Differential

The differential is part of the rear-axle-housing assembly, which includes the differential, rear axles, wheels, and bearings.

The rear axles are attached to the wheels and have bevel side gears on their inner ends. The differential case is assembled on the left axle but can rotate on a bearing independently of the axle. The differential case supports the differential-pinion gear on a shaft, and this gear meshes with the two bevel gears. The ring gear is attached to the differential case so that the case rotates with the ring gear when the latter is driven by the drive pinion.

Most vehicles are sold with open differentials, which do not offer any additional traction off-road. Power-transfer in an open differential follows thae path of least resistance, supplying power through the spider-gears to the wheel with the least amount of traction. This is most evident off-road when one tire breaks traction and spins (like in the mud or when one tire lifts off the ground).

Lables the components of an open differential.

When a car is driving straight down the road, both drive wheels are spinning at the same speed. The input pinion is turning the ring gear and cage, and none of the pinion within the cage are rotating-both side gears are effectively locked to the cage.

Note that the input pinion is a smaller gear than the ring gear. You may have heard terms like rear axle ratio or final drive ratio. These refer to the gear ratio in the differential. If the final drive ratio is 4.10, then the ring gear has 4.10 times as many teeth as the input pinion gear.

When a car makes a turn, the wheels must spin at different speeds.

The pinions in the cage start to spin as the car begins to turn, allowing the wheels to move at different speeds. The inside wheel spins slower than the cage, while the outside wheel spins faster.

The open differential always applies the same amount of torque to eace wheel. There are two factors that determine how much torque can be qpplied to the wheels:equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; in a low traction situation, such as when driving on ice, the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions. So, even though a car may be able to produce more torque, there needs to be enough traction to transmit that torque to the ground. If you give the car more gas after the wheels start to slip, the wheels will just spin faster.

If you’re ever driven on ice, you may know of a trick that makes acceleration easier: If you start out in second gear, or even third gear, instead of first, bbecause of the gearing in the transmission you will have less torque available to the wheels. This will make it easier to accelerate without spinning the wheels.

Now what happens if one of the drive wheels has good traction, and the other one is on ice?

This is where the problem with open differentials comes in.

Remember that the open differential always applies the same torque to both wheels, and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip. It doesn’t take much torque to make a tire slip on ice. And when the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction, your car isn’t going to move very much.

Another time open differentials might get you into trouble is when you are driving off-road. If you have a four-wheel drive truck, or an SUV, with an open differential on both the front and the back, you could get stuck. Now, remember-as we mentioned on the previous page, the open differential always applies the same torque to both wheels. If one of the front tires and one of the back tires comes off the ground, they will just spin helplessly in the air, and you won’t be able to move at all.

The solution to these problems is the limited slip differential (LSD), sometimes called positraction. Limited slip differentials use various mechanisms to allow normal differential action when going around turns. When a wheel slips, they allow more torque to be transferred to the non-slipping wheel.

参考译文：

差速器是后桥壳总成的一个部件，后桥壳总成包括差速器、后桥、车轮和轴承。

后桥与车轮相连，内装有一个半轴锥齿轮。差速器壳支承在左侧后桥上，而且能够在轴承上做独立运转。差速器壳支承在行星齿轮轴上，行星齿轮与两个半轴齿轮相啮合。冠状齿轮与差速器壳相连，这样当冠状齿轮由传动齿轮驱动时，差速器壳也转动。

大部分市场上的差速器都是开式差速器，不能在越野时提供额外的附着力。开式差速器的分动器在小阻力的道路上，通过十字轴，将动力按最小附着力分配给车轮。最明显的越野路况是一个轮胎丧失附着力或转不动。

当一辆轿车沿着一条路直线行驶时，两侧车轮以同一转速转动。主动齿轮带动冠状齿轮和壳体，壳体内的小齿轮都不转动——两边的齿都有效地将壳体锁住。

注意到主动齿轮的齿数比冠状齿轮少，你可能已经了解了后桥减速比或主减速比。在差速器中，这些都是指主减速比。如果主减速比为4.10，冠状齿轮的齿数就要比主动齿轮的齿数多4.10倍。

当一辆汽车转弯时，车轮必须以不同的转速旋转。壳体内的小齿轮在车辆转向时开始转动，以此实现两侧车轮以不同转速旋转。内侧车轮要比壳体转得慢，但外侧车轮就要转得相对快点。

开式差速器一般都是将大小相同的扭矩分配到两侧车轮上。有两个因素决定分配到车轮扭矩的多少：设备和牵引力。在干燥的环境，有充足的牵引力的情况下，分配到车轮的扭矩受到发动机及齿轮的限制：在牵引力较小的情况下，诸如在冰面上行驶，在这种情况下，扭矩的大小受限于车轮以至于不打滑。所以，即使一辆车可以产生更大的扭矩，同样需要足够的牵引力用以将这些扭转力传输到地面上。如果当车轮开始打滑时，你用力踩油门，只会使车轮转得更快。

如果你曾经在冰面上开过车，你可能知道使加速变得容易的方法：那就是不以一挡起步，而是用二档起步，甚至是三档，因为变速器的档位越高，传到车轮的扭矩越少。这样就会让车轮在不转的情况下加速更快。

当一个汽车主动轮在附系数较高的路面上，而另一个主动轮却在冰面上，会发生什么样的情况，这就是开式差速器的问题所在。

记住，开式差速器总是运用于两轮转矩相等的情况下，最大扭矩受限于最大防滑系数的限制。它并不会给在冰面上的车辆以更大的扭矩。而且牵引力好的那个车轮仅获得和差的车轮一样的很少量的扭矩，此时，你的车就不能正常前进。

除此之外，开始差速器可能在你越野的时候给你带来麻烦。如果你有一辆前后都有差速器的四轮驱动车或越野车，你可能被卡住。现在，记得——就如我们之前已经提到过的一样，开始差速器一般都是给两轮传递相等的扭矩。如果一侧前轮及一侧后轮都陷入地中，那么其他两轮只能在空中无助的旋转，汽车根本无法移动。

这类问题只能通过防滑式差速器来解决。防滑差速器使用多种机械技术来实现常规差速器使车辆转弯的行为。当一侧车轮打滑时，它提供更多的扭矩给不打滑的车轮。

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