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Thread: Finding power to move cart

  1. #1
    Associate Engineer
    Join Date
    May 2016
    Posts
    1

    Finding power to move cart

    I am interning at a company this summer and my boss has asked me to design a cart that will move on rails to move heavy equipment. I believe I have all the calculations worked out, but I don't want to waste company money if I am incorrect. I am trying to find the size of motor required to complete this task. Also in this design I had to make a few assumptions and I want to make sure they weren't far fetched.

    I am to move a cart weighing 24000 lb. The final velocity of this cart is to be around 1.9 in/sec (very small and one assumption that was made). The radius of the wheels that are being used is 6in. The equations that I have come to are P_max= 2*x^2*m/t^3. T_max=m*v*r/t. So the assumptions I made were that it would take about 5ft for the cart to come to speed and it would do so in about 5 seconds. Using these assumptions I came to P_max=0.543hp. and T_max=142in*lb.

    My boss told me to ignore friction in this case because we are just going to round way up for motor size anyway, but after reading through other forums, 1.) I think it would be good practice for me to include friction in this and 2.) because of the size of this load, a lot of people have said not to ignore friction. If I were to add friction to this It would change the power eqn to P_max=2*x^2*m/t^3 + m*g*(cof of fric) *x/t. If i kept friction what would be a good assumption for friction? Other forums have said 0.01 would be a good number for this application.

    Any help would be appreciated. This is my first post so please be nice, and I hope I haven't violated any rules.

  2. #2
    Technical Fellow jboggs's Avatar
    Join Date
    Mar 2011
    Location
    Myrtle Beach, SC
    Posts
    908
    Welcome to the forum, bro.
    Rule #1 - The boss is always right.
    And you know rule #2.

    Your curiosity and initiative are admirable and will enhance your value to employers in the future. Keep that up! But I can tell you this - your boss does not want to hear "some guys on some forums said I should consider friction." Not a good career move. Feel free to run your calculations with and without, at whatever level you might want. If you can find a good reason to consider friction, bring it up to him, but don't base your suggestion on comments you read on line. Base it on your own research and calculations.

    This is one of those early lessons engineers learn after school. You can calculate all you want, and you should, but when it comes to applying it in the real world (unless you're in something like aerospace), you're probably going to end up doubling everything. You'll learn that oversizing is generally less costly than unexpected failures.

    You'll also learn that no matter what you do as an intern, they probably aren't going to implement any of it without a thorough review by one of the staff engineers.

    You'll also learn that they WANT you to ask if you are unsure. If you can bring your research with you when you ask, that helps a lot.

    Oh, and to your original question about whether or not to consider friction? That will depend on the conditions of the bearings and the contact surfaces, which is different in every plant. Do not forget that whatever conditions you see there now will change in the future. And when that cart stops because of some little crap on the track, they aren't going to blame the crap on the track. They're going to blame the "damned engineer" that undersized the drive.

    So, the boss might be telling you to ignore friction, but he's also going to tell you to oversize the drive significantly. So, in his mind he is accounting for it, without spending time calculating it.

    One more thing - my big congratulations and praise for not falling prey to the common and infuriating practice of so many in the young "texting" generation: that of thinking all written communication is the same as texting! Business writing should always exhibit proper grammar, spelling, capitalization, and punctuation. Thank you!

  3. #3
    Principle Engineer Cragyon's Avatar
    Join Date
    Feb 2013
    Location
    Newark, NJ
    Posts
    299
    This page will get you in the neighborhood..

    Electric Motor Sizing -> http://www.engineersedge.com/motors/...tric_motor.htm

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