Direct drive mechanism
Encyclopedia
A Direct drive mechanism is one that takes the power coming from a motor
without any reductions (such as a gearbox).
. Usually motors are built to achieve maximum torque
at high rotational speeds, usually 1500 or 3000rpm. While this is useful for many applications (such as an electric fan
), other mechanisms need a relatively high torque at very low speeds, such as a phonograph turntable
, which needs a constant (and very precise) 33 1/3 rpm or 45 rpm. A 1500 rpm motor couldn't even start rotating if fed the necessary voltage
to make it spin at 33 rpm, and it wouldn't be precise keeping that speed.
The slow motor also needs to be physically larger than its faster counterpart. For example, in a belt-coupled turntable, the motor diameter is about 1 inch (2.5 cm). On a direct-drive turntable, the motor is about 4" (10 cm).
Also, direct-drive mechanisms need a more precise control mechanism. Low voltage variations on a high-speed motor, which is reduced to low rpms can go unnoticed, but in a direct-drive, those variations are directly reflected on the rotational speed.
to the bicycle
, around 1860, incorporated a direct drive mechanism, where the pedals were directly connected to the front wheel
hub, and one rotation of the pedals produced one rotation of the front wheel. This mechanism is still used on most unicycle
s and children's tricycle
s.
The use of these mechanisms in high-power devices is under progress:
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
without any reductions (such as a gearbox).
Advantages
- Increased efficiency: The power is not wasted in friction (from the beltBelt (mechanical)A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the...
, chainRoller chainRoller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire and tube drawing machines, printing presses, cars, motorcycles, and simple machines like...
, etc, and especially, gearboxes.) - Reduced noise: Being a simpler device, a direct-drive mechanism has fewer parts which could vibrate, and the overall noise emission of the system is usually lower.
- Longer lifetime: Having fewer moving parts also means having fewer parts prone to failure. Failures in other systems are usually produced by aging of the component (such as a stretched beltBelt (mechanical)A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the...
), or stress. - High torque at low rpm.
- Faster and precise positioning. High torque and low inertia allows faster positioning times on permanent magnet synchronous servo drives. Feedback sensor directly on rotary part allows precise angular position sensing.
- Drive stiffness. Mechanical backlash, hysteresis and elasticity is removed avoiding use of gearbox or ball screw mechanisms.
Disadvantages
The main disadvantage of the system is that it needs a special motorElectric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
. Usually motors are built to achieve maximum torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
at high rotational speeds, usually 1500 or 3000rpm. While this is useful for many applications (such as an electric fan
Fan (mechanical)
A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...
), other mechanisms need a relatively high torque at very low speeds, such as a phonograph turntable
Phonograph
The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
, which needs a constant (and very precise) 33 1/3 rpm or 45 rpm. A 1500 rpm motor couldn't even start rotating if fed the necessary voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
to make it spin at 33 rpm, and it wouldn't be precise keeping that speed.
The slow motor also needs to be physically larger than its faster counterpart. For example, in a belt-coupled turntable, the motor diameter is about 1 inch (2.5 cm). On a direct-drive turntable, the motor is about 4" (10 cm).
Also, direct-drive mechanisms need a more precise control mechanism. Low voltage variations on a high-speed motor, which is reduced to low rpms can go unnoticed, but in a direct-drive, those variations are directly reflected on the rotational speed.
High-speed
- FansFan (mechanical)A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...
: Imprecise, depending on the fan, between 1000 and 12000 rpm. - Hard drives: Very precise, 5400, 7200, 10000, 15000 rpm and others.
- VCR heads: Very precise, 1800 rpm (NTSC) or 1500 rpm (PAL).
- Sewing MachineSewing machineA sewing machine is a textile machine used to stitch fabric, cards and other material together with thread. Sewing machines were invented during the first Industrial Revolution to decrease the amount of manual sewing work performed in clothing companies...
s: 3000 rpm to 5000 rpm depending on machine type. - Turn Tables: CNC machines with fast and precise turning tables
- Washing machineWashing machineA washing machine is a machine designed to wash laundry, such as clothing, towels and sheets...
s: up to 1400 rpm spin speeds
Medium or variable
- Floppy disk drives
- CD drives: the CD is directly coupled to the rotorRotor (electric)The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...
; 250 to 500 rpm for audio, higher speeds in portable and computer drives.
Very low rotational speeds
- Phonograph TurntablePhonographThe phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
s: Very precise, 78, 33 1/3 and 45 rpm.
Other uses
The original introduction of pedalsBicycle pedal
A bicycle pedal is the part of a bicycle that the rider pushes with their foot to propel the bicycle. It provides the connection between the cyclist's foot or shoe and the crank allowing the leg to turn the bottom bracket spindle and propel the bicycle's wheels...
to the bicycle
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....
, around 1860, incorporated a direct drive mechanism, where the pedals were directly connected to the front wheel
Wheel
A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...
hub, and one rotation of the pedals produced one rotation of the front wheel. This mechanism is still used on most unicycle
Unicycle
A unicycle is a human-powered, single-track vehicle with one wheel. Unicycles resemble bicycles, but are less complex.-History:One theory of the advent of the unicycle stems from the popularity of the penny-farthing during the late 19th century...
s and children's tricycle
Tricycle
A tricycle is a three-wheeled vehicle. While tricycles are often associated with the small three-wheeled vehicles used by pre-school-age children, they are also used by adults for a variety of purposes. In the United States and Canada, adult-sized tricycles are used primarily by older persons for...
s.
The use of these mechanisms in high-power devices is under progress:
- Washing machineWashing machineA washing machine is a machine designed to wash laundry, such as clothing, towels and sheets...
: Various manufacturers, including ArçelikArçelikArçelik A.Ş. is a household appliances manufacturer in Turkey. The Company engages in the production and marketing of durable goods, components, consumer electronics and after-sale services...
, Fisher & PaykelFisher & PaykelFisher & Paykel is a major appliance manufacturing company based in East Tamaki, New Zealand.Originally an importer of domestic refrigerators, Fisher & Paykel now holds over 420 patents and bases its identity on innovative design, particularly in the areas of usability and environmental...
, LGLGLG may refer to:*LG Corp., a South Korean electronics and petrochemicals conglomerate*LG Electronics, an affiliate of the South Korean LG Group which produces electronic products* Lawrence Graham, a London headquartered firm of business lawyers...
, SamsungSamsungThe Samsung Group is a South Korean multinational conglomerate corporation headquartered in Samsung Town, Seoul, South Korea...
, Whirlpool and ToshibaToshibais a multinational electronics and electrical equipment corporation headquartered in Tokyo, Japan. It is a diversified manufacturer and marketer of electrical products, spanning information & communications equipment and systems, Internet-based solutions and services, electronic components and...
have released washing machines where the drum is driven directly by the motor, replacing the usual belt or gearbox-driven system which increases noise and decreases efficiency.
- TrainTrainA train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...
: The Milwaukee Road class EP-2Milwaukee Road class EP-2The Milwaukee Road's class EP-2 comprised five electric locomotives built by General Electric in 1919. They were often known as Bipolars, which referred to the bipolar electric motors they used. Among the most distinctive and powerful electric locomotives of their time, they epitomized the...
electric locomotives introduced in 1919 had the driving wheels mounted directly to the traction motor shafts. East Japan Railway CompanyEast Japan Railway Companyis the largest passenger railway company in the world and one of the seven Japan Railways Group companies. The company name is officially abbreviated as JR East in English, and as in Japanese. The company's headquarters are in Yoyogi, Shibuya, Tokyo....
(JR East) built an experimental E993 series electric multiple-unit (EMU) called the "AC Train" in January 2002 to test the feasibility of direct-drive motors on commuter trains. The technology was later incorporated in the production E331 series EMU which entered service on the Keiyō LineKeiyo LineThe is a railway line connecting Tokyo and Chiba, Japan, running mainly along the edge of Tokyo Bay. It is operated by the East Japan Railway Company .It provides the main rail access to the Tokyo Disney Resort and the Makuhari Messe exhibition center...
in 2007.
- VehicleVehicleA vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....
: Dutch e-Traction Company produce a direct drive bus wheel for elecrical vehicles. The buses produced have 2 direct driven wheels, each 400kg, with electronics and water cooling. They drive the bus without any central motor, gearbox or other devices. The bus internal space is then fully used, except for rechargeable batteries.
- Wind turbines: Various companies have developed direct drive generators for wind turbines, which are designed to improve efficiency and decrease maintenance costs.
- Speed dome cameras: JVC Professional developed and produced a direct drive mechanism for use in speed dome cameras in CCTV systems. Direct drive enables perfect control of camera mechanism speeds. Even at high 36x magnification, the camera can quickly and accurately capture images, at rates from 500 degree/s to 0.04 degree/s.
- Double bass pedal : Allows a drummer to have full control of his double bass pedals. This is different than normal double pedals which connect the pedal to the beater via a rubber strip or a metal chain.
- Telescope Mount Drive : Allows a telescope to follow the apparent daily rotation of the heavens around earth, for photographic and visual application. The absence of a periodic error associated with gear drives is an advantage.