USS New Orleans (CA-32)

A differential is a system Mécanique which has as a function to distribute a speed of Rotation by distribution of the kinematic effort, in an adaptive way, immediate and automatic, with the needs for a mechanical unit.

For example, it is very useful on a motor vehicle where it makes it possible the driving wheels to turn has speeds different at the time of the passage from a curve: the Roue S located outside turn more quickly than that located inside.

Operation

On these two images motricity coming from the engine is represented by the large red arrow and is exerted on the blue unit:
  1. On the image of left, both trees of exit (on the left/red and on the right/yellow) turn at the same speed. The effort of rotation is thus distributed equitably and the pinion intermediate green (called satellite) does not turn.
  2. On the image of right-hand side, the red tree is slowed down. The intermediate pinion is then actuated and transfers an additional rotation to the yellow tree which turns then at an high speed.

The system functions in an adaptive way: when one slows down “slightly” one of the two trees (pinching, friction…) the number of revolutions of this tree is slowed down and the speed of the other increases by as much.

This process presents the disadvantage to transfer the couple, i.e. the effort in rotation, in the same direction, the majority of the cases of use, it would be rather desirable to apply a higher couple to the tree opposing greatest resistance.

In its most frequent form the differential is composed of a bevel gear (interdependent of the driving shaft) involving a conical crown wheel perpendicularly supporting a cage of bevel pinions, those actuating two planetary pinions interdependent of the driven shafts.

The mechanical differential was invented in 1827 by the French mechanic Onésiphore Pecqueur (1792-1852). It was employed since 1860 on the first road vehicles with vapor.

Case of use

The most widespread case of use is the drive of a vehicle. In turn, Wheel located inside (on the side where one turns), having a weaker distance to traverse, turns less quickly than the wheel located outside. Thanks to the differential, motricity is maintained while authorizing the difference in speed between the wheels. It thus ensures better a Handling (without differential, a vehicle tends to going straight) and makes it possible to limit the pneumatic wear of the S.

There also existed of the vehicles without differential, choice sometimes dictated by the saving in weight: the Lancia Lambda of 1921, the Close Laboratory to 1923, the Isetta of 1953. In these the last two cases, the problem was solved by a very reduced way back. The Karting is another example of four-wheeled vehicle without differential, but as the axles are very short, the defect induced in turn is less important than on the ordinary cars. Another solution is to motorize only one wheel. However, the vehicle is likely to deviate a little, and this solution is almost used only for the toys.

Differential with mechanical limited slip

The weakness of the ordinary differential lies in the fact that if a wheel of the driving train does not adhere (for example wheel on snow, in mud), the whole of the transmission of the couple is done on this wheel, and conveys it does not advance more correctly, even any more. To solve this problem, certain differentials are designed to be limited starting from a certain threshold of slip (for example detecting a variation of couple between the two half trees fixed at the satellites), they are the differentials with limited slip (French DGL or English LSD). The cross-country vehicles often use these devices because they are designed to go in very movable grounds (mud, snow, etc). For the powerful vehicles which develop much couple in general, it can be necessary to use one (or of) DGL. In this case, the vehicle can modify its behavior in turn, and instead of skidding, it preserves its adherence but can modify the foreseeable trajectory appreciably. It is necessary then more “to control” that to lead!

There exist several marks of differentials with limited slip (DGL) mechanical. Many patents are still active, the solutions of the market thus comprise notable differences. Among the most current models, one can quote the differentials:

  • Torsen (Gleason Works American manufacturer of pinions and gears, deposited the Torsen mark, for “TORque SENsing”, using an endless screw, principle invented in the middle of XXème century by American engineer Vernon Gleasman),
  • Quaife (English manufacturer, either in OEM at Ford in particular, or in part of replacement),
  • GKN Driveline (Japanese manufacturer of DGL and visco couplers to limited slip).

One often says that the DGL is damaged to xx %, to mean that it tolerates a variation of couple of 100-xx % between the half trees. A DGL does not force the halfshafts to be interdependent in the event of ice-skating of a driving wheel, which would be an operation mechanically very constraining (risk of important breakage).

Other complementary systems

There exist other systems with limited slip placing upstream differential. A remarkable example is celebrates it Quattro system of Audi and Volkswagen with transverse engine which rests on a sophisticated clutch:
  • the system Haldex Limited Coupling Slipway of the company Haldex Traction AB, multiple-disc clutch turning in an oil bath whose pressure is controlled electro-hydraulically by an electronic system.
It makes it possible to couple partially or completely the axles before and back (in general it is assembled just upstream of the rear axle). This architecture is extremely powerful (some go until saying too much) and makes it possible to manage the adherence of the four wheels in perfect compatibility with ABS, EPS and other electronic systems. The essential performance of the system lies in its reaction time extrèmement short (a wheel patinates only approximately a tenth of turn before the reaction of Haldex). This system is also able to take into account, in real-time, of the subtle parameters as the diameter different from each wheel (each tire is never exactly same diameter, a “wafer” spare wheel can be used).

The principal criticism carried against the systems of the Haldex type is the consumption of power more important than in the mechanical systems without fluid. However, contrary to the mechanical systems which are always active (and always dissipate of this fact of the power), it can not be active in normal condition and preserve an average output higher than the DGL.

See too

  • Bridge-engine

Random links:Canton of Montauban-of-Brittany | La historia de Nueva Zelandia | Michael Snow | Cracmol | Farming community in the French revolution | Soricomorpha | USS_la_Nouvelle-Orléans_(CA-32)