A structure provides rigid support for a cryogenic component but transmits minimal heat. The structure includes two beams of Invar (or equivalent low-thermal expansion iron/nickel alloy) that are held in alignment, without touching each other, by pretensioned strands of KevIar® (or equivalent) aromatic polyamide. The strands have a small cross section, low thermal conductivity, high stiffness, and high tensile strength.
The concept of mounting a cryogenic component on thin, low-thermal-conductance tension members is not new; what is new is the particular rigid configuration of this structure, which is illustrated schematically at the top left of the figure. The Invar beams, with their low coefficient of thermal expansion, minimize contraction at low temperatures, which contraction would reduce the tension on the strands and thereby reduce the strength of the support.
As shown in more detail on the bottom part of the figure, the strands are anchored by epoxy in grooves in end plates bolted to the beams; this prevents the weakening effect of knotted or crimped terminations. It also prevents the sudden slackening and consequent loss of tension that can occur when a high tensile load is applied to a strand wrapped in several turns around a terminating shaft or spool.
When a load is applied, for every strand in which the tension increases, there is another strand in which the tension decreases by the same amount. Because increasing tension leads to failure by breakage and the decrease of tension past zero leads to failure by buckling, the structure can be made to support loads over the widest range by pretensioning the strands to about half their breaking strength. (Thus, one ensures that failures in both modes are approached simultaneously.)
The first step in assembling the fixture is to clamp a temporary spacer between the two Invar beams to hold them in alignment. Two strands are rinsed several times in acetone and then dried. The grooves in the end plates are cleaned and roughened by bead blasting, and a small amount of epoxy is applied to them. The assembly is mounted on a lathe between a four-jaw chuck and a live center in the tailstock.
Each strand is anchored to the fixture, and the fixture is rotated by hand while the strands are guided into the appropriate grooves. The tension is determined by special couplings that slip at a predetermined torque. Before going to the fixture, each strand is wrapped several times around a brass shaft connected to the coupling; the coupling slips and feeds the strand when the correct tension is reached. To prevent the strands from advancing along the shaft as it turns, the shaft has a 15° taper that opposes this tendency. Two slip couplings (one for each strand) are mounted on pivots to allow each strand to be properly positioned as the fixture rotates.
The fixture is wound in multiple rotations so that each link is actually built up of more than one strand. The multistrand approach greatly reduces the stress on the free ends that must be anchored in the epoxy. More epoxy is added to the grooves during winding to cover the strands. The assembly is left under tension until the epoxy hardens. Then the excess lengths of strand are cut off, the assembly is removed from the lathe, and the spacer is removed.
For testing, the fixture was wound with four turns of KevIar 29 of 50-lb (223-N) breaking strength, which was tensioned to 20 lb (89 N). This resulted in a total cross-section of 0.52 mm² and a breaking strength of 200 lb (890 N) for each link. The force and deflection of the fixture were measured at 77 K for an axial compressive load. The reciprocal of axial stiffness was found to be 2.9 × 10-4 in./lb (1.7 × 10-6 m/N). The strands broke at a load of 441 lb (1,962 N).
This work was done by Pat Roach of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Mechanics category,or circle no. 170 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).