Turning Corners on the Space Station

Ventilation manual overrides rely on flex shafting

The International Space Station is composed of a variety of modules used for living and workspace. Al-though automated life support systems are in place, manual overrides offer additional safety. As part of these systems, about 35, five-inch butterfly valves are strategically placed inside a variety of modules connected to the International Space Station for intermodule ventilation.

Each module is a large cylinder that houses equipment either for specific experiments or for use as living space for the astronauts. Because of the rounded shape of the modules, two or more coupling devices such as universal joints would have been required between the manual-control handle and the valves. The distance between each valve and the nearest point for hand operation suggested a low-torque system; however, manual overrides using universal joints would have required three times the torque than that needed to ro-tate the valve using the alternative solution: flexible shafting.

In selecting a flexible shaft, one was needed that could accurately operate with extremely low differen-tial rotational torque in both reverse and forward directions. If the shaft were to loosen due to the unwind-ing effect of reverse-operation, control accuracy would be reduced and the life support system would not operate as efficiently as required. In fact, if the shaft were to loosen, the motors would not have the extra torque to operate both the valves and the shafts as required by the system design.

All components, lubricants, and materials of construction of a manned space vehicle are specially se-lected to meet rigorous and critical requirements. Any weight savings in the module translates directly into cost when it comes to the amount of launch thrust necessary to lift the unit from Earth’s atmosphere. In the case of selecting flex shafting, weight savings of wound wire versus solid-metal mechanical linkages made from stainless steel were significant. Additionally, the lighter weight flexible shaft mechanism allows more laboratory equipment to be installed into the module. Because of the lack of failures to the units, the entire system has proven to be both rugged and reliable, including the butterfly valves themselves, as well as each unit’s remote manual override system.

SS White Technologies, Piscataway, NJ, supplies the flexible shafts used in the life support butterfly valves. Their low-loose design combats unwinding during reverse operation. The company’s flex shafts are used in other extreme environments, including business- and commercial-class aircraft, medical systems, harsh industrial operations, and MIL-spec applications. Lubricated and non-lubricated flexible shafts are available with numerous mounting arrangements to fit tip sizes from 0.130- to 0.187-in.

—RM