4. TRANSMISSION GEARBOX OVERHAUL
In the previous lesson, we learnt about the removal, dismantling, inspection and re-assembly of the gearshift control mechanism which is used to shift gears in the transmission. In this lesson, we will study about the transmission itself. Before dismantling the transmission, it is removed from the engine. Only after that, the transmission is dismantled. After dismantling the transmission we will study how the various gears provide different speed ratios and the gear selecting and the gear-shifting mechanisms. Finally, the countershaft will be dismantled for inspection of its various components.
4.0
Objectives After going through this lesson you will be able to: Dismantle the transmission. Explain the flow of drive through transmission. Describe the working of the synchronizer. Dismantle the countershaft.
i. ii. iii. iv.
4.1 •
Disassembly of Transmission Components Remove the clutch release bearing from the release fork after moving the clutch fork to the extreme forward position by moving the clutch release arm.
The input shaft bearing retainer is removed next. For this, the following is sequence of steps to be followed: • •
Remove the four screws with the help of a Philips head screwdriver. Tighten two screws, out of the four removed, in the two threaded holes provided in the retainer in order to pull out the retainer from the gear case. During this time, keep the fork lifted upwards and out of the way by pushing the release arm backward.
After the input shaft-bearing retainer has been removed, the clutch release arm is removed. • •
Loosen the clutch release arm nut with the help of a 12 mm open-end spanner. Pull out the arm along with the return spring. The upper gear case is removed next.
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Loosen all 18 bolts on the upper gear case. Remove one bolt by 10 mm socket at the rear end of the gear case. Remove all the 17 bolts of No 12 size with a 12 mm socket and speed handle. Note that the bolts are of varying lengths.
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Tap the upper cover with a plastic hammer and lift the upper cover. Leave all the parts in the lower case.
Fig. 4.1 shows the reverse idle gear in the upper case. Its purpose is to reverse the rotation of the countershaft so that the car moves in the reverse direction. • • • •
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Remove the lock pin from the upper case. Take out the reverse gear and the shaft. Remove the speedometer driven gear. Remove the clutch release shaft and fork along with the return spring. Remove the countershaft end plug. Set the upper case aside.
Fig. 4.1: Reverse idle gear.
The input shaft drives the countershaft and the final gear of the differential. There are splines on the side gear where the drive shaft is connected. The drive shaft drives the wheel. The various gears on the countershaft are the first speed gear, the reverse gear over the low speed synchronizer sleeve, the second speed gear, the third speed gear, the high-speed synchronizer sleeve and the top speed gear. There is also a final drive pinion, which drives the final gear of the differential.
4.2
Flow of Drive through Transmission
The gears on the input shaft and the countershaft are always in mesh. The low speed synchronizer on the countershaft is engaged either with the low, that is the first driven-gear or the second driven-gear. The high-speed synchronizer is engaged either with the third driven-gear or the top driven-gear. The reverse idlergear is of the sliding mesh type and is engaged with the low speed synchronizer sleeve on the countershaft and the reverse drive-gear on the input shaft. The lower case of the transmission has the three fork shifting mechanism built in it. The upper case houses the reverse gear shift.
Fig. 4.2: (a) First speed gear. (b) Reverse gear.
The low or the first speed driven-gear on the countershaft rotates freely on this shaft, being driven from the low drive-gear of the input shaft. Shifting the lever into “low” causes the low-speed-gear shifter fork to push the low speed synchronizer towards low driven-gear and mesh with it through the dogteeth, thus driving the countershaft, since the synchronizer is mounted on the countershaft on splines. In this condition, the drive is transmitted through the low drive-gear on the input shaft and the low driven-gear on the countershaft to the final gear of the differential. In order to obtain the reverse drive, shifting the lever into “reverse” causes the reverse gear shifter fork to mesh the reverse idle-gear with the reverse gear on the input shaft and the low speed synchronizer sleeve on the countershaft. In this condition, the drive is transmitted through the reverse gear on the input shaft, reverse idle-gear and the low speed synchronizer on the countershaft to the final gear of the differential.
Fig. 4.3: (a) Second speed gear. (b) Third gear.
In the second speed drive, shifting the lever into “second” causes the same low speed gear shifter fork to push the low speed synchronizer to the other direction that is, towards the second driven-gear and mesh it with this gear, thereby driving the countershaft. In this condition, the drive is transmitted through the second drivegear on the input shaft and second driven-gear on the countershaft to the final gear of the differential.
In the third speed drive, shifting the lever into “third” actuates the high-speed shifter fork to engage the high-speed synchronizer with the third driven-gear on the countershaft. This gear, like the low and second driven-gears, is free on the shaft and merely rotates being driven by the third drive-gear of the input shaft when the gearshift lever is in any other position. However, when the lever is shifted to “third” position, the drive is transmitted through the third drive-gear on the input shaft and the third driven-gear on the countershaft to the final gear of the differential.
Fig. 4.4: (a) High speed sleeve. (b) Top speed gear.
In the top speed drive, shifting the lever into “top” causes the high-speed shifter fork, which is also used for the third speed, to mesh the top gear with the highspeed synchronizer on the countershaft. In this condition, the drive is transmitted through the top drive-gear on the input shaft and the top driven-gear on the countershaft to the final gear of the differential. Fig. 4.5: Final gear.
Self-check Questions 1. 2. 3. 4.
Name the 2 shafts in the transmission. Name the various gears on the countershaft. What is the purpose of the idle gear in the gearbox? The gears on the input shaft and the countershaft are always in mesh. State whether True/False 5. How many speeds are provided by the transmission of the Maruti 800 car?
4.3
Working of the Synchronizer
A synchronizing device is used in the Maruti 800 transmission in order to match the speeds of the gear and the countershaft so that meshing of the dog clutch is smooth. The synchronizing device consists of a splined hub, which fits on the splined counter shaft. The hub carries three keys in slots and these keys are forced outwards by two expanding springs. This hub and keys assembly is contained within a sleeve, which has a collar in which the shifter fork is carried. On both ends of the sleeve, outside the Fig. 4.6: Components of synchronizer hub, there are two synchronizer rings, assembly. which are made of gunmetal. These synchronizer rings have dog teeth on their outer periphery and an internal cone with suitable groove pattern. When the sleeve slides towards the gear whose rotation is to be transmitted to the countershaft, the internal cone of the synchronizer ring first matches with the external cone of the gear. This gear is always in mesh with the driver gear on the input shaft. It can freely rotate on the countershaft on a needle bearing which is carried on a splined bush. The gear also carries dogteeth on its periphery behind the cone. When gears have to be shifted, it is done by sliding the sleeve in the desired direction with the help of the shifter fork, which causes the keys to push out the synchronizer ring. The internal cone of the ring matches with the external cone of the gear and their speeds are synchronized, that is, both start rotating at nearly the same speed. The keys, which fit in the slots of the ring, move with the ring and thereby rotate the hub, which in turn rotates the sleeve and the countershaft. By this time, the sleeve is pushed forward further so that its teeth ride over the dogteeth of the ring and the gear. The sleeve, hub, and the countershaft are then positively driven at the speed of the gear.
Self-check Questions 6. Synchronizer is used in the Maruti 800 transmission in order to match the speeds of the _____ and the ______________ so that meshing of the dog clutch is smooth. 7. Name the parts of the synchronizer.
4.4 • • • •
Dismantling of Gearbox Remove the input shaft end plug and the countershaft end plug. Lift out the input shaft and the countershaft from the lower case. Remove the two axle seals from the side of the differential. Lift out the differential unit.
The three-fork shifting mechanism built in the lower case of the transmission consists of the high speed shifter fork, the low speed shifter fork and the reverse gear shifter fork. The first two forks are directly mounted on their respective shifter fork shafts. The reverse gear shifter fork is moved by a lever fixed to the reverse gear shifter fork shaft. The forks are moved sideways by moving the shifter fork shafts by the gear shifter shaft through the gear selecting mechanism. The gear shifting mechanism has levers which engage forks of the appropriate shifter block which is further locked to the shaft to move the shaft and hence, the fork. • • •
Remove the five C-rings from their grooves in the lower case by pushing them out with a screwdriver. Loosen the No 14 size special bolt with a ring spanner and open it with a Philips head screwdriver. Pull the spring out from the hole and tap the case to remove the steel ball. The spring loaded ball helps to locate the gear selector shaft in position.
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Overturn the transmission case and loosen the bolts holding the plate enclosing the gear selecting mechanism with a No 10 socket and handle and then open them with a ring spanner. Pry open the plate with a screwdriver and plastic hammer and remove it. Push out the spring pin, which holds the selector block to the selector shaft with a pin remover and a hammer.
The spring pin, which holds the selector block to the selector shaft, is split to provide the spring action. • •
Remove the selector shaft from the block and the case. Remove the gear selecting lever and shaft along with the two springs.
To remove the shifter forks and the blocks from the shafts from inside the gear lower case, the following is the sequence of steps: • • • • •
Push out the lock pin holding the low speed shifter fork and block to its shaft with a pin remover and a hammer. Push out the stopper pin also on the shaft. Push out the lock pin of the high-speed shifter fork and its stopper pin in a similar manner. Push out the lock pins of the shifter block and the fork lever on the reverse gear shaft. Remove the stopper pin on the reverse gear shaft.
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Push the low speed shifter fork shaft to one side and remove its one end from the lower case with the help of a screwdriver. Pull out the shaft completely from the lower case, holding the other end with a water pump plier. Push out the high-speed shifter fork shaft similarly. Remove the fork and the block from the gear case. Remove the reverse gear shifter fork shaft, simultaneously removing the shifter block and the shift fork lever from the shaft. Loosen the No 14 bolt and remove the reverse gear fork. The lower case would be stripped of all its components by now.
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Turn the case upside down. Tap out the steel balls.
The bigger balls are for interlocking the shifter shafts and the smaller ones are for locking the position of a gear after meshing. • • •
Push the idler shaft with a screwdriver and remove the idler gear. Remove the idler shaft from the upper case. Loosen the screw and remove the speedometer gear case. The countershaft assembly is dismantled next.
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Hold the countershaft in the countershaft holder and clamp it in a vice. Unscrew the countershaft nut with the help of a No 22 ring spanner. Remove the countershaft nut. Pull out the speedometer driven gear from the countershaft.
A bearing puller is used to remove the countershaft bearing from the countershaft. • • • •
Slide the legs of the bearing puller under the outer face of the bearing. Place the center bolt of the bearing puller at the end of the countershaft. Screw in the center bolt, but prevent the rotation of the bearing puller at the same time. The bearing will slide out smoothly. Lift out the other parts one by one. The washer has a cut for a steel ball, which prevents its rotation.
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Remove the first gear and the needle bearing. The gear rotates freely on the countershaft due to this needle bearing.
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Remove the steel ball and the synchronizer ring. The ring has a wire spring mounted on it.
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Push out the bush. This bush has the needle bearing mounted on it.
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Dismantle the components of the low speed synchronizer.
The low speed synchronizer also acts as the reverse gear. Within its sleeve is installed a hub which is mounted on the countershaft on splines. There are three keys that position the hub in the sleeve. •
Lift out the second synchronizer ring and the second speed gear.
The anti-rattle spring prevents the sideways movement of the second and third speed gear. • • •
Remove the needle bearing, the splined bush and the washer. Lift out the third gear and its needle bearing, the splined bush and the synchronizer ring. Remove the splined washer and then the synchronizer.
The high-speed synchronizer also has a sleeve and a hub. There are three keys, which fit in the hub. • •
Remove the synchronizer ring for the top gear. Lift off the top gear.
The top gear is mounted on a needle bearing and has a washer placed beneath it. This washer has a cut for a lock pin to prevent its rotation. • •
Remove the lock pin with a pair of tweezers. Remove the roller bearing.
Self-check Questions 8. Spring loaded ball helps to ______________________________. 9. Spring pin, which holds the selector block to the selector shaft, is ____ to provide the spring action. 10. The bigger balls are for _______________________ and the smaller balls are for _______________________. 11. The anti-rattle spring is installed between the ___________________.
4.5
Assignments
4.5.1 Class Assignment 1. Discuss the necessity of transmission in the car.
4.5.2 Home Assignment 1. Write a note on the construction and working of the synchronizer.
4.6
Summing Up
We studied about dismantling of transmission in this lesson besides studying how drive flows through the transmission to provide different gear ratios. In the next lesson, we will inspect the components so dismantled. .
4.7
Possible Answers to Self-check Questions
1. input shaft and the countershaft 2. first speed gear, reverse gear over the low speed synchronizer sleeve, second speed gear, third speed gear, high-speed synchronizer sleeve and the top speed gear; final drive pinion, which drives the final gear of the differential 3. to reverse the rotation of the countershaft so that the car moves in the reverse direction 4. True 5. 4 forward and one reverse 6. gear, countershaft 7. splined hub, three keys with outward expanding springs, sleeve, synchronizer ring. 8. locate the gear selector shaft in position 9. split 10. interlocking the shifter shafts, locking the position of a gear after meshing 11. second and third speed gears on the countershaft
4.8
Terminal Questions
1. Describe the working of the transmission of the Maruti 800 car. 2. Explain the working of the synchronizer 3. Write a note on flow of drive through transmission.
4.9
References
1. Maruti 800 Service Manual
4.10
Glossary
Countershaft A shaft that is below the main shaft in a conventional manual transmission having three shafts. It is also called a lay shaft. The gears on the countershaft are called counter gears. In the Maruti 800 transmission, it is the second of the two shafts and is the output shaft C-ring C-shaped steel ring used to hold bearing in casing as the C-groove is cut in both Interlocking balls Steel balls used to interlock gear shifter fork shafts Spring pin Split pin with spring action so that it does not get loose in the shifter fork shaft Stopper pin Solid pin used to limit the motion of shifter fork shaft Synchronizer An assembly containing the sleeve, hub, keys, and ring used to make a clash-free change of gear ratio in a manual transmission/transaxle. Transmission Mechanical system that transmits the torque to the driving wheels; it usually consists of clutch, gearbox, propeller shaft, final drive and half-shafts.