var bannersCurrentNum = 0;
var secondOnBanner = 20000 //20 sec
$().ready(function(){
    $('.banner_on_main').hide();
    changeBanner();
    setInterval(changeBanner, secondOnBanner);
    
})

changeBanner = function(){
    if ($('.banner_on_main').eq(bannersCurrentNum+1).size())
        bannersCurrentNum++;
    else
        bannersCurrentNum = 0;
    
    $('.banner_on_main').hide();
    $('.banner_on_main').eq(bannersCurrentNum).show();
        
}