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).

Contents

[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.
  • Faster and precise positioning. High torque and low inertia allows faster positioning times on permanent magnets sychronous 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 mechnisms.

[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).
  • Sewing Machines: 3000 rpm to 5000 rpm depending on machine type.
  • Turn Tables: CNC machines with fast and precize turning tables

[edit] Medium or variable

[edit] Very low rotational speeds

  • Turntables: Very precise, 78, 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. This mechanism is still used on most unicycles.

The use of these mechanisms in high-power devices is under progress:

  • Train: The Milwaukee Road class EP-2 electric locomotives introduced in 1919 had the driving wheels mounted directly to the traction motor shafts. East Japan Railway Company (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 Line in 2007.
  • Vehicle: Dutch e-Traction Company Produce a direct drive bus wheel for elecrical vehicle. Bus has 4 wheel each 400kg, with electronics and water cooling driving completely the Bus without any central motor, gearbox or other things. Bus space is than fully used only for a accumulator batteries.
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