Brett Fraser

As in fact has an automatic transmission - How does it work?

As in fact has an automatic transmission - How does it work?

Posted in Tech, asked by Brett Fraser, 5 years ago. 725 hits.

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An automatic gearbox effectively does the same job as a manual one by supplying a selectable range of gear ratios between the engine and the wheels. However, there are key differences in their methods. For a start, a manual gearbox physically moves between gears to select them where as an automatic simply utilities different combinations of parts that are already connected to give the desired ratio. In its most basic form, the whole setup revolves around a planetary gear set, which sits in the centre of the gearbox whilst the other parts work around it. The gear set is made up of the central sun gear and what are known as planet gears and a carrier that then rotates around it within a larger outside ring gear housing.
There are a series of clutches and bands in place that then clamp down to stop movement of the parts that make up the gearset or open up to allow it. The gearbox is then able to switch between gear ratios by altering whether each of these parts remain stationary or receive input and/or output. The clutches that restrict or allow movement to the gearset are controlled using hydraulics that are governed by valve bodies. Until recently, these valves were all pressure controlled but modern automatics tend to use more complex electro-mechanical servos to control the valve bodies and these, in turn, are controlled by a computer. Of course it gets more complicated the more gears there are; for example, BMW’s eight-speed uses four planetary gear sets and five shift elements made up of two brakes and three clutches!

Bob Harper
5 years ago
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