Sprag design adds new dimension

Greenbelt, MD--It was the partial vacuum of space that launched this patented design. NASA needed roller locking sprags, but the special lubricants required for in-orbit operation proved too slippery: standard 2-D sprags would not hold.

To increase the apparent coefficient of friction, John Vranish, a design engineer at NASA/Goddard Space Flight Center, turned his attention to a geometric solution. Two-dimensional sprags typically comprise a pair of semi-cylinders joined back-to-back, with cylinder centers slightly displaced from each other. Contact between each sprag and its inner and outer races describes a line on the perimeter of each semi-cylinder.

Three-dimensional sprags, in comparison, make four-point area contacts with their races in a way that effectively doubles the torque they can withstand. They also introduce a wedging angle not found in 2-D geometries.

The new geometry, Vranish states, has been shown to increase the performance of roller locking devices about four-fold. As an example, he cites the use of 2-D sprag-type one-way clutches in the automatic transmissions of the auto industry.

"Installing 3-D sprags in these one-way clutches," Vranish says, "could increase torque by a factor of two, and halve the size at the same time. If volume remains constant, torque can be quadrupled."

Another potential benefit is longer life. Because their contact stresses are less, and because they have a natural lubrication path around the sprag due to the grooves in their races, 3-D sprag clutches should wear much better than conventional 2-D designs.

Additional details...Contact NASA Technology Transfer Program, Goddard Space Flight Center, Technology Utilization Office, c/o Dr. George Alcorn, Greenbelt, MD 20771, (301) 286-5810, or [email protected].

'Anti-lash wrench'

While development of the 3-D sprag is still underway, a 3-D sprag-based replacement for hand ratchet tools in space--also developed at NASA's Goddard Space Flight Center--illustrates the technology's versatility.

Current ratcheting devices have limited indexability, established by the gear pitch which determines the movement required to set the ratchet's mechanical diode into the next tooth. Attempts to make a ratchet without the gear pitch limitation have failed. Reason? In order to withstand an increasing load with no slipping, designs using 2-D sprags would have to be much larger than current ratcheting devices.

Using a 3-D sprag mechanism gives an infinitely indexable ratcheting capability. Tests demonstrate that the NASA "Anti-Lash" reversible 3-D sprag socket wrench does not slip under load. Additionally, the wrench to be made the same size as a conventional socket wrench, yet provide impressive torque.