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What HP/torque Electric Motor to lift 1K lbs w/ sprokets

  1. May 19, 2017 #1
    I am building a lift using 4 threaded spindles and sprockets to travel 12 feet.

    The motor needs to have 56 mount as it will be mounted vertically (Shaft down) attached to a sprocket that drives a chain to the 4 supporting sprockets.

    The load (1,000 lbs, 250 lbs at each corner) will be lifted by rotating 4 sprockets each turning at about 1800 RPM, traveling up a (2 inch) threaded shaft at 4.5 turns per inch for about 144 inches between 10-20 seconds.

    I believe (based on my novice calculations) I will need: (4.5 turns = 1 inch, need 650 turns for 144 in, 650/20 seconds = approximately 1900 RPM.)

    I think I will end up with a 3-phase motor as I will need to have an inverter to control the start and stop speed. My crude guess (I am a software guy, so I don't really know about Mechanical Engineering) is that I will need 1.5 HP based on life experience.

    Is there anyone who can help me find the answer to:
    How much torque (and HP motor) I need to turn four (4) 4.5 inch (outer diameter - 2 inch threaded inside) sprockets each lifting 250 lbs at about 1800-1900 RPM?

    Thank you very much for your input.
     

    Attached Files:

  2. jcsd
  3. May 19, 2017 #2
    Have you made any actual calculations, or is this all just seat-of-the-pants guess work? If you have made some calcs, please post them. If not, that would be a good place to start.
     
  4. May 19, 2017 #3

    JBA

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    First, you indicate a gear ratio of .93 from the sprockets to the motor, is this really necessary or would the slightly slower lift speed at 1800 rpm be acceptable?

    Given the desired motor rpm, then,what is left to be determined is the motor torque required.

    For the torque calculator for a standard acme thread lead screw design and, at the bottom, the equations (which are representative for your application) see:

    http://www.amesweb.info/Screws/LeadScrewCalculationsAcmeThreads.aspx

    PS: The thread angle (α) = atan [(1 / thread pitch) / (thread pitch diameter x π)]

    Calculate for one screw load, next multiply this by 2/4 (the thread to sprocket diameter ratio and then by 4 (the number of screws) to get the estimated minimum required motor torque; then HP = torque x motor rpm.

    At the same time, there is much more than this for the complete, safe and durable design of such a lifting system, so I strongly recommend you not try to design such a system yourself without the assistance of a professional familiar with such lifting system designs.
     
  5. May 19, 2017 #4
    Dr. D & JBA

    Thank you for your generous time to respond and I appreciate the input.

    The calculations I have made are posted in the spreadsheet. I believe they are accurate in terms of rotation and distance traveled per seconds, but I have been wrong before. The only seat-of-the-pants estimate is the motor power estimate, hence my post here for your contribution.

    I am a (retired/recovering) software engineer with some (school) physics background – amazingly enough, after 40 years I know what atan is! Otherwise, I am ignorant of this science. This is a one-time limited budget private project – not meant to be scalable or reproducible. That doesn’t mean I will be cheap about it; I just haven’t been able to secure the appropriate professional Mechanical Engineering help yet.

    The .93 ratio is not necessarily required. It came from the calculation of needing 648 revolutions for about 20 seconds (1944 RPM) to travel 144 inches and given commercially available 1800 RPM motors. I would prefer 15 seconds but 20 would work.

    I recognize there is more to these calculations. I have not taken into account the strength of the sprocket (wear or friction), starting load inertia, temperature, etc.

    My intent is to over-estimate the components; for example, I am using a 2-inch threaded rod (120,000 lbs rating), where it can probably be 1.5-inch and using a #60 chain, where #40 would probably be fine, and using steel tapered roller-bearings that support 4000+ lbs for 250 lbs.

    I know I need a 3-phase inverter based motor (to control speed) but I only have 220v Single Phase power. So, I intend to use a VFD controller to start, accelerate, decelerate, and stop the motor – perhaps with AGIS protection. I am not sure if a PLC unit or a Sensorless Vector Mode to manage PWM is necessary at this point. I am just learning about this stuff.

    Having said all this, I really do appreciate your input and will seek professional help… maybe even for this project!

    If you have any other input, I will greatly appreciate it. Thank you again.
     
  6. May 20, 2017 #5
    One other comment comes to mind regarding the motor. You have not specified the motor type (at least, I don't recall such), but most industrial applications use induction motors. You mentioned the figure of 1800 rpm as the motor speed, but the only way to truly get this is with a synchronous motor (usually a much more expensive option). For an induction motor with nominal speed of 1800 rpm, the rated speed (speed under rated load) is typically between 1725 and 1775 rpm, depending on the design. So, when you think you have a motor selected, note the rated speed and use that figure, rather than 1800 rpm, for motor speed.
     
  7. May 20, 2017 #6

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    What is your load? Where does the shaft go?
     
  8. May 21, 2017 #7
    Considering safety issues in raising 1000 pounds 12 feet, and the power requirements involved to do this in 10-20 seconds, have you looked at commercially available lifts? They would be designed for reliability at these loads and have self-braking and other safety features.

    It's not clear to me that all the various safety issues are being addressed in this DIY design.
     
  9. Jun 1, 2017 #8
    If you are lifting 1000lbs to a height of 12' and doing it within 20 seconds, then you will be doing at least 600 ft-lbs of lifting per second. 1 horsepower is 550 ft-lbs per-second. You typically only want to run the electric motors at a 70% duty cycle. So the absolute minimum that I would even try to lift this with is 1.5 horsepower. In the name of durability, I would use a 3.5hp motor and lift the load in 10 seconds.
     
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