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DESIGN COMPONENTS Springs, Gas Springs & Dampers Four spring rates in one part A medical manufacturer thought it was testing spring capabilities to the limits with a requirement for a unique characteristic. But Abssac was able to meet the company’s needs with a machined spring Probably one of the most demanding machined spring applications to date for Abssac involved a medical equipment manufacturer wanting to utilise precision compression, torsion, lateral bending and lateral translation rates – all within one individual spring. In addition to this, the springs required easy attachment and would have to be lightweight and relatively inexpensive – something that previously was unobtainable from a standard wound spring format. To add to the difficulty, Abssac’s customer also requested three other springs, identical in size but with different spring rates. But that was easy compared with the real challenge of being able to accurately supply not just one spring elastic rate, but all four in the single part. Precise linear deflection Machined springs are quite different from conventional wound products. Rather than deforming a wire around a set form to produce the spring, machined springs are – as the name suggests – machined from a solid piece of material. In use, two main advantages are immediately apparent. Machined springs can provide very precise, linear deflection rates because virtually all residual stresses are eliminated. And the machined spring also enables the designer to incorporate the way in which the spring attaches into a single piece design, often incorporating parts of the spring assembly into the single part construction. But what is most compelling about machined springs is the fact that it is possible to have more than one spring coil in the single piece construction or multi start configurations, which deliver quite outstanding performance advantages. In fact, the machined spring’s capability to be supplied in multiple start spring coil configurations takes the performance and reliability to levels not achievable by the traditional wound spring format. Naturally, the most common configuration is the single start spring, which consists of a single continuous coil element, which starts at one end and terminates at the other end, much like its wire wound counterpart. The double start or indeed a triple start spring has two or three intertwined continuous coil elements, still within the same single part construction. In effect, this puts multiple independent helixes in the same cylindrical plane, which provides totally enhanced spring performance. On multiple start machined springs, virtually all internal moments are resolved within the spring itself and the double start machined spring configuration is the closest single Cartesian co-ordinate deflection part available with today’s technology. Five start configuration Pulling on extensive product knowledge it was known that the triple start Heli-Cal beam configuration delivered very high lateral stiffness. Combining this with the requirement of a high lateral translation rate became the starting point for analysis. Next, came the remaining three rates. Using first-class finite element analysis software, these three were then quantified. A trend also appeared, showing that as the number of starts increased, the ratio of the remaining three rates converged. In fact, at five starts, the ratio was near perfect. At six or more, divergence was experienced, hence no benefit. All that was left was to select a coil thickness, and outer diameter to inner diameter ratio that would best fit the customer’s needs. Combine this with the use of 7075-T6 aluminium for light weight and lower machining cost, and the design was successfully completed. The advantages of the machined spring product are numerous, having proven itself in medical, aerospace, semiconductor and motor-sport industries to name but a few. Wherever a wound spring is not able to meet your performance criteria or a new spring design requires ultimate accuracy and repeatability, the machined spring is the ideal partner. So the next time you require a spring in a critical or high duty cycle environment, the machined spring from Abssac may have the answer to your design problems. MORE INFORMATION: Enter D345 on the enquiry card or visit ‘latest issue stories’ at www.industrialtechnology.co.uk for further details. You can also find more stories from Abssac and more news on springs and dampers 34 INDUSTRIAL TECHNOLOGY • May 2008
More details: Write in 350 on the free information card More details: Write in 351 on the free information card whatspringstomind? Stock springs? We’ve 15,500 different types and sizes of springs. New for 2008 Constant force springs REDUX wave springs Millions in stock and custom options. Call us on 0118 978 1800 or visit www.leespring.co.uk Lee Spring Limited, Latimer Road, Wokingham, Berkshire, RG41 2WA Telephone: 0118 978 1800 Fax: 0118 977 4832 Email: info@leespring.co.uk More details: Write in 352 on the free information card 35
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