Direct drive mechanism

A direct drive mechanism is one that uses the torque coming from a motor without any reductions (such as a gearbox).

Direct drive mechanisms can be categorized in groups

(A) Torque motor mechanisms. Output shaft is driven either by brush torque motor or brushless torque motor. The brushless torque motor requires commutation. Battery driven devices such as cordless hand drills are generally use brush torque motors.

(B) AC drives. Generally driven by 60 cycle voltage. Examples- turntables low speed, fans medium speed, hard drives high speed.

The major alternative to direct drive mechanism is geared stepper motor mechanism driven by current pulses.

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 do not have that things 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 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 motor. Usually motors are built to achieve maximum torque at high rotational speeds, usually 1500 or 3000 rpm. 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) 3313 rpm or 45 rpm.

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 25 mm (1 in). On a direct-drive turntable, the motor is about 100 mm (4 in).

Also, direct-drive mechanisms 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 present in several products:

High-speed

  • Fans: Imprecise, depending on the fan, between 1000 and 21000 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 precise turning tables
  • Washing machines: up to 1600 rpm spin speeds

Medium or variable

Very low rotational speeds

  • Phonograph turntables: Very precise, 78, 3313 and 45 rpm.
  • Telescope mount: Very precise, e.g. 1 revolution in 24 h.
  • Ski lift drive: up to 30 rpm depending upon line speed.

Other uses

See also

References

  1. Patel, Prachi. "GE Grabs Gearless Wind Turbines". Technology Review (MIT). Retrieved 7 April 2011.
  2. Dvorak, Paul. "Direct drive turbine needs no gearbox". Windpower Engineering. Retrieved 7 April 2011.
  3. "Fanatec Release Details On Their(sic) Direct Drive Wheel - Inside Sim Racing". 4 June 2017.
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