I am a very practical person, and am familar with engines and manual gearboxes/clutches. But automatic transmission is a new ball game to me.
Can one of you guys explain how an automatic works. Are there clutch plates to wear out? How does it know when to change gear?
ps. I managed to successful fit the kit cruise control on my 740 automatic, taking the pulses from 3 magnets and pickup coil on the prop shaft. Absolutely magic!on this my first automatic transmission car. Delighted to find out that one can press the resume button at say
15-20 miles per hour and it goes up thro' the gears to the preselected cruising speed. I've lost count of how many cars this kit cruise control has been on, and it is still in working order.
Typically, an autobox will use an epicyclic gear set, sometimes called planetary gear. But there have, over the years, been various other arrangements. I'm not up on the lastest developements, but I would imagine that computer control and 'fly-by-wire' engine management is set to revolutionise (ahem!) transmission systems.
Most conventional autoboxes do have friction surfaces of some sort to control the changes - old ones would have brake bands, but modern ones have a type of clutch. These are used to control which parts of the gear set are able to rotate. They differ from the conventional clutch that you find on a car, in that they are immersed in oil, so wear is minimal, to the extent that you can expect an autobox, with proper care, to achieve 200k miles. Usually they outlast the car, but with Volvo's reputation for high mileage this can become an issue.
The link between engine and gearbox is most often by a torque converter, (also called a fluid flywheel, though in the strictest sense this is not necessarily correct), which is detailed here:
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One of the most unlikey arrangements I came across was by Renault, probably around the late 50s or early 60s. They used a conventional gearbox with some sort of rams to effect the gear changes (I can't remember what controlled them). Instead of a clutch, they had a drum filled with iron filings bolted to the flywheel. The output shaft to the gearbox was finned, and the drive was engaged by passing a current through a coil round the outside of the drum, to magnetise the iron filings. I think history tells us that this can't have been a success.
The first ones made by Oldsmobile and were like a clutch where the plates never touched. There was a viscous fluid that transfered force between the plates. Then they went to hydraulic pump designs. The newer ones all have several things in common. They need engine rpm sensors of some sort and speed sensors of some sort. They can change their ratios depending upon load and speed. They need the ability to slip when stopped and to lock up at cruising speed. Check the following for details:
Have a look at the service notes for a Borg-Warner 35 transmission, as fitted to a Triumph 2000 about 30 years ago
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It gives a good idea of the principle of operation.
More modern transmissions have more gears and electronic (rather than hydraulic) control logic - but they still have a torque converter and several cascaded sets of epicyclic gears. Some have lock-out clutches in parallel with the torque converter to eliminate slip in the cruise condition.
Gear change points are a function of both speed and throttle position - so at high throttle openings, they hang on to the lower gears for longer - boy racer style!
The Magnetic Clutch was also used by NSU in their Ro80 car. The three-speed automatic transmission was engaged/disengaged by light pressure on the gear knob which operated the electric current for the Magnetic Clutch which allowed the transmission to work. The three ratios used were 0 to 40mph, 0 to 80mph and 0 to 120mph.
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