cs.ROSep 19, 2026

SHAFT: A Slack-Compensating, Helical-Buckling-Attenuating Flexible-Shaft Transmission for Lightweight Multi-DoF Manipulation

Authors: Tomoya Takahashi, Moses Gladson Selvamuthu, Richiro Tadakuma, Kazutoshi Tanaka

Organizations: OMRON SINIC X Corporation, Hongo 5-24-5, Bunkyo-ku, Tokyo, Japan · Yamagata University, 4 Chome-3-16 Jonan, Yonezawa, Japan · OMRON SINIC X

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Abstract

Lightweight and slim manipulators enable safe operation in human living environments. Proximal actuation using remote transmission mechanisms, such as wire-driven or Bowden cables, effectively reduces inertia and arm size by relocating motors near the base and transmitting torque to distal joints. Existing approaches either increase mass through additional components, such as pulleys for direction changes, or suffer from reduced transmission efficiency due to friction losses. Flexible shaft transmission avoids both mass increase and excessive friction losses, but faces increasing angular transmission error due to helical buckling caused by slack generated at joint bending. To address this problem, we propose SHAFT: a Slack-compensating, Helical-buckling-Attenuating Flexible- shaft Transmission mechanism. This mechanism compensates for slack through a proximal tensioner, improving the angular transmission error and efficiency of flexible shaft transmission without increasing the moving mass of the arm section. In a transmission path containing four 90-degree bends, the proposed mechanism demonstrated approximately 30% higher efficiency and approximately 65% lower angular transmission error compared to a flexible shaft transmission without a tensioner. Using this mechanism, we fabricated a 6-Degree-of-Freedom (DoF) arm with a 1-DoF gripper manipulator consisting of a rotary module housing motors with a tensioner, and a 280 g weight for the arm module. The proposed manipulator represents a novel remote actuation system for achieving lightweight construction with high efficiency, contributing to the acceleration of safe robot deployment in human environments.

Figures & tables

Fig. 1: (a) Helical buckling of flexible shafts caused by slack, (b) Proposed method: Slack compensation suppresses angular transmission error, (c) Fabricated flexible shaft-based manipulator.
Fig. 2: (a) straight position of link, (b) Constant curvature shape of flexible shaft.
Fig. 3: Schematic of single remote actuation unit of SHAFT.
Fig. 4: Tensioner mechanism: (a) Released position, (b) Contracted position, (c) Rotation motion.
Fig. 5: Basic design of manipulator driven by SHAFT.
Fig. 6: Schematic of joints, motors, and flexible shaft configuration of manipulator.
Fig. 7: Design of shoulder module and J1 motor.
Fig. 8: Design of coupled drive mechanism of gripper and J6.
MethodInner shaft diameterOuter tube diameterJoint reduction ratio
W/ tensioner (Proposed)3 mmNone10:1
W/ conduit3 mm10 mm10:1
W/o tensioner3 mmNone10:1
Bowden cable3 mm6.5 mmNone (1:1)
TABLE I: Comparison of mechanical parameters of each transmission method using experiment.
Fig. 9: Setup for single-unit transmission experiment.
Fig. 10: (a) Flexible shaft with tensioner, (b) Flexible shaft with conduit, (c) Bowden cable.
2d2dSlack length [mm]EfficiencyAngular transmission
[mm]LtotalL_{\text{total}}Lmax⁡L_{\max}Measured[%]error [deg]
3025.7535.153551.306.0
5042.9258.575560.937.2
7060.0882.017761.459.4
TABLE II: Slack, Efficiency, and Angular transmission error vs. Support Bearing Distance dd .
Fig. 11: Result of bending experiment.
Fig. 12: Transient motion of the manipulator to the predefined setpoints.
 S1S2S3S4S5S6
Distance Error [mm]3.283.28 ±0.47\pm 0.473.593.59 ±1.65\pm 1.653.063.06 ±0.71\pm 0.712.582.58 ±1.14\pm 1.141.651.65 ±0.85\pm 0.852.592.59 ±1.51\pm 1.51
TABLE III: Distance error of each checkpoint. (mean ±\pm std) [mm]
Fig. 13: Result of moving speed experiment: (a) Velocity, (b) Acceleration of end-effector.
Fig. 14: Motion of manipulator and tensioner carrying 500 g payload.
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