cloyd wrote:Gerrit,
Mostert na die maal, maar wat is jou siening (en verduideliking) vir sleep met 'n automatiese ratkas - 2004 3,8 GLS.
Cloyd.
Hi Cloyd,
Sorry for the late reply.
Most autoboxes in automotive applications use planetary gear sets (though there are some true sequential manual transmissions that work on the countershaft principal, just like a normal manual and of course the CVTs). They operate in a very different way to a manual transmission and they don't really have a "special" gear (such as the direct drive ratio in a manual transmission).
Simple planetary gear sets have a sun gear in the middle with three or more planet gears that mesh with it as well as with a larger ring gear around the whole lot. The shafts that the planet gears spin on are all connected together by what is called the planet carrier. Either the sun gear, the ring gear or the planet carrier can be driven and is then called the input. If one of the other components is prevented from rotating, the remaining component (the output) will rotate at a different speed to the input. Depending on which component is selected as the input and which is the output, different speed ratios between input and output are achieved (i.e. different gear ratios). For a simple planetary gear set, two forward ratios and one reverse ratio can be obtained by these various combinations of input and output. In addition, direct drive can be obtained by locking any two components to each other (this effectively prevents planetary motion). This would be fine if one wanted to build a gearbox with three forward speeds and one reverse.
In modern cars, more than three forward ratios are normally required, so compound planetary gearing is employed. The basic principle is identical to simple planetary gearing, but the the gear set typically has two different sun gears and double planet gears (i.e. two planet gears of slightly different sizes connected together on one shaft. One of these engages the one sun gear and the ring gear directly and the other one engages the other sun gear. This all sounds very confusing, but it works in a similar way to a simple planetary gear set, except that there are more options of bits to drive and bits to hold. Direct drive is achieved in a similar way as in the simple gear set by connecting bits together in such a way that planetary action is blocked (e.g. by locking the small and large sun gears together).
Because of the way direct drive is achieved, there is no rotation of the planet gears about their shafts, so perhaps there is some slight benefit in terms of wear reduction, but the drive torque is still transmitted via the gears in the planetary train, so the gear teeth are still loaded. One of the great things about planetary gearsets, though is that the loads are reacted internally to a great extent, i.e. the planet gear pushes itself away from the sun gear when torque is transmitted, but it also pushes itself away from the ring gear so that these equal and opposite forces cancel thus relieving many of the bearings from side loads.
Automatic transmissions have the added bonus of having a control system that ensures they select the appropriate ratio based upon driveline load and vehicle speed. This factor in addition to the fact that there is not really as strong a case to regard any gear as "special" in terms of transmission wear mean that one can sit back and relax and let the box do the thinking in most situations.
There are a couple of exceptions, for example:
1) The autobox control system can not predict that a hill might be looming ahead, so sometimes it helps to force a downshift before it is deemed necessary by the box.
2) In conditions where the transmission shuttles in and out of torque converter lock-up mode in 5th gear, it makes sense to consider adapting the travel speed to one where less shuttling occurs or to force selecting 4th gear (and possibly adapting travel speed.
3) Despite innovations like grade logic (the transmission's ability to adapt shift strategies based on the descent angle of the road) it sometimes is useful to force selection of a lower gear for enhanced engine braking on downhills.
