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Found 3 results

  1. I had many people ask me in the Wildcat 6x6 topic and it's video if I really need 12 motors, couldn't I get simillar performance with a lighter model? So in order to test and showcase my reasoning I built this very simple 4x4, it uses only 555 parts and is powered by "only" 2 motors: Originally the driveline and gear ratio was as following: Motor's fast outputs to the 2 speed gearbox - 12 tooth bevel gears - via cardan joints to the planetary hubs But than I soon noticed the model lacked sufficient torque, especially in high gear, so I changed the driveline to the following: Motor's fast outputs to the 2 speed gearbox - 20:28 normal differential - via small CV joints to the planetary hubs This driveline ended up having a very simillar gear ratio as the Wildcat 6x6, I think it's some 3% faster, which is negliable IMO. One thing that I did have to do is lubricate the main drive axle, as it was prone to melting in high gear: Because this light 4x4 has open differentials, the front suspension was designed in such a way that it imitatates a pendular front axle by having shock absorbers on a pivot. This allows the front axle to basically act like a pendular axle, easily adjusting to the terrain amd keeping the weight even on both left and right side. So...What about the results? Well, the outdoor test showed that the model performs good in low gear, it has plenty of torque to climb and works really smooth and efficient. But where it lacked was in high gear. While it did reach 15 km/h as the Wildcat 6x6, the acceleration itself was much lower and both motors were drawing a lot of current. Any prologned steering in high gear at full power caused the motors to overheat and cut out, no powersliding on gravel like the Wildcat 6x6. Also when driving on the pump track, it was able to go over big hills in high gear, but it was lacking the speed to actually get significant air, again unlike the Wildcat 6x6. So... Why does it seem more motors and a heavier model is preferable? The answer comes to one factor - Power To Weight ratio: Wildcat 6x6 uses 12 drive motors and weighs around 2,5 kg - this means each motor has to move around 208 grams of weight This light 4x4 offroader uses 2 drive motors weighs around 0,8 kg - in this case each motor has to deal with 400 grams of weight, almost double of the Wildcat 6x6 So does that mean bigger is always better? Well when it comes to math, it seems to point that way. But bigger and heavier models suffer from another problem - the Inverse Cube Law. Basically a bigger model needs more structural reinforcments to hold itself together which means it's heavier and therefore needs more power, etc... It's a typical problem in aviation and rocket science. In our case, a bigger model if built efficiently (using frames and such for a light, strong chassis) will generally perform better, but it will also cause a greater load on the individual parts such as suspension components and wheels in the case of my Wildcat 6x6 (when going to quickly over a large bump, the front wheels have a tendency to fall off to to the higher forces in relation to higher weight). So what's the take from all this? I would summarize it like this: If you want a robust, reliable model go light and slower If you want a very high performance model, pack it full of motors, though individual components may suffer more and fail under their own weight. Also remember there are some heavy elements such as steering motor, wheels, tyres, hubs that you will have to use regardless of the number of drive motors - which weight favours being spread among more motors And also that some components such as planetary hubs do come with some inherent friction that will have to be overcome - which again favour it's friction being spread among more drive motors. I'd love to to hear your opinion about this. Also if anyone is interested, i can make a seperate topic/video about the Light 4x4 offroader.
  2. (Warning: lot of text, dry theory and zero nice looking pictures ahead) Sorry for those horrible pictures - my phone from 2014 has a nice size but the camera is showing its age Whenever i see big and/or powerful model i wonder about the geartrain design, efficiency and reliability. I always assumed that it's better to transfer power with high rpm, rather than high torque. It took some time to figure out how to put this assumption to the test, but here it finally is: my geartrain testing stand V3.1: The idea is to measure the torque loss across the tested geartrain at a given rpm for a known resistance. Those measurements can then be compared for different geartrains. There are several challenges i had to solve and that still can be optimised - input is very welcome! Adjustable and reliable power source: i opted for a switching mode power supply on PF motors reliable torque measurement: originally i planned to get the input torque from electrical measurements on the motor and trust the brake's torque. Instead i built sensors based on differentials. rigid test-geartrain mounting: i think i used more 5x7 frames than 42055 but i'm still not satisfied (more on that in the results) consistent braking torque that doesnt change over time: i wanted to be able to run the testing stand for extended time, so no weight lifting or friction. Instead of an electric brake i opted for a fluid/air brake The assembly consists of: 2x PF L motors as power source with the added rpm sensor on a 1:3 ratio to get a better sensor resolution (from 20 rpm to 6,7 rpm) A rotating set of weight blocks on a 1:3 ratio as high inertia rpm-buffer The input torque sensor based on an inline planetary gearset and a lab scale The test-geartrain The output torque sensor The aerodynamic brake as power sink Let's take a closer look at the seperate modules: The power sorce are these two PF-L motors, as the brake is powerful enough to drive one motor alone into thermal shutdown - even without a testing geartrain. They are regulated by a switching mode power supply. Idealy i'd set the driving torque via the current (an electric motor's torque is proportional to its input current). My current-dial isn't nearly acurate enough, but fortunately those armchair-engineer thoughts don't matter in the real world As the rpm sensor only has a resolution of 20 rpm, i attached it with a 8t gear to to get a 1:3 ratio and therefore a 6,7rpm resolution (not visible in the picture). The inertia rpm-buffer was added because i had rpm oscilations, especially at higher torques. It's build from two weight blocks which spin at a ratio of 1:3 to store more energy (kinetic energy is proportional to rpm squared). This ratio is achived by 36t and 12t gears instead of the simpler 24t and 8t gears as bigger gears generally cause less bearing loads (longer levers with the same torque). They are hidden in the dual 5x7 frames to the left and right. To minimize the bearing losses of the weight block assembly, i didn't mount it directly in stud holes but instead layed it on four black discs. This way the weight lies on 8 holes (2 per disc) instead of 2 and has a lower rpm. Another advantage of this bearing system would be a way lower breakaway-torque, but that's irrelevant in this case. The torque sensors work by changing the direction of the roation and bracing the idler gears on lab scales. I built my own planetary differentials because they are more efficient than bevel geared differentials like those from lego and i expected a lot of torque. I had a hard time figuring out the angle between the gears so they don't have any preload. Then i realised that i could have made my live a lot easier by switching the 20t and 16t gears around The input torque sensor (red) is mounted on small turntables which transfer the force from the high torques. As the output torque sensor (green) only has to transfer the torque from the air brake, i mounted it directly on the turning axles to get more precise readings (no stiction from the turntable) The lab scales have a resultion of 0,05g up to 1kg, which is far to precise for this application. Here's a cutaway of the gears in the sensor. The input (red) and output (green) turn in opposite directions through the idler gears which are mounted on the yellow sensor beam, which then presses onto the scale. The tested gear train get's mounted in a big compromise of stiffness, space and removability. This part of the whole assembly has the most potential for optimization. For the beginning i made tests with a 3:1 ratio (power transfer with high torque) or 1:3 ratio (power transfer with high rpm). The power sink was the part of the project i experimented the most with, until i settled on this air brake. It's surprising how much energy it takes to turn those dishes. I can change the resistance by moving the dishes further inside, changing the gear ratio to the air brake, or change the dishes for some different airfoil. It has a permanent 1:3 ratio to provide some resistance even at low rpm. The size of the airbrake is also the reason i had to mount all the other components higher The connection between the modules is achieved with universal joints to prevent potential resistance from misaligned axles, the modules are braced against each other with two 5x7 frames: one immediately under the universal joint to assure correct allignment and one further below for torque transfer. Testing is unfortunately a pretty involved process: First i have to set the rpm via the voltage of the power supply (this can be a challenge even with the added inertia) Then i have to note the voltage, rpm and the min. and max. values on the scales manually. This is necessary (and difficult) because the scales have a high readout frequency and there's a lot of vibration in the system leading to readout variations from 2 to 7g. Then it's off to the next measuring point. I do this once with rising and faling rpm to prevent measuring errors by hysteresis. Automating this with a mindstorms system would be great but i have neither the sets nor any idea how to implement the torque sensors. First results showed that the variations in the testing runs with the same gear assemblies are bigger than the differences between the different gear trains I don't trust those results because of this (and because it goes against my intuition ) (green is the high torque transfer, violet the high rpm transfer) Now i'm not sure how to proceed. I could make more measurements in the hope that they'll even out eventually. Or rebuild the modules to be more stiff? I'm looking forward to your comments and helpfull insights
  3. Hi. I am thinking about updating my computer to improve my designs with LDD, but since I read that the LDD software is old, it is not updated, etc., I do not know if I will really see an improvement in performance. I do not mind buying a very powerful computer, but I'm afraid to buy it and the LDD improvement is very small. My current computer: -Intel Core i-5 (2013) -Memory RAM: 16 gb -Solid hard disk of 125 GB -Disk hard disk 1 TB -Graphic card 2 GB Envy Powerful computer: (possible purchase) -Intel Core i-7 9700K 3.6 Ghz -Motherboard MSI z370-A Pro -Memory RAM: 32 gb (2x16) Corsair V. RGB pro DDR4 -Solid hard disk of 500 GB wd blue 3d Nand SSD SATA -Disk hard disk 1 TB -Graphics card 6 GB Gigabyte Aourus Geforce GTX 1060 GDDR5 The problem is for models with more than 20,000 pieces. Now I'm with a model of 24,000 pieces, and I need to select sets, to move them, etc, and when I do that, you have to wait at least 1 minute, and when you make the move, it almost always gives errors. Do you think that with a powerful computer I will see an important improvement with models of more than 20,000 pieces? I would not mind (if the performance of LDD improve a lot) even buy an i9, or instead of 32 GB of Memory Ram, buy 64 GB of Memory Ram, but here comes my second doubt: -What elements are the most important for the performance of LDD? Processor (CPU), RAM, Graphic Card?, Maybe?, In that order? What do you think?. I would be willing to invest in a large, powerful computer, but not if the difference in performance will be small. Greetings.