One common but challenging problem in cryogenic engineering is to produce a mount that has excellent thermal isolation but is also rigid. Such mounts can be achieved by suspending the load from a network of fibers or strings held in tension. Kevlar fibers are often used for this purpose owing to their high strength and low thermal conductivity. However, Kevlar presents challenges since it expands on cooling and tends to creep after initial tensioning, causing reductions in the resonant frequencies and a shift in the position of the suspended element, which can lead to misalignment and thermal short circuits. With existing designs, such as the Kevlar suspension used on the Herschel SPIRE instrument, it is difficult to re-tension the Kevlar or measure the tension because parts of the Kevlar string are staked with epoxy. Non-cryogenic designs used on a larger scale, such as tensioning reels on sailboats, use turnbuckles and fixed eyebolts that cannot be scaled down to a small-scale structure without a significant addition of mass.
A suite of compact design elements has been developed to improve the reliability of suspension systems made of Kevlar. The Kevlar is anchored to the load via a pair of tensioning stars whose arm stiffness is optimized to ensure that the Kevlar spans remain sufficiently taut during and after thermal cycling. All other anchor points for the Kevlar are designed to have much higher stiffness to maintain the optimal geometry when under load. Pulleys are used at each anchor point allowing the tension to equalize between all spans. The resulting symmetry allows the load to remain fixed in space even as the suspension elements undergo thermal strain, and the tension buffering provided by the tensioning stars reduces the concomitant changes in the structure’s resonant frequencies.
The tension is adjusted by means of a capstan made of an array of stainless steel dowel pins to which the Kevlar string is belayed. Each pin has a large diameter compared to the Kevlar string so there are no machined corners that might initiate fraying of the Kevlar. The capstan is locked with an integrated mechanical clamp, so no epoxy is needed to secure the Kevlar or the capstan in place. Thus, the tension of the Kevlar can be adjusted after the initial creep or easily reworked if needed. The tension can be measured in situ by measuring the flexure of a single arm of the tensioning star.
This work was done by Joseph B. Young, Bret J. Naylor, and Warren A. Holmes of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47940
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Robust Tensioned Kevlar Suspension Design
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Overview
The document discusses the Robust Tensioned Kevlar Suspension Design developed by the Jet Propulsion Laboratory (JPL) at the California Institute of Technology, under NASA's sponsorship. This design addresses the challenges associated with using Kevlar in suspension systems, particularly in environments subject to thermal cycling, such as in adiabatic demagnetization refrigerators.
The primary focus of the design is to improve the reliability of Kevlar suspension systems, which are known to experience issues like loosening due to cooling and creep after initial tensioning. These problems can lead to a reduction in resonant frequencies and shifts in the position of suspended loads, potentially causing thermal short circuits. The document outlines a suite of design elements that mitigate these issues.
Key features of the design include:
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Tensioning Stars: The Kevlar string is anchored using symmetrical hubs called tensioning stars, which allow for flexibility and maintain tautness as the string elongates during thermal cycling. This design enables in situ measurement of string tension by assessing the flexure of the star arms.
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Fixed Anchorages: These are designed with low compliance relative to the tensioning stars, ensuring that all spans have equal length and tension. This uniformity prevents shifts in the load's rest position due to temperature changes or creep.
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Capstan Mechanism: The tension of the Kevlar string is adjusted using a capstan made of stainless steel dowel pins. This design avoids sharp corners that could fray the Kevlar, making it more reliable and easier to construct. The capstan is secured with a mechanical clamp, eliminating the need for epoxy, which allows for easier adjustments after initial tensioning.
The document also includes a demonstration of this design in a prototype adiabatic demagnetization refrigerator, which features 300 mK intercept stages and a 50 mK cold finger. The design elements presented aim to enhance the performance and reliability of Kevlar suspension systems, making them suitable for various aerospace applications.
Overall, the Robust Tensioned Kevlar Suspension Design represents a significant advancement in the field, addressing critical challenges and improving the functionality of suspension systems in demanding environments.
