A cable-driven instrument was developed, which can be used through a working channel of a robotic flexible endoscope. This paper describes the required tensile force to handle a payload force of 3 N deriving specifications for an actuation unit. Therefore, the way the force ratio behaves when the entire length of the instrument’s active section of 75 mm is used, and how the force ratio behaves when a length of 50 mm is used, were investigated. It is intended to provide a simple and powerful instrument to enable more complex surgery with robotic flexible endoscopes. A tensile force of 254 N is needed in practice to achieve a 4.8 N payload. Much more than theoretically expected. It turns out that friction has a great influence on wired instruments.