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Suitable metal for flooring in an industrial lift + power

  1. Nov 27, 2015 #1
    • Thread moved from the technical forums to HH, so no HH Template is shown
    Hi all, first post here.

    I'm currently faced with an assignment in a class where we really haven't been taught anything and have to figure it out on our own. We've been asked to design an industrial lift system to raise approximately 3000 kg up 25 stories. I have done a lot of research and have been able to design a suitable platform frame with less than 3mm of deflection at maximum loading. I've also been able to calculate the required torque and kW rating of the motors I want to use. However, I am left with a few problems left to figure out and I'm not sure where to begin looking for solutions.

    My frame is made from standard UK beam segments in a simple criss-cross pattern as below:
    10xambr.jpg

    The assignment says:
    "The floor needed to be constructed from the suitable expanded metal as per the given cage dimensions. The construction company would like the floor to be made of suitable light metal to keep the Material Cost to the minimum."

    now, I could just slap a 200 mm steel plate across the frame and i'm sure that would be sufficient, but I honestly have NO idea how to begin to select a suitable material and thickness for this application. The most relevant teaching i've had was 2 years ago doing simply supported beam bending analysis, but this strikes me as a more complex type of calculation. Considering that it is a lift, I imagine that it should be able to take a "point load" of the maximum rated load of 3000 kg. This seems like the sort of thing a lightweight aluminium sheet might not be happy with. Can anyone suggest a way to calculate this?

    also, the assignment gives us this:
    "Power Supply Range : 370- 460 V,50 or 60 Hz,3 Phase
    Rated power: kW ?
    Power supply Fuse ?
    Starting Current : 130 A
    Power Consumption :65 kVA"

    I've calculated that I would need about 17 kW with safety margin to lift this load with my design. I've never been asked to decide what fuse to use. Any pointers on that would be appreciated as well :)

    Thanks in advance for any help you can offer!

    -Chris
     
  2. jcsd
  3. Nov 27, 2015 #2

    billy_joule

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    The spec asks for expanded metal so you shouldn't use plate. If you do some googling on expanded metal you'll find load ratings tables etc which should help guide your selection.
    Remember the lift must accelerate so you need to account for the dynamic loading - not just the static.
     
  4. Nov 27, 2015 #3
    Thanks for that! I had no idea what "expanded metal" meant until i just googled it! It helps if you have a teacher who would explain these kinds of things! The guy teaching this module was in serious trouble last year and nearly costed my group our HNCs for not covering all the learning objectives for the CAD module, we were actually teaching HIM how to use the software... there are 6 of us in this group and i'm the only one so far who has produced anything for the assignment because i'm determined to prove my worth :)

    Today we had a temp cover our class, we learned more in an hour than we had all semester!

    also, he said the lift should move at 12 metres a minute, so in my calculations I assumed an "instantaneous" acceleration of 0.2 m/s force plus gravity just to be safe
     
  5. Nov 30, 2015 #4
    Well since the questions seems geared towards the frame side, there's a few important things to remember:
    1) Even if you think it would be super simple to just throw a thick plate floor to lift stuff up, that plate itself will carry significant weight and will change your frame design (i.e. make it much heavier) and it will snowball into a gigantic motor and generally a messy situation. A 200 mm plate that big weights 20,000 kg by itself.

    2) How spread out is your 3000kg load? If you assume they will center the load and distribute it well across the floor, then you can use the span tables provided by expanded metal flooring providers. This will minimize the additional strain on your motor from the floor.

    http://www.nilesexpandedmetals.com/nem/grating-load-tables.asp [Broken]

    3) Your scenario shows a working load of roughly 50 lbs/ft2 (250kg/m2), which is probably too low in the event that the client wishes to lift something compact but heavy (such as a pallet of bricks or a piece of machinery on a dolley). I would design your floor grating for 200psf (1000Kg/m2) minimum or a point load of 3000 lb (1500kg) at the center of the lift, whichever is worse.

    Using that load table and assuming that you're only putting the grating on the beams shown, your span will be about 48". This means that even using the 7lb/sf grating, you're only going to achieve a working load of about 175lb/sf and that will cause a .25" deflection (6mm). You should add additional rows of beams to your frame as needed to make sure deflections stay low. This is especially important because the frame is going to deflect, too, not just the grating.

    4) For the design of connections/welds/cables, be sure to use appropriate safety factors (look up applicable codes) and impact factors. Lifting cables are generally 3:1 or 5:1 safety factor on ultimate vs working strength, while impact factors due to moving load, uneven lifting, crane movement, etc. range from 1.05 to 1.6 in the extreme case of a sudden stop in the lifting trolley.

    Let me know if there's anything else specific you don't understand (structurally).
     
    Last edited by a moderator: May 7, 2017
  6. Dec 3, 2015 #5
    Hi, I have no idea what the spread of the load is, it isn't specified. I designed the frame to cope with a point load of 3000kg at the farthest point from where it is connected to the rack and pinion rail because of this.

    I've found an alternative material to expanded metal for the flooring that is relatively lightweight and wouldn't require additional supports, heavier than the expanded metal, but able to cope with a loading like this. I'll pitch it at the teacher tomorrow and see if he bites. Who knows, may get credit for thinking outside the box.

    I really wish this design teacher would actually do something other than show a 15 min video of engineers at dyson designing a vacuum cleaner and then tell us to go design a lift with no guidance. I've looked at some of the legislation regarding lifts but its a bit difficult to get your head around unless you already know something about lifts :)

    If anything comes up, I'll update tomorrow. But, as it stands, I've done far more than anyone else in the group anyway, nobody even had a platform design as of last week. And nobody's given any instructions on how to calculate deflections and whatnot of a frame, we only know simply supported beam bending of a single linear beam. I used Autodesk Inventor's frame analysis to find my deflections. And not a single mention of how to go about choosing an appropriate motor, but I found that on my own and ran it by a different teacher who used to do this for a living and he said my calculations seem reasonable.

    Thanks for the input!
     
  7. Dec 3, 2015 #6
    You can conservatively estimate your deflection in your long center beam as W*L^3/48*E*I, where W is your weight, L is the length of the beam, E is Young's Modulus, and I is the section's moment if inertia.

    You can solve for I and look up cross sections in a beam table to find this. IF we assume W is 6.2kip, L is 157.5", E is 29000ksi, and max deflection is 0.12", then:

    I = 6.2(157.5)^3/(0.12)(48)(29000) = 145 in^4. You can accomplish this with a section such as a W12x26 (W305x165x40 in British Units).
     
  8. Dec 3, 2015 #7

    SteamKing

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    Designing for a 3000 kg point load is not the same as designing for a 3000 kg distributed load. Your floor frame is going to become quite heavy from being over-designed in this manner.

    This is not to say that situations like this don't occur in the real world. Often engineers are given vague or ambiguous instructions about how something is supposed to be designed. It shouldn't stop the prudent engineer from asking follow-up questions to try to resolve the ambiguities, rather than plunging ahead and designing something unsuitable for the task.
    While, you're at it, you should discuss the point above, about designing the frame for a point load v. a distributed one. This has more influence on the overall design than selecting the flooring.
     
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