Lee Spring carries over 23,000 catalogued items, but it is just as common to find that a variant or complete custom item is required to suit a new project or a revised assembly process where a special spring design may contribute added quality to the operation of a product or to its longevity, often with significant savings in component cost and/or assembly time.
The Lee Spring in-house technical capability enables it to develop project specifications to international standards, such as RoHS, REACH and DFARS. This means the company can guide customers through the specification prototyping, development and trials process to final sign-off and mainline production.
From long experience, it has been found that, while spring technology is a well-established and well understood engineering discipline, its design is both complex and variable. This means it is difficult for design engineers to be aware of everything - and it is therefore important to consult the spring manufacturer at an early stage, in order to decide on the function of the spring and its detail design before working on the rest of the application; otherwise, there is a risk that retro-fitting to suit space already designed may not be possible or prove much more expensive.
Of course, the first step is often to explore a typical custom variant from the standard spring range and this will include varying finishes, alternative materials for enhanced performance, polishing or coating, non-standard loadings, non-standard sizes, matching to suit exceptional environments, demands for extreme longevity or specific concerns regarding materials matching etc.
Equally, a common request is often to meet tighter tolerances which may be required for specialist housing designs. The broader custom process frequently encompasses the meeting of special standards, non-standard finishes – for aesthetic or functional reasons, such as colour matching or colour coding, also special shapes, special end forming, retaining clips – wire forms and stampings or other formed metal parts (fourslide parts), trim clips etc.
A vital part of custom design of course is the selection of secondary operations, such as assembly, colour coding, electro-polishing, grinding, heat treating, looping, passivation, powder coating, shot peening, spring setting, plating – eg, with nickel or zinc, and use of other special finishes, such as black oxide.
Although the development of springs could be described as evolution, rather than revolution, in recent years manufacturing processes and changes in end use criteria have impacted on the specification of these ubiquitous products. For example, medical, aerospace, food and toy applications demand special load characteristics and treatments, such as ultrasonic cleaning, while RoHS, WEEE and REACH have influenced the types of finishes offered.
The most popular types are compression, extension, torsion, wave and disc springs. Added to these are conical, swivel hook, battery and drawbar springs. More recent additions include continuous-length extension springs, light-pressure springs, as featured in Lee Spring's latest stock spring catalogue and the Mini Compression BANTAM type.
Probably the most important consideration for a custom spring design is the choice of material that can greatly influence the size, suitability, performance and longevity of the finished item. Most standard stock springs are manufactured in music wire, stainless steel, oil-tempered MB, chrome silicon steel or Elgiloy, a cobalt/chrome nickel alloy, and these are of course options for custom variants and for full one-off designs.
Key factors affecting material choice for a particular application include: meeting stress conditions, either static or dynamic; capability of functioning at a required operating temperature; compatibility with surroundings – eg corrosive environment; and special requirements such as conductivity, constant modulus, weight restrictions, magnetic limitations, etc.
Music wire springs are normally supplied with a zinc plating baked for hydrogen embrittlement relief, while die springs are painted different colours to denote duty.
Battery springs are produced in music wire and nickel coated, as most alkaline batteries use nickel-plated containers. Here, the use of similar materials removes the possibility of galvanic corrosion and enhances resistance to wear. Additionally, nickel helps to break down the oxide that forms on the surfaces of batteries.
All of Lee Spring's 316 stainless steel springs are passivated and ultrasonically cleaned to offer medical – and food – grade levels of cleanliness. Other special finishes may be specified.
Spring performance is affected by temperature, which should not exceed 120°C for music wire, 260°C for stainless steel and 245°C for chrome silicon steel.
However, the specification of full custom springs or components can involve production in an extremely wide range of materials, such as: beryllium copper, brass, hard drawn carbon steel, oil tempered carbon steel, oil tempered chrome silicon, oil tempered chrome vanadium, Elgiloy, Hastelloy, Inconel, Monel, music wire, phosphor bronze, plastic composites, 300 series stainless steel and 17-7 stainless steel.
Over recent years, some innovative spring materials have been brought into use for mainstream catalogue products and are available for custom designs. Plastic composite springs, chosen for their strength, non-toxicity and anti-corrosion properties, are used in medical applications, and have even been considered for food and drink machinery, and for aviation seats. Elgiloy, a lightweight malleable material, is often found in applications such as smart phone production or medical instruments.
So what's next? ‘Saving the planet’ is a recurring theme for everyone, so recyclable or ecologically-friendly materials being put into new products is good news. For many industries, especially automotive or aerospace, robustness, lightness, and resistance to corrosion and flammability are also critically important.