Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help in design

  1. Feb 9, 2007 #1
    well,im thinkin of designing an eraser,specifically the conventional chalk eraser, that can act like a vacuum cleaner,in the sense that i would have to put a motor in it,for the reason that the user would have to use a lesser effort for him to erase something,nd no chalk dust...well u could say that it is nonsense,but is this possible to do,and definitely,it must be small like other eraser and no plugs,just battery..
    if its possible,where shud i start the design..tnx a bunch..
  2. jcsd
  3. Feb 9, 2007 #2


    User Avatar
    Science Advisor
    Gold Member

    Have a look at miniature vacuum cleaners used for photographic applications.
  4. Feb 9, 2007 #3


    User Avatar
    Science Advisor

    As design inspiration, take a look at the vacuum set ups they use with drywall sanders in home improvement stores. They work pretty well and it would serve the same purpose.
  5. Feb 14, 2007 #4
    well,i actually do have a design now..but would just like to ask wat r the calculation that i shud have with this one..because my prof requires us to show calculations of our design..
    i made an eraser,just like the conventional in shape and size, has a roller and a brusher, that would eliminate the dust in the fabric's surface and the vacuum setup on top, but really cant frigure out, if it would work,what r the things that i cud compute and how should i do it..how wud i know,if the pressure is enough, and bout the motor speed,coz i was thinkin if it can b powered by a battery,,..is 4 AA enough, to power the motor and the fan as well..what other things should i consider?tnx..
  6. Feb 14, 2007 #5
    What you are trying to calculate is "capture velocity." This is basically the speed that the air needs to travel in order to "grab" the chaulk dust and hold it until it is moved through a channel or passage to where you want to deposit it.

    The capture velocity of very fine powder and dust is 2000 FPM.

    If you do not produce this velocity of air the dust will fall out of the air and accumulate on the duct or passage, or won't be picked up at all.

    The calculation is simple enough: Q=AV

    Q = Quantity of air in Ft cubed per minute (CFM)

    A = Cross sectional area of orifice in square feet

    V = velocity of air in feet per minute

    Perhaps with a beater-bar type arrangement lifting and throwing the dust and short enough run to the depository for the dust this velocity could be reduced some.
    Last edited: Feb 14, 2007
  7. Feb 14, 2007 #6
    tnx a lot..its really a big help..do i have to compute the right sizes of shafts for the motor..or its not needed..do i really have to have a powerful capture velocity,that large..since i do have a brusher..all i need is something that can suck the dust..up to the compartment.
  8. Feb 14, 2007 #7
    Considering you are capturing the particles in a enclosed area, close to the orifice, I discovered that you can reduce the rate down to only about 150 fpm. The higher velocity capture rate was for dust in open air.

    Check out the fan laws and relate them to the fan you are planning to use to determine the capacity it will move the air at what RPM and HP.

    Last edited by a moderator: Apr 22, 2017
  9. Feb 15, 2007 #8
    well..bout the formula u've given..i was actually luking for the right cross sectional area,since i don't have an idea bout the value of Q...
    and would just like to ask,can a fabric in a roller erase a chalk from a blackboard,moving it laterally,coz someone tells me it can't..but i still insist it can..maybe depending on the fabric..is it possible?
  10. Feb 16, 2007 #9
    I would say this would be a good thing to test with a prototype or model. A prototype doesn't have to be exactly like the invention, just needs to test the function of the components. Create a roller and try different types of cloth.
    Even just moving it manually, just to see if the cloth will remove the chaulk. Even just moving the cloth laterally across the blackboard by hand will give you an idea if it could do what you intend.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook