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Mechanism for self-unloading trucks (engineering question)

  1. Feb 20, 2008 #1
    I would like to know if it is possible to use the weight of a loaded truck to unload it, and if so by what kind of mechanism.

    Imagine a truck loaded with a shipping container.

    A "fork" (as in forklift but without the lift) is inserted in holes between the truck trailer and the container. The truck parks on a special platform. This platform is now "unlocked" so that it gets depressed by the weight of the truck. The fork with the container now rests on "stools" (I don't find the appropriate English word) on each side of the truck. The truck has now been unloaded with no consumption of energy, and can drive off.

    Later an unloaded truck reverses in under the container which rests on the fork which in turn rests on the "stools" on both sides. Now the stools are "unlocked" so that the weight of the container pushes down until it rests on the truck trailer. Again, no energy has been consumed. The fork can be removed and the truck drive off loaded.

    The platform and the "stools" can be raised when unloaded, which could be done with quite little energy. Possibly by manual labor.

    I wonder what kind of mechanism would be suitable for the "depression" movement under the heavy load of the container and truck. Hydraulic or counter weight or screws or what? I also wonder what concepts could be useful for the "locking and unlocking" mechanism? The parking of the truck between the "stools" should take place in locked mode, without vertical movements.

    The bakground is a vague idea for an inland cargo terminal in a developing country. Trucks can only take one container each to and from a seaport because of steep mountains just inside the coastline. If containers could be reloaded on the other side of the mountains, then each truck could take two (or even three on trailers with enough space) containers for the thousands of kilometers of flat roads in the vast inland. THis would drastically reduce the soaring fuel costs. Since the economy is severly depressed, traffic is low and so there is little need for speedy reloading. Also, low cost is extremely important. The use of forklifts (and the large paved areas they require for manouvering) would be too expensive. Especially investment costs must be kept at a minimum since the probability of total loss is high due to the corrupt government. An attractive piece of equipment such as a forklift is very likely to get stolen (by the police!)
  2. jcsd
  3. Feb 20, 2008 #2


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    Have you considered small 4-wheeled trailers that can be strung together? Take one at a time over the mountains with a truck that can handle the steep grades, and another truck on the inland side of the mountains could tow several of these trailers, dropping them off and picking up others along the way (if materials need to be delivered in that direction) or picking them up on the return trip. Unloading the trailers would require no more than a small platform with a ramp at each freight depot, and possibly some hand trucks, dollies or just manual labor.
  4. Feb 20, 2008 #3
    Yes, but unfortunately that doesn't seem to be feasible in large scale any time soon. If I were to start a trucking company, I would seriously consider starting out with such a system for my trucks. But there's not one single such trailer around today. It would require substantial investments for the existing trucking companies to set such a standard in any scale of relevance (we need some kind of semi-regular flow of trucks capable of towing such trailers in order for the system to be practical. One such truck would a little like have the only telephone in the world...)

    And even if the existing trailers were modified so that they could tow an additional trailer, there is still need for an inland cargo terminal where those towed trailers can be safely parked and reliably matched with the right trucks. The extra cost of offering a reloading service of containers on existing old trailers, cannot be that large. So I think that the concept you mention has potential for the future, but that it will be a complement rather than a substitute for reloading service, for quite some time ahead.

    Actually, modern powerful trucks can carry two containers each across the mountains. One idea would be to switch trailers rather than reloading them. Then one could use the existing newer trucks in the mountain area and the stock of old trucks on the flat inland. But the trailers are rarely compatible across different truck models... Also, the mountains cause wear and tear which is more costly for the newer trucks when they carry two containers, than for the old ones carrying only one container each. There are plenty of trucks left over from some UN mission in the 1980s. They often break down and are constantly repaired, until finally taken out of service to supply spareparts to the other trucks. But the capital structure is such that this is more economically than using newer equipment, inspite of the delays. Shipping and logistics work quite different in destitute developing countries, than in busy great port in the developed world where time has such a high value.
  5. Feb 20, 2008 #4
    Another phenomena is that full containers only come from the port with destination inland, never the other way around. Imported goods is containerised. Exported goods (agricultural products and ore) are not conteinerised. Trucks can take two empty containers over the mountains without problem. This also means that full containers need to be reloaded from one per truck to two per truck, but very rarely the other way around.

    Couldn't one use some kind of lever and counterweight in order to make the truck and container separate by using gravity? I need to know if the concept is silly or if it just might be possible. I doubt there are solutions available "off the shelf". It would be madness to use anythin but a forklift or a crane in any busy port. There isn't much of a market for developing low cost alternatives for developing countries.
  6. Feb 20, 2008 #5


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    Welcome to PF, Inflation.
    If you stop and look at your proposal, you'll notice that you're overcomplicating what is basically a good idea. There's no need for any kind of jacks ('stool' means something else here). Just back the truck up until the forks engage, drop the ramp a few inches, and drive away. Have another truck back in and crank the ramp up again.
    That was a wee bit facetious, but what I mean to say is that the power to do this has to come from somewhere. You initially mentioned the weight of the truck doing it, which is feasible. On the other hand, more energy has to be expended to get the truck back out of the pit. Since that would normally be provided by the truck's engine, the efficiency would probably be lower than just using an electric hoist to lift the cargo in the first place.
  7. Feb 20, 2008 #6


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    I do not think any idea is silly, if it is the product of trying to find a solution to a problem.
    I would be very surprised if your trucks are not equipped with air brakes, and this gives you a means to design a lifting device that can be activated from a supply tank on the truck.
    Just lowering tire pressure can give a 2 or 3 inch advantage. If you have scrap iron that can be used, there are several ways to build a lift system using the simplest of common materials, that generally are laying around. Of course i'm used to being in a country where people throw things away at an alarming rate, and without much consideration to repair.:frown:
  8. Feb 20, 2008 #7

    "Crank", like how one does it manually when one changes tires on a normal car? I just wonder if that works on a truck with an (average 10 ton, maximum 20 ton) container on it? If it does work, and doesn't take some hard to come by (or hard to make) jack, then it's straight forward. Locals will be happy to have it as their job to "crank", and if it takes 15 minutes per truck, that's okey too.

    Yes, that would be nice. Maybe the wheels of the truck could be positioned on som "rolls". When the truck is in position and those rolls are unbreaked, the truck would accelerate which would drive the rolls and that rotation could be transfered to do the lifting work needed (like cranking the jack, for example)?

    But for sure, having a motor around to do the lifting is within a feasible budget. Only a forklift is not. Partly because it will destroy the ground if it isn't paved, which is really expensive, and it is attractive to thiefs.
  9. Feb 20, 2008 #8
    That's really worth looking into!

    Yes, and since it can't be a problem to inflate the tires of a loaded truck, this should be able to give lift too. Different trailers have different heights over ground, but than might be fine tuned by adding or removing slices of metal or concrete "sheets" on the structure upon which the "fork" will rest. Tires could be in- or deflated before offloading or loading operations to give maximum vertical range. It might work.

    Yes, there is scrap iron around. It is slowly and steadily produced from the many amazingly over-used trucks and other machinery around here... (Or "there" I should say since I'm back in the industrilized world for now). And people are handy with making good use of it. Reparations is a big employer.
    Last edited: Feb 20, 2008
  10. Feb 22, 2008 #9
    Container lift made out of used tires

    I have enclosed pictures (quickly made in Sketchup) of how a dirt cheap design of a container lift might look like in principle.

    I elaborated on the brilliantly simple idea mentioned by RonL, to lift and lower by adjusting the tire pressure. The lifts will work in pairs, on each side of the trailer/container. They will basically consist of used tires which are inflated and deflated by a compressor. In addition, the trucks own tire pressure could be adjusted too if needed.

    What would the caveats and weaknesses be from an engineering point of view?

    By connecting the vents of all the tires in the lift with a hose, their air pressure can be adjusted simultaneously. One could allow for trailers of different height by placing e.g. planks on the ground between the lifts. The drawings I made looks very flimsy for carrying a container weighting up to 20 ton. But by using more tires and attaching them in a good way should allow for enough stability. A hard structure could be used in the lift to stop the container from destroying the tires when they are completely deflated. And after the truck has left, hard structures could be placed directly under the container to carry some of the load and stop the container from accidently falling into the ground. Since the lifts can be moved between uses, ground wear can be distributed too. And so on.

    Used tires are available very cheap locally. Air compressors are around too. They will almost be needed anyway in order to service the tires of the trucks before and after they cross the mountains. Compressors are cheap enough to allow for a backup. It feels much better to use money for salaries and purchasing stuff second and, than for importing expensive capital goods. You know, most of the governmental "aid budgets" end up at home in the donating countries as payments for goods and services exported to the "aid project". I believe that doing sound business is the best way to develop poor countries.

    This starts to look like an interesting low investment cost project. The tranportation cost savings it could achieve, would stimulate more trade and transport on this route. The economic externalities could be disproportionatly large.

    A project to install a "real" cargo terminal has been investigated by a large international shipping company, I have heard. But since it would cost several $100,000 to start up (pavement, forklifts, consultants), it would be ridiculously uneconomical with todays small traffic. It seems that they can't think outside the "thousands of containers per day" box.

    Attached Files:

    Last edited: Feb 22, 2008
  11. Feb 22, 2008 #10


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    Very good, and almost what i was thinking, also consider locking the upper plate, at the outside, to the base and have it hinge, you will get a lever action that will make the lift quicker acting.:wink:
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