A direct-drive mechanism is a
mechanism design where the
force
In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
or
torque from a
prime mover is
transmitted directly to the effector device (such as the
drive wheels of a
vehicle
A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered land vehicle, human-powered vehicles (e.g. bicycles, tricycles, velo ...
) without involving any intermediate
couplings such as a
gear train or a
belt.
[Asada, H., & Kanade, T. (1983) ]
Design of direct-drive mechanical arms
' in ''Journal of Vibration, Acoustics, Stress, and Reliability in Design'', Volume 105, Issue 3, pp.312-316
History
In the late 19th century and early 20th century, some of the earliest locomotives and cars used direct drive transmissions at higher speeds. Direct-drive mechanisms for
industrial arms began to be possible in the 1980s, with the use of
rare-earth magnetic materials.
The first direct-drive arm was built in 1981 at
Carnegie Mellon University
Carnegie Mellon University (CMU) is a private research university in Pittsburgh, Pennsylvania, United States. The institution was established in 1900 by Andrew Carnegie as the Carnegie Technical Schools. In 1912, it became the Carnegie Institu ...
.
Today the most commonly used magnets are
neodymium magnets.
Design
Direct-drive systems are characterized by smooth torque transmission, and nearly-zero
backlash.
[Bruno Siciliano, Oussama Khatib (Eds., 2008) ]
Springer Handbook of Robotics
', p.80[Robotics Technology Abstracts](_blank)
Volume 4, Cranfield Press, 1985, p.362, quote: "direct drive. The direct coupling of motors eliminates backlash completely"
The main benefits of a direct-drive system are increased efficiency (due to reduced power losses from the drivetrain components) and being a simpler design with fewer moving parts. Major benefits also include the ability to deliver high torque over a wide range of speeds, fast response, precise positioning, and low
inertia.
The main drawback is that a special type of electric motor is often needed to provide high torque outputs at low
rpm. Compared with a multi-speed transmission, the motor is usually operating in its optimal
power band for a smaller range of output speeds for the system (e.g., road speeds in the case of a motor vehicle).
Direct-drive mechanisms also need a more precise control mechanism. High-speed motors with speed reduction have relatively high inertia, which helps smooth the output motion. Most motors exhibit positional
torque ripple known as
cogging torque. In high-speed motors, this effect is usually negligible, as the frequency at which it occurs is too high to significantly affect system performance; ''direct-drive'' units will suffer more from this phenomenon unless additional inertia is added (i.e. by a
flywheel) or the system uses feedback to actively counter the effect.
Applications
Direct-drive mechanisms are used in applications ranging from low speed operation (such as
phonographs,
telescope mounts,
video game racing wheels and
gearless wind turbines) to high speeds (such as
fans,
computer hard drives,
VCR heads,
sewing machines,
CNC machines and
washing machines.)
Some electric railway locomotives have used direct-drive mechanisms, such as the 1919
Milwaukee Road class EP-2 and the 2007
East Japan Railway Company E331. Several cars from the late 19th century used direct-drive
wheel hub motors, as did some concept cars in the early 2000s; however, most modern
electric car
An electric car or electric vehicle (EV) is a passenger car, passenger automobile that is propelled by an electric motor, electric traction motor, using electrical energy as the primary source of propulsion. The term normally refers to a p ...
s use inboard motor(s), where drive is transferred to the wheels, via the
axles.
Some automobile manufacturers have managed to create their own unique direct-drive transmissions, such as the one
Christian von Koenigsegg invented for the
Koenigsegg Regera.
See also
*
Belt-drive
*
Chain-drive
*
Direct-drive sim racing wheel
*
Drive shaft
A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power (physics), power, torque, and rotation, usually ...
*
Hubless wheel
*
Linear motor
*
Individual wheel drive
References
{{Reflist
Mechanisms (engineering)
Gearless electric drive