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

Tomoya Takahashi; Moses Gladson Selvamuthu; Richiro Tadakuma; Kazutoshi Tanaka

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

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.

Method	Inner shaft diameter	Outer tube diameter	Joint reduction ratio
W/ tensioner (Proposed)	3 mm	None	10:1
W/ conduit	3 mm	10 mm	10:1
W/o tensioner	3 mm	None	10:1
Bowden cable	3 mm	6.5 mm	None (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.

$2d$	Slack length [mm]			Efficiency	Angular transmission
[mm]	$L_{\text{total}}$	$L_{\max}$	Measured	[%]	error [deg]
30	25.75	35.15	35	51.30	6.0
50	42.92	58.57	55	60.93	7.2
70	60.08	82.01	77	61.45	9.4

TABLE II: Slack, Efficiency, and Angular transmission error vs. Support Bearing Distance

d

Fig. 11: Result of bending experiment.

Fig. 12: Transient motion of the manipulator to the predefined setpoints.

	S1	S2	S3	S4	S5	S6
Distance Error [mm]	$3.28$ $\pm 0.47$	$3.59$ $\pm 1.65$	$3.06$ $\pm 0.71$	$2.58$ $\pm 1.14$	$1.65$ $\pm 0.85$	$2.59$ $\pm 1.51$

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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A Compact Robotic Finger with 2-DoF MCP Joint Embedding DoF-Selective Passive Continuously Variable Transmission for Wide Force-Speed Operating Range

JaeHyung Jang, Jee-Hwan Ryu

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Robotic Hand Design

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This letter presents a compact two-degree-of-freedom (DoF) robotic finger with a flexion-selective passive continuously variable transmission (CVT) to achieve a wide force-speed operating range. Inspired by the functional differentiation of the human metacarpophalangeal (MCP) joint, the proposed mechanism realizes DoF-specific transmission differentiation by selectively assigning passive CVT to the flexion-extension DoF while preserving direct transmission for abduction-adduction. For a wide force-speed operating range, a force-responsive passive CVT is embedded in the flexion pathway, while direct transmission is preserved for the abduction-adduction pathway. To selectively realize transmission adaptation within a multi-DoF MCP mechanism, an output-side passive CVT employing a moving-pulley-inspired wire-routing structure is introduced. The resultant force generated by the wire tensions acting on the pulley that passively increases the flexion moment arm and transmission ratio according to the applied load without additional actuators, sensors, or control. Experimental results demonstrate a maximum output-force amplification of 4.19-fold and a mean amplification of 3.6-fold across the tested flexion angles ranging from 15 degrees to 75 degrees through moment-arm adaptation, thereby substantially expanding the achievable force-speed operating range. Furthermore, dexterous ball-rolling experiments verify that passive transmission adaptation can be achieved while preserving abduction-adduction functionality. These results demonstrate a scalable transmission design strategy for compact multi-DoF robotic hands.

Abstract

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