Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

166 Chapter 5 Tracked Vehicle Suspensions and Drivetrains
On a more practical scale for mobile robots, urethane belts with
molded-in steel bars for the drive sprocket and molded-in steel teeth for
traction are increasingly replacing all-metal tracks. The smaller sizes can
use solid urethane belts with no steel at all. Urethane belts are lighter and
surprisingly more durable if sized correctly. They also cause far less
damage to hard surface roads in larger sizes. If properly designed and
sized, they can be quite efficient, though not like the mechanical efficiency
of a wheeled vehicle. They do not stretch, rust, or require any
maintenance like a metal-link track.
The much larger surface area in contact with the ground allows a
heavier vehicle of the same size without increasing ground pressure,
which facilitates a heavier payload or more batteries. Even the very
heavy M1A2 has a ground pressure of about eighty-two kilo pascals
(roughly the same pressure as a large person standing on one foot). At
the opposite end of the scale the Bv206 four-tracked vehicle has a
ground pressure of only ten kilo pascals. This low ground pressure
allows the Bv206 to drive over and through swamps, bogs, or soft snow
that even humans would have trouble getting through. Nevertheless, the
Bv206 does not have the lowest pressure. That is reserved for vehicles
designed specifically for use on powdery snow. These vehicles have
pressures as low as five kilo-pascals. This is a little more than the pressure
exerted on a table by a one-liter bottle of Coke.
When compared to wheeled drivetrains, the track drive unit can
appear to be a relatively large part of the vehicle. The sprockets, idlers,
and road wheels inside the track leave little volume for anything else.
This is a little misleading, though, because a wheeled vehicle with a drivetrain
scaled to negotiate the same size obstacles as a tracked unit would
have suspension components that take up nearly the same volume. In
fact, the volume of a six wheeled rocker bogie suspension is about the
same as that of a track unit when the negotiable obstacle height is the
baseline parameter.
The last advantage of tracks over wheels is negotiable crevasse width.
In this situation, tracks are clearly better. The long contact surface allows
the vehicle to extend out over the edge of a crevasse until the front of the
track touches the opposite side. A wheeled vehicle, even with eightwheels,
would simply fall into the crevasse as the gap between the
wheels cannot support the middle of the vehicle at the crevasse’s edge.
The clever mechanism incorporated into a six-wheeled rocker bogie suspension
shown in Chapter Four is one solution to this problem, but
requires more moving parts and another actuator.
To simplify building a tracked robot, there are companies that manufacture
the undercarriages of construction equipment. These all-in-one
drive units require only power and control systems to be added. They are