What is a differential, what is it for, and how is it designed
The differential as an automobile mechanism will soon celebrate its two-century anniversary, but its design over these many years, although improved, has retained its key features. So what is the differential, and what role does it play in a car?
1. WHAT IS A DIFFERENTIAL?
A differential in a car is a mechanism that allows power and therefore rotation to be transferred from the gearbox to the wheels by dividing the flow of this power into two, for each of the wheels of one axle, with the ability to change the ratio of power transferred to them, and therefore allowing the wheels to rotate at different speeds. Simply put, the differential divides 100% of the power transmitted by the gearbox into two streams for each of the wheels on the same axle, and these streams can be redistributed depending on driving conditions from 50:50 to 100:0.
2. WHAT IS A DIFFERENTIAL FOR?
The main purpose of a differential is to allow the wheels on the same axle to rotate at different speeds while maintaining an uninterrupted torque flow. For a car, this is primarily important in corners: after all, when driving around an arc, the wheels on the outside of the turn travel a longer distance than the wheels on the inside, and therefore must rotate at a higher speed to maintain the stability of the car.
If the wheels on the axle are connected rigidly, the inner wheel will slip in the turn. For a rear-wheel drive car, this increases the risk of skidding, and for a front-wheel drive car, it radically worsens the car’s handling and control in a turn. Thus, ensuring free and independent rotation of the wheels on the same axle while maintaining constant torque transfer to them from the engine has been one of the fundamental tasks since the creation of the car – and this task has been successfully solved.
3. HOW IS THE DIFFERENTIAL DESIGNED?
The differential is a special case of the planetary gear. Physically the differential is normally a set of four pinions, the rotation to which is transmitted a fifth pinion, the idler pinion of the main gear, coupled to the differential housing, which acts as the driver. The main gear is a set of two gears: the driven one receives rotation from the gearbox and transmits it to the driven one. The driven pinion of the main gear transmits the rotation through the housing to the satellite gears, which, in turn, are meshed with the sun gears, rigidly mounted on the drive half-axles of the wheels.
When the car moves in a straight line, the satellite gears are stationary, and the rotation speed of the main gear is equal to the rotation speed of the sun gears: the wheels rotate at the same speed. In a turn, however, the satellite gears begin to rotate, providing the difference in speeds of the sun gears and, therefore, the wheels on the outside and inside of the turn.
4. WHAT ARE THE DISADVANTAGES OF THE DIFFERENTIAL?
The main disadvantage of the differential is at the same time its main advantage – the ability to transfer up to 100% of the power to one of the wheels. On this basis, in conditions where one wheel has insufficient traction, most of the power will be transferred to it. Thus, sometimes even with one wheel on a surface with sufficient grip, the vehicle cannot move.
To eliminate this problem have been developed a variety of designs – differentials with increased internal resistance (so-called self-locking) and differentials with forced locking, manual or automated. Depending on their design and purpose, they can either change the redistribution of power flow in favor of the wheel with good traction, or completely lock the differential, forcing the wheels on the axle to rotate at the same speed. We’ll look at the different types of these differentials in separate articles.
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What is a differential and what are locks for.
A differential is a mechanical device that transfers torque from one source to two independent consumers in such a way that the angular speeds of the source and the two consumers can be different relative to each other. This torque transfer is made possible by the use of a so-called planetary mechanism. In automotive engineering, the differential is one of the key components of the transmission. It primarily serves to transmit torque from the gearbox to the wheels of the drive axle.
Why do we need a differential? In any corner, the path of an axle wheel traveling on a short (inner) radius is less than the path of another wheel on the same axle traveling on a long (outer) radius. As a result, the angular velocity of the inner wheel must be less than the angular velocity of the outer wheel. In the case of a non-driving axle, this condition is fairly easy to fulfill, since the two wheels may not be connected to each other and rotate independently. But if the axle is leading, it is necessary to transmit torque simultaneously to both wheels (if you transmit torque only to one wheel, the ability to control the car according to modern concepts will be very poor). With a rigid connection of the wheels of the leading axle and the transfer of torque to a single axle of both wheels, the car could not turn normally, as the wheels, having equal angular velocity, would tend to take the same path in the turn. The differential solves this problem: it transmits torque to the separate axles of both wheels (half-axles) through its planetary mechanism with any ratio of angular velocity of the half-axles. As a result, the vehicle can move and steer normally both on a straight track and in a corner.
However, due to the physics of the device, the planetary mechanism has a very bad property: it tends to transmit the resulting torque to where it is easier. For example, if both wheels of an axle have the same grip and the force required to spin each wheel is the same, the differential will distribute the torque evenly between the wheels. But as soon as there is a noticeable difference in traction (for example, one wheel hit the ice, and the other remained on the asphalt), the differential will immediately begin to redistribute torque to the wheel, the effort to unwind which is the least (that is, the one on the ice). As a result, the wheel on the asphalt will stop receiving torque and stop, and the wheel on the ice will take all the torque and rotate with increased angular velocity, and the planetary mechanism will play the role of a reducer, which increases the rotation speed of this wheel. Naturally, this phenomenon greatly worsens the passability and drivability of the car. After all, logically, in the situation considered, it is desirable to transfer the torque to the wheel, located on the asphalt, so that the car could continue to move.
In all-wheel-drive vehicles, the differential is usually equipped with two axles, and often the differential can be found also between the axles (inter-axle differential). Thus, we get a transmission scheme, in which there are as many as three differentials: two axles and one inter-axle differential. The last one is necessary for constant driving with all-wheel drive and torque transfer to all four wheels. In fact, in a turn, the wheels of the steering axle (usually the front one) have quite different angular velocities than the wheels of the rear axle. The axle differential is designed to transmit torque from the gearbox to both drive axles with different ratios of angular velocities. This scheme with three differentials is one of the most common schemes for permanent four-wheel drive (Full time 4WD).
However, this is the subject of another section. In this section, we are interested in the differential and its properties. Returning to the above-described problematic properties of the planetary mechanism, it is interesting to consider the situation when the four-wheel drive car with an inter-axle differential one of the four wheels hit the same ice (or a slippery pit). What happens then? The differential of the axle, which wheel is on the ice, will give all the torque received to this wheel. The inter-axle differential, in turn, also tends to transfer the torque to where it is easier. Naturally, it is easier for the center differential to transmit torque to an axle with a wheel spinning on ice than to an axle whose wheels have good traction and can move the car. As a result, all the torque from the engine and transmission will go to spin the only wheel on the ice. The other three wheels will stop and receive no torque from the differentials. The result: out of the four driving wheels, there is only one wheel left, which slips on the ice – the four-wheel drive vehicle is “stuck”. So how to make the differentials transfer torque to the wheels with better road grip? For this purpose, various ways of partial and full, manual and automatic differential locks have been developed, which will be discussed below.
The main purpose of the differential lock is to transfer the necessary torque to both of its consumers (half axles or gimbals). There are fundamentally different methods of solving this problem.
With this type of locking, the differential actually ceases to perform its functions and turns into a simple clutch, rigidly linking the axles (or gimbals) and transmitting them the same torque with the same angular velocity. In order to fully lock the classic differential, it is sufficient to either block the ability to rotate the satellites, or to rigidly connect the differential cup to one of the half axles. This locking is usually realized by pneumatic, electric or hydraulic actuator, controlled by the driver from the passenger compartment of the vehicle. It is used for both axle and center differentials. Picture shows ARB locking diagram for axle differential, in which satellites are locked.
This kind of interlocks may be engaged only when the car is completely stopped. You should be very careful when using them, as the force of the motor is enough to “break” the locking mechanism or to break the half-axle. It is better to use such locks only at low speed for driving in difficult terrain, as while using them in axles (especially in steering) the car loses a lot in steerability. As a rule, rigid axle and center differentials locks are used on all-wheel drive vehicles, such as Toyota Land Cruiser, 4Runner (Hilux Surf), Mercedes G-Class, etc.
Limited Slip Differentials – differentials with limited “slip” (one axle in relation to the other).
In this case, the locking of one of the axles with the differential cup is used. The viscocoupler is mounted coaxial to the half axle so that one of its drives is rigidly attached to the cup of the differential, and the other to the half axle. In normal motion, the angular velocities of the cup and the half-axle are the same, or slightly different (in a turn). Accordingly, the working planes of the clutch have the same slight divergence in angular velocities and the clutch remains open. As soon as one of the axes begins to receive noticeably more torque and higher angular velocity relative to the other, there is friction in the clutch and it begins to block. The greater the difference in velocity, the greater the friction inside the fluid coupling and the degree of its blocking. As the degree of blocking increases and the cup and axle angular velocities equalize, the friction inside the fluid coupling begins to drop, leading to a smooth release of the fluid coupling and de-locking. This scheme is used for the center differential, because its design is too massive to be mounted on an axle gearbox. (Scheme on the picture) This locking mechanism is well suited for use in poor road conditions, but in real off-road conditions its abilities are far from outstanding: the clutch can not cope with the constant change of the axle clutch states with the ground, lags when switching on, overheats and fails. This type of inter-axle differential lock can be found on “parket” SUVs: Toyota Rav4, Lexus RX300, etc.
The principle of operation of these interlocks is quite simple. Instead of the classical gear planetary mechanism, cam or gear pairs are used, which at a slight difference in angular velocity of the half axles have the ability to mutually rotate (jump), and when slipping, they jam and block the half axles with each other. It is not difficult to imagine what happens to the car when such a lock is triggered in a corner.
Some copies simply disconnect one of semi-axles at the moment of occurrence of a small difference of speeds. That is why only military and special equipment differentials (APCs, etc.) are normally equipped with such locks.
The design of such differentials is quite simple and does not differ in principle from the device of an ordinary open differential. Between the half-axles and the differential cup are added sets of friction plate units (which are marked with red dots in the picture on the right). This is why these differentials are often referred to as “friction based LSD”. When the differential tries to redistribute torque to one of the half axles and a difference in the angular velocity of the half axles and the cup begins to occur, the plates hold back the occurrence of this difference by the friction force. Of course, when the amount of torque exceeds the friction force of the plates, all of the rotation is transferred to the more easily rotated half-axle. Such locks operate within a relatively small range of torque ratios.
Quite often the friction blocks are spring loaded. Such differentials are standard installed in rear axle of many SUVs – Toyota 4Runner (Hilux Surf), Nissan Terrano, Kia Sportage, etc. The American company ASHA Corp. has gone further, providing the LSD differential friction package with a locking device consisting of a pump with a piston (Gerotor differential). When a difference in angular velocity between the axle and the cup occurs, the pump presses oil (fluid) onto the piston and squeezes the friction unit, thereby locking the differential. This design is called Gerodisk (Hydra-Lock) and is standard on Chrysler SUVs (pictured left). For almost all friction based differentials, it is necessary to use a special oil which contains additives that allow the friction blocks to work properly.
This is one of the most interesting, efficient, technologically advanced and practically applicable forms of differential locks. The principle of operation is based on the property of the hypoid pair to “unlock”. In this regard, the main (or all) gearing in such differentials hypoid (worm, or in common parlance – screw). There are not many variations of these designs – you can distinguish three basic types.
The first type is produced by company Zexel Torsen. (T-1) The hypoid pairs are the drive axle gears and satellites. In this case, each half-axle has its own satellites, which are paired with the satellites of the opposite half-axle by conventional spur gear. It should be noted that the satellite axis is perpendicular to the semi-axis. During normal motion and equality of torques transmitted to the semi-axles, the hypoid satellite/drive pinion pairs are either stopped or rotated, providing the difference in angular velocity of the semi-axles in the rotation.
As soon as the differential tries to give torque to one of the half axles, the hypoid pair of that half axle begins to unclip and block with the differential cup, resulting in partial differential lock. This design operates in the largest torque ratio range, from 2.5/1 to 5.0/1, which is the most powerful in the series. The operating range is adjusted by the angle of the worm teeth.
The author of the second type is the Englishman Rod Quaife. In this case, the axes of the satellites are parallel to the semi-axes. The satellites are located in peculiar pockets of the differential cup. The paired satellites are not spur gear engagement, but form another hypoid pair which also takes part in locking process by expanding (on the second picture). Tractech’s True Trac differential has a similar device. Even in Russia, we have the production of similar differentials for the domestic cars UAZ, etc.
And the company Zexel Torsen in its differential T-2 offered a slightly different layout, in fact, the same device (in the picture on the right). Due to its unusual design, the paired satellites are connected to each other on the outside of the sun gears. Compared to the first type, these differentials have a smaller locking range, but they are more sensitive to differences in transmitted torque and engage earlier (starting at 1.4/1). Tractech has recently released an Electrac axle differential with an electrically actuated positive differential lock.
The third type is made by Zexel Torsen (T-3) and is used mainly for axle differentials. The planetary structure of the design allows the nominal torque distribution to be shifted in favor of one of the axles. For example, the T-3 differential used on the 4th generation 4Ranner has a nominal torque distribution of 40/60 in favor of the rear axle. Accordingly, the entire range of partial lock operation is shifted as well: from (front/rear) 53/47 to 29/71. In general, the nominal torque distribution between the axles can be shifted from 65/35 to 35/65. Partial locking is triggered at 20-30% difference in transmitted torque to the axles. This structure also makes the differential compact, which in turn simplifies the design and improves the layout of the transfer case. The torque sensitive differentials described above are very popular in motorsport. Moreover, many manufacturers install these differentials in their models as standard, both as an axle differential and as an inter-axle differential. For example, Toyota installs such differentials both in passenger cars (Supra, Celica, Rav4, Lexus IS300, RX300, etc.), and SUVs (4Runner / Hilux Surf, Land-Cruiser, Mega-Cruiser, Lexus GX470) and buses (Coaster Mini-Bus). These differentials do not require special oil additives (unlike friction-based differentials), but it is better to use quality oil for loaded hypoid gears.
Differentials are controlled by electronic brake force control systems (Traction Control, etc.)
In today’s automotive industry, more and more electronic traffic control systems are being used. It is rare to find cars that are not equipped with ABS (which prevents the wheels from locking during braking). Moreover, since the late 80’s of the last century, advanced manufacturers began to equip their flagship models with traction control systems – Traction Control. For example, Toyota installed the Traction Control system on the Lexus LS400 in 1989 (90). The principle of operation of this system is simple: the universal (also serve ABS) rotation sensors installed on the controlled wheels, fix the beginning of slippage of one wheel axle relative to the other, and the system automatically brakes stalled wheel, thereby increasing the load on it and forcing the differential to give the moment to the wheel with good traction. In case of severe slipping, the system can also limit the fuel supply to the cylinders. The operation of such a system is very effective, especially on rear-wheel drive cars. As a rule, if desired, this system can be forcibly deactivated with a button on the dashboard.