Performance Testing of Rachetless EVAHand Tools

University of Colorado - Boulder

The Hubble Space Telescope (HST) Servicing Missions have demonstrated that Extravehicular Activity (EVA) power tools need a hand (non-powered) ratcheting capability to accomplish delicate and unforeseen tasks. Depending on the pitch or spacing of the teeth on conventional ratcheting hand socket wrenches, sometimes more than 1/8 th of a turn in the backward direction can be required to engage the next tooth. This minimum motion requirement of the ratcheting device causes the wrench to work inefficiently in confined spaces. Furthermore, the bulkiness of an astronaut's spacesuit and gloves causes additional range of motion problems when ratcheting wrench tools are used for EVA tasks. For example, during the HST First Servicing Mission while changing out the Solar Array Drive Electronics, the Mini Power Tool designed for the task failed, so a bigger ratchet tool had to be used. Unfortunately, the confined space combined with the range-of-motion problems caused by the ratcheting action made the task difficult to perform. Therefore, the usability of future EVA tools can be increased by removing the ratchet from the design.

No successful ratchetless wrench has been developed to date. However, the NASA Goddard Space Flight Center (GSFC) has developed and patented a special three-dimensional (3-D) roller locking sprag technology that, when used in place of the traditional ratcheting mechanism, results in a device with infinite indexibility and infinitely small range of motion. Locking occurs because of the wedging action that occurs between the tapered periphery of the 3-D roller locking sprag and a grooved race, as shown in Figure 1. This unique geometry permits construction of compact locking mechanisms that can withstand large loads because the sprags are very small but have large contact radii and therefore, low contact stresses. Thus, a wrench incorporating 3-D sprags is an essential part of EVA tool evolution!

Figure 1 Ö GSFC 3-D Roller Locking Sprag Geometry

During the summer of 1997, GSFC developed and tested the use of this 3-D sprag technology in commercial hand "ratcheting" tools to create a commercial "ratchetless" sprag wrench. The Space Systems Laboratory (SSL) at the University of Maryland is extending this commercial sprag wrench development to EVA tools. Testing of various EVA sprag tools is being conducted in the SSL's Neutral Buoyancy Research Facility (NBRF), a 50 foot diameter, 25 foot deep water tank used as one simulation of the weightless environment of space. This proposal will continue the EVA tool development by testing the wrenches in another simulation of the weightless environment of space aboard the KC-135.

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Last Modified: Tue May 19 1998
CSR/TSGC Team Web