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Torque calculation based on weight and wheel size

  1. Dec 27, 2013 #1
    hey,

    So i need a formula that will help me calculate the torque required to run 2 wheels that support a certain weight. The things i know are follows:

    Total weight of the vehicle: 650-700kgs
    Wheel Diameter: 16-17 inches
    Average Velocity i will require the vehicle to move at: 10m/s


    Basically, I want to choose a motor that will power the two wheels and will provide forward and backward motion and a switch for on/off action. To do that, i will first need to calculate the torque I require. If I left out any details please let me know:)
     
  2. jcsd
  3. Dec 28, 2013 #2

    Simon Bridge

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    Welcome to PF;

    The torque required for a constant speed is just that required to overcome losses - i.e. friction, deformation of the tires etc. Any extra torque results in acceleration. It does not matter if you are going backwards or forwards - any differences will be due to the gearbox.

    What you need is the maximum power required - and, probably, the mass of the wheels.
     
  4. Dec 28, 2013 #3
    hey simon,
    Thank you:) So basically this set up is required for basic movement of a machine with the overall weight i mentioned. Im not really sure what kind of acceleration ill need, but the primary function of the machine is not motion so acceleration required will be moderate.
    Also, I don't know the power required to run the set up. I want to know how i can calculate it. What other parameters do i need to calculate the torque?
     
  5. Dec 28, 2013 #4
    hey simon,
    Thank you:) So basically this set up is required for basic movement of a machine with the overall weight i mentioned. Im not really sure what kind of acceleration ill need, but the primary function of the machine is not motion so acceleration required will be moderate.
    Also, I don't know the power required to run the set up. I want to know how i can calculate it. What other parameters do i need to calculate the torque?
     
  6. Dec 28, 2013 #5

    Simon Bridge

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    Define "moderate"... working out the power to just move the machine along requires very detailed information about the machine. This is the sort of thing that is usually measured: push it with something else up to a known speed and then let it free-wheel to a standstill - the distance tells you about the drag forces, but the kinetic energy over time can back-of-envelope the minimum power you need.

    The usual trick is to deliberately overpower whatever you want to drive, and add a throttle.
    Any engine with that power or more should keep it rolling - the minimum torque would just be to overcome the initial rolling resistance - which you work out from the minimum force needed to start the thing moving.

    That's with a direct drive - there are complications if a gearbox is involved.
    I take it you don't want to post a schematic?
     
  7. Dec 28, 2013 #6
    Okay so considering i want to move a container containing various components that weighs a total of 650 kgs, what are the specifics i require to calculate how much torque i require? The machine is basically some sort of medical equipment that is mobile so im tring to figure out what kind of motor i have to buy to provide basic motion. I need it to be powerful enough to move from one point to another. So i want to know how i have to go about this task. Please excuse my limited understanding of these concepts. Im kinda new to this:)
     
  8. Dec 28, 2013 #7

    Simon Bridge

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    You want the rolling resistance and the drag - like I said.
    These are things that you measure.

    rolling resistance: the force needed to get the object moving - depends on things like how much the wheels deform and the viscosity of lubricants when they are cold - as well as the specifics of it's construction.

    drag: the resultant retarding force through the center of mass - but you can get away with the rate of energy loss when free-rolling without power.

    If there were no rolling resistance (hardened steel wheels on a similar track say) and no drag (magnetic bearings and operated in a vacuum) then you could get it moving (albeit invisibly slowly) with the slightest nudge, and it would keep moving for ever.

    The easiest thing to do is look for similar weight machines that already have motors and see what they use - that way someone else has already done the hard work.

    If you loaded the whole thing on an electric wheelchair - would that move it about OK enough for you?
    Then use an electric wheelchair motor.
     
  9. Jan 2, 2014 #8
    Okay thanks a lot for the info! Really helped. I wanted to ask you... Also, based on my application, should i go with a geared motor or not?. If yes then why?
     
  10. Jan 2, 2014 #9
    I used a motor sizing tool to determine the kind of torque i require and heres what i got:
    Angular Velocity= 5.5 rad/s
    Torque: 400Nm
    so based on the information i have, how do i go about choosing the type of motor?
    Thanks.
     
  11. Jan 2, 2014 #10
    I used a motor sizing tool to determine the kind of torque i require and heres what i got:
    Angular Velocity= 5.5 rad/s
    Torque: 400Nm
    so based on the information i have, how do i go about choosing the type of motor?
    Thanks.
     
  12. Jan 2, 2014 #11

    Simon Bridge

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    YOu are going to find advise very pragmatic and down-to-Earth:

    You should use whatever motor you can afford that fits your requirements (see below)

    You should choose a motor that you can afford that beats those specs.
    The specs are usually written on the motor someplace.
     
  13. Jan 2, 2014 #12
    Okay so at the moment i need a motor that is compact cause im limited in terms of space. Money isnt a problem at the moment. I just want to get an idea of the kind of motor i require so i look for vendors. There are so many different kinds of electric motor, choosing one that suits my needs isnt an easy task. Any help would be much appreciated:)
     
  14. Jan 2, 2014 #13

    Simon Bridge

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    Since money isn't an object, you go to an electric motor manufacturer and tell them what you need.
     
  15. Jan 10, 2014 #14
    Hey thanks for all the help. I figured out what exactly my specs are. I have one last question. Do i go with a Permanent Magnet DC Motor or a Servo Motor and how exactly are they different from each other?
    Thanks!
     
  16. Jan 10, 2014 #15
    Hey thanks for all the help. I figured out what exactly my specs are. I have one last question. Do i go with a Permanent Magnet DC Motor or a Servo Motor and how exactly are they different from each other?
    Thanks!
     
  17. Jan 10, 2014 #16

    Simon Bridge

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    Go with the cheaper one.

    The servomotor has additional engineering so you can control the angular position precisely but you don't really need that control. All else remaining equal - get the other one.
     
  18. Jan 10, 2014 #17
    Okay so here's the thing. I need the trolley to move in the forward direction with a slight push and in the backward direction with a slight pull. So by applying slight force in either front or backward direction, the motor takes over and the trolley moves. I want to know if this is possible and if so, how can i go about achieving it?
     
  19. Jan 10, 2014 #18

    Simon Bridge

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    It can, there are a number of different ways - i.e. use a friction block on the wheels with a much higher static than kinetic friction - some sort of switch on the wheels detects the initial direction.

    But it is simpler just to use a dead-mans switch on the handle with a thumb switch for forward/backwards.
     
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