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Help my dad and I settle a argument

  1. Feb 8, 2012 #1
    My dad has a car lift in his garage with arms that swing out and extend under the car. He says that when you are lowering the lift without a vehicle on it if you stand on the end of the arm then it will apply more downward force causing the lift to lower faster. I think that it doesn't matter where you stand on the arm. If you weigh 200 pounds you are going to be applying 200 pounds no matter if you stand on the end or on center. I do agree that standing on the end of the arm applies more torque where the arm meets the upright. Am I wrong or right? I've attached a picture of the lift so you can see what I'm talking about. Thanks for the help.
    http://www.superlifts.com/images/s9f-big.jpg [Broken]about.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Feb 8, 2012 #2
    You are correct, however standing near the edge of the arm applies more stress in the arm itself to hold you up. (this does not effect the downward force however)
     
  4. Feb 8, 2012 #3
    So if the question is about where on the arm to stand to lower the lift faster, then I agree with you, you apply the same force downward. In fact, standing on the end will give more torque like you said and may squeeze the arm against the vertical part causing more friction and may make it even worse for lowering. I would say you should stand closer to the connected end.
     
  5. Feb 8, 2012 #4

    DaveC426913

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    Well, considering they're designed to hold several tonnes of car out there on the ends of those arms, I'm sayin' torque binding is a problem they've solved.
     
  6. Feb 8, 2012 #5
    If that is a hydraulic lift then standing on an arm should not increases the speed of lowering. In a hydraulic circuit that is designed to accommodate varying loads, a pressure compensated relief valve is used. This valve ensures that the speed of the actuator is constant under varying loads.
     
  7. Feb 8, 2012 #6
    I would say you should stand closer to the connected end.http://www.bosin.info/g.gif [Broken]
     
    Last edited by a moderator: May 5, 2017
  8. Feb 8, 2012 #7

    DaveC426913

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    Why?
     
    Last edited by a moderator: May 5, 2017
  9. Feb 9, 2012 #8
    They obviously solved it, but still standing at the ends should increase the friction by some non-zero amount. So strictly theoretically speaking, I would agree that standing on the end of the arm would (or could) be slower.
     
  10. Feb 9, 2012 #9

    sophiecentaur

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    It's probably a worm drive with a threaded rod up inside each arm. They are very inefficient (the friction is desirable so you don't need to lock them) so nothing you can do will make much difference. Listen to the tone of the motor to see if you are actually having any effect when you step on and off.
    You can both stand out in the garage and argue about that sound too. Then go down the pub and the one who's wrong can but the other one a drink.
     
  11. Feb 9, 2012 #10
    Do an experiment, (and post the results).
     
  12. Feb 9, 2012 #11

    DaveC426913

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    It is a big assumption that increased friction translates into slower speed. If the motor and gearbox have a fixed output, then it may well proceed at a fixed rate regardless of any small changes to friction. The friction may simply result in more heat for example.
     
  13. Feb 9, 2012 #12

    turbo

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    My friend and mechanic has one of these lifts in his shop. They don't rely on friction to hold the arms in position - instead there are mechanical stops that ratchet into position as you raise the arms. It wouldn't be safe to work under the vehicle otherwise.
     
  14. Feb 9, 2012 #13

    sophiecentaur

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    Does the car really creep down, once it's been raised? I bet the mechanical stops are only there as belt and braces. I never knew a screw jack come unscrewed whilst I have changed a wheel.
     
  15. Feb 9, 2012 #14

    turbo

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    There are stops in the lift-arms that ratchet into position as the lift is raised.
     
  16. Feb 9, 2012 #15

    sophiecentaur

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    So what do you have to do with the stops to lower it?
     
  17. Feb 9, 2012 #16

    turbo

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    I think he has to raise the lift slightly and release the stops before lowering.

    For those looking at the image and imagining lots of binding torque, those arms are not ever in those positions in use. The arms are swung fore and aft on each side to allow the pads to be placed under structurally-sound points under the frame of the vehicle. Those pads are much closer to the posts than you might think, looking at that image. They are not physically closer to the posts, but they are much closer to the center-line of each post.
     
  18. Feb 9, 2012 #17
    I agree, that's why I specified it "could" be slower. I guess my point is that since there was an argument about this, the dad is less right than the son. It doesn't hurt it to stand on the end of the arm, but from some point of view...perhaps efficiency, perhaps stress, it's not going to be better, but it MIGHT be worse.
     
  19. Feb 9, 2012 #18
    My hoist is hydraulic as is the one in the picture. Although the speed range is fairly narrow, there is a noticable difference in lowering rates depending on the weight. Having a person on one of the arms helps speed things up (down) when there is no car.
     
  20. Feb 9, 2012 #19
    Thanks for the help everyone. I think I have him convinced I am right. Using some of my daughters toys and a scale, I performed a small scale experiment last night and sent him the pictures. It is a hydraulic lift and standing on the arms does cause it to lower a little faster if there is no car on it.
     

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  21. Feb 9, 2012 #20

    DaveC426913

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    OK.

    1]
    Your stance was that it would not change. So how can you be right if your experiment showed you to be wrong?

    2]
    Wait, what? It lowers faster with no car?

    So, you're both wrong?

    Finally, and most egregious:
    3]
    How does a completely different device that uses a completely different mechanism say anything at all about the real-world behavior of the full-scale hydraulic lift?
     
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