Direct drive mechanism
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A Direct drive mechanism is one that takes the power coming from a motor without any reductions (such as a gearbox).
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[edit] Advantages
- Increased efficiency: The power is not wasted in friction (from the belt, chain, 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 belt), or stress.
- High torque at low rpm.
[edit] Disadvantages
The main disadvantage of the system is that it needs a special motor. 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 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.
[edit] Applications
Direct drive mechanisms are present in several products:
[edit] High-speed
- Fans: 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).
[edit] Medium or variable
- Floppy disk drives, and many other stepper motor-driven devices.
- CD drives: the CD is directly coupled to the rotor; 250 to 500 rpm for audio, higher speeds in portable and computer drives.
[edit] Very low rotational speeds
- Turntables: Very precise, 33 1/3 and 45 rpm.
[edit] Other uses
The original introduction of pedals 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.
The use of these mechanisms in high-power devices is under progress:
- Washing machine: Toshiba released direct-drived washing machine on the market in 1997, however sales was limited to Japan. LG presented a washing machine which drives the drum through a direct-drive mechanism. Samsung also introduced a washing machine which drives the drum with a direct-drive motor in spring of 2006.
- Train: Electric Multiple-Unit (EMU) Train, An experimental car "Type E993" called "AC Train", It has direct-drived motor which made by East Japan Railway Company (JR East) in Jan. 2002. Service EMU "Type E331" which harnessed this experience is under manufacture and trial run on the Keiyo Line.
