OVERDRIVE

Overdrive is a device to step up the gear ratio in the car. It is fitted in between transmission and the propeller shaft. It enables a high cruising speed to be attained with a comparatively low engine speed (upto 20 – 25%) on long journeys. This results in less wear of the engine and decreases vibration and noise. As the friction lows at lower speeds is less, there is a saving of fuel also with the overdrive. Overdrive is generally fitted on top gear only. But in some sport cars, over drives are also fitted on gears other than the top gear which increases the torque ratios available. For examples, when overdrive is fated on top, third and second gear, seven forward speeds or torque ratios are available. The overdrive may be operated either manually or automatically at a predetermined speed.

To understand the working of an overdrive, consider the above figure. It consists of an epicyclic gear train in which the sun gear is free to rotate on the input shaft, while the carrier can move on splines, on the input shaft. A freewheel clutch is also fitted on the input shaft splines. The output shaft is connected to the ring. When the sun gear is locked with the casing i.e., it becomes stationary, of the output shaft is increased i.e., overdrive is engaged.

When the sun gear is locked to the carrier or to the ring, solid drive through n is obtained. Thus depending on the locking of the sun gear with ring gear or with carrier the overdrive or the normal direct drive is obtained. There is another possible control of the mechanism i.e., when the sun wheel is kept free to rotate on the input shaft. In this case there is direct drive through the freewheel clutch when the engine develops power. However when the accelerator is brought to zero position and the engine is simply idling, the output shaft tends to override the input shaft. The roller of the freewbeel clutch in this case no longer remains wedged and the car free wheels. Thus for gear changing one simply has to lift his foot off the accelerator pedal, the clutch need not be operated.

SYNCHROMESH GEAR BOX

Synchromesh gear box is an automatic means for matching the speeds of engaging dogs. Synchromesh gear box is a device which facilitates the coupling of two shafts rotating at different speeds. Synchromesh unit is used in most of modern gear boxes. In synchromesh gear box, sliding dog clutches are replaced by synchromesh device. The synchromesh devices are used to simplify the operation of changing gear. Synchromesh device helps unskilled drivers to change gears without the occurrence of clashes and damages.

By synchromesh device, the members which ultimately are to be engaged positively are first brought into frictional contact and then when the friction has equalized their speeds, the positive connection is made.

The basic requirements of synchromesh device are:

(1) A braking device such as cone clutch.

(2)  To permit easy meshing means of releasing pressure on the clutch before engagement of gears.

The engine shaft carries a pinion which meshes with a wheel fixed to the layshaft, while the gear on the mainshaft is free to rotate and is permanently meshed with another wheel fixed to the layshaft. Both the pinion and the wheel on the mainshaft have integral dog tooth portions and conical portions. The synchronizing drum is free to slide on splines on the mainshaft. This drum has conical portions to correspond with the conical portions on the gearbox shaft pinion and on the wheel that rotates freely on the mainshaft. The synchronizing drum carries a sliding sleeve. In the neutral position, the sliding sleeve is held in place by the spring loaded balls which rest in the dents in the sliding sleeve (or ring gear). There are usually six of these balls.

In changing gear, the gear lever is brought to the neutral position in the ordinary way, but is immediately pressed in the direction it has to go to engage the required gear. When a shift starts, the spring loaded balls cause the synchronizing drum and sliding sleeve, as an assembly to move toward the selected gear. The first contact is between the synchronizing cones on the selected gear and the drum. This contact brings the two into synchronization. Both rotate at the same speed. When the speeds of the two have become equal, a slightly greater pressure on the gear lever overcomes the resistance of the balls. Further movement of the shift fork forces the sliding sleeve on toward the selected gear. The internal splines on the sliding sleeve i.e. the dog portion, match the external splines on the selected gear the dog teeth are locked up, or engaged, and thus positive connection is established. The gear shift is completed.

CONSTANT MESH GEAR BOX

In constant mesh gear box all the gears are always in mesh and the engagement between the gears which are freely rotating on the transmission main shaft and the transmission main shaft is effected by moving the dog clutches, as explained below.

The engine gear box shaft is integral with a pinion. The pinion meshes with a wheel on the layshaft. The layshaft is therefore driven by the engine shaft. Three more wheels are fixed to the layshaft as in the sliding mesh gearbox. These gears rotate with the layshaft. The transmission main shaft is just above the layshaft and in line with the engine shaft. The three gears (first gear, second gear and reverse gear) on the main shaft are perfectly free to turn on the main shaft. These three gears are in constant mesh with the three wheels on the layshaft. One of these three gears meshes with a wheel on the layshaft through an idler wheel which is mounted and freely rotating on a pin fixed to the gearbox casing. 


The three main shaft gears are, therefore constantly driven by the engine shaft, but at different speeds. The first gear and the second gear rotate in the same direction as the engine shaft while the reverse gear rotates in the opposite direction to the engine shaft.

If anyone of the gears on/the mainshaft is coupled up to the main shaft, then there will be a driving connection between the main shaft and the engine shaft. The coupling is affected by the dog clutch units. The dog clutch members are carried on splined (or squared) portions of the mainshaft. They are free to slide on those squared portions, but have to revolve with the shaft.

If one of the dog clutch members (l) is slid to the left it will couple the wheel (first gear) to the main shaft giving the first gear. The drive is then through the wheels and this dog clutch member. The other dog clutch is meanwhile in its neutral position.

If, with the above dog clutch member in its neutral position, the other dog clutch member (2) is slid to the right, it will couple the wheel (second gear) to the mainshaft and give second gear. If this dog clutch member is slid to the left, it will couple the mainshaft directly to the pinion fixed to the engine shaft. This will give a direct drive, as in the sliding mesh gear box.

The reverse gear is engaged by sliding the dog clutch member (which gives the first gear) to the right. Then it will couple the wheel (reverse gear) to the mainshaft. The drive is then through the wheels, the idler and the dog clutch member.

In the constant mesh gear box, the gears on the mainshaft must be free to revolve. For this, they are either be bushed or be carried on ball or roller or needle bearings.

The main advantages of the constant mesh gear box over the sliding mesh type are as follows:

1. Helical or double helical gear teeth can be used for the gears instead of spur gears. Then gearing is quieter.
2. Synchronizing devices can be used for smooth engagement.
3. Any damage that results from faulty manipulation occurs to the dog clutch teeth and not to the teeth of the gear wheels.
4. Once the dog clutches are engaged, there is no motion between their teeth. But when gear teeth are engaged, the power is transmitted through the sliding action of the teeth of one wheel on those of the other. The teeth have to be suitably shaped to transmit the motion properly.
5. If the teeth on the wheel are damaged, the motion will be imperfect and noise will result.
6.  Damage is less likely to occur to the teeth of the dog clutches, since all the teeth engage at once, whereas in sliding a pair of gears into mesh the engagement is between two or three teeth.

SLIDING MESH GEARBOX

Sliding mesh gearbox is the oldest and simplest form of gear box. In order to mesh gears on the splined main shaft with appropriate gears on the layshaft for obtaining different speeds, they are moved to the right or left. It derives its name from the fact that the meshing of the gears takes place by sliding of gears on each other.

A three speed sliding mesh gear-box is shown in Figure. Splines are provided on the main shaft. For meshing the pinions with the matching gears on the layshaft, the pinions are slided along the spline. When the main shaft is driven from the layshaft the gear reduction is provided by the first pair of gears which are always in mesh. They are usually known as constant mesh gears. For changing gear the clutch is depressed and the gear lever is moved till the selector pinion on the main shaft engages with its mating gear on the layshaft. The drive from the engine will be again transmitted through the gear-box when the clutch is released. To obtain three forward speeds, reverse and neutral, the relative position of the gears will be as below:

First gear: The largest gear on the main shaft is driven by the smallest gear or pinion on the layshaft. With corresponding increase torque, the speed reduction is quite high. When climbing and moving off steep hill, starting the vehicle from rest this gear is usually used.

Second gear: In this gear, there is less speed reduction and smaller torque increase.

Third or top gear: In order to revolve primary or main shaft at the same speed without any charge in the torque the main shaft is driven through a dog clutch in this gear.

Reverse: In this gear, the peed reduction is usually same as that in the first gear. But the direction of rotation of the main shaft will be reversed by introducing an idler in it. It is due to this change in the direction of rotation of the driving wheels provided by the idler that the motor vehicle moves in reverse direction.