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Elevators and speed

  1. Jul 1, 2016 #1
    I know that there are really fast elevators out there that can travel between 10 meters/second to 16 meters /second. These are the passenger elevators at the luxury buildings that are really high high up ( > 380m).

    I assume that since the elevators have a counterweight that basically makes both side weight approx the same...most of the speed is achieved from a controller. Can these speeds be achieved with extremely large mass counterweights...like the boat lifts which have 8000 ton counterweights on each side?? Would it be possible to reach 10m/s or even something reasonable like 5m/s? What would prevent this for such large masses?

    I
     
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  3. Jul 1, 2016 #2

    billy_joule

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    The counter weight is just there to balance the weight of the lift, not provide the force for acelleration
    https://en.wikipedia.org/wiki/Counterweight

    Adding any weight beyond balancing means, for a given drive motor power, you'll go slower in one direction. The overall effect is that average lift travel times get worse as the motor has to accelerate more mass and do more work on average.

    Lifts could go much faster than they do at present, the reason they don't is because experiencing high acceleration isn't very comfortable, people would rather wait a few extra seconds than enter the office feeling nauseous like they've just been on a theme park ride.
     
  4. Jul 2, 2016 #3
    Ok - thanks :) So from this I am conclude these points:

    1. If each counterweight is like 10,000 metric tons, we have balance and a relatively powerful motor than we can achieve speeds of 5m/s or we can lift one counterweight from the ground up to 100 meters in about 20 seconds -- and vice versa?

    2. Beyond overcoming friction, a taller height (i.e 200 meters), doesn't factor into the ability not to reach 5m/s?

    I was thinking at such weights, the higher friction would have an exponential effect on the motor's ability to reach 5m/s??
     
  5. Jul 2, 2016 #4

    billy_joule

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    Each counterweight? There is only one. To counter balance the weight of the lift and the people in it.
    http://www.explainthatstuff.com/how-elevators-work.html
    http://science.howstuffworks.com/transport/engines-equipment/elevator.htm

    Of course the height of the building is critical. That's why taller buildings have elevators that can reach higher velocities.
    With the same acceleration (limited by passenger comfort) travelling a greater distance will lead to a higher velocity.
    Like driving in a car, riding a bike or running, it takes time and distance to increase your velocity. If you want to drive at 300km/hr you need a longer road than if you want to drive at 50 km/hr.


    Friction isn't relevant. Modern ball bearings practically eliminate friction (relative to the other forces present).
    Greater weights leads to greater inertia. Put simply: Heavier things are harder to move!
    https://en.wikipedia.org/wiki/Inertia

    Adding extra weight to a lift system and expecting it to go faster makes as much sense as putting bricks in your backpack and hoping you'll be able to run faster
     
  6. Jul 2, 2016 #5

    jack action

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    @billy_joule : I think the OP is asking if a high speed elevator is possible with large weight (and counterweight), not if we can increase speed with counterweight.

    When you increase the weight, you increase friction linearly and not exponentially (friction force ##\propto## mass). It also increases the inertia of the system, which is also proportional to mass. Inertia will be a much greater factor than friction.

    Basically, if you double the mass, you will need to double the power to achieve the same speeds. This is also true for decelerating the elevator (braking). Braking is probably what deters designers of heavy elevators to reach high speeds as, if something goes wrong, the destructive potential is enormous because the kinetic energy gained by the elevator is so large. For example, you will probably be more scare if you see a a fully loaded semi-trailer out of control at 70 km/h going towards you in your rear-view mirror, than a car out of control at 140 km/h.
     
  7. Jul 5, 2016 #6
    @jack action -- thanks for that insight. I guess it makes a lot of sense when you talk about the inertia. I just wasn't sure how much inertia applied in this situation because on both sides, there is the same mass if it was a perfect counterweight. So any large weight going down on one side has movement only because of the motor that overcomes friction and gives it an incremental push. Any large inertia of the weight going down is cancelled by the large weight on the other side which pulls downward.

    So my pondering comes from looking at these boat lifts :

    1. Fallkirk wheel in Scotland :
    2. Strepy Thieu boat lift in Belgium :
    3. 3 Gorges Dam boat lift in China :


    Here are the details of each operation :

    upload_2016-7-5_23-5-43.png


    I see that Falkirk Wheel in the UK can raise 24 meters in 4 minutes; the Strepy Thieu 73 meters but it takes 7 minutes; but the 3 gorges dam in china takes almost 35 minutes to reach 113 meters???

    So I was concluding that maybe it's the weight that makes the lift slower...but with 3 Gorges Dam it takes much longer just to go less than half of the Strepy Thieu lift?! I am guessing because of the power needed to slow down a weight that large (over 2 times that of Strepy Thieu).

    One thing I was trying to figure out is this:

    The Falkirk wheel takes 1.5 Kilowatt hours only to power one lift rotation 24 meters high but the weight is 1200 tons (about 600 each caisson) The Strepy combined weight is 15,000 tons (each caisson and water is 7500 tons). Does this mean the power for strepy is roughly about 36 times greater to complete the lift (15000 tons/1200 tons = 12.5 ; then multiply 12.5 times 3 to get 72 meters instead of the Falkirk 24 meter rise? So...for Strepy boat lift, the power utilized will be 1.5KwH * 36 = 54KwH ?

    A little crude on how I deduced this...but is that a fair approximation?
     
  8. Jul 6, 2016 #7
    Mining Elevators are pretty quick, not sure on the motor specs however.
    Quite a nice video of one here in South Africa.
     
  9. Jul 6, 2016 #8

    jack action

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    When you're talking about displacing water, then having slow accelerations becomes critical. You don't want to create waves inside the containers.
    First, kWh is a unit of energy or work, not power. Power would be energy/work per unit time (kW).

    The work done is the force times the displacement (Strepy case) or, in rotation, torque times the angular displacement (Falkirk case). The force/torque here is the one represented by the friction, which is dependent on the weight somehow. Does the other friction characteristics compare one with another? That is hard to tell just by looking at the lifts. But if you were comparing 2 boat lifts of different sizes with similar construction, your assumption would probably be right.
     
    Last edited: Jul 7, 2016
  10. Jul 7, 2016 #9
    @jack action
    @Billy Joule

    Thanks for all the great input guys! Another angle I was thinking about:

    The Anderton Boat Lift in the UK is not using a Pulley System of Counterweights but the Hydraulic system using cylinders and pistons. These are my questions on the system:

    1. Isn't there less friction since there is no weight on the pulley wheel; only friction on the seals and accounting for viscosity?
    2. No need to add power to account for increasing rope length on the lower counterweight?

    So given this -- if we used this system raise equal mass on each side by 24m (like Falkirk) - would it be about the same or just slightly lower than the 1.5 Kwh of energy to run this one turn?

    upload_2016-7-7_12-23-40.png
     

    Attached Files:

  11. Jul 7, 2016 #10

    jack action

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    Viscosity may be more important than one might think. We're talking hydrodynamics so pressure drag and skin friction applies.
    I'm not sure why we're talking about power. I'm no expert on the subject, but the only boat lift I saw was the one in Peterborough, Canada:
    And this system works since 1904, in a place with cold winters. Why other boat lift designs choose to use motors instead of gravity is beyond my understanding.
     
  12. Jul 7, 2016 #11

    rbelli1

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    Inertia is not cancelled at all. You can see this yourself really easily. Just set up two buckets and a pulley. Try to move them empty and then try full of sand. In both cases the forces all cancel out but the inertia certainly doesn't.

    BoB
     
  13. Jul 10, 2016 #12
    I think some Canals have to conserve water so the local conditions don't allow the constant loss of water from the upper stream..I think I read that somewhere. Falkirk uses balanced caissons and a small power load of 1.5Kwh to move the counterweights around and control the speed.
     
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