Direct drive mechanism
A direct drive mechanism is one that takes the power coming from a motor without any reductions (such as a gearbox).
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 phonograph turntable, which needs a constant (and very precise) 33 1⁄3 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 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. 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.
Direct drive mechanisms are present in several products:
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