Gear Box: Principle of Gearing and Types of Gear Boxes

Gear Box

A gearbox is a mechanical method of transferring energy from one device to another and is used to increase torque while reducing speed. Torque is the power generated through the bending or twisting of a solid material. This term is often used interchangeably with transmission.Located at the junction point of a power shaft, the gearbox is often used to create a right angle change in direction, as is seen in a rotary mower or a helicopter. Each unit is made with a specific purpose in mind, and the gear ratio used is designed to provide the level of force required. This ratio is fixed and cannot be changed once the box is constructed. The only possible modification after the fact is an adjustment that allows the shaft speed to increase, along with a corresponding reduction in torque.In a situation where multiple speeds are needed, a transmission with multiple gears can be used to increase torque while slowing down the output speed. This design is commonly found in automobile transmissions. The same principle can be used to create an overdrive gear that increases output speed while decreasing torque.

Principle Of Gearing

Consider a simple 4-gear train. It consists of a driving gear A on input shaft and a driven gear D on the output shaft. In between the two gears there are two intermediate gears B, C. Each of these gears are mounted on separate shaft.We notice that:

Gear A drives Gear B

Gear C drives Gear D

Therefore, the over all speed ratios are:

1.Types of Gear Boxes:  

The following types of gear box are used in automobiles:

·        Sliding Mesh

·        Constant Mesh

·        Synchromesh.

i. Sliding Mesh Gear Box

It is the simplest gear box. The following figure shows 4-speed gear box in neutral position.

4 gears are connected to the lay shaft/counter shaft. A reverse idler gear is mounted on another shaft and always remains connected to the reverse gear of countershaft. This “H” shift pattern enables the

driver to select four different gear ratios and a reverse gear.

Gears in Neutral:

When the engine is running and clutch is engaged the clutch shaft gear drives the countershaft gear. The countershaft rotates opposite in direction of the clutch shaft. In neutral position only the clutch shaft gear is connected to the countershaft gear. Other gears are free and hence the transmission main shaft is not turning. The vehicle is stationary.

First or low shaft gear:

By operating the gear shift lever the larger gear on the main shaft is moved along the shaft to mesh with the first gear of the counter shaft. The main shaft turns in the same direction as that of the clutch shaft. Since the smaller countershaft is engaged with larger shaft gear a gear reduction of approximately 4:1 is obtained i.e. the clutch shaft turns 4 times for each revolution of main shaft.

Second speed gear:

By operating the gear shift lever the third gear on the main shaft is moved along the shaft to mesh with the third gear of the counter shaft. The main shaft turns in same direction as clutch shaft. A gear reduction of approximately 3:1is obtained.

Third speed gear:

By operating the gear shift lever, the second gear of the main shaft and countershaft are demeshed and then the third gear of the main shaft are forced axially against the clutch shaft gear. External Teeth on the clutch shaft gear mesh with the internal teeth in the third and top gear. The main shaft turns in same direction as clutch shaft. A gear reduction of approximately 2:1is obtained i.e. the clutch shaft turns 2 times for each revolution of main shaft.

Fourth speed gear/ Top or High-Speed Gear:

By operating the gear shaft lever the third gears of the main and countershaft is demeshed and the gears present on the main shaft along with the shaft is forced axially against the clutch shaft gear. External teeth present on the main shaft engage with the internal teeth present on the main shaft. The main shaft turns along with the clutch shaft and a gear ratio of approximately 1:1 is obtained.

Reverse gear:

By operating the gear shift lever, the last gear present on the main shaft is engaged with the reverse idler gear. The reverse idler gear is always in mesh with the counters haft gear. Interposing the idler gear between the counter-shaft reverse gear and main shaft gear, the main shaft turns in the direction opposite to the clutch shaft. This reverses the rotation of the wheels so that the wheel backs.

ii.Constant Mesh Gear Box:

In this type of gear box, all gears of the main shaft are in constant mesh with the corresponding gears of the countershaft (Lay shaft). Two dog clutches are provided on the main shaft- one between the clutch gear and the second gear, and the other between the first gear and reverse gear. The main shaft is splined and all the gears are free on it. Dog clutch can slide on the shaft and rotates with it. All the gears on the countershaft are rigidly fixed with it.

When the left hand dog clutch is made to slide to the left by means of the gear shift lever, it meshes with the clutch gear and the top speed gear is obtained. When the left hand dog clutch meshes with the second gear, the second speed gear is obtained. Similarly by sliding the right hand dog clutch to the left and right, the first speed gear and reverse gear are obtained respectively. In this gear box because all the gears are in constant mesh they are safe from being damaged and an unpleasant grinding sound does not occur while engaging and disengaging them.

iii.Syncromesh Gear Box:

In sliding Mesh Gear box the two meshing gears need to be revolve at equal peripheral speeds to achieve a jerk less engagement and it is true for constant mesh gear box in which the peripheral speeds of sliding dog and the corresponding gear on the output shaft must be equal. The

peripheral speed is given byWhere d1 and N1 are pitch circle diameter and r.p.m. of gear and d2 andN2 diameter and r.p.m. of attached dog respectively. Now N1 ≠ N2 since d1 ≠ d2 . Thus there is

a difference in gear and dog which necessitates double declutching. The driver has to disengage the clutch twice in quick succession therefore it is referred as double declutching. There are two steps involved in this process:

The clutch is disengaged i.e. first declutching and the gear system is placed in its neutral

position. Now the clutch is reengaged and acceleration pedal is pressed to adjust the engine speed according to driver’s judgment.The clutch is disengaged(i.e. second declutching) again the

appropriate gear is engaged and then the clutch is reengaged

It is that gear box in which sliding synchronizing units are provided in place of sliding dog clutches as in case of constant mesh gear box. With the help of synchronizing unit, the speed of both the driving and driven shafts is synchronized before they are clutched together through train of gears. The arrangement of power flow for the various gears remains the same as in constant mesh gear box. The synchronizer is made of frictional materials. When the collar tries to mesh with the gear, the synchronizer will touch the gear first and use friction force to drive the gear to spin at the same speed as the collar. This will ensure that the collar is meshed into the gear very smoothly without grinding.Synchromesh gear devices work on the principle that two gears to be engaged are first bought into frictional contact which equalizes their speed after which they are engaged readily and smoothly.

The following types of devices are mostly used in vehicles:

·        Pin Type

·        Synchronizer ring type

A synchronizing system is used for smooth meshing. Synchromesh works like a friction clutch. In the following figure two conical surfaces cone-1 is the part of the collar and the cone-2 is the part of the gear wheel. Cone1, 2 are revolving at different speeds. While cone-2 is revolving, cone-1 gradually slides into it. Friction slows or speeds up the gear wheel. Finally both the cones revolve at same speed.

revolving at different speeds. The internal cone comes in contact with the outer cone of the gear wheel. Friction slows or speeds up the gear wheel.

And when the collar and gear wheel rotate at same speed the spring loaded outer ring of the collar is pushed forward. The dog slide smoothly into mesh without clashing. The collar and gear wheel lock and revolve at same speed. This the principle of synchromesh.