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  • Help! Design a constuction lift?

    Discussion in 'The main mechanical design forum' started by thakao16, Jan 10, 2014.

    1. thakao16

      thakao16 New Member

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      Good day mates! This is my first time posting a thread here so please be kind to me. I'm sorry for wasting your time but I would like to ask some help here if you would please. The problem that I will state here is from Chapter 23 of M.E.D 4th Ed. by Mott. The problem is as stated.

      Design a constuction lift. The lift will raise building materials from ground level to any height up to 40.0 ft (12.2m). The lift will be at the top of a rigid scaffold that is not a part of the design project. It will raise a load of up to 500 lb. (2.22kN) at the rate of 1. 0ft/s (0.30 m/s). The load will be on a pallet, 3.0 ft. By 4.0ft. (0.91mX1.2 2m). At the top of the lift , means must be provided to bring the load on to a platform that supports the lift.Design a constuction lift. The lift will raise building materials from ground level to any height up to 40.0 ft (12.2m). The lift will be at the top of a rigid scaffold that is not a part of the design project. It will raise a load of up to 500 lb. (2.22kN) at the rate of 1. 0ft/s (0.30 m/s). The load will be on a pallet, 3.0 ft. By 4.0ft. (0.91mX1.2 2m). At the top of the lift , means must be provided to bring the load on to a platform that supports the lift.

      I would like to know how will I start there? Haha. I mean seriously, Im kind of lost here.


      Thanks friends!
       
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    3. Lochnagar

      Lochnagar Well-Known Member

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      One thing you should always do when you are tasked with a new design - is look at what your "competitors" have done - since it is very rare for something to be completely new.

      I had a look at what Scanclimber do who are one of your "competitors" - and tracked down this very good video on Youtube (see hyperlink below) - of one of Scanclimber's products - being brought onto site and "assembled" - and put into action.

      From this video - you can see that they are using a rack and pinion to drive "the cage" up and down the pillar. My best guess is that the electric motor drives through a worm gear - before going to the spur gear - since a worm drive can be made non back drive-able - which I would say in this application would be advantageous - so that you don't have a "run away" cage - whizzing down to the ground at break neck speed - with the obvious dire consequences of that.

      If it was a hydraulic drive for example - you would use a counterbalance valve - to control the rate of descent - to avoid this "run away" situation I have described above.

      If you go and look at Scanclimber's website - and look at the various models they have - you will see a host of information there - on for example the emergency braking system, etc, etc. In addition you will get weights and dimensions too - which will help you home in on the size of motors and brakes etc, etc.

      Although this sounds like a university project you are doing - if you were designing this for real - you would need to think about things like wear of the rack and pinion - and the consequences of that to the safe operation of the system. Additionally, the transverse float of the cage on the pillar would be a critical issue too - since you would not want the spur gear to become disengaged from the rack (the consequences of this would be very dire) - if there was too much float in the system.

      Hope this helps.

      https://www.youtube.com/watch?v=DM3cWY2G4oY
       
    4. thakao16

      thakao16 New Member

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      Thanks mate! That's what I've been thinking, a rack and pinion but would it not be an overkill because the maximum height is just 40ft.? And may I ask also if it's okay with you, where to start the computations?
       
    5. Lochnagar

      Lochnagar Well-Known Member

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      You can use crane hoist forks (see the first link below) - and couple that to a small scaffold hoist on a swing arm (see the second link below) up at the top of the building. However, I think 40 feet is probably a bit too much for this method - particularly if the wind catches the load that is being hoisted.

      http://www.whitneyeng.com/pdf/GER AU.pdf

      http://www.brandontoolhire.co.uk/en/attachment.php?id_attachment=48

      The alternative - and I believe "safer option" - would have a tower or pillar that is secured to the building - and on which a carriage slides up and down on this pillar (see the third link below).This is like I detailed in the youtube video yesterday. The advantage of this method - is that the load is fully guided - so it can't swing about.

      http://www.rapidplatforms.co.uk/sites/default/files/Geda-Hoists-850ZZP-Electric-Hoist.pdf


      The place to start the design is with the load.
      If you go for the carriage option - you need to size the carriage up to carry the load - so the load on the rack and pinion = (the weight of the load + weight of the carriage) x dynamic factor. I would suggest a dynamic factor of 2 - since in engineering we usually assume the load is applied gradually - but if it is applied suddenly - that is usually equivalent to 2 x the load. In addition, you need a safety factor to yield - and I would suggest a minimum safety factor of 1.5 -to yield to the worst case loading. Remember, you have wind loading too.

      Engineers quite often design things from a stress perspective - and forget about deflections - that is a mistake. Deflections are just as important. Usually, if it is a simply supported load (i.e. two supports and a beam with a load inbetween the two supports) - max deflection recommended is span over 360 - and if it is cantilever supported - max deflection recommended is span over 180.

      If you go for the tower option - remember it will be subject to buckling - due to the axial load. Don't forget the offset load (centre of tower to centre of pallet (which is where the centre of the load is) - which is creating a moment load.

      Hope this helps.
       
    6. thakao16

      thakao16 New Member

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      Thanks! This is a really huge help because I'm quite lost at how to start things up. Im very thankful! Btw, if I may ask again, do I need to use a counterweight or the motor can do all the work? Im sorry if my linguistic skill is not that refined hahaha. Thanks again mate! God bless!
       
    7. Lochnagar

      Lochnagar Well-Known Member

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      In lifts in buildings - there is a weight that moves in the opposite direction to the lift - in exactly the same way as a ski lift - it is a continuous loop - so the motor has less work to do - it just has the weight of the people to raise - not the weight of the lift carriage.

      However, in this application - that should not be necessary. But don't forget brakes. With both systems (the scaffold hoist - or the rack and pinion system) - the worm drive at the end of the electric motor - will provide a mechanical lock - but you will still need a fail safe brake.
      Hope this helps once again.
       
    8. thakao16

      thakao16 New Member

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      Thank you once again sensei! Hahaha.
       

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