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Guessing gearbox arrangements?

  1. Mar 13, 2007 #1
    Hi guys,

    I am trying to do an analysis of an electric carjack, i know the load, i know the torque require at the end of the jack(ive calculated it to be 5Nm). i know the typical electric car jack uses 12V 10A pmdc motor (about 3500rpm no load speed and stall torque 0.4Nm).
    I know that to maximize the output torque of the motor i need a gearbox that'll give out the optimum mechanical advantage. I dont have any clue how the design of an eletric car jack gearbox which is on the market, but i know the dimensions of it,15x2x4cm, below is the link of my design base on the dimensions.
    units are in cm and all the gears are fix to the shaft/motor.


    Anyway, the gear arrangements will increase the motor torque 25times(assume super efficient).and so this will give out 25*0.4=10Nm. Well its larger than the required torque,so it'll work but wait..thats assuming if the gears are 100% efficient, and at stall torque..

    So my question is can anybody like suggest any other gear arrangements which gives out more than mine or speculate what are the gear arrangements on the electric car jack on the market right now?

    thank you
    Last edited: Mar 13, 2007
  2. jcsd
  3. Mar 25, 2007 #2
    I don't think you could cut teeth strong enough in a pinion with a diameter of 1 cm.

    Have you thought about worm gears? http://science.howstuffworks.com/gear5.htm

    These gears also have the advantage that you don't need a brake, as they are irreversible transmissions, so you can switch off the motor and the car won't come down. And with a bevel gear you can keep the motor in the position you've drawn.

    The efficiency of spur gears is about 90%, but bevel, and worm gears have lower efficiencies
  4. Mar 25, 2007 #3


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    I agree that a worm & roller set is the proper way to go. The mechanical advantage is awesome, and there is the bonus of irreversibility that sunday pointed out.
    For an even higher ratio, epicyclic trains are available or buildable that can give you over 1,000:1 gearing.
    Last edited: Mar 26, 2007
  5. Mar 25, 2007 #4
    Wait. I realized there is a image of the jack in the pic you posted, and that type of "scissors" jack is irreversible as is. So you don't need an additional worm gear, and, yes, your gear arrangement could do.

    Which level of detail you need in the solution? If this is for some courses of machine design you'll have to produce the # of teeth in every gearwheel, and in advanced courses, the modulus of the teeth.

    Have you got more information about the gearbox? e.g. # of intermediate shafts, distances between shaft centers, etc.
  6. Mar 26, 2007 #5


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    Gold Member

    I never saw that picture the first time around either. The scissors jack is indeed based upon a lead-screw. It was my impression that you were going to build the whole thing from scratch. If you have that final-stage preassembled as in the picture, then your design is fine.
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