Gearshift Mechanisms


Gearshift Mechanisms

There are three separate shift rails and forks of a typical transmissions shift linkage for a five-speed transmission. Each shift rail/shift fork is used to control the movement of a synchronizer, and each synchronizer is capable of engaging and locking two speed gears to the mainshaft. This shift rails transfer motion from the driver controlled gearshift lever to the shift forks. The shift forks are semicircular castings connected to the shift rails with split pins. The shift fork rests in the groove in the synchronizer sleeve and surrounds about one-half of the sleeve circumference.

The gearshift lever is connected to the shift forks by means of a gearshift linkage. Linkage designs vary between manufacturers but can generally be classified as being internal or remote.

Internal gearshift linkage controls are positioned at the side of or on top of the transmission/transaxle housing. They are most often used in console (floor) mounted, rear-wheel-drive applications.

Internal Linkage Design

When the driver selects a specific gear range, the hooked end of the shift lever fits into the notch in the selected gear gate. The gates and shift forks are held to shift rails with pins. Any movement of the shift lever is transferred to the selected gate, rail, and fork.

To hold the shift rail in position and prevent other rails from moving, a special locking system is used. The shift rails are designed with detent and interlock notches. Each shift rail has a detent ball that drops into the detent areas on the shift rail as a gear selection is made. This effectively locks the gear into position. There is also a detent notch for neutral. For example, when a shift from first to second gear is made, the detent ball moves from the first gear detent notch on the first/second rail to the second gear notch on the rail. A spring is used to hold the ball firmly in the notch and lock the transmission in second gear. At the same time the first to second shift is being made, the interlock pin moves out of its notch on the first/second shift rail. The interlock pin now rides on the outside diameter of the first/second rail until it is forced into the third/fourth interlock notch. The seated interlock pin holds the third/fourth rail, stopping it from moving. The only way the third/fourth rail can be moved is to return the first/second rail to the neutral position. When this is done, the interlock pin drops into piece, permitting third/fourth shift rail movement.

Remote Linkage Design

Remote gearshift linkages must be used when the shifting lever cannot or is not positioned at the transmission/transaxle location. A remote linkage can be divided into two parts: the external section required to transmit motion from the remote gearshift lever location to the transmission or transaxle, and the internal, which included all linkage components contained inside the transmission/transaxle housing.

The external section of a remote linkage can consists of rods or cables, which transmit motion to the control shaft that is connected to the internal components inside the housing.

The control shaft passes through the transmission/transaxle housing. The internal end of the control shaft is connected to the selector block. The crosswise slot on the selector block operates the outer guide lever. The lengthwise slot on the selector block transfers rotary action from the control shaft to the inner guide lever. The movement of these parts results in the movement of the guide shaft selector block, which engages the proper shift fork to move the required synchronizer.

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