Differentials..Full

If you've read How Car Engines Work, you understand how a car's power is generated; and if you've read How Manual Transmissions Work, you understand where the power goes next. This article will explain differentials -- where the power, in most cars, makes its last stop before spinning the wheels.

The differential has three jobs:

• To aim the engine power at the wheels
• To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
• To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)

In this article, you'll learn why your car needs a differential, how it works and what its shortcomings are. We'll also look at several types of positraction, also known as limited slip differentials.

Why You Need a Differential

Car wheels spin at different speeds, especially when turning. You can see from the animation that each wheel travels a different distance through the turn, and that the inside wheels travel a shorter distance than the outside wheels. Since speed is equal to the distance traveled divided by the time it takes to go that distance, the wheels that travel a shorter distance travel at a lower speed. Also note that the front wheels travel a different distance than the rear wheels.

For the non-driven wheels on your car -- the front wheels on a rear-wheel drive car, the back wheels on a front-wheel drive car -- this is not an issue. There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. If your car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components.

 

What is a Differential?

The differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed.

The differential is found on all modern cars and trucks, and also in many all-wheel-drive (full-time four-wheel-drive) vehicles. These all-wheel-drive vehicles need a differential between each set of drive wheels, and they need one between the front and the back wheels as well, because the front wheels travel a different distance through a turn than the rear wheels.
Part-time four-wheel-drive systems don't have a differential between the front and rear wheels; instead, they are locked together so that the front and rear wheels have to turn at the same average speed. This is why these vehicles are hard to turn on concrete when the four-wheel-drive system is engaged.

Main function of diffrential in automobiles(Brief)

Differential gearing allows power to be split to two shafts (let's just say wheels), traveling at different speeds, with equal TORQUE going to each wheel. The differential gear carrier rotates at the AVERAGE speed of the two wheels.

You can see how handy this would be going around a curve. The two wheels turn at different speeds, and the differential keeps the torque equal to them while they're doing that.

Iron Carbon Diagram

A study of the microstructure of all steels usually starts with the metastable iron-carbon (Fe-C) binary phase diagram (Figure 1). It provides an invaluable foundation on which to build knowledge of both carbon steels and alloy steels, as well as a number of various heat treatments they are usually subjected to (hardening, annealing, etc).



Figure 1. The Fe-C phase diagram shows which phases are to be expected at metastable equilibrium for different combinations of carbon content and temperature. The metastable Fe-C phase diagram was calculated with Thermo-Calc, coupled with PBIN thermodynamic database.

At the low-carbon end of the metastable Fe-C phase diagram, we distinguish ferrite (alpha-iron), which can at most dissolve 0.028 wt. % C at 738 °C, and austenite (gamma-iron), which can dissolve 2.08 wt. % C at 1154 °C. The much larger phase field of gamma-iron (austenite) compared with that of alpha-iron (ferrite) indicates clearly the considerably grater solubility of carbon in gamma-iron (austenite), the maximum value being 2.08 wt. % at 1154 °C. The hardening of carbon steels, as well as many alloy steels, is based on this difference in the solubility of carbon in alpha-iron (ferrite) and gamma-iron (austenite).

At the carbon-rich side of the metastable Fe-C phase diagram we find cementite (Fe₃C). Of less interest, except for highly alloyed steels, is the delta-ferrite at the highest temperatures.

The vast majority of steels rely on just two allotropes of iron: (1) alpha-iron, which is body-centered cubic (BCC) ferrite, and (2) gamma-iron, which is face-centered cubic (FCC) austenite. At ambient pressure, BCC ferrite is stable from all temperatures up to 912 °C (the A₃ point), when it transforms into FCC austenite. It reverts to ferrite at 1394 °C (the A₄ point). This high-temperature ferrite is labeled delta-iron, even though its crystal structure is identical to that of alpha-ferrite. The delta-ferrite remains stable until it melts at 1538 °C.

Regions with mixtures of two phases (such as ferrite + cementite, austenite + cementite, and ferrite + austenite) are found between the single-phase fields. At the highest temperatures, the liquid phase field can be found, and below this are the two-phase fields (liquid + austenite, liquid + cementite, and liquid + delta-ferrite). In heat treating of steels, the liquid phase is always avoided.

The steel portion of the Fe-C phase diagram covers the range between 0 and 2.08 wt. % C. The cast iron portion of the Fe-C phase diagram covers the range between 2.08 and 6.67 wt. % C.

The steel portion of the metastable Fe-C phase diagram can be subdivided into three regions: hypoeutectoid (0 < wt. % C < 0.68 wt. %), eutectoid (C = 0.68 wt. %), and hypereutectoid (0.68 < wt. % C < 2.08 wt. %).

A very important phase change in the metastable Fe-C phase diagram occurs at 0.68 wt. % C. The transformation is eutectoid, and its product is called pearlite (ferrite + cementite):

gamma-iron (austenite) —> alpha-iron (ferrite) + Fe₃C (cementite).

Some important boundaries at single-phase fields have been given special names. These include:

• A₁ — The so-called eutectoid temperature, which is the minimum temperature for austenite.
• A₃ — The lower-temperature boundary of the austenite region at low carbon contents; i.e., the gamma / gamma + ferrite boundary.
• A_cm — The counterpart boundary for high-carbon contents; i.e., the gamma / gamma + Fe₃C boundary.

Sometimes the letters c, e, or r are included:

• Ac_cm — In hypereutectoid steel, the temperature at which the solution of cementite in austenite is completed during heating.
• Ac₁ — The temperature at which austenite begins to form during heating, with the c being derived from the French chauffant.
• Ac₃ — The temperature at which transformation of ferrite to austenite is completed during heating.
• Ae_cm, Ae₁, Ae₃ — The temperatures of phase changes at equilibrium.
• Ar_cm — In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling, with the r being derived from the French refroidissant.
• Ar₁ — The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling.
• Ar₃ — The temperature at which austenite begins to transform to ferrite during cooling.
• Ar₄ — The temperature at which delta-ferrite transforms to austenite during cooling.

If alloying elements are added to an iron-carbon alloy (steel), the position of the A₁, A₃, and A_cm boundaries, as well as the eutectoid composition, are changed. In general, the austenite-stabilizing elements (e.g., nickel, manganese, nitrogen, copper, etc) decrease the A₁ temperature, whereas the ferrite-stabilizing elements (e.g., chromium, silicon, aluminum, titanium, vanadium, niobium, molybdenum, tungsten, etc) increase the A₁ temperature.

The carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content (0.68 wt. % C in case of the metastable Fe-C phase diagram). The ferrite-cementite phase mixture of this composition formed during slow cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite lamellae that coarsens (or "spheroidizes") into cementite particles dispersed within a ferrite matrix after extended holding at a temperature close to A₁.

Finally, we have the martensite start temperature, M_s, and the martensite finish temperature, M_f:

• M_s — The highest temperature at which transformation of austenite to martensite starts during rapid cooling.
• M_f — The temperature at which martensite formation finishes during rapid cooling.