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How would you calculate this?

  • 10-04-2010 7:23pm
    #1
    flashforward
    Registered Users, Registered Users 2 Posts: 325 ✭✭


    Calculating power required to drive plunger:

    Motor rotates at 75rpm

    Diagram shown in attachment.

    The maximum depth under water (Plunger profile) of 600mm x588 x 1219.2 displaces 215kg of water.

    My (too) simple solution:

    Force = 215 x 9.81 = 2109.15N

    Work done = F x d = 2109.5 x.6 = 1265.5J

    Power = Work/Time = 1265.5/(.4) = 3164 W or 3.164kW


    Should I look at solving this with some form of force integral?


    Anyone have any other ideas?


Comments

  • #2
    steifanc
    Registered Users, Registered Users 2 Posts: 235 ✭✭steifanc


    when you say calculate power do u mean watts or force ? as in power needed to drive a moter at xrpm or what force is needed to be created to force the plunger into its application ?


  • #3
    flashforward
    Registered Users, Registered Users 2 Posts: 325 ✭✭flashforward


    steifanc wrote: »
    when you say calculate power do u mean watts or force ? as in power needed to drive a moter at xrpm or what force is needed to be created to force the plunger into its application ?

    I mean I want to choose a suitable motor to drive this plunger for this application.

    In my op I have attempted to calculate this but I fear I am doing it wrong.


  • #4
    steifanc
    Registered Users, Registered Users 2 Posts: 235 ✭✭steifanc


    OK , may be the maths forum would be a better help to u thats not exactly a easy calculation , there are loads of things to consider depending on how accurate you need to be , things like drag , generated momentum in the water , water composition , related surface tension of the water ,
    once you put the plunger into the water everything changes, and its related with time approaching limits .
    so you need someone thats into calculus and integration ,
    or maybe make up a transfer function and model it on matlab let the comp do the work ,
    the relationship between mechanical and electrical energy is

    In terms of mechanical energy, one watt is the rate at which work is done when an object is moved at a speed of one meter per second against a force of one newton.
    1W = 1Js-1 = 1kgm2s-3 = 1Nms-1


  • #5
    crosshair1
    Registered Users, Registered Users 2 Posts: 555 ✭✭✭crosshair1


    Probably too late a reply but anyways heres my two cents worth!
    There are too many variables to accurately predict the answer to your question.
    The problem is the speed at which the plunger descends!!
    If the plunger is very slow then your calcs with buoyancy is relevant and also the weight of the plunger is relevant but mechanical friction is also an issue due to the speed of motor/gears and reductions available.
    If the plunger is to descend rapidly (very rapidly) the larger forces will be due to the surface tension and viscous friction of the liquid and this will be much larger than mechaincal friction.
    The simplest eqn applying to this is Power(w)=F(N)xV(m/s), you know the force of buoyancy but no others so really an approximation will have to be made.


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