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Ari Holopainen / double_clutch | |
| Q. Double Clutch Gearboxes: Progressive gear steps Inspirer: Personal cars Double clutch gearbox or Direct shift gearbox (DSG) is an automatic personal car transmission with parts of manual transmission. It has 2 independent drive lines which are engaged alternately with double clutch unit. One drive line contains odd gears and another even gears. While one drive line is free of torque you can always preselect next gear before shifting. Shifting itself is only switching from one clutch to another so double clutch gearbox shifts very fast. Like other personal car transmissions also double clutch gearboxes have progressive gear steps. Next double clutch gearboxes need remote control shifting system but I don’t have parts to build and test such systems. So if you build remote control system for any of these gearboxes, let me know how to do it. :-) Essential in shifting system is that it shifts only free drive line but doesn’t affect to driven drive line. This way gears can be preselected which is the idea of double clutch gearbox. Q.1. 4-speed, overall gear ratio 2,38:
This 4-speed double clutch gearbox is remake that has sport car’s close ratio ratio set. It’s also shorter and simpler design than original version. Original version is wide ratio design that suits better to diesel powered car so it’s moved to page Junkyard if you want to check it. Porsche has had 4-speed double clutch gearboxes in the 80’s but I haven’t found ratios of those gearboxes. While Porsche’s gearbox was race transmission my guess is that it’s close ratio design.
While this lego version is not automatic you have to shift gears manually. Here is how it works: First you shift 1st gear with shift lever 1-3 and then move off with engaging drive line 1 in double clutch unit which is in input axle. While driving with 1st gear you preselect 2nd gear with shift lever 2-4. When it’s time to shift you only shift double clutch unit. Now in 2nd gear drive line 2 is active but 1st gear is still engaged. It’s time to disengage it and preselect 3rd gear. And again shifting happens with double clutch unit. 4th gear is shifted with same manner. Lego version is not so handy as real double clutch gearbox but it’s remote control friendlier than truck or tractor gearboxes I have designed. With 3 small motors it’s possible to build remote control shifting system. Shifting mechanisms don’t have to stop in neutral while gears are preselected. Only time you need a neutral in double clutch unit is if you want to stop without stopping driving motor. Building note: use only two bricks per axle (three with universal joint). There are coaxial axles in this design. Q.2. 4-speed version II, overall gear ratio 3,33:
Operational principle of this version II is same as in previous double clutch gearbox so if you need information how it works check previous chapter.
Also this one is close ratio design but gear steps and overall gear ratio are larger than in version I and suit to personal car transmission. Also this one is remake: ratio set is not changed but design is shorter and simpler than original which is available on page Junkyard. Building note: use only two bricks per axle (three with universal joint). There are coaxial axles in this design. Q.3. 7-speed, overall gear ratio 5,00: Here is 7-speed version of double clutch gearbox. I found proper ratios for this transmission some time ago but it took some time before I finally designed it. I knew this gearbox wouldn’t be simple but final design is longer and more complicated that I wanted it to be. Actually gear connecting is as complex as in real double clutch gearbox but real version has automated use. While gears are divided to odd (drive line 1) and even (drive line 2) gears 6- and 8-speed versions need same number of synchro parts as this so they will be as complex as this 7-speed version.
Gear steps are small and overall gear ratio is only 5,00 so this gearbox is pure close ratio version. This gearbox is used same way as previous 4-speed versions so check 4-speed double clutch gearbox version I to know how to use this. While this transmission has more gears it means that gear connecting synchro parts must be stopped also in neutral positions. For example in drive line 1 you can’t shift to 5th or 7th gear if 1st or 3rd gear is engaged, otherwise driveline will become locked. This is important feature if you want to build remote control version of this transmission. Note: you may not see this in pictures but every axle has only two bearing points except output axle that has three bearing points due to universal joint. If you have difficulties to figure out this gearbox from pics download LDraw file. Q.4. 6-speed, overall gear ratio 5,00:
This lego 6-speed double clutch gearbox is quite interesting construction: double clutch is made from synchro parts like in 4-speed versions but drive lines 1 & 2 are sequential sliding axle solutions. Compared to real double clutch gearbox this sequential structure is first difference, second is that drive lines don’t have neutral positions at all. Funny fact is that four ratios of this gearbox are integers and rest two have only one decimal.
Using this gearbox is almost same as shifting 4-speed versions. Here is how it goes: Before you move off make sure that 1st and 2nd gear are engaged. To move off engage drive line 1 with double clutch. While 2nd gear is already engaged you are ready to shift to 2nd gear, shifting itself is switching from one clutch to another. In 2nd gear you preselect 3rd gear in drive line 1 and then shift with double clutch. While in 3rd gear you preselect 4th gear in drive line 2 and again shift with double clutch. Then you repeat this until you reach top gear. One notable thing is that drive line that is free of torque is rotated by output axle. So in low gears there is significant overdrive situation while rotating happens from rear to front. This can mean bad efficiency so after shifting to 2nd gear preselect 3rd gear immediately to get better efficiency. Maybe in 3rd gear is also wise to preselect 4th gear but in upper gears problem doesn’t occur. And I found later that this gearbox could be 1 stud shorter: 16 teeth gear of drive line 2’s sliding axle won’t mesh with 16 teeth gear of output axle so they could be parallel. So you can also move drive line 1’s rear gears and rear brick segment 1 stud forward. One interesting fact is that 6-speed sequential gearbox is not possible with legos because it’s too long construction, not even 5-speed is practical. But 2 parallel 3-speed sequential subgearboxes allow you to build 6-speed version. With this technic also 7- and 8-speed double clutch gearboxes are possible. Q.5. 6-speed version II, overall gear ratio 5,00: Here is another 6-speed double clutch gearbox that has synchro parts in double clutch but drive lines are sliding axle solutions. This time gearbox is double overdrive version. Conversion is done by changing gears that mesh with clutch gears from 24 teeth to 16 teeth. Other gear pairs are same as in previous version so this gearbox has same gear steps and overall gear ratio. Gears’ order is different because I first thought that I was creating totally new gearbox. This gearbox is used same way as previous 6-speed version so check it if you need more info about shifting.
While this is overdrive gearbox, ratios in lower gears are smaller than in direct drive version. This diminishes overdrive situation in lower gears in free drive line which is rotated by output axle. So possible efficiency problem in lower gears is not so essential. On the other hand overdrive top gear has poorer efficiency than direct top gear. Q.6. 8-speed, overall gear ratio 6,75: I got finally ready this 8-speed double clutch gearbox. It’s based on previous 6-speed gearboxes so check 6-speed version I for more information. This gearbox is a little bit long because I had difficulties to get all gear wheels into this construction. For example in output axle two 24 teeth gears where needed sequential by drive line 2 but drive line 1 needed gap between two 24 teeth gears so you actually needed three 24 teeth gears and lots of space.
Gear steps are not pure progressive while gear step 1-2 is same as 2-3 and again gear step 3-4 is same as 4-5. But otherwise there is progression so all in all you can say this gearbox has progressive gear steps. One notable thing is too small gear step 7-8 but without it I couldn’t get 8 gears. I have searched through many many gear pair combinations but it’s very difficult to get 8-speed gearbox with progressive gear steps. Other notable thing is that this gearbox has 3 overdrive gears so it’s possible that efficiency of the top gear is not very good while its ratio is as small as 0,67. While input axle is supported by two technic brick 1x2 with 2 holes you may have difficulties to attach studded lego motor to the input axle. If you need hints to solve this problem check 4-speed version II in personal car transmissions (not in double clutch gearboxes). Reason for this arrangement is to get simpler shifting devices for drive line 1 and 2 by moving whole gearbox half stud sideways. Q.7. 4-speed version III, overall gear ratio 3,27: This 4-speed double clutch gearbox uses same technic as previous 6-speed versions so it has synchro double clutch and sliding axle drive lines. But this time there is a new feature: shifter that handles shifting of both drive lines.
Shifter’s idea is that it shifts only free drive line while driven drive line stays in gear. Shifter has positions 1-2, 3-2 and 3-4, and these positions are preselected while shifting itself happens with double clutch. Here is how it goes: Shifting from 1st gear to 2nd doesn’t need preselection, only switching from 1st clutch to 2nd. When you are in 2nd gear in upshifting procedure you change shifter’s position from 1-2 to 3-2. This changes gear in drive line 1 but doesn’t affect to drive line 2 and 2nd gear. When ready you shift with double clutch. While in 3rd gear you move shifter from position 3-2 to 3-4 (affects only to drive line 2) and you are ready to shift to 4th gear with double clutch.
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