jQuery.fn.onlyNumbers = function() {
    return this.each(function() {
        jQuery(this).keydown(function(e) {
            var key = e.charCode || e.keyCode || 0;
            return (
        	key == 13 ||  // Enter 
	        key == 189 || // Minus symbol (-)
                key == 8 ||   // Backspace
                key == 9 ||   // Tab
                key == 46 ||  // Delete
                key == 186 || // Colon (:)  
                (key >= 37 && key <= 40) || // Arrows
                (key >= 48 && key <= 57) || // Numbers
                (key >= 96 && key <= 105)   // Numbers (Num Lock)
            );
        });
    });
}; 

// Usage
// jQuery( el ).onlyNumbers();