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NASA Tech Briefs - Engineering Solutions for Design & Manufacturing
April 1999 - Vol. 23, No. 4
Machined Springs: a Very Good Solution
The storing of energy in mechanical spring-like devices started long ago, and today springs are everywhere around us, in scales, mechanical pens, valve closures precision instruments, and so forth. Traditional springs such as wire-wound and Belleville washers fulfill many spring applications. But for designers with special and/or precise design requirements for their applications, a helical machined spring can be a very good solution.
Helical Products Company has reinvented spring design by applying its HELI-CAL® Flexure technology, with its inherent versatility, to the design and manufacture of one-piece machined springs. The HELI-CAL Flexure is a flexible helix-curved beam machined from one piece of material. It can take the form of a precision spring, coupling, or U-joint, and can be manufactured into a wide variety of shapes and sizes for almost any application. To date, more than 20,000 unique machined Flexures have been created by Helical Products Company.
During the development of such a spring, the following criteria are normally considered:
- desired spring performance;
- envelope size;
- integration of multiple features; and
The Helical machined spring combines the elastic properties of various high-strength materials (typically corrosion-resistant steels, aluminum, and titanium) with the curved-beam helix to provide precise deflection rates for compression, extension, torsion, lateral bending, and lateral translation-spring functions.
Because Helical’s springs are machined for a specific application and purpose, there are no stock machined springs. Applied to either high-precision applications or commercially graded needs, the versatile HELI-CAL Flexure, used as a spring, provides superior elastic performance in a wide variety of applications. Machined springs can be designed to specifically address requirements for rate, reactions at desired deflections, combined rates, modal properties, weight, inertial limits, and more.
This application-specific approach facilitates an unusually good correspondence between what is desired and what is economically possible. Helical springs can also support such design objectives as reliability, repeatability, and multiple part integration.
Wire-wound springs and Helical’s machined springs share many areas of common functionality. There are four venues, however, where the machine spring excels:
- integration of specifically desired attachments;
- pure couple reactions for torsional applications;
- wide variation of attachments for extension springs; and
- incorporation of multiple start configurations to eliminate extraneous reaction forces and moments.
The ability to integrate multiple features into a machined spring is another notable benefit. An integration of flanges, geometric shapes (squares, hexes, rounds, etc.), bearing seats, valve seats, gears, splines, bell-crank arms, internal and external threads, and other machinable shapes is available to the machined spring designer.
Wire springs provide only bent wire in the form of internal or external tangs for torsional spring attachments. Such a configuration is prone to high stress at the maximum bend of the attaching wire, plus, because of the configuration, the torque is the product of a force at a distance. Whenever torque is created at this technique, a resolution of the force is required. Machined spring attachments allow couples to be used to create the desired torque, and hence eliminate the need for force resolution.
Like the torsion wire-wound spring, the extension wire-wound spring is limited to a bent wire form (hoops and hooks) for attachment. Since nearly any machined form is available, the attachment for extension machined springs is limited by only the creativity of the design engineer.
In single-start springs used in compression or extension, whether they be wire-wound or machined, forces and moments in addition to the compression or extension forces are required to keep the springs at rest. Multiple starts, available only to machined springs, resolve these additional forces and moments within the given spring. The result is that ends of compression and extension springs naturally remain normal to the line of motion. Multiple start machined springs are available in two, three, four, and five start configurations.
There are, certainly, cost differences between wire-wound and machined springs. The wire product can be produced complete in usually less than a minute of process time, which is many times faster than that required for the machined springs. So, from a cost standpoint, the wire-wound product appears to be the spring selection of choice, and there are millions – likely billions – of successful wire-wound spring applications. In addition to the four direct benefits of machined springs cited previously, however, they can also allow for component integration, in which the reduction of overall part count reduces assembly time, conserves assembly space, and reduces inventory space and related purchasing activities and costs. Once designers become familiar with this concept, the many benefits of a multifeatured component become apparent. The HELI-CAL Flexure spring is a perfect foundation for integrating multiple features and functions into a single part.
Whether it be cross slots, double tangs, spline, bolt circles, flanges, geometric shapes, threads (internal or external) or simply machined flat ends, HELI-CAL machined springs provide a robust technical solution path to many elastic problems that are generally not solvable with traditional wire-wound springs.
For the Mars Pathfinder mission, NASA’s Jet Propulsion Laboratory was charged with developing a Martian data-gathering vehicle that could deploy the Alpha Proton X-ray Spectrometer (APXS). The solution, the Sojourner rover, is a small radio-controlled cart that can roam around the surface of Mars, back up against a rock, and enable the APXS transducer head to align to the rock, radiate it, gather the reflected radiation, and determine the rock’s composition.
This application required tight compliance of the transducer head to the rock surface. The most common way to accomplish a compliance motion is by using a gimbaled spherical joint, but this requires a clean sliding surface and is relatively heavy. One of the development engineers decided to try to fashion a totally elastic device. The plan was to use three Helical Products machined springs situated at the three corners of an equilateral triangle. The springs are subject dominantly to lateral bending, but small compression deflations are also possible. The HELI-CAL Flexure spring designed is made from 15-5PH CRES for an optimized weight of 6 grams.
Currently, Helical is building high-strength, Beta-C titanium springs to be used in connector assemblies in the International Space Station Alpha. These springs will be used dominantly in lateral translation, with launch for the components scheduled in 2001. Additionally, high-strength stainless compression and extension springs that each incorporate right- and left-handed Flexures are being readied for NASA’s Shuttle Radar Topographical Mapper (SRTM) mission scheduled for launch in late 1999.
For more information, contact Gary Boehm, senior research engineer at Helical Products Co., Inc.