Manual Transmission Typesby Joanne Robitaille
Prior to 1938, all vehicles were manual transmission. Transmissions are necessary to transfer engine power to either the driveshaft or axle halfshafts and propel the vehicle. Manual transmissions are categorized into two basic types: sliding-gear and constant-mesh. While these two are most often used, before the introduction of automatic transmissions, car-makers created other types of manual transmissions.
Sliding-gear transmissions are only found on older model cars. When the transmission is in neutral, the only things that move within the transmission case are the main drive gear and cluster gear. In order to send power to the drive wheels (either front or rear), the clutch pedal must be depressed so that the shifter handle can be moved. Moving the shifter handle changes the position of the shift linkage and forks and slides a gear along the mainshaft directly above the cluster gear. Once these two gears have meshed, the clutch can be released. To change gears again, drivers are required to unmesh the current gear before synching two new gears. With this type of transmission, not all of the gears have the same diameters and tooth numbers. The different diameters cause the gears to rotate at different speeds and this can cause gear clash. This problem is one of the main reasons this type is no longer used.
Also known as synchronized transmissions, constant-mesh transmissions keep the drive gear, cluster gear and mainshaft gears in constant motion. This is possible because the gears are free to spin around the mainshaft (these are locked into place on a sliding-gear transmission). A dog clutch is used to lock these gears into place when they're needed. When the shift linkage moves, teeth on the dog clutch and mainshaft gears lock onto each other and hold the gear stationary. Synchronizers are used in constant-mesh transmissions to prevent any clashing or grinding while shifting.
As with all things, manual transmissions went through a series of evolutions and variations as car manufacturers experimented with design. One manual transmission type developed prior to the introduction of automatic transmissions was called the Wilson Preselector. Introduced in 1930, this transmission used a planetary gear system to preselect gear ratios using a small lever on the steering column. To shift gears, the driver presses down on a foot pedal that calls up one of the preselected gears. When this happens, the previous gear is disengaged at the same time that the new gear engages.
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