In theory, each moveable (as opposed to fixed) pulley doubles the simple machine's mechanical advantage (the required force decreases as the line distance required increases). In practice, however, the friction of the lines and pulleys counteract this advantage. The effect of friction is negligable at first, but it gradually increases as more pulleys are added, ultimately canceling out any mechanical advantage gained — thus placing limits on the machine's effectiveness, no matter how many additional pulleys are added.

Mouse over (or click on) the image to see the research team demonstrate their rig!