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Question about motors please advise

  1. Oct 3, 2011 #1
    hi all

    i came here just to get a little advice. i am building a custom lift for the back of my work van. i need to lift up my tool box (which is quite heavy {about 125 lbs}) 3-5 times a day. putting it in and out is a pain. anyway i was wondering which electric motor i should use or what the tech specs should be on such a device.


    thanks in advance for the replies and insight.


    also how would i control speed?
     
  2. jcsd
  3. Oct 3, 2011 #2

    berkeman

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    Welcome to the PF.

    You might look at existing wheelchair lifts for vehicles, to get ideas for motors and lift mechanisms. I've seen a very clean arm/crane arrangement in the back of a van, similar to the one on the top of this Google Images page:

    http://www.google.com/search?tbm=is...40l5506l0l8876l15l14l0l5l5l0l188l1079l3.6l9l0

    .
     
  4. Oct 3, 2011 #3
    you know i had looked at those but can't seem to find any part numbers to the motors. or the tech specs.
     
  5. Oct 3, 2011 #4

    berkeman

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    I followed the Scooter Store link from the in-van lift in the pictures, and got to the manufacturer's website. The Owner's Manual for the lift doesn't list the specifications for the motor(s), but it does have some pretty good scale drawings of the motors. They look to be 12Vdc gear motors. Have a look to see if they look familiar:

    http://www.harmarmobility.com/products/itemDetail.cfm?proID=23&tabName=Documents

    .
     
  6. Oct 3, 2011 #5
    i didn't think to go to the scooter store. i only need to go up/down. i was thinking of just a manual swing arm with truck wheel bearings. how many amps do you think i would need to pull this off?



    btw thanks for the help.
     
  7. Oct 4, 2011 #6
    Why you need it necessarily motorized if you lift the box 5 times and not 55 times a day? Look through the diy depots in your vicinity for what they offer - for lifting, such as hoists - and turn on your imagination.
     
  8. Oct 4, 2011 #7

    berkeman

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    I can try a quick approximation...

    125 pounds is about 57 kg. http://www.metric-conversions.org/weight/pounds-to-kilograms.htm

    To lift 57kg about 1 meter takes energy E = m * g * h = 57kg * 9.8m/s^2 * 1m ~ 570 Joules

    The power calculation depends on the time it takes to do the lifting. If you do the lift in 10 seconds,

    P = E / time = 570J / 10s = 57 Watts.

    So 57 Watts for 10 seconds, or half that power for 20 seconds, etc.

    If you use the 57 Watt power number (to lift 125 pounds 1 meter in 10 seconds), and you use a 12V battery for the power source, that is then:

    P = 57W = V * I = 12V * I, so I = 4.75A

    That all assumes high efficiency, so you may need to increase that to more like 7A or more in a real situation, especially depending on motor and gear efficiencies.

    You can use the motor RPM and gear ratios and whatever to figure out how long it takes to lift your box, and ratio the power calculation accordingly.

    Hope that helps. Be sure to check all of my numbers...
     
  9. Oct 4, 2011 #8

    sophiecentaur

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    I googled 12V winches and found a Champion Winch
    (http://dl.dropbox.com/u/5846105/Winch%20Manuals/Champion/c3500a%20Spec.pdf" [Broken])
    for £100 ish. This spec sheet for the motor agrees more or less exactly with what Berkeman suggests (not surprisingly - he can do his sums properly).
    Something off the shelf could save you a lot of time and trouble and may not be more costly than buying the components individually. There may be cheaper things available that could suit you just as well - this one is severely meaty.
     
    Last edited by a moderator: May 5, 2017
  10. Oct 4, 2011 #9

    jim hardy

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    i just bought a DC gearmotor at salvage yard that looks about right flavor for what you describe
    nameplate says "1/2 hp 80 amps"
    planning to use it for a winch on my trailer


    one hp will lift 550 pounds one ft per second, by definition

    you want to lift 125 lbs maybe three feet in maybe three seconds?

    That's 125lbs X3ft/3sec/550 = 0.23 hp
    so a 1/2 hp gearmotor sounds reasonable, maybe a tad oversized
    so figure on at least 40 amps probably a little more.

    was that the question? i wander sometimes.....
     
    Last edited: Oct 4, 2011
  11. Oct 4, 2011 #10

    sophiecentaur

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    Also, you need to see what speed it runs at so that you can choose the right diameter of drum for the wire. If you under-gear it with a small drum you can reduce the current draw by taking a bit longer to raise the box.
     
  12. Oct 5, 2011 #11

    NascentOxygen

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    A 12 volt electric drill coupled to a pulley? Raising the load is one thing, lowering it is another. Do you envisage it lowering without electric power? Maybe it's okay to drive the drill motor as a generator?
     
  13. Oct 6, 2011 #12

    sophiecentaur

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    If you just short the connections, you should get enough braking to avoid smashing your foot on descent. The gearing of the motor would help a lot in this respect. Heating effects would hardly be a problem over such a short braking time.
     
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