Technic Dump Truck This MOC was started in October of 2007, and I worked on it off and on until August 2008. Features ======== This model is electrically powered and has 4 main functions. Drive, steer, tray raise/lower, and load cover/uncover. This is achieved with two older style motors (2838) each powering two functions, and a micromotor to switch between the functions. I was also able to restrict myself to the monochromatic colours, with every piece (aside from the micromotor and associated brackets, of course) either Black, White, Old Gray or Old Dark Gray, with the vast majority of parts Black and Old Gray. The tray lift mechanism is worthy of note. A rack-and-pinion system pulls a liftarm against the pivot point, raising the tray. However, when the tray is close to the bottom of its travel, the liftarm is almost parallel to the tray, which means that a large amount of torque is required to lift the tray. To counter this, I implemented a vertical rack-and-pinion lifter at the front of the tray to lift the tray directly. This can be seen in the sequence of photos that shows the tray being lifted. Challenges ========== There were two really significant challenges to overcome in the construction, the gear system, and the tray lifting system. The lifting system, as I mentioned above, had to cope with the fact that a large amount of torque was required to lift the tray in the early stages of its lift cycle. I wanted to devise a system that, with the input shaft continuously turning one way, would activate a vertical lifter to push the tray up from the front for a short distance, then transfer movement to the primary lifting mechanism that would take the lift the rest of the way. Ideally the system would have to work in reverse for the lowering of the tray i.e. lower most of the way with the liftarm, and then take the last few degrees of rotation from the front of the tray. To achieve this, I experimented with differential transmissions, and Michael Powell's sliding-worm-gear directional transmission, but they both required the front lifter to be sprung so that its 'natural' state was up, and able to support the weight of the tray. This meant that a large force was needed to activate the mechanism, necessitating additional gearing down, as well as structural bulk to stop it tearing itself apart. I then tried to come up with a way to lift the front of the tray by rotating a small liftarm underneath the tray, with the idea that the liftarm could continue to rotate as the tray was lifted the whole way, thus removing the requirement to transfer movement between the two systems. I put a lot of effort into finding a gear ratio that would turn the liftarm such that the tray was resting on its supports when the liftarm was flat underneath it, and when the liftarm was vertical, the slack in the rest of the gear chain had been taken up, and the primary lifting mechanism would take over the lift. Unfortunately, the ratio that enabled that meant that not enough torque was delivered to actually support the weight of the tray! As mentioned above, I finished up with a vertical rack-and-pinion system that was connected to the lifting motor. The sequence runs as follows: 1) (Lifting the tray) Motor drives the vertical lifter up via the rack-and-pinion system, while the primary lifter takes up the slack in the considerable gear train. 2) The lifter reaches the end of its travel, and the rack dis-engages from the pinion. 3) The lifter remains in its guides while the primary lifting mechanism lifts the tray to full tilt. 4) (Lowering the tray) Motor reverses direction. The pinion re-engages with the rack through gravity. The primary mechanism begins to bring the tray down. 5) The lifter reaches the other end of its travel. Just at the point where the rack would dis-engage from the pinion again, a rubber band prevents it from going too far. 6) The primary mechanism brings the tray all the way down. Motor is turned off. 7) The rubber band keeps the rack poised to re-engage with the pinion when the motor is once again turned on to lift the tray. I had to sacrifice the abovementioned ability to exactly reverse the lifting process when lowering, but I decided that it was better than having the extra complexity that achieving this would have required. The second of the large challenges was to get the gearbox right. I had to come up with a way to transfer motion between two different outputs. It had to be able to do that for two inputs at the same time, it had to be done with a micromotor, and it had to be able to transfer a large amount of torque - any solution that moved a gear in its own plane to mesh with another gear would not work, because the forces between the gears would tend to push them apart. The final solution was to stack 5 of the 14 tooth bevel gears (4143) together on the same shaft, and have a 16 tooth spur gear able to slide along them, engaging one of two output gears (one at each end of the gear's travel). Making use of the 16 tooth clutch gear (6542) meant that I could mount the 'travelling' gears on the same shaft, and slide that back and forth using an arm connected directly to the micromotor. There were also difficulties due to the fact that three of the four output shafts needed to pass in close proximity with each other, en route to the rear of the vehicle. Minor challenges included finding a way to drive the load covering mechanism that could deliver enough torque to do the job, but could also be disengaged when the tray is lifted [This I achieved by placing two sets of the 14 tooth bevel gears (4143) just wide enough apart that a 40 tooth gear could sit on (and be driven by) them. The tray is heavy enough to keep the teeth from slipping (mostly) and when the tray is raised, the gear just lifts right off], ensuring that the aforementioned system could actually drive the load covering mechanism [a strategically placed elastic band assisted in overcoming gravity, and made for an easier-to-move mechanism] and making sure there was room for everything - keeping the vehicle as compact as it was something of an achievement. Shortcomings ============ As good as it is, it is far from perfect. There are several areas where improvement could be made. The vehicle is very heavy. Even though it only drives at a snail's pace, the motor struggles to propel it. Also, the steering is ineffective when the wheels are turned through more than about half their angle. I suspect that lengthening the distance between the steered wheels and the first pair of driving wheels would help this, but the truck is already longer (compared to its other dimensions) than I would prefer. The mechanism that switches between the drive/steer functions and the tray functions (load coverer and tray lifting) is not always smooth. Also, for the switch to work at all, both motors need to be turning, because unless you're very lucky, the spur gears won't be in the correct rotational alignment to mesh properly. This could have been avoided with the use of double-bevelled gears, but 1) the spacing required would have been very difficult to achieve and 2) I didn't think of it until just now! Conclusions =========== I am quite happy with the way it turned out. I got all the functionality that I wanted, and the end result even looks kind of like what it's supposed to represent. It's just a shame that the electric wires I have are too short to make a reasonable remote control!