmission gears are classified with their associated shaft. The four types include the main drive gear, countershaft gear, mainshaft gears, and reverse idler gear. See Figure 8-3. Main drive gear The transmission input shaft and main drive gear are a one-piece unit, Figure 8-4. The shaft is splined to the clutch disc. The clutch disc turns the shaft and main drive gear whenever the clutch is engaged. When the transmission is in Neutral (clutch engaged), the input shaft turns, but the output shaft does not turn. Figure 8-5 shows an input shaft, main drive gear, and related parts in an exploded view. Countershaft gear The countershaft gear, or cluster gear, as it is also called, is a series of helical gears machined out of a single piece of steel. An exploded view of a typical countershaft Two shafts fit together and can rotate independently Input shaft Countershaft Reverse idler shaft Output shaft Figure 8-2. This shows the major shafts that are found in manual transmissions. Note that needle bearings are used between input and output shafts since they often rotate at different speeds.