I have installed window grilles from my contractor. they are the modern horizontal bars design. when i try to slide the aluminium grilles, i realised that they do not run on rollers or there isnt any ball bearings on it, it seems like the entire frame of grille relies on the metal sticking out that is on the track. I feedback to the contractor but he mention that nowadays noone uses rollers as rollers will spoil easily and this kind of aluminium grille is more firm... can anyone verify?