Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

140 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains
Figure 4-9 The common and
unpredictable differential
tion surfaces, and the vehicle is going straight, the wheels rotate at the
same rpm. If the vehicle turns a corner, the outside wheel is traversing a
longer path and therefore must be turning faster than the inside wheel.
The differential facilitates this through the internal gears, which rotate
inside the large gear, allowing one axle to rotate relative to the other. This
system, or something very much like it, is what is inside virtually every
car and truck on the road today. It obviously works well.
The simple differential has one drawback. If one wheel is rolling on a
surface with significantly less friction, it can slip and spin much faster
than the other wheel. As soon as it starts to slip, the friction goes down
further, exacerbating the problem. This is almost never noticed by a
human operator, but can cause mobility problems for vehicles that frequently
drive on slippery surfaces like mud, ice, and snow.
There are a couple of solutions. One is to add clutches between the
axles that slide on each other when one wheel rotates faster than the
other. This works well, but is inefficient because the clutches absorb
power whenever the vehicle goes around a corner. The other solution is
the wonderfully complicated Torsen differential, manufactured by Zexel.
The Torsen differential uses specially shaped worm gears to tie the
two axles together. These gears allow the required differentiation
between the two wheels when turning, but do not allow one wheel to spin
as it looses traction. A vehicle equipped with a Torsen differential can
effectively drive with one wheel on ice and the other on hard dry pavement!
This differential uses very complex gear geometries. The best
explanation of how it works can be found on Zexel’s web site:
www.torsen.com.