American Machinist - Helical Products Co.
Three small machined springs are letting NASA scientists close in on Mars rocks with names like Yogi, Boo Boo, Scooby Doo, and Barnacle Bill. The springs, which are found on the Mars Rover Sojourner undercarriage, are used to soft mount a device called an Alpha Proton X-Ray spectrometer. The spectrometer analyzes soil and rock composition on the Martian surface.
Manufactured by Helical Products Co., Santa Maria, Calif., the P/N 2421 springs permit lateral bending – with a slight to moderate amount of compression. In essence, this permits the spectrometer to "float" and conform to the irregular surface of whatever object it presses against. Consequently, scientists can get plenty of information on whatever the Rover slides up to.
In 1994, NASA's Jet Propulsion Laboratory asked Helical to design two machined springs because it provided the desired elastic properties and room for special end attachments. In this case, the end attachments were threaded bores with external holes for a hookspanner wrench to adjust the spring position.
Helical came up with the P/N 2421 spring, made of the 15-5PH CRES, and a stiffer spring, P/N 2425, made for 38644 Beta C Titanium. The P/N 2425 had the same mass, 6gm, as the 2421, but it was shorter in length and larger in diameter. The 2425 has twice the lateral bending stiffness of the 2421, but testing at JPL indicated that the 2421 was best suited for the mission.
From a manufacturing standpoint, both springs were a straightforward size and configuration. The springs were machined from a single piece of metal into rectangular shaped coils, a configuration that provides exceptionally precise operational performance.
The most difficult part, says Helical, was determining which spring geometries were necessary to obtain the desired performance. To accomplish this, the company used large-scale, finite-element-analysis program to model the spring when subjected to large displacements. Consequently, the resulting spring design incorporates specific lateral bending, lateral translation, and compression spring rates.
Helical has created springs for several space applications including the Hubble Space telescope and the future International Space Station. Machined springs let designers alter coil size and number, reaching different, mathematically specified, elastic rates.