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Load on castors - can't calculate.

  1. Jul 27, 2004 #1
    Hi all:

    I've been working on this for few hours, and I've given up. I'm hoping that you can help!

    I've included a diagram of what I have to work with. Basically it's an existing engine crane with a modified base that I'm trying to design it to pick up a 600-700# electric motor, (but a 1000# limit is what I'd like to design to). But I can't determine the loads on the 4 castors, in order to purchase a strong enough pnuematic castor.

    I think that a 12" castor rated at 450#/caster should be more than sufficient, but I'd like to go to an 8" castor rated at 300#/castor if I can.

    Because I'm only a first year engineer, and only have basic statics under my belt, I'm really confused trying to solve this one. Can anyone help and show me how to calculate this?


    Thanks a bunch,
    Shaun
     

    Attached Files:

  2. jcsd
  3. Jul 27, 2004 #2

    turin

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    I am having a hard time reading your diagram. Can you attach an enlarged version?
     
  4. Jul 28, 2004 #3

    brewnog

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    The key thing to remember for castor usage is that if you have a 4-wheeled trolley, you can only guarantee that 3 wheels will be on the floor at any one time. So your castors must be able to support the trolley if a wheel lifts up due to an uneven floor.
     
  5. Jul 28, 2004 #4
    Link To Larger Diagram

    Turin: I know, but this site wouldn't let me upload unless it was a 400x400 pixel image. :grumpy:
    So I uploaded a larger image for you here. :wink: click on "My Documents" and click on "Diagram"

    brewnog: Good point, I never even thought of that. :rolleyes:
    The ground will be packed gravel, so thee are dips and holes that it will encounter.

    Thanks Guys. :approve:

    Turin: Will you be getting back to me on this? Let me know. :confused:
     
  6. Jul 28, 2004 #5
    You should be able to do this using statics. Assign a point A to the joint where the boom is joined to the base. Use [itex]\vec{r}[/itex] from A to the CM of the mass. Calculate the moment at A due to the mass. Translate the moment into a set of forces at the Wheels. Use a worse case scenario where all of the force 2 is concentrated on one wheel. Also, include the force required to move the hoist. Determine the force required to overcome static friction and apply that force along the x-axis originating from the motor CM.

    Don't worry too much about the funky angles of the boom. Use some basic trig to find the x/y location of the motor CM and use that point as the location of the two acting forces. Then simply convert those forces to a Moment. I'd be surprised if your statics text (if you still have it) doesn't have a problem similar to this in the moments section.

    Why do you want to change the wheels on the hoist? Most hoists are designed to lift multiple tons of weight (mine can lift up to 4 tons) and use steel wheels because of the forces developed at the wheels.

    Well, good luck.
     
    Last edited: Jul 28, 2004
  7. Jul 28, 2004 #6

    brewnog

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    If the hoist isn't designed to lift these kinds of weights (it should really, because your diagram looks like the kind of engine hoist which would, as faust9 said, easily manage a few tonnes) then it is unlikely to be the castors which are the 'weak point' of the system. Be sure that all the joints are strong enough otherwise you might get very sore feet!
     
  8. Jul 28, 2004 #7
    faust9: I've tried several attempts at using statics to solve this but, I'm really confused, :uhh: I'm afraid that I'm coming up with bad figures. :yuck: The statics text that I'm using is based on building construction and I can't locate a simular example. :confused:

    faust9 & brewnog:I've included a few more pics (Look in the My Documents folder) which include the motor that needs to be moved and the crane I'd be using. :surprise: As you can see the crane has no base, which is why I chose it. I need a very wide wheel base to fit around the existing concrete stand (>50"), and because the ground is packed gravel/dirt mix, I think the pnuematic castors would work best. :redface:

    I'm gonna try another attempt with faust9's instructions. But I was confused just reading his tutorial. :yuck: Let me know what you guys are thinking. :approve:

    Thanks,
    Shaun
     
  9. Jul 28, 2004 #8

    Cliff_J

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    Gravel and dirt?!?! You may want to consider using two casters at each point to distribute the weight over an even larger surface area to avoid sinking in. Just moving an engine hoist around on packed gravel while completely unloaded is a chore and I don't think pneumatic tires are going to help much.

    You may want to start with testing the ground. Lets start simple and say that force #2 is going to be 500lbs per castor (think of the impulse for each pump of the jack raising the load and it could exceed this but I said we'll do simple now) and each castor has a contact patch of 2 in^2 so we've got 250lbs/in^2 going on. How much will this sink in? What will the horizontal force needed to move this hoist be? Do you really have 2 in^2 of contact patch?

    A very simple solution would be to use something like a trailer wheel/tire combo or something from agriculture that's designed for high loads and soft terrain. Something like a small riding lawn mower front tire - get where I'm going? My two cents...

    Cliff
     
  10. Jul 28, 2004 #9
    Don't know if this helps but....

    Here's a link to the catalogue that I'm choosing my casters from:

    NOTE:

    You need Adobe Acrobat or a .PDF veiwer to read it. :devil:


    (you might need to right click this link and hit "save target as")

    When it opens, The castor I'm looking at is the 7900 Series Pneumatic Castors, 450lbs Capacity on page 85. :approve:

    Do you still recommend that I double up on casters? And if so, just on the front legs, or front and rear? :rolleyes:

    Thanks guys, I'm learning alot here. :biggrin:
    Shaun
     
    Last edited: Jul 28, 2004
  11. Jul 28, 2004 #10

    Cliff_J

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    Doubling up may not be enough nor even tripling up. This is an engineering forum, shouldn't be too much of a guessing game now should it? :smile:

    I wouldn't know the acceptable amount of sinkage for your application. My advice was (indirectly) for you to investigate your ground conditions and consider options that distributed the load better rather than waste money on a solution that may not work.

    Your solution might be as simple as cutting up strips of plywood and placing them on the ground to make a smooth hard surface that will definitely spread the load over a large surface area and allow you to move 1000lbs by just pushing/pulling. Then you could use metal casters too. But I have no idea how far you need to move this weight or other restrictions. But when I lived on a farm, we'd only use the hoist just enough to get the load (be it an engine block or pump or whatever) to a place where we could hook up the tractor to move across anything other than smooth concrete. We'd even use the tractor to move the hoist because it was just too much work to try to push/pull it across anything other than smooth concrete.

    Cliff
     
  12. Jul 29, 2004 #11
    Cliff_J: The ground around the area of the motor is very dense/packed - dirt/clay/gravel. I don't know how to compare it, but like a fairly smooth, hard packed hiking trail. As we move away from the motor the ground does become softer with gravel and sinking is an issue; however, we can transport the motor into the back of a truck or trailer or something before we get to that point.

    We just want to get that motor down safely (without having to use prybars and manual lifting), and move it out of it's location to a truck/what not.

    To calculate the force at the front castor:

    Would I basically draw a sketch of the hoist and simulate the "mast&boom" from the intersection of the "mast&legs" to the "Center of Gravity of the motor"? Because that would be simple to figure out, but I think that I'm wrong.

    Please let me know.

    Shaun
     
  13. Jul 29, 2004 #12

    Cliff_J

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    KISS

    I'd say to then just use some 2x10s or plywood to move the hoist on top of as this would be very simple. Metal castors would suffice and sinking would not be of any concern. You could re-use the existing crane's wheels and make them work (assuming the crane was rated for 1000lbs).

    For calculating only the static weight on the castors, its a combination of the weight of the hoist and the weight of the load, each can be calculated seperately. If you know the location of the cg you can use the distance to each wheel in only the horizontal direction to find the static loading. If it were 1/2 way between the wheels obviously it would be distributed evenly, if it were directly above one set of wheels then all its weight would bear on them.

    But this is dynamic. You're going to jack it up - that means acceleration vertically which means more force which will add to the load on the casters. You're going to move it, forces laterally but with a high cg that means that as the load swings then more load is tranferred to the caters. The weight rating of the castors likely has a nice safety margin before failure, but since people will be around and this is a great deal of weight more safety margin is better than less.

    At my uncle's equipment dealership a new mechanic was moving a freshly overhauled tractor engine on a hoist with the boom extended out about even with the front casters. He caught a groove in the concrete floor and WHAM! it tipped forward. Absolutely amazing how much potential energy in 1000lbs 3ft off a floor, be careful and plan in your safety margin accordingly.

    Cliff
     
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