AUTOMOBILE RUNNING SYSTEM AND POWER TRAIN 10. DIFFERENTIAL AND REAR AXLES
In the conventional layout, power from the propeller shaft has to be turned by 90O to be given to the wheels. This is accomplished by the differential. It is placed between the two rear wheels. There are two more functions performed by the differential. It differentiates the speed of the wheels at turns and also increases traction by reducing the gear ratio by about 4:1. In the conventional layout, the differential is a set of bevel gears assembled in a housing. In front wheel drive cars such as the Maruti 800, where the axis of wheels is the same as that of the differential, bevel gears are not used. Instead helical gears are used. In the conventional layout, drive from the differential is transferred to the wheels via the rear half shafts also called rear axles. These are of different types and are classified on the basis of the loads carried by them and the placement of wheel bearings. We shall study about rear axles also in this lesson.
10.0
Objectives After going through this lesson, you will be able to: i. ii. iii. iv. v. vi.
10.1
Explain the necessity of a differential Differentiate between the differential used in a conventional layout and that used in a front wheel drive car. Describe the construction and working of a differential Differentiate between various types of rear axles Tell why full-floating axles are used in heavy vehicles Understand how the axles are held in place
Introduction
In the conventional layout, power from the engine is transmitted along the length of the vehicle till its direction needs a change by 900. The differential performs this function. It is generally mounted near the longitudinal center line of the vehicle between the powered wheels (rear and/or front as the case may be). In fact the differential has three functions to perform:
• • •
To transmit the engine power to the powered wheels To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it reaches the wheels To transmit the power to the wheels while allowing them to rotate at different speeds on turns. This function is the one that earned the differential its name.
The different types of rear axles are classified on the basis of loads carried by them and the placement of wheel bearings. They are respectively called semi-floating, three-quarter floating, and full-floating. The loads that rear axles have to bear are driving torque, side thrust during cornering, and vehicle weight. Wheel bearing could be placed between the axle and the casing or between the casing and the wheel hub. The axle could be tapered on the outside i.e. toward the wheel side and the wheel mounted on this taper or it may have a flange to which the wheel is bolted. These details and more are discussed in the following sections.
10.2
Function of the Differential
When the vehicle is moving straight, the speed of both the powered wheels is equal but when it takes a turn, the speed of outer wheel has to be more than that of the inner wheel in order to obtain pure rolling motion of the wheels on the road. This function is performed by the differential. The differential also serves to increase the traction at the driving wheels by providing a gear reduction, that is, speed reduction of around 4:1. In other words, the speed of the driven wheels is only about one-fourth the speed of the gearbox output shaft or the propeller shaft. This is accomplished by keeping the size of the crown wheel (final gear in case of differential of front wheel drive car) four times that of the driving pinion.
• • •
Thus, the three functions of the differential are: to increase the speed of the outer wheel and decrease the speed of the inner wheel at turns. to turn the flow of power by 90O (in conventional layout). to increase the torque (traction) at the driving wheels.
Self-check Questions 1. How does the differential gets its name? 2. How many functions does a differential perform in a conventional layout? 3. What is the ratio of speeds of the propeller shaft and the road wheels?
10.3
Placement of the Differential
The differential is always used between the wheels that are given engine power. Hence it is placed between the rear wheels in rear wheel drive vehicle, between the
front wheels in front wheel drive vehicle, and between both the front and rear wheels in an all-wheel drive vehicle. Hence in an all-wheel drive vehicle 2 differentials are used. The front and the rear differential are given a proportion of the engine power through the transfer gearbox (called the centre differential in Fiiig. 10.1).
Fig. 10.1: Placement of differential(s) in rear, front, and all wheel drive cars
10.4
Construction of the Differential
10.4.1 Differential of Rear Wheel Drive Car The differential gear system is enclosed in a housing and consists of a crown wheel (called final gear in the differential for front wheel drive car), cage, two or more planet gears and two sun gears. Axles or drive shafts are connected to the sun gears via splines. The crown wheel (or final gear) gets drive from the drive pinion and rotates the cage which is bolted to it. The cage carries the two planet gears which revolve around the sun gears and rotate them at the same speed, and in the same direction, when the car is moving straight. These, in turn, rotate the half shafts and the wheels. Fig. 10.2: Bevel gears.
Fig. 10.2 shows some bevel gears. Such bevel gears are used for sun (side) gears and planet pinions in the differential. The crown wheel and drive pinion are also bevel gears in the conventional layout.
10.4.2 Differential of Front Wheel Drive Car In the case of a front wheel drive car, the final gear of the differential unit through which power enters the differential and the wheels are on the same axis. Hence power has not to be turned by 90O. The final gear here is a helical gear. In this differential bevel gears are not used. The transmission here is a compact unit containing the gearbox and the differential units in the same housing. In the case of a Maruti 800 car, since the differential and wheels are on the same axis, bevel gears are not used as the power is not required to be turned through 90 O.
Sun Gear
Cage
Fig. 10.3: Differential (a) Complete unit. (b) Final gear.
Self-check Questions 4. Give the placement of the differential. 5. How many differentials are used in a 4-wheel drive car?
10.5 Working of the Differential When a car is driving straight down the road, both the driven wheels rotate in the same direction at the same speed. The input pinion turns the crown wheel to which cage is bolted and none of the planet gears within the cage turn about their axis. Therefore both the sun gears are effectively locked to the cage. The sun gears have
internal splines to which the half-shafts are mated. At the ends of the shafts are the wheels on both sides.
Fig. 10.4: Working of differential.
When the car takes a left turn, some of the weight of the car is transferred to the left wheel due to the inclination of the road, and it represents an extra load. The right side wheel, on the other hand, experiences a decrease in the load. Due to this, the two planet gears rotate about their axis also, besides revolving about the sun gears, and transfer the speed of the left hand side wheel to the right hand side wheel. Thus the inside wheel rotates slower than the cage while the outside wheel rotates faster.
Self-check Questions 6. Planet pinions spin about their axis when the car takes a turn/moves straight ahead. Choose the correct alternative. 7. Sun gears and planet pinions are _____ gears. 8. The ______ wheel rotates at a slower speed.
10.6
Rear Axles
Power from the differential is transmitted to the rear wheels through the rear axles. The rear axle is made up of two pieces, called the half shafts, each of which transmits power to one or two wheel(s), as in heavy vehicles, on either side. The rear axle is housed in the axle casing or housing as shown in Fig. 10.5.
Fig. 10.5: Rear axle in casing.
Self-check Questions 9. On what basis are rear axles classified? 10. Why is the rear axle in 2 pieces?
10.7 Types of Rear Axles Rear axles may be classified as semi-floating, three-quarter-floating, or fullfloating, depending on the loads carried by them and the position of the wheel bearings vis-Ă -vis the axle casing and the wheel hub. The semi floating type axle is used in light vehicles like cars and the full floating type in heavy vehicles like trucks etc. 10.7.1 Semi-floating Rear Axle A semi-floating axle is very common in rear wheel driven light vehicles. It consists of an axle shaft (half shaft) on each side that is splined on the inner end where it fits in the sun gear of the differential and thus gets rotation. The axle shaft rides in a large roller or ball bearing at the outer end of the axle housing. The bearing is placed between the axle and the casing. The axle is tapered at the outer end on which the wheel hub is mounted and its movement relative to the axle restricted by a key. At the end of the axle, wheel nut screwed.
Fig. 10.6: Semi-floating type rear axle.
In another semi-floating rear axle from a late-model Jeep, the shaft and flange that carries the wheel studs are in one piece. It is enclosed within a flanged outer axle housing (brake backing plate and components removed).
Fig. 10.7: Semi-floating rear axle with flange.
The semi-floating rear axle bears part of the weight of the vehicle, as the load carried by the casing is transferred to the axle, and it also has to bear side thrust on corners by virtue of the wheel hub being mounted on the taper at the end of the axle, besides transmitting driving torque which is the main function of the axle. Thus it has to carry the maximum of loads carried by any axle and hence it is used for light vehicles only. 10.7.2 Three-quarter Floating Rear Axle In the three-quarter floating type rear axle, ball bearing is placed between the axle casing and the wheel hub. As in the case of the semi-floating type rear axle here also the axle is tapered at the outer end, the wheel hub being mounted on this taper and its movement relative to the axle restricted by a key. At the end of the axle wheel nut is screwed. This axle bears side thrust on corners by virtue of the wheel hub being mounted on the taper at the end of the axle and transmits driving torque Thus it does not have to carry the weight of the vehicle and has to bear loads only due to side thrust and driving torque. The inner end of the axle is splined and is carried in the sun gear of the differential for transferring driving torque.
Fig. 10.8: Three-quarter floating rear axle.
10.7.3 Full-floating Rear Axle A full-floating axle is generally used in vehicles that are designed for heavy duty, or are intended to carry heavy loads. This axle neither bears side thrust on corners nor does it have to carry the weight of the vehicle. It only transmits driving torque. The full-floating axle uses an axle shaft on each side that is simply splined at both ends or splined on the inner end and has a drive flange on the outer end. The shaft mates to the differential in the same way as a semi-floating axle. However, the outer end of the shaft differs. Here, the splined end of the shaft slides into a locking hub or an internal splined steel drive plate that bolts to a hub cap. In some cases, the drive flange may be part of the shaft itself. In either case, the axle shaft is allowed to float in the system. For a full-floating system, the axle shaft only serves to transmit the driving torque from the differential to the wheel. It does not carry the weight of the vehicle like a semifloating does. This means the axle load capacity is greatly increased with a full-floating axle. Further advantages of a full-floating axle include being able to remove a broken axle shaft, yet still have the ability to keep a functional rolling tyre on that corner of the vehicle. This can be done since the wheel actually bolts to the hub that rides on the spindle attached to the axle housing. In the full-floating type rear axle, bearings are placed between the axle casing and the wheel hub. Unlike the semi-floating and three-quarter floating type rear axle here the axle is not tapered, rather it ends in a flange through which it is bolted to the wheel hub. Axial movement of the bearings is prevented by locknuts.
Fig. 10.9: Full-floating type rear axle.
Fig. 10.10: Full-floating axle with locking hub.
A full-floating axle is easily recognized externally by the drive flange or locking hub that is evident in the center hole of the wheel. In Fig. 10.11 you can see the axle shaft and end plate of a typical full-floating axle that uses an attached drive flange.
Fig. 10.11: Full-floating axle with attached drive flange.
Self-check Questions 11. 12. 13. 14. 15. 16. 17. 18.
Which axle is used in light vehicles? Where is the bearing placed in case of semi-floating axle? What all loads does a semi-floating axle take? Which axle is used in heavy vehicles? Name the 2 methods of fixing the wheel hub to the axle in case of full-floating axle. How will you recognize a full-floating axle by looking at it on the outside? Which load do all axles have to carry? How are axles mated to the differential?
10.8 Methods of Holding Axle in Housing - C-Clip v Pressed Bearing In order to hold the axle shafts in a semi-floating axle housing, there are two methods. One uses a C-clip inside the differential assembly, and the other uses a pressed bearing out at the wheel end of the axle shaft. On a C-clip-style axle, the axle shaft rides on roller bearings and is held in the axle housing by a C-clip in the differential assembly. The clip fits in a small groove machined near the end of the axle shaft. To remove this clip requires the removal of the differential inspection cover, and may require partial disassembly of the carrier itself depending on the specific type of lock used in the axle. Once the clip is removed, the axle shaft can be slid out of the axle housing. Many differentials use C-clips to retain the axle shafts in the housing. To remove the axle shafts you must remove the housing cover, the differential cross pin, and these C-clips.
Fig. 10.12: C-clip.
The C-clip that retains the rear axle shaft fits on the machined groove near the end of the shaft.
Fig. 10.13: Machined groove for C-clip
On an axle using a pressed bearing setup, the axle is held in place by the pressed-on wheel ball bearing and possibly a pressed collar or retaining clip adjacent to the bearing. The bearing assembly usually fits into a flanged cup that bolts to a mating flange on the outer axle housing. This type of axle uses ball bearings because the bearings must support both radial and axial loads (perpendicular and parallel to the axle shaft). There are advantages and disadvantages to disassembling each type of axle. The C-clip variety requires access to the differential area, but the press bearing variety requires brake line work and brake bleeding. Another difference is that when an axle shaft on C-clip assembly breaks, there is nothing left holding the axle shaft in the housing so the tyre and wheel assembly will readily separate from the vehicle. On a pressed bearing-type axle, the wheel and tyre will usually remain intact, with the bearing pressed to the axle shaft holding the assembly together.
Self-check Questions 19. How are semi-floating axles held in the housing?
10.9
Class Assignment
1. Compare the differential of a front wheel drive car with that of a conventional layout. 2. Discuss why a heavy vehicle will not drop down on that side at which its axle breaks while a light vehicle will do so.
10.10 Activity 1. Lift the drive wheels of a car. Rotate one wheel by hand in clockwise direction. What happens to the other wheel? Can you explain? 2. Go to workshop and see different types of rear axles. Note the method of attaching the wheel to the axle. Also study how the wheel hub is mounted
10.11 Summing Up We need to make the wheels of the vehicle rotate at different speeds at turns. This is because the inner wheel has to travel a shorter distance and the outer wheel a longer distance on turns. Hence differential is placed between the powered wheels. It differentiates the speeds of the wheels at turns. It also improves traction by providing a final gear reduction. In the conventional layout it also turns the power by 90O. Rear live axles (half shafts) transmit power from the differential to the wheels. They are of 3 types viz. semi-floating, three-quarter floating, and full-floating. This classification is done on the basis of the loads carried by each and the placement of bearing. The semi-floating axle is used in light vehicles only as it has to carry all the possible loads whereas the full-floating axle is used in heavy vehicles as it has to transmit only driving torque. The full-floating axle also has the advantage that the wheel on the side that the axle breaks does not fall down. Thus the vehicle is not required to be jacked up for replacing the axle.
10.12 Possible Answers to Self-check Questions 1. It differentiate speed of the inner and outer wheel at turns 2. 3 3. 4:1 4. It is placed between the powered wheels 5. 3 6. takes a turn 7. bevel 8. inside 9. Rear axles are classified on the basis of loads and placement of bearings 10. So that both wheels may be rotated at different speeds on turns. 11. Semi-floating axle. 12. Between the axle and casing. 13. Vehicle weight, side thrust, and driving torque. 14. Full-floating axle. 15. Through locking hub or drive flange. 16. By the drive flange or locking hub that is evident in the center hole of the wheel. 17. Driving torque. 18. Through splines. 19. Through C-clip or pressed bearing.
10.13 Terminal Questions 1. Give the functions of a differential in a conventional layout and a front wheel drive car.
2. Discuss the placement of a differential in various layouts. 3. Differentiate between differentials of a conventional layout and a front wheel drive car. 4. Describe the working of a differential. 5. What are the various types of the rear axles? Describe them with figures. 6. Give the advantages of the full-floating type rear axle and state where it is used. 7. What are the various methods of holding the semi-floating axle in its housing? Give the advantage and disadvantage of each method.
10.14 References and Suggested Further Reading 1. 2. 3. 4.
AA BOOK OF THE CAR. Drive Publications Ltd, London. Maruti 800 Service Manual http://www.howstuffworks.com http://www.off-roadweb.com
10.15 Glossary All-wheel drive
Vehicle in which power is given to all wheels at front and rear.
Axle
Shaft, here live axle i.e. which rotates.
Bevel gear
Pair of gears whose axes intersect.
Cage
Carrier of sun and planet gears.
Conventional layout
Front engine rear wheel drive car.
Crown wheel
Large bevel gear mated to the drive pinion in conventional layout.
Final gear
Large helical gear mated to the drive pinion in front wheel drive car.
Gear reduction
Speed reduction.
Helical gear
Gear whose teeth are at an angle.
Key
Piece of steel, rectangular in shape, which fits in grooves, cut in 2 parts thus preventing relative motion between them.
Planet gear/planet pinion
Gear which revolves about the axis of sun
gears and can also rotate about its own axis . Powered wheel / drive wheel / Wheels (front or rear) to which power train is driven wheel connected.
Splines
Straights grooves which help lock two parts one having internal and the other external splines
Sun gear/side gear
Gear to which half-shafts are connected and which rotates about its axis only
Wheel hub
Part which carries the bearing and on which is mounted the wheel