AUTOMOBILE RUNNING SYSTEM AND POWER TRAIN 9. POWER TRANSMISSION SHAFT AND JOINT
The power from the engine is delivered to the wheels by the power train. In the conventional layout, power from the gearbox is transmitted to the wheels via the propeller shaft and the universal joints. There is also a slip joint, or the sliding joint as it is sometimes called, incorporated in between. In the front wheel drive car with transverse engine, drive shaft and constant velocity joints are used instead. The function of the universal joint is to transmit power at an angle. The slip joint serves to accommodate changes in length of the propeller shaft when the rear wheels go up and down over bumps and in potholes. The constant velocity joint serves the dual purpose of the universal joint and the slip joint.
9.0
Objectives After going through this lesson, you will be able to:
i. ii. iii. iv.
9.1
Explain the necessity of the universal joint and the slip joint. Explain the necessity of constant velocity joints Describe the construction and working of the constant velocity joint. Recognize failure symptoms of CV joints.
Introduction
In the conventional layout, the engine along with the clutch and the transmission gearbox is at the front while power is given to the rear wheels. This necessitates use of a long shaft called the propeller shaft to transmit power between the gearbox and the wheels. Further, the axis of the engine, clutch, and the gearbox is at a different level than that of the differential, which is the same as that of the wheels. This requires that the shaft be inclined while at the same time it is required to rotate at high speed. Hence universal joints are used which permit transmitting power at an angle. Now, when the rear wheels move up or down, due to a bump or a pothole, the length of the propeller shaft needs to change. If the connection between the gearbox and the differential were to be rigid, the propeller shaft would bend or extend. In order to overcome this challenge, we use a slip joint at the front end of the propeller shaft
before the universal joint. The construction of the propeller shaft and the universal joint is described in subsequent sections. In the front wheel drive car equipped with transverse engine, drive shaft and constant velocity joints function to transmit drive at an angle and also accommodate changes in length.
9.2
Universal Joints
Universal joints are used to connect the gearbox to the propeller shaft that is placed after the gearbox. Universal joints are used at both ends of the propeller shaft. They are useful where two rotating shafts are connected at an angle. The propeller shaft is inclined to the axis of gearbox as well as that of the differential.
Fig. 9.1: Universal joint (a) Use. (b) Construction.
The two shafts which are to be connected have forks at their ends and the two forks are connected by means of a cross which is also called the spider. In the figure, the two pairs of shafts to be joined are the gearbox output shaft and the propeller shaft at the front end, and the propeller shaft and the differential input shaft at the rear end.
9.3
Propeller Shaft
The propeller shaft is a hollow cylindrical shaft that is used to transmit power from the gearbox to the differential by rotation. It is hollow to keep it light in weight as well as strong enough to transmit power. The propeller shaft has to be perfectly balanced otherwise it results in violent vibrations. The gearbox end of the propeller shaft has a sliding joint which is also called the slip joint. The gearbox and differential are mounted on different members of the
Fig. 9.2: Propeller shaft.
chassis and the distance between them keeps changing when the vehicle moves on uneven roads. To compensate for this change in distance, length of propeller shaft also has to change and this is where the slip joint performs its function. If the distance between gearbox and differential is very large, a two-piece propeller shaft is used, as in the case of trucks and buses. The figure shows a propeller shaft with the slip and universal joints.
Self-check Questions 1. 2. 3. 4.
9.4
Why is the axis of the engine and the differential not at the same level? When is the universal joint required to be used? How many universal joints and slip joints are used? Name the pair of shafts joined by the universal joints.
Front-Wheel Drive Set-up
The layout in which the engine is mounted between the front wheels across the length of the vehicle and power is given to the front wheels is used in most of the small and medium sized cars that we see on Indian roads, including the Maruti 800, Alto, and Esteem.
Fig. 9.3: Transversely mounted engine driving the front wheels.
In such a front wheel drive car the propeller shaft, universal joints and slip joint are not used. Instead, a drive shaft and constant velocity (CV) joints are used for each front wheel. CV joints are employed at both ends of each drive shaft.
Fig 9.4: Typical front-wheel drive vehicle set-up.
The front wheels are connected to the differential side gears with a drive shaft having a constant velocity joint (CV joint) at both ends - the outer i.e. the wheel side and the inner i.e. the differential side. The outer joint can turn through a wide angle when the wheel is steered. The inner joint can slide in or out, as required, when the wheel goes over bumps etc. and the shaft needs to change its length. Each joint is composed of an outer race (also called cup or dome) which is grooved on the inside, cage, splined inner race and steel balls – six in number in the Maruti 800 car. The cup of the outer joint has a splined and threaded stub shaft for mounting the wheel. The dome of the inner joint has a splined hub to accommodate the transmission shaft from the differential side gear. During a turn, the outer race of the wheel side joint turns with the wheel and also transmits drive because of the six balls locking the outer race and the inner race which in turn is fixed to the drive shaft via splines. This joint has a splined stub axle at the outer end of the cup on which the wheel hub is mounted and held in place by a nut and split pin. At the inside joint, the drive shaft which is fixed to the inner race can slide through the balls in the grooves of the outer race (dome) along the axis of the drive shaft. Thus the pair of constant velocity joints carries out the function of both the universal joints and the slip joint used in the conventional layout.
Fig 9.5: Drive shaft with inner and outer CV joint.
9.5
Drive Shaft and Constant Velocity Joint
Due to bumps and uneven surfaces in the road, a car's wheels tend to move up and down continuously while driving down the road; as a result, drive shafts cannot be made up of a solid shaft. The CV joint's precursor, the universal joint, was used with the propeller shaft of rear wheel drive cars because of its ability to transmit power at an angle. With the advent of front wheel drive cars, however, car manufacturers had a new problem: the joints in the drive shafts needed to account not only for the up-and-down movement of the wheels, but Fig. 9.6: Constant velocity joint.
also for the back-and-forth motions due to steering. CV joints have the function to transmit torque at constant speed to the steered wheels as well as to accommodate the up and down movement of the suspension. At one end of the inside CV joint is the transmission output shaft and at the other end is the drive shaft which gives power to the wheel via another CV joint. You will find CV joints in all front-wheel drive cars.
Fig 9.7: Location of CV joint.
The CV joint is packed with grease and encased in a rubber boot. It does not require any maintenance and has a long life provided the protective boot does not get damaged.
Fig 9.8: Rubber boot.
Self-check Questions 5. How many CV joints are used in a front wheel drive car? 6. How is the CV joint lubricated?
9.6
Types of Constant Velocity Joints
Constant Velocity Joints (also called homo-kinetic joints) allow a rotating shaft to transmit power through a variable angle, at constant rotational speed, without an
appreciable increase in friction or play. They are mainly used in front wheel drive and all wheel drive cars. However, rear wheel drive cars with independent rear suspensions typically use CV joints at the ends of the rear axle half-shafts. Early front wheel drive systems used Hardy-Spicer joints, where a cross-shaped metal pivot sits between two forked carriers. These are not strictly CV joints as they result in a variation of the transmitted speed except for certain specific configurations. In 1928 Alfred H. Rzeppa designed the Constant Velocity joints. They allowed a smooth transfer of power despite the wide range of angles they were bent in. Drive shafts using CV joints are self-supporting along their length, and do not need additional supports except for very long shafts. There are two most commonly used types of CV joints: the Rzeppa ball type joints and tripod joints. Different variations of ball-type CV joints are commonly used on the differential side of the drive shaft, while tripod-type CV joints are mostly used on the wheel side. The Maruti 800 car uses ball type CV joint at both the ends.
Splines
Axle Spindle Threads
Oil Seal Journal
Fig, 9.9 Ball type CV joint used at the wheel side in M 800 car
In the Rzeppa ball type CV joint (Fig. 9.10), the end of the driveshaft (1) is splined which fits in the center of a large, steel, star-shaped ‘gear’ (3) that nests inside a circular cage in the joint and is held in place by a circlip. The cage is spherical but with ends open, and it typically has six openings around the perimeter. This cage and gear fit into a grooved cup that has a splined and threaded shaft attached to it. Six large steel balls (2) sit inside the cup grooves and fit into the cage openings, nestled in the grooves of the star gear. The outer shaft on the cup (4) then runs through the wheel bearing and is secured by the axle nut. This joint is extremely flexible, and can accommodate the large changes of angle when the front wheels are turned by the steering system. The differential side joint has a splined hub into which fits the transmission shaft from the differential side gear.instead of the shaft on the cup
Fig 9.10: Constant velocity joint (Rzeppa ball type) 1-Driveshaft from transmission. 2-Steel balls (in this case 6) in a 'cage' nestled in a star shaped gear (3) which is splined to the driveshaft. The balls run in grooves in the dome (4). Spherical 'dome' and outer axle shaft are fixed in the hub of the wheel.
Outer Race Grooves Fig 9.11 Construction of Rzeppa ball type joint
These joints are very strong, and are usually highly over specified for a given application. Maintenance is usually limited to checking that the rubber gaiter (dust/weather boot) that covers them is secure and not split. If the gaiter is damaged, the MoS2 (molybdenum disulfide) grease that the joint is packed with, will be thrown out. The joint will then pick up dirt, water, and road deicing salt (sprinkled on the road in cold countries to melt snow) and cause the joint to overheat and wear, and the grease can also contaminate the brakes. In worst case, the CV joint may disjoin causing the vehicle to stop moving. The constant-velocity (CV) shaft, boot and joints are all part of the same assembly and are used on front-wheel-drive and many four-wheel-drive vehicles. One shaft assembly is used per side, and is sometimes dubbed a ‘half-shaft’. The transmission’s output shaft connects to the inner CV joint. The inner joint has the ability to slide in and out and therefore make minor changes in the overall length of the shaft assembly. This is important to allow for changes in suspension travel and ride height. From there, the main portion of the axle shaft connects the outer CV joint. Both the inner and the outer joints are covered with a protective boot. The outer CV joint has the inherent ability to provide constant torque transfer, even while the wheels are turned
sharply. The CV joint then connects to a stub shaft that joins with the wheel hub and bearing assembly. Older front-wheel-drive vehicles used to use unequal length shafts, which led to a driving characteristic known as ‘torque steer’- pulling to one side while accelerating. Today, most cars have equal length shafts on both sides, which neutralizes the effects of torque steer. Sometimes, at the wheel side, where the shaft moves only up and down with the movement of the suspension, a ‘Triax’ (also known as ‘Tripod’) joint is used. This has a three-pointed yoke attached to the shaft, which has barrel-shaped rollers on the ends. These fit into a cup with three matching grooves, attached to the differential. Since there is significant movement only in one axis, this simple arrangement works well.
Fig 9.12: Tripod joint
Self-check Questions 7. Who designed the CV joints and when? 8. Is there any difference between the joints used at the outer and inner end of the drive shaft? 9. Which joint is used at the inboard end? 10. How many balls are used in the outer joint? 11. In Maruti 800 car are the 2 drive shafts of equal length? What is the consequence?
9.7
Constant Velocity (CV) Joint Failure
9.7.1 CV Joint and Boot Service A common cause of CV joint failure is cracks in the CV boot. When the boot is cracked, it allows dirt and grit to mix with the grease and wear down the parts of the joint. A clicking noise heard when turning is the most common symptom of CV joint problems. The CV boots should be replaced as soon as cracking is visible in their rubber folds.
CV joints should be inspected periodically and may require replacement as a car ages. A CV joint is covered with a bulbous rubber boot that tends to deteriorate over time. When a CV boot cracks or tears open, the CV joint is left exposed to the elements, which will quickly damage the joint. Most common problem with the CV joints is when the protective boot gets damaged. Once this happens, the grease comes out and the moisture and dirt come in, eventually causing the CV joint to fail due to lack of lubrication and corrosion. If the CV axles are inspected periodically, torn boots can be replaced as needed, potentially extending the life of the joints; however, if torn boots are left unattended, the joint or the entire axle may soon need to be replaced. 9.7.2
Fault Finding and Diagnosis
The symptom of a bad outer CV joint is usually a clicking noise while turning. A shudder, vibration or clunking sound when accelerating or decelerating usually means trouble in the inner CV joint. Don’t ignore the warning signs of a bad CV joint; you could lose steering or be stranded. Signs of worn or damaged CV joints include lubricant leaks, clicking sounds during turning, clunks during acceleration or abrupt deceleration and front end vibrations. Servicing the Constant Velocity (CV) joints is a maintenance task that should be performed at least every 50,000 miles or so. Why? Because the grease in the CV joints will harden and lose its lubricating properties over time. This will lead to premature failure of the joint. A properly maintained joint will easily outlast most street-driven vehicles. While not difficult, it is a messy and time consuming task. If a damaged boot is caught early, simply replacing the CV joint boot and repacking the CV joint with fresh grease may fix the problem. If one continues driving with a broken boot, the CV joint or a whole drive shaft will need to be replaced. In worst case, the CV joint may disjoin causing the vehicle to stop running. Constant velocity joints are usually reliable and largely trouble-free. The two main failures are wear and partial seizure. Wear in the outer joint usually shows up as vibration at certain speeds, a bit like the vibration caused by an unbalanced wheel. To determine if the joint is worn, find a big empty car park and drive the car slowly in tight circles, left and right. Worn joints will make a rhythmic clicking or cracking noise. Wear in the inner joints shows up as a "clunk" or "pop" when applying power, or if severe, when lifting off the throttle. Partial seizure causes a strange "pattering" sensation through the suspension. It is caused by the joint overheating, which in turn is usually caused by the outer joint boot having split, allowing the joint to throw out its grease. If caught in time, you can clean the joint carefully, repack with grease and replace the boot.
9.7.3 Maintenance Tips CV boots should be inspected at every oil change. CV boots can be easily damaged from rocks, sticks, ice and just normal wear. When a CV boot tears or cracks, the lubricant packed inside is free to move out of the CV joint and dirt is now allowed into the joint. Unless the damage to the boot is discovered quickly, it’s likely that the joint will also need to be replaced along with the boot. A replacement axle may be the wisest choice, depending on cost.
Self-check Questions 12. What is the usual cause of CV joint failure? 13. How would you recognize a bad outer CV joint? 14. How would you recognize a bad inner CV joint? 15. At what time interval should the CV joint be serviced? 16. What is the cause of partial seizure of the joint? How is it recognized?
9.8
Class Assignment
1. Discuss differences between Universal joint and CV joint and suitability of the latter for front wheel drive cars.
9.9
Activity
1. Visit a workshop (if required) and locate the CV joints in a car. Inspect the boots and say whether they need replacement.
9.10
Summing Up
Propeller shaft transmits power between the gearbox and the wheels. Since it is inclined while at the same time it is required to rotate at high speed therefore universal joints are used which permit transmitting power at an angle. Slip joint accommodates changes in length at bumps and pot holes when the wheel moves up and down. CV drive shaft assemblies fulfill the demanding role of having to supply torque to the wheels while turning and while the suspension is moving up and down. The CV joint is packed with grease and encased in rubber boot. It does not require any maintenance and has a long life provided the protective boot does not get damaged. Different types of CV joints are used on drive shafts at the outer and inner endthe Rzeppa ball type joint and tripod joint respectively. The outer CV joint has the inherent ability to provide constant torque transfer, even while the wheels are turned sharply. The inner joint has the ability to slide in and out and therefore make minor changes in the overall length of the shaft assembly. Even though some front-wheel-
drive vehicles use unequal length shafts, which leads to a driving characteristic known as ‘torque steer’- pulling to one side while accelerating, now cars have equal length shafts on both sides, which neutralizes the effects of torque steer. A common cause of CV joint failure is cracks in the CV boot. When the boot is cracked, it allows dirt and grit to mix with the grease and wear down the parts of the joint. A clicking noise heard when turning is the most common symptom of CV joint problems. The CV boots should be replaced as soon as cracking is visible in their rubber folds.
9.11
Possible Answers to Self-check Questions
1. the engine crankshaft is at a higher level because of crankcase and oil pan below it and to provide sufficient ground clearance below the engine; if the differential is also at the same level the wheels will have to be very big and also the centre of gravity of the car will be high; this will decrease stability 2. to join 2 rotating shafts at an angle 3. 2,1 4. gearbox output shaft and the propeller shaft at the front end, and the propeller shaft and the differential input shaft at the rear end 5. 4 (1 at each end of both the drive shafts) 6. It is packed with MoS2 grease and encased in a rubber boot 7. Alfred H. Rzeppa in 1928 8. Yes 9. Triax or Tripod type CV joint 10. Six 11. No. Pull to one side while accelerating called ‘torque-steer’ 12. Crack or tear in rubber boot 13. Wear in the outer joint usually shows up as vibration at certain speeds, a bit like the vibration caused by an unbalanced wheel and a clicking noise while turning. 14. A shudder, vibration or clunking sound when accelerating or decelerating usually means trouble in the inner CV joint. 15. 50,000 km. Because the grease in the CV joints hardens and loses its lubricating properties over time. 16. Partial seizure is caused by the joint overheating, which in turn is usually caused by the outer joint boot having split, allowing the joint to throw out its grease. It causes a strange "pattering" sensation through the suspension.
9.12 1. 2. 3. 4. 5.
Terminal Questions Why are universal joints not suitable for cars with front wheel drive? Describe the location of CV joints with the help of a simple layout. Describe the inner joint. Describe the outer joint. What is the common cause of CV joint failure? How would you recognize a failed outer/inner CV joint?
9.13
References and Suggested Further Reading
1. AA BOOK OF THE CAR. Drive Publications Ltd, London. 2. Maruti 800 Service Manual 3. http://www.howstuffworks.com 4. http://www.en.wikipedia.org 5. http://www.samarins.com 6. http://www.extremehowto.com 7. http://www.autorepair.about.com 8. http://www.carcare.org 9. http://www.wisegeek.com 10. http://www.meineke.com
9.14
Glossary
Boot
Bellows type flexible cover or case made of rubber.
Constant velocity joint
Joint used in front wheel drive car which permits torque transmission at constant speed to the steered wheels and also accommodate the up and down movement of the suspension.
Drive shaft
Shaft joining differential and wheels in a front wheel drive car.
Front wheel drive car
Car in which engine power is given to the front wheels.
Grease
Thick lubricant.
Propeller shaft
Hollow cylindrical shaft that is used to transmit power from the gearbox to the differential.
Sliding joint
Same as slip joint.
Slip joint
Splined shaft end which can move linearly in a hollow splined sleeve.
Transverse engine
Engine laid at 90O to the length of the car.
Universal joint
Joint having 2 forks connected by means of a cross thus permitting movement in 2 planes.