Gear Box Mechanical subject
Gear Box
B.tech ( mechanical Engineering)
“The gear box which is also known as the transmission system. It is the second element of the power train in an automobile. It is used to change the speed and torque of vehicle according to variety of road and load conditions.”
Function of Gear Box-
1.Provide the torque needed to move the vehicle under a variety of road and load conditions. It does this by changing the gear ratio between the engine crankshaft and vehicle drive wheels.
2. Be shifted into reverse so the vehicle can move backward.
3. Be shifted into neutral for starting the engine.
4. To provide neutral, so that a plant vehicle can be operated with the vehicle at rest, but with the engine running at high revolutions in order to provide power for units such as hydraulic pumps.
5. To provide a means to take power away from the gearbox to power auxiliary components (PTO).
Main Components of Gear Box-
1. Shaft- Shaft is a cylindrical member made of cast iron, alloy steels. It's transmitted power from one place to another place. There are 3 shafts present in this gearbox which are :
a. Counter shaft or Lay Shaft:-Counter shaft is a shaft which connects with the clutch shaft directly. It contains the gear which connects it to the clutch shaft as well as the main shaft. It may be run at the engine speed or at lower than engine speed according to gear ratio.
b. Main shaft:-It is the shaft which runs at the vehicle speed. It carries power from the counter shaft by use of gears and according to the gear ratio, it runs at different speed and torque compares to counter shaft. One end of this shaft is connects with the universal shaft.
c. Clutch Shaft:- The clutch shaft carries the engine output to the gearbox but act as input for the gearbox. It is also known as input shaft.
2. Gears:- Gears are used to transmit the power from one shaft to another. They are most useful component of gear box because the variation is torque of counter shaft and main shaft is depends on the gear ratio. If gear ratio is large than one, the main shaft revolves at lower speed than the counter shaft and the torque of the main shaft is higher than the counter shaft. On other hand if the gear ratio is less than one, than the main shaft revolves at higher speed than the counter shaft and the torque of the main shaft is lower than the counter shaft. A small car gear box contains five speed gear ratio and one reverse gear . There are three types of gears are used in gearbox namely-
a. Spur Gears
b. Helical Gears
c. Bevel Gears
3. Bearings:- Whenever the rotary motion encounters, bearings are required to support the revolving part and reduce the friction. In the gear box both counter and main shaft are supported by the bearing.
Types of Gear Box-
1. Manual Transmission
i) Sliding Mesh Gearbox
ii) Constant Mesh Gearbox
iii) Synchromesh Gearbox
2. Automatic Transmission
i) Hydramatic Gearbox
ii) Torque Converter Gearbox
3. Epicyclical Gearbox
Manual Transmission
In this type of transmission different speed ratio or gear ratio is selected by the driver manually. Some special skill of driving is required to operate this type of gear box. This is subdivided into three types.
i) Sliding mesh gearbox
ii) Constant mesh gearbox
iii) Synchromesh gearbox
i) Sliding Mesh Gearbox-
It's Simplest type of gearbox & Simple in construction. In this gearbox, spur gears are used. The gears in the layshaft is rigidly fixed shaft. The gears in main shaft alone can move along the spline. The selector fork moves the gear and moves the perfect mesh with the required gear in lay shaft when the gear lever is shifted. A idler gear is provided for reverse gearing. Thus the power form the clutch shaft is transmitted to the main shaft through the lay shaft.
History-
The credit for the development of first modern manual transmission was given to French inventors Louis-Rene Panhard and Emile Levassor.
Parts of Sliding Mesh Gear Box-
It consist of following parts.
1. Output shaft
2.Low and reverse sliding gear
3. Second sliding gear
4. Clutch
5. Input shaft
6. Clutch gear
7. Counter shaft drive gear
8. Counter shaft
9.Low speed gear
10. Second gear
11. reverse gear
12. Reverse idler gear
13. Gear shift fork
Working of Sliding Type Gear Box
First Gear - In the first gear, the gearbox provides maximum torque at low speed. The smallest gear on the lay shaft engages with the biggest gear in main shaft, thus transmitting high torque.
Second Gear - In the second gear, the gearbox provides low torque and high speed when compared to1st gear .
Third Gear - In the third gear, the gearbox provides low torque and high speed when compared to 2nd gear.
Fourth Gear - For fourth gear the clutch shaft and the main shaft is engaged by a dog clutch, thereby the main shaft rotates at maximum speed as that of clutch shaft.
Reverse Gear - When selecting reverse, the direction of rotation of the output shaft is changed This is achieved by using an idler gear. The idler gear is meshed between a counter shaft gear (layshaft or 2nd motion shaft) and an output shaft gear(main shaft or 3rd motion shaft).
Advantages of Sliding Type Gear Box
1. Since only one gear is in mess in sliding mesh gearbox so less fluctuating loads on shafts causing less vibration and noise unlike the constant mesh gearbox in which all gears are in constant mesh.
2. Its efficiency is more than constant gearbox as only one gear is in mess unlike the constant mesh gearbox in which all gears are in constant mesh.
3. Its manufacturing is easy as compared to constant mesh gearbox.
4. Its mechanism is simple.
1. Only spur gears can be used as gears are not in constant mesh like constant mesh gearbox in which helical or herringbone gears can be used.
2. More effort is required to engage the gear as the gear has to be slide in sliding mesh gearbox unlike constant mesh gearbox where only dog clutch has to be slide for engagement of different gears.
3. Less life of gear as more wear and tear of gear is caused in sliding mesh gearbox due to friction.
4. It takes more time and money to replace the gears if the gearbox fails but in constant mesh gearbox only dog clutches are to be replaced at failure which takes less time and money.
Applications of Sliding Type Gear Box
1. Sliding Mesh Gearbox is used in Alfa 12HP with 4-speed manual transmission.
2. It is also used in Fiat 6HP with 3-speed manual transmission.
3. It is also used in Mercedes 35 HP and Renault Voilturette.
ii) Constant Mesh Gearbox-
This is gearbox which was invented to overcome the limitations of sliding mesh gearbox .In this gearbox all the gears are always in mesh. The gear remains fixed and not slide like the sliding mesh gearbox. This gearbox has different parts like counter shaft, main shaft, clutch shaft, gears and dog clutch. In this gearbox all the gears in the main shaft and the counter shaft are always engaged with each other . Different transmission ratio or speed ratio are obtained by using the dog clutch .
Parts of Constant Mesh Gear Box
1 Shafts – There are 3 shafts present in this gearbox which are :
i) Main Shaft- It is also known as output shaft. It is the splined shaft over which the dog clutches along with gears are mounted. Gears on this shaft are free to rotate.
ii) Lay Shaft or Counter Shaft- It is an intermediate shaft between the Main Shaft and Clutch Shaft. The gears of counter shaft are in constant mesh with gears of main shaft. Also the gears of counter are shaft are not free to rotate as they are directly connected to the Counter Shaft.
iii) Clutch Shaft:- The clutch shaft carries the engine output to the gearbox but act as input for the gearbox. It is also known as input shaft.
2 Dog Clutch:- The dog clutch couples the lay shaft and main shaft by interference and not by friction. Dog clutches are used to transmit appropriate gear ratio to the main shaft or output shaft by coming in interference with pair of gears with suitable gear ratio. There are usually two dog clutches in a Constant Mesh Gear Box.
3) Gears:- Gears of constant mesh gearbox come in pairs. All gears of lay shaft or counter shaft are always paired with gears of main shaft or output shaft.
This paired gears of counter shaft and main shaft provide different gear ratio which can be transmitted to main shaft by engaging dog clutch with appropriate gear ratio required. Two type of gears are used in constant mesh gearbox:-
i) Helical Gears:- These gears have angular cut teeth over cylindrical cross-section metal body.
ii) Bevel Gears:- These gears have angular cut teeth over conical cross-section metal body.
Working of Constant Mesh Gearbox
When the dog clutch is engaged with different gears of main shaft different gear ratios are obtained as gears of main shaft are always paired with gears of counter shaft to form different gear ratios. If the dog clutch is not in contact with any gear of main shaft the gears of main shaft rotates freely and does not rotates the main shaft as they are connected with main shaft using bearings. The main shaft rotates only when one of the dog clutch is engaged with any of the gear of the main shaft. Reverse gear is obtained in this gearbox using the same technique that was in sliding gearbox i.e. using the idle gear between main shaft gear and counter shaft gear.
First Gear:- First gear is obtained in constant mesh gearbox when dog clutch gets engage by interference with the largest gear of main shaft which is in constant mesh with smallest gear of main shaft. This gear provides maximum torque and minimum speed to the main shaft.
Second Gear:- Second Gear is obtained when dog clutch gets engage with second largest gear of main shaft which is in a constant mesh with second smallest gear of lay shaft. This gear provides higher speed and lower torque than first gear.
Third Gear:- Third gear is obtained when dog clutch engages with second smallest gear of main shaft which is in constant mesh with second largest gear of lay shaft. This gear more speed and less torque than second gear.
Fourth Gear:- This gear provides the highest or maximum speed in a vehicle using constant mesh gearbox. This gear is obtained when dog clutch engages with smallest gear of main shaft which is in constant mesh with largest gear of lay shaft.
Reverse Gear:- In this gear the vehicle goes in reverse direction. Like sliding mesh gearbox, an idler gear is also used in constant mesh gearbox between the main shaft gear and lay shaft gear to form reverse gear. Reverse gear is obtained when dog clutch engages with gear in main shaft which is paired with idler gear.
Advantages of Constant Mesh Gearbox
1. Quicker gearing can be achieved.
2.Only damage occurs on dog clutch teeth not teeth of gear wheels.
3.Synchronizing devices can be used for smooth engagement of gears.
4.Once the dog clutches are engaged there is no motion between their teeth but the power is transmitted through the sliding action on teeth on one wheel on those of other when the gear teeth are engaged.
5.To transmit the motion properly, the teeth are suitably shaped.
Disadvantages of Constant Mesh Gearbox
1. It is less efficient than the others due to higher mesh teeth. Skill is required for it.
2. The double clutch mesh is required. This is required to have the spinning movements of the shaft.
3. Noise & Vibration problem during changing of gears.
Applications of Constant Mesh Gearbox
1. Constant mesh gearbox was mainly used in farm trucks, motor bikes and heavy machinery.
2. It is also used in cars like Ford Model T.
3. Constant Mesh Box was used in motor bikes before the introduction of synchromesh gearbox in 1928 by General Motors.
iii) Synchromesh gearbox
1. Main shaft Gears:- A diaphragm shaft is used to connect the syncing devices and gears to the output shaft.
Synchromesh gearbox is a modification of the constant mesh gearbox for smooth gear changes. The main difference between the constant mesh gearbox and synchromesh gearbox is that synchronizer is used in place of dog clutch in synchromesh gearbox. In constant mesh gearbox lots of noise and vibration is created during the gear changing process. To overcome this synchromesh gearbox was developed which synchronizers are used which has conical grooves cut over its surface that provide frictional contact to the gears which is to be meshed in order to equalize the speed of main shaft, lay shaft and clutch shaft which in turn provides smooth shifting of gears. This gearbox is used in nearly 52% of the vehicle on the road today.
Parts of Synchromesh Gearbox
2. Lay Shaft Gears:- It is the mid shaft over which rotational motion from the collision shaft to the final output shaft is placed on the appropriate sized gears.
3. Clutch Shaft:- It is the shaft used as an inserted shaft in the transmission box when the output of the engine is brought to the transmission box.
4. Cone Synchromesh:- There are two buttons on the side of the device to be used. The first is a hollow cone and the other is the ring of the dog’s teeth. The gear is made of the cones and teeth in contact with the synchromesh.
5. Synchronizers:- These are special shifting equipment in synchromesh boxes which have conical grooves cut into their surface, which provide frictional contact with the equipment to be meshed so that the main shaft, lay shaft and embrace shaft speed is equalized, which, in effect, ensures a smoother shifting of the gears.
6. Gear lever:- It is the shifting lever operated by the driver and is used to select the appropriate gears.
Advantages of Synchromesh Gearbox
1. Smooth and Noise free shifting of gears which is most suitable for cars.
2. No loss of torque transmission from the engine to the driving wheels during gear shifts.
3. Double clutching is not required.
4. Less vibration.
5. Quick shifting of gears without the risk of damaging the gears.
Disadvantages of Synchromesh Gearbox
1. It is extortionate due to its high manufacturing cost and the number of moving parts.
2. When teeth make contact with the gear, the teeth will fail to engage as they are spinning at different speeds which causes a loud grinding sound as they clatter together.
3. Improper handling of gear may easily prone to damage.
4. Cannot handle higher loads.
Applications of Synchromesh Gearbox
It is used in bikes like KTM duke 390cc. & most of the race cars like formula-1.
Automatic Transmission
A transmission in which various speeds are obtained automatically is known as automatic transmission. In this type of gear box driver merely selects the general car condition such as forward or reverse. The selection, timing and engagement of gear for the required gear speed are accomplished automatically when the accelerator is pressed or depressed. Automatic transmission needs no gear change lever and clutch pedal since clutch and transmission is a combined unit and works automatically.
i) Hydramatic Gearbox
It's is an automatic transmission developed by both General Motors Cadillac and Oldsmobile divisions. Introduced in 1939 for the 1940 model year vehicles, the Hydramatic was the first mass-produced fully automatic transmission developed for passenger automobile use.
The Hydramatic was designed to combine hydraulic operation of a planetary gearbox (allowing much shifting to be automated) with a fluid coupling instead of a friction clutch, eliminating the need for de-clutching. The transmission would have four forward speeds (3.82:1, 2.63:1, 1.45:1, and 1.00:1) plus reverse, with all acceleration provided by gearing; its fluid coupling did not multiply the engine output as a torque converter does. It incorporated a parking pawl which was engaged when the shift selector was placed in reverse with the engine off. There was no separate Park position as found with modern transmissions.
The Hydramatic transmissions are termed as the fully automatic transmissions. They have essentially three or four speed and reverse epicyclic gearboxes with brakes and clutches operated by oil pressure. These are controlled by the joint action of a governor, whose speed is proportional to that of the car. Valve actuated transmission are currently in use. The Hydramatic drive does not have a clutch pedal to engage and disengage the transmission system from the engine. The motion of the vehicle is controlled entirely by accelerator and brake. The Hydramatic drive combines a four-speed forward and reverses automatic geared transmission with a fluid flywheel.
1. Hydramatic drive
2. Controlled-coupling Hydramatic transmission
3. Three-speed Hydramatic transmission
Gear Transmission System
The geared transmission consists of three sets of constant mesh helical planetary gears placed in series. Two planetary gear sets provide the four forward gear changes, and the third is used for reverse. The fluid flywheel acts on the cushion the impact of the automatic shift as well as to reduce the torque reactions of the engine. A centrifugal governor in the transmission selects the proper gear for each speed and throttle position.
The change from one gear to another is accomplished through hydraulically operated pistons. In some cases assisted by springs, within control planetary unit. The speed at which the various shifts occur is governed by the throttle position as well as by the centrifugal governor. When the throttle is opened, the gear shifts at higher and higher speed is obtained. Driver control is provided by a lever and segment mounted on the steering column beneath the steering wheel. This adjustment can be done to any one of the four positions-neutral, drive, low and reverse. In the drive, four forward transmission shifts automatically for any one of the four forward speeds. The low position is for hill climbing or muddy road.
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