Friction Characterization of Threadless Linear Transmission System Using Rolling-Element Bearings to Create Imaginary Helical Pattern

Linear transmissions are commonly used to convert the rotation of rotary actuators to the translation in diverse robotic systems. As an alternative to conventional ball screws, the Roh'Lix linear transmission applies six rolling bearings preloaded on a threadless shaft with an angle between their rotational axes to create an imaginary helical pattern along the shaft. Concerning the traveling distance per shaft revolution, the Roh'Lix can offer the lead length up to three times of its shaft diameter, which tends to provide higher backdrivability than power screws. The normal force at the contacting surface between the bearings and the shaft can be adjusted through the spring force, which allows setting the slip limit of the bearings against the shaft. When the thrust force is over the desired limit or the movement of the Roh'Lix is constrained, the shaft can keep rotating while the bearings slip. The overload protection is useful for safe physical human-robot interaction. In this study, constant speed tests were conducted to observe the position-dependent and speed-dependent friction of the Roh'Lix linear transmission at different thrust force limits adjusted by the spring force. The effect of the actuation torque for compensating friction on the transparency at the endpoint was also evaluated through the human interaction force. Better knowledge about the friction characteristics of this threadless linear transmission can be applied for further adaptation in collaborative robots operating at high force.