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TECHNICAL BRIEFS

# Design of Lightweight Lead Screw Actuators for Wearable Robotic Applications

[+] Author and Article Information
Kevin W. Hollander

Department of Mechanical and Aerospace Engineering,  Arizona State University, Tempe, AZ 85287-6106

Thomas G. Sugar

Department of Mechanical and Aerospace Engineering,  Arizona State University, Tempe, AZ 85287-6106kevin.hollander@asu.edu

J. Mech. Des 128(3), 644-648 (Jul 29, 2005) (5 pages) doi:10.1115/1.2181995 History: Received March 21, 2005; Revised July 29, 2005

## Abstract

A wearable robot is a controlled and actuated device that is in direct contact with its user. As such, the implied requirements of this device are that it must be portable, lightweight, and most importantly safe. To achieve these goals, an actuator with a good “power to weight” ratio, good mechanical efficiency, good “strength to weight” ratio, and that is safe is desired. The design of the standard lead screw does not normally perform well in any of these categories. The typical lead screw has low pitch angles and large radii, thereby yielding low mechanical efficiencies and heavy weight. However, using the design procedure outlined in this text, both efficiency and weight are improved; thus yielding a lead screw system with performances that rival human muscle. The result of an example problem reveals a feasible lead screw design that has a power to weight ratio of $277W∕kg$, approaching that of the dc motor driving it, at $312W∕kg$, as well as a mechanical efficiency of 0.74, and a maximum strength to weight ratio of $11.3kN∕kg$$(1154kgf∕kg)$.

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## Figures

Figure 1

Lead screw geometry; as drawn, pitch (p) and lead (l) are equivalent in a single helix screw

Figure 2

Mechanical efficiency of lead screw systems: Shaded part of the graph is the typical design region for the majority of lead screws. α is small, radius is large, weight is large, and efficiencies are lower. Designs in the unshaded region of the graph, where α is large, implies smaller radii, lower weight, and higher efficiencies. a: NTN Bearing Corporation (17). b: Serway, Physics textbook (18). c: MatWeb Material Database (19).

Figure 3

Prototype actuator, high efficiency lead screw

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