A direct-drive mechanism is a

mechanism Mechanism may refer to: *Mechanism (economics), a set of rules for a game designed to achieve a certain outcome **Mechanism design, the study of such mechanisms *Mechanism (engineering), rigid bodies connected by joints in order to accomplish a ...

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 ...

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

from a

prime mover Prime mover may refer to: Philosophy *Unmoved mover, a concept in Aristotle's writings Engineering * Prime mover (engine or motor), a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc.) into ener ...

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 A gear train or gear set is a machine element of a mechanical system formed by mounting two or more gears on a frame such that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each oth ...

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 magnet A nickel-plated neodymium magnet on a bracket from a hard disk drive file:Nd-magnet.jpg">Nickel-plated neodymium magnet cubes Left: high-resolution transmission electron microscopy image of Nd2Fe14B; right: crystal structure with unit cell mar ...

Design

Direct-drive systems are characterized by smooth torque transmission, and nearly-zero

backlash Backlash may refer to: Literature * '' Backlash: The Undeclared War Against American Women'', a 1991 book by Susan Faludi * ''Backlash'' (Star Wars novel), a 2010 novel by Aaron Allston * Backlash (Marc Slayton), a comic book character from ...

.Bruno Siciliano, Oussama Khatib (Eds., 2008)
Springer Handbook of Robotics
', p.80Robotics Technology Abstracts
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 Inertia is the natural tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes the velocity to change. It is one of the fundamental principles in classical physics, and described by Isaac Newto ...

. The main drawback is that a special type of electric motor is often needed to provide high torque outputs at low

rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 def ...

. Compared with a multi-speed transmission, the motor is usually operating in its optimal

power band The power band of an internal combustion engine or electric motor is the range of operating speeds under which the engine or motor is able to output the most power, that is, the maximum energy per unit of time. This usually means that maximum ...

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 Torque ripple is an effect seen in many electric motor designs, referring to a periodic increase or decrease in output torque as the motor shaft rotates. It is measured as the difference in maximum and minimum torque over one complete revolution, ge ...

known as

cogging torque Cogging torque of electrical motors is the torque due to the interaction between the permanent magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable pro ...

. 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 A flywheel is a mechanical device that uses the conservation of angular momentum to store rotational energy, a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, a ...

) 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

phonograph A phonograph, later called a gramophone, and since the 1940s a record player, or more recently a turntable, is a device for the mechanical and analogue reproduction of sound. The sound vibration Waveform, waveforms are recorded as correspond ...

telescope mount A telescope mount is a mechanical structure which supports a telescope. Telescope mounts are designed to support the mass of the telescope and allow for accurate pointing of the instrument. Many sorts of mounts have been developed over the year ...

s, video game racing wheels and gearless wind turbines) to high speeds (such as fans, computer hard drives, VCR heads,

sewing machine Diagram of a modern sewing machine Animation of a modern sewing machine as it stitches A sewing machine is a machine used to sew fabric and materials together with thread. Sewing machines were invented during the first Industrial Revolutio ...

s, CNC machines and

washing machine A washing machine (laundry machine, clothes washer, washer, or simply wash) is a machine designed to laundry, launder clothing. The term is mostly applied to machines that use water. Other ways of doing laundry include dry cleaning (which uses ...

s.) 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 motor A wheel hub motor, hub motor, or in-wheel motor is a motor that is incorporated into the Wheel#Hub, hub of the wheel. Wheel-hub motors are commonly found on electric bicycles. Electric hub motors were well received in early electric cars, but ha ...

s, 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

axle An axle or axletree is a central shaft for a rotation, rotating wheel and axle, wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In ...

s. 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.

References

{{Reflist Mechanisms (engineering) Gearless electric drive

History

Design

Applications

See also

References