Manual Transmission Gears
The gears in a manual transmission are available in different types,in order to fit a specific vehicle. Within these types of gears, there are different gear ratios available,in order to accomplish specific performance goals. The types of manual transmission gears that are available are sliding-mesh gear, constant-mesh gear, and synchromesh gear.
Sliding Mesh Gears
This style of transmission is rarely used today in a regular car. It is more commonly seen in a race car, agricultural equipment and older heavy duty trucks. In a sliding mesh gearbox, straight cut spur gear sets spin freely, and must be synchronized by the driver. This is done by trying to match engine revolutions to the speed of the vehicle to avoid gear clash.
In a common configuration there are three shafts inside the sliding mesh transmission. The gears are mounted to the shafts which are called the clutch shaft, counter shaft and transmission main shaft. The gear on the clutch shaft is known as the clutch gear and it is rigidly fixed to the clutch shaft. The gear on the counter shaft is known as the drive gear and it is rigidly connected to the clutch gear. Counter shaft carries three other gears which are rigidly fixed to it. These gears include first gear, second gear and reverse ear. The transmission main shaft is a splined shaft which carries two gears. It can slide through the shifter yoke when the shaft lever is operated by the driver. There is a reverse idler gear that is mounted on another shaft and always remains connected to the counter shaft reverse gear.
Since this transmission type was easy to design it was most commonly used in the early days of cars. Due to the gear clash which sounds like grinding gears and the amount of friction created between the gears it is not commonly used today.
Constant Mesh Gears
In order to reduce the grinding noise heard from the transmission helical gears were used. By using a helical gear, which is a gear that has the teeth cut at a diagonal, the gears are always in contact or mesh which is why it is called constant mesh. In this type of transmission, the gears on counter-shaft and clutch shaft are rigidly fixed to it. The gears on the main shaft, which is splined, are held with the help of bushes. So the gears on main shaft can freely rotate without rotating the main shaft. The dog clutches are provided on the main shaft which is used to engage the gears. These dog clutches can move to the left or right freely but cannot rotate freely on the main shaft.
Since the gears are in constant mesh and the use of helical gears instead of spur gears, the grinding sound is almost completely removed during the transmissions operation. However, the big drawback is the necessity of double clutching. Double clutching is when the driver needs to wait until the same speed of the counter shaft and main shaft is achieved to change the gear.
The synchromesh transmission is now used in most modern manual transmission vehicles. The gears are still in constant mesh with one another but synchronizer cones are used to equalize the speed of the gears by bringing them into frictional contact when changing gears. By having the synchronizer cones the driver is no longer required to double clutch, and operation of the transmission is much smoother.
In a synchromesh transmission the gears on the main shaft are provided with the cones which are connected to the conical synchronizer on the dog clutch. When engaging the gears, the conical faces meet first to equalize the speed through the frictional contact and then with the further movement of the selector lever the dog teeth and gear locking drive gets engaged.
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