Among
all other speed changing mechanisms, the most simplest type of mechanism is
the Traction Drive. These drives are the most frequent choice for many drive
systems due to their simplicity and having the ability to adjust the speed
smoothly and in excellent way. For more than 100 of years, these drives have
been used in industrial services. In the recent period, these drives are
considered as rivals of the best gears and the best gear belts, because
these contain best quality traction fluids and super quality bearing steels
which are highly fatigue resistant. Two conical shape, rotating rollers are
used in a common type of Traction drive. These rollers are used in
transferring torque between themselves through the adhesive metallic sheet
of a thin film of special lubricants. In another basic type of Traction
Drive, pivotal traction rollers are used. These rollers in this type of
Traction Drive, come in between an output disc and an output disc.
Functional Principles of Traction Drives
This Drive is a kind of fixed-ratio mechanical adjustable speed drive.
There are two types of mechanical adjustable speed drive. Along with
variable pitch drive, this is the second type of it. Through metal rollers
running, These Drive transmits power against the mating metal rollers. The
ratio between the input and output speed is controlled to keep to remain
constant by moving the rollers and thus changing the diameters of the
contact path. Also, different shape of rollers and mechanical designs have
also been used for the same purpose. The shapes of the rollers may be either
cylindrical, conical, rings, discs, toroids or spherical. With the help of
rotational radius of the driven roller on the driven member, the speed ratio
is being determined by these drives. To maximize the efficiency of these
drives as well as their life, the roller's normal load to the torque should
be adjusted. For this adjustment, a torque responsive roller loading
mechanism is applied in these Drives. The performance of these Drive in
adjusting the mechanical speed with high effectiveness, depends upon
following thing: the friction that is generated between specially shaped
output & input plates and the speed adjustment mechanism.
Types of Traction Drives
- Lubricated Traction Drives – Synthetic fluids are used
in these Drives. These fluids reduce the friction (contact pressure)
between the power capacities and the rolling members. The fluids stat to
behave like plastic, when the viscosity of these fluids increases
tremendously under constant high pressure. In transmitting power, these
plastic like fluids help the drives without appreciable metal to metal
contact.
- Dry Traction Drives – These drives have removed the use
of fluids for lubrication purpose. Without using any fluids, these
drives permit about cent percent efficiency in power transmission. In
place of any fluid lubricants, spiral loaded system is used, which
provides the slippery between the various driven and driving members
inside the Dry Traction Drive.
Limitations of Basic Traction Drives
The limitation of these two basic types of Drives is their low potential to
resist the high throughput power and the torque impetuses which are common
in the automotive drive-trains. Another shortcoming of these basic Traction
Drives is their low durability. This low durability arises from the high
surface weary stresses generate in the elements of the Traction Drive.
Although, the fluid/lubricant technology is improving day by day, but still
it is the major source of concern for these drives. The third type of
problem which is found in these Drive is: the rotating components in these
drives are bulky. This reduces the speed of rotation of these components
along with the desired acceleration rate. This in effect, increase the
emissions of the vehicles.
Configurations of Traction Drives
There is a great possibility that the configurations of these drives may
differ from each another. But, most of the design differentiations lie in
their respective power transferring speed to the outer members. Some
examples are:
- Perbury, Torotrak, Tractionmatic
- Full toroid, half toroid, nutating
- Traction Tec, Excelermatic, Jaguar
- Kraus, Arter, Forster
Applications of Traction drives
In the power transmission industry, it was believed that these Drives would
not make any noticeable impact. The reason was that, most of the experts did
not believe that two smooth rollers of a Traction Drive could transmit force
through a fluid. But, today, with the changing advance technology, people
start to believe that traction drives can do this thing. Therefore, in the
engineering industry, these drives have made an intense popularity. These
drives are used in various applications in the engineering industry,
especially when it is the requirements of high efficiency and sound
operations. These drives are also widely used in engineering industry, when
it is concerned to speed control, bringing compactness and consistency in
performances. The inherent accuracy of these drives is proved by the
micrometer speed-setting dials which are fitted on these drives. Such speed
setting dials also provide extra torque even at low speed.
In the planetary vehicles, these drives have proved themselves as the best
alternatives of the gear drives. Contact geometry can be formed in the low
wear to the solid lubricants through these drive rollers which are present
in these drives. In contrast, in other traditional drives, to bring smooth
torque at low speed, additional components are required in these.
