Machined springs prove to be a well oiled solution
Machined springs are becoming components of choice within the oil and gas industry. Peter Wright of Abssac explains the reasons why.
Oil and gas exploration applications represent extremely challenging and inaccessible environments – certainly places where you do not want failures in equipment to occur. This is exactly why the machined spring is such an important component, offering unique performance and proven reliability.
In many cases wire wound springs are used for these applications, incorporating brazed end attachments, such as tangs or ground captivated end seats. In each case a single spring coil is utilised and by the nature of the wound wire design they have some performance limitations. But the machined spring is a unique alternative to the wire wound spring. By machining a finite analysed spring form into a solid piece of material, machined springs are available in compression, extension, torsion and lateral bending formats. The machined spring can deliver a predicated linear force in any of the spring formats. In some cases and depending on the geometry of the spring design tolerance on rate can be as good as ±1%. 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 unique performance characteristics unobtainable by wire wound springs. 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 the 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 is without the relative stresses these type of connection can harbour.
The availability of a choice of materials such as stainless steel, titanium as well as other exotic alloys adds strength to the argument that machined springs are a formidable product that can be used where reliability and performance is paramount
A recent application called for a machined spring to be used as part of a load measuring system within a downhole tool for the oil and gas industry. The requirement was to measure a 5000lbs/22241N 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 actual 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.
The final design would use three machined springs attached end to end together to achieve a 10mm deflection for a 5000lbs load. The springs would be constrained within a tube and the ±10mm of linear movement would be 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 17-4ph stainless steel providing an axial spring rate of 6370N/mm with a variant on rate of ±10% 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 test were then carried out at the customers within a test rig.
At the same time another identical spring was produced using CC445 stainless steel, which provided the same rate and performance of ±10% but 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 a the final single spring system, which would have a total length of 650mm.
Whether it’s a new spring application or an enhancement of an existing spring design, the machined spring product is relatively unknown by design engineers and yet has so much to offer. Many applications are in critical application areas where reliability, and longevity of performance requisite, and the machined spring can offer the ideal solution.
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