7. DIFFERENTIAL OVERHAUL
In the conventional layout, the differential is a separate unit placed between the two rear wheels. In a four-wheel drive car, there are two differentials one each between the front and the rear wheels. In the case of Maruti 800 and such other front wheel drive cars, however, the transmission gearbox and the differential are assembled together as one compact unit. In this lesson, we shall study about removal, dismantling, inspection and assembly of the differential of a Maruti 800 car.
7.0
Objectives After going through this lesson you will be able to: Dismantle the transmission. Understand the working of the differential. Dismantle, inspect its components and re-assemble the differential.
i. ii. iii.
7.1
Dismounting the Transmission
The transmission case and the clutch housing, as shown in the figure, is one compact unit in a Maruti 800 car. The transmission case houses the gearbox and the differential. To be able to overhaul the differential, the transmission has to be first dismounted from the engine as per the procedure outlined in lesson 1 of this course. After the transmission has been dismounted, we proceed further as detailed in the succeeding sections. Fig. 7.1: Clutch housing and transmission case.
7.2 •
Disassembly of Transmission Components (Repeat from section 4.1 of Lesson 4 of CAF 104) 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 sequence of steps is followed:
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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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7.3
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. The bolts are of varying lengths. Tap the upper cover with a plastic hammer and lift the upper cover.
Working of the Differential
After the upper gear case has been removed, the differential unit in the transmission case becomes visible as shown in the figure. The figure 7.2 shows a large helical gear, which is called the final gear of the differential unit. In a conventional differential, this final gear is known as the crown wheel. Drive is transmitted to the wheels via the cage and the side gears from this final gear. The side gears are also called the sun gears. The front axles, which drive the wheels, are mounted on the splines of the two side gears. Sun Gear
Cage
Fig. 7.2: Differential (a) Complete unit. (b) Final gear.
When the transmission input shaft rotates and the transmission is in gear, the final drive pinion drives the final gear of the differential. This rotates the cage, which,
in turn, moves the side gears and the axles. At this time, the planet pinions do not rotate about their axes. When one axle (that connected to inner wheel) is loaded due to transfer of weight of the car on turns, as the road is inclined inwards, the side gear of the inner wheel slows down due to additional force. The planet pinions start rotating about their axes and transfer rotation to the side gear to which the outer wheel is connected. When the car turns towards the other side, phenomenon is reversed. This is because the planet pinions then turn in the opposite direction.
Self-check Questions 1. In the conventional layout, the differential is a separate unit placed between the ________________. 2. In a four-wheel drive car, there are _____differentials. 3. In front wheel drive cars the transmission gearbox and the differential are assembled together as one unit. State whether True or False. 4. The differential unit in Maruti 800 is located at the rear axle. State whether True or False. 5. The final gear of the differential is a ________ gear 6. In a conventional differential, the final gear is known as the ___________. 7. The side gears are also called the __________. 8. The front axles, which drive the wheels, are mounted on the splines of the __________. 9. The planet pinions do not rotate about their axes when the car ______________ 10. The planet pinions start rotating about their axes when the car ____________
7.4 • • • • •
Dismantling Remove both the oil seals that are mounted on the side bearings, one by one. Hold the cage in a bench vice. Remove the bearing with the help of a bearing puller. Hook the legs of the bearing puller on the outer race of the bearing. Screw in the center bolt on the shaft center. This will pull the legs outwards and the bearing will slide out.
The side bearings used in the differential are of No. 6207 Z. The letter Z in the bearing number designation denotes that the bearing is sealed on one side. The seal is kept towards the inside i.e. away from the axle side. •
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Remove the other bearing by holding the final gear in the bench vice. Make sure not to clamp on the bearing journal or the splines of the sun gear, otherwise, these may get damaged. Hold the cage in the vice and loosen and remove the eight bolts of No 14 size, which hold the final gear to the cage. Either a ring spanner or a socket with handle can be used.
Planet Pinion
Planet Pinion Pin
Fig. 7.3: (a) Planet pinions and pin. (b) Planet pinion pin.
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Tap the final gear with a plastic hammer and remove it from the collar of the cage. Lift out the sun gear from the cage. The rotation of the puller is prevented by inserting a screwdriver between the
legs. • •
Remove the lock pin from the planet pinion pin. Push the planet pinion pin out and remove the two planet pinions along with their thrust washers.
The planet pinion pin has a flat surface at both ends so that oil may seep in for lubrication. •
7.5 •
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7.6 • •
Remove the second sun gear from the cage.
Inspection Inspect the final gear visually by examining the teeth of the gear for wear. The surface of the gear where thrust washer of sun gear bears should be smooth and shiny. Also the bearing journal of the final gear should be smooth and shiny. There should be no evidence of scratch marks etc. Inspect the cage for wear at the thrust faces of planet pinions and side gear and bearing journal. Inspect the side gears for wear at teeth, journals, and splines. Inspect the planet pinions for wear at the thrust face and teeth. Inspect the planet pinion pin for wear.
Assembly Apply lubricating oil to a side gear and thrust washer and place it in the cage. Rotate the gear and check its free rotation.
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Smear some oil on the planet pinion pin and insert it in the cage. Keep the lock pinhole in the pin towards the lock pinhole in the cage. Apply oil at the back and inside of a planet pinion, thrust washer and the thrust face of cage. Push the pin inside after placing the pinion in the cage. Normally, it is a push fit. Place the other planet pinion on the opposite side, after applying oil, and keep it in proper position. Align the hole in the pinion with the hole in the cage and push the planet pinion pin keeping the lock pin hole in the correct position. Push the pin, by a hammer if needed. Match the lock pinholes and insert the lock pin in place. Check the free rotation of pinions and side gear on one side in the cage. The half assembly is now ready. Similarly, the other side gear is fitted.
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Apply lubricating oil on the thrust faces of the sun gear and washer, the cagehalf in the final gear, and on the bearing journal of the final gear. Fit the cage-half over the final gear after matching the holes of cage and the final gear. Screw in the bolts of No 14 size by hand. Tighten the bolts, on opposite sides, one by one, followed by tightening of the bolts to a torque of 8-10 mkg by a torque wrench. Check the bolts again for proper torque. Check the assembly by moving the side gears. The pinions should move freely but there should be no play between the gear and the pinion. Check the backlash with the help of a fuse wire. Fit one side bearing with the sealed side of the bearing towards the inside, that is, the final gear side. Fit the bearing with the help of a bearing installer and hammer. Fit the other side bearing with the sealed side of the bearing towards the cage side and the open side of the bearing towards the outside, with the help of a bearing installer and hammer. Check bearing for free movement and for any noise and stickiness after fitment. The whole assembly must move freely in the bearings. This completes the assembly of the differential unit.
Self-check Questions 11. 12. 13. 14.
The side bearings used in the differential are of No. __________. The letter Z in the bearing number designation denotes that ________________ The seal of the side bearing is kept towards the ____________________. The planet pinion pin has a flat surface at both ends so that__________________________.
7.7
Inspection of Components and Transmission Re-assembly
When the differential assembly is overhauled, the components of the transmission are also checked for proper functioning. • • • • •
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Inspect the transmission input shaft and counter shaft visually for any apparent defect. Place both the gear shafts in the lower case of the transmission taking care to properly mesh the gears. Check the proper meshing between gears by rotating the transmission input shaft with hand. Place the differential in its proper place in the transmission housing. Check the free rotation of all the gears and the final gear assembly by moving the input shaft. Apply Suzuki Bond No 1215 over the matching faces of the lower case and the upper case before bringing them together. Place the upper case over the lower case, ensuring that the clutch fork shaft is held in the lower case and the idle gear is installed in the upper case. At the time of fitting the upper case, care should be taken that the reverse idle gear should fit properly in the reverse fork lever. Tap the upper case with plastic hammer for proper fitment. Insert all the bolts in proper place. The bolts are of unequal length. Also, one bolt is of No 10 size and the rest of the seventeen bolts are of No 12 size. Tighten the bolts holding the upper case to the lower case in the prescribed sequence.
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Place the oil seal over the side bearing and fit the end plugs over the countershaft and the input shaft of the transmission with a plastic hammer. Insert the oil gauge. Insert speedometer gear bush and lock it with a bolt. Install the second oil seal over the other side bearing. Fit the end plug on the countershaft-bearing retainer at the clutch side after applying Suzuki Bond No 1215. Tighten the bolts holding the upper case to the lower case to a torque of 7.5 mkg. Fit the input shaft-bearing retainer by four screws with a Philips screwdriver. Ride the spring over the clutch fork. Fit the release bearing along with the retainer over the fork. Fit the clutch release fork arm along with spring and match the punch mark on the fork and release arm. Tighten the nut.
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Mount the transmission on the engine.
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Self-check Questions 15. Apply ___________________over the matching faces of the lower case and the upper case before bringing them together. 16. Tighten the bolts holding the upper case to the lower case to a torque of ________.
7.8 Assignments 7.8.1 Class Assignment 1. 2. 3. 4.
Discuss the consequences if differential in not installed in a 4-wheeler. Is a differential installed in a 2-wheeler and a 3-wheeler? Discuss. Discuss the absence of differential in a cycle rickshaw. Is a differential used in a locomotive engine and train coach? Discuss.
7.8.2 Home Assignment 1. Write the working procedure of differential. Why it is used in a vehicle?
7.9
Summing Up
The differential is also a gearbox but it has a different function to perform. It should be assembled to a high degree of accuracy and any laxity in assembly may give rise to a whining noise at high speeds. When assembling the differential, special care should be taken to ensure proper tooth contact and proper backlash between the mating gears. After the transmission has been assembled it is mounted on the engine and the whole unit is installed in the car. We will study engine re-mounting in the next lesson.
7.10 1. 2. 3. 4. 5. 6. 7.
Possible Answers to Self-check Questions two rear wheels two True False helical crown wheel sun gears
8. side gears 9. moves straight 10. takes a turn 11. 6207 Z 12. the bearing is sealed on one side 13. inside i.e. away from the axle side 14. oil may seep in for lubrication 15. Suzuki Bond No 1215 16. 7.5 mkg
7.11
References
1. Maruti 800 Service Manual
7.12
Glossary
Backlash
Clearance between the meshing teeth of two gears
Crown wheel and pinion
Part of the final-drive assembly; a pair of bevel gears, one large (crown wheel) and one small (pinion). The longitudinal propeller shaft is connected to the pinion, and the crown wheel is connected to the transverse half-shafts through the differential.
Differential
Gear assembly between axles that permits one wheel to turn at a different speed from the other, while transmitting power from the drive shaft to the wheel axles
Final gear
Maruti transmission has final gear instead of crown wheel through which differential gets the drive
Helical gear
Gears that have teeth cut at an angle.
Planet pinion
Pinion gear attached to a planet carrier (cage) that surrounds a sun gear in a planetary gear set.
Sun gear
The center gear in a planetary gear set.
Suzuki bond No. 1215
Proprietary adhesive