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Machined springs for demanding applications

19 October 2014

Abssac is claiming great success with its Heli-Cal machined spring product in the oil and gas industry, where it is used in a variety of applications to store energy, deliver a constant force, measure accurately for deflection or act as a fail-safe device.

By machining a spring form from a solid piece of material, springs can be manufactured in compression, extension, torsion and lateral bending formats. The machined spring can deliver a predicted linear force in any of these formats, and in some cases (and depending on the geometry of the spring design) tolerance on rate can be as good as +/-1 percent. Since the parts are machined it is then simple to incorporate end attachments into the design, which can allow pure moment attachment. 

Most importantly, the machined spring product can offer multi-start springs within the single piece spring system. Offering single, double and triple start configurations as standard, the product delivers novel performance characteristics unobtainable from wire wound alternatives.

For example, in compression, a double or triple start spring significantly unifies the lateral bending and lateral translation forces and moments around the spring’s circumference, given a lateral deflection. In torsional mode, the machined spring can allow pure moment attachment, by incorporating specific end attachments into the single piece design.

When using integral tangs the machined spring avoids the relative stresses that these types of connection can impose. These springs are also offered in a choice of materials such as stainless steel, titanium and other exotic alloys to ensure reliability and high performance.

A recent application undertaken by Abssac called for a machined spring to be used as part of a load measuring system within a downhole tool.  The requirement was to measure a 5,000lbs/22,241N compressive/tension load. The maximum operating temperature was 200°C and the springs would be exposed to corrosive bore hole fluids. In addition to this harsh environment the spring would have to perform within a very constrained space envelope where the diameter of the spring was fixed but the length was not an issue.

Machined spring design using finite element analysis techniques

The final design used three machined springs attached end-to-end to achieve a 10mm deflection at 5,000lbs. The springs were constrained within a tube and the +/- 10mm of linear movement was measured with a precision potentiometer.    

Samples for test and evaluation were manufactured with a spring coil geometry designed to deliver the compression spring characteristics required. Two materials were used for the test samples, one using a certain grade of stainless steel providing an axial spring rate of 6,370N/mm with a variant on rate of +/-10 percent between 0.10mm and 0.60mm compression from free length.

This lower load test and rate performance theoretically proved that the required spring performance at the higher loadings would be possible. With confidence, the maximum loading tests were then conducted at the customer's premises within a test rig.

At the same time another identical spring was produced using a different grade of stainless steel, which provided the same rate and performance of +/-10 percent, but which was to be used to compare spring rate tolerance over time in a higher yield material.

In each case the spring design incorporated a threaded turned end at one end and a tapped hole at the opposite end. which allowed three springs to be assembled together to form the final single spring system with a total length of 650mm.

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