Springs & Pressings
A Designers' Guide
‘Nobody ever thinks about springs at all, until they’re broken!’ It’s true. And, anyway, how do you choose which spring I get many visitors to my home in Bolton,
is right for your application when they
wanting to see my steam workshops. As
vary so much in type and design? There’s
well as my steam engines, there’s also a
wire or flat strip, the tolerance to specify,
good bit of industrial stuff and
and will the heat treatment lead to dimensional variation? What about
engineering tackle that I’ve saved from the scrap man and put to good use. You
surface finish: do you need to paint,
see, engines have always fascinated me
electroplate, shot-peen or what? It’s all
right from being a kid.
rather interesting once you get into it, but you do need advice you can trust.
When I was approached to write the foreword for this book, I started to
Now take this grand little book here. It
consider all the machines that I’ve come
provides guidance on all these points and
across over the years. Take for instance, the Victorian steam-driven machinery
you’ll find it useful to keep and refer
that I’m into. To make the operation run
to as I will. The spring engineers who
smoothly and safely it uses springs on the set of governor balls. Most machines,
produced this book are real experts in their field and are happy to provide any
in fact, rely on springs to make them
further technical information you need.
lots of other information as well. I reckon
function properly even this newSo now you know what to do when
fangled, high-speed stuff such as the jackhammers that I use when
you’re in a bit of a pickle - ‘Just give
demolishing a chimney. In fact, without
them a ring, for advice on spring!’
springs, most machines, would be no use. Then over a pint or two later that night, the thought suddenly dawned on me.
Spring 1. a spring is a device for storing mechanical energy when displaced. 2. a good spring is one which under load can take considerable deflection, and
return to its equilibrium without undergoing any lasting dimensional change.
Springs are everywhere! Almost every m a c h i n e t h a t i s developed incorporates some form of spring, from telephones, and domestic appliances through to engines and medical devices and unless the spring is working correctly, the application will fail. Spring reliability is crucial, and statistics show that correct spring design is the most important factor in ensuring long life. Made from any of a wide variety of materials from plastic through to metal, spring design is a complex business. Involving a spring designer throughout the design process, from initial concept ideas through to prototyping and to production, pays dividends: any design modifications required as the process progresses, can be achieved quickly and efficiently with a spring specialist on board. This saves the design engineer both time
and redesign costs and ensures that the spring design is the most reliable, cost-efficient and long-lasting it can be. The spring designer is a valuable partner to engineering designers in both streamlining new product and machine design as well as reducing risk, and this book will take an engineering designer’s perspective. Springs and pressings manufactured primarily from ferrous and non-ferrous alloys form the focus.
SECTION 1: Wire Components SECTION 2: Flat Strip Components British Standards quoted are under review at the time of publication.
Spring Materials -
The right material for the job
Spring materials are chosen for their strength and are amongst the strongest materials used in industry. Springs are designed to work to far greater working stresses than virtually any another component. For instance, helically wound compression springs are able to be stressed to 70% or greater, than the ultimate tensile strength of the material. Also spring materials have to be able to work in extreme environments such as elevated or low temperatures and corrosive solutions and be able to undergo extreme dynamic loading, and shock loading. Spring materials are also utilised for their electrical and magnetic capabilities. There are many different types of materials available to the spring designer. In this section ‘we will deal with the more commonly used spring wire materials. Strip materials will be discussed later.
For general engineering purposes spring steels are the best choice for the designer, due to their relative low cost and their wide availability. They also are the strongest materials that the designer can choose. For sizes lower than 2.00mm cold drawn carbon steels specified under standards such as BS5216, 5202, DEFlO6 are the highest strength. These materials come in a range of strengths and surface finishes that can be matched to the spring’s end requirements. For instance, for the wire size 1.60mm the range of material grades taken from BS5216 generally used are as follows:
The grades also refer to the material surface finish and therefore dynamic qualities as follows:
Due to the fact that the mechanical strength is obtained through the drawing process, as the size of wire increases, so the ultimate tensile strength of the material decreases. Some of the above grades are available pre-drawn with a zinc or aluminium/zinc coating that will give sufficient corrosion protection for non-arduous applications. Otherwise the above materials, like all carbon or low alloy steels will require some form of corrosive protection.
Other types of spring material are low alloy or carbon pre-hardened and tempered steels. These materials are drawn annealed and are then hardened by the wire manufacturer to produce a high strength material. These are stronger than cold drawn materials above the size of 2.00mm. The mechanical strength for these materials is obtained through the hardening process, so the ultimate tensile strength does not depend on the wire size. In fact, it is possible to obtain a higher ultimate tensile strength with larger section materials than lower section. They have excellent static and dynamic properties, but are prone to corrode readily without surface protection. There are many standards covering pre-hardened and tempered materials, depending on whether it is a carbon steel or one of the many low alloy steels. Alloys such as silicon chrome (BS2083 685A55) or chrome vanadium (BS2083 730A65) are among the most widely used.
Stainless steels are used widely throughout manufacturing where the corrosive or relaxation resistance requirements are too great for normal spring steels, or the working temperatures are too high. There are many grades of stainless steel varying in their
mechanical properties and corrosion protection. Generally stainless steels are about 20% weaker than spring steels of the same size, but there are precipitation hardening grades that are nearly of equivalent strength. Stainless steel grades are covered by BS2056 1991, and the grades generally used are 301526, 302526 both similar having 17%/18% chromium and 7%/8% nickel respectively. These grades are used widely, but for greater corrosion resistance especially salt water, grades 316533 and 316542 are used, having molybdenum added for improved resistance to chlorides. The stainless grades detailed above all get their strength from the cold drawing process. This process makes the materials slightly magnetic. If very low magnetic permeability is required there are two stainless grades that can be used. These are 305511 and 904514, which are virtually free from residual magnetism. If greater strength is required, precipitation-hardening stainless steels can be used. After the springs are manufactured they are heat treated at 480°C. This causes small precipitates to grow through the material, increasing the ultimate tensile strength. For example, in the as drawn condition, l.OOmm wire has a minimum ultimate tensile strength of 1710 N/mm2, while after heat treatment this is increased to 2030N/mm*. This increase is at the cost of a slightly inferior corrosion performance than 302526 and 301526.
Copper-based alloys are used where high electrical and thermal conductivity, nonmagnetic or good atmospheric resistance are required. There are three alloys that find a place in spring manufacture covered by BS2873. They are CZ107, a spring brass wire, PB102, PB103 phosphorus bronze and CB102 beryllium copper. Spring brass wire CZ107 and phosphorus bronze PB102, PB103 get their material strength from the work performed in cold drawing. Phosphorus bronze, with its high tin content, has the higher tensile strength, and due to this it is the most widely used copper alloy. Beryllium copper CB102 is a precipitation hardening material. It can be purchased in a variety of hardnesses, depending the amount of heat treatment carried out at the mill. It is the most expensive copper alloy, but as it can be hardened it can be used to greater working stresses than the other copper alloys.
It is generally recommended that all spring materials are subjected to a stress-relieving operation after forming. In the case of cold drawn spring steel this would be at a temperature between 220°C and 375°C for 10 minutes to 1 hour depending on the type of spring and its application. The object of this is to reduce the stresses introduced during coiling, especially in the case of compression and extension springs, as these stresses are not beneficial. Stress relieving also slightly increases the elastic limit of the material and stabilises the spring’s dimensions. The problem with stress relieving is that as the ‘coiled in’ stresses are removed, the spring will move and this leads to dimensional change. This dimensional change has to be taken into account by the spring maker before coiling. Stress relieving is often not carried out on extension springs as the heat treatment reduces the amount of initial tension.
Compression springs are widely used throughout industry as they are relatively simple to produce and have excellent static and dynamic properties. Given that compression springs are the most widely used helically wound springs, more detail will be given here than in the next chapter on extension or torsion springs. However, many of the design details listed below are equally relevant to extension and torsion springs. Compression springs, like extension springs, are stressed in torsion. In effect, these springs can be likened to a torsion bar, wound helically to reduce the space taken up. There is a limit to which materials can be stressed in torsion and this is related to the materials ultimate tensile strength. In the case of an unprestressed compression spring manufactured from a BS5216 spring steel, the stress limit in torsion is 49% of the ultimate tensile strength.
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Compression springs are generally designed so that the minimum working position is, at most, 85% of the total deflection available. Beyond this position, coils will begin to contact each other, reducing the effective number of active coils, thus increasing the spring rate. In addition, when operating dynamically, fretting (wear) can occur between the faces of the contacting coils (see section on fatigue, page 21).
When defining the free length tolerances are generally not specified when there are two or more working positions, as they are determined by the tolerances on the working loads. This is unless they are required for assembly purposes. There are a number of choices of end coil formation available for compression springs (see diagram 7). A closed and ground spring requires more manufacturing processes than a simple closed or open spring. The ground end of a spring does give much better stability than the other end formations, but when the wire diameter to mean diameter ratio is high, the end formation can be considered as relatively stable. The end formation will affect the springs tendency to buckle, which in many applications is unacceptable. Where the end formation has closed coils, this reduces the number of active coils. Active coils are those that deflect and contribute to the spring’s rate. There is an uncertainty in measuring the number of active coils within a spring, and because of this the British Standard tolerances do not apply for a spring with less than 3.5 total coils. The choice of spring type can also be dictated by a number of other factors: the spring’s anticipated working requirements; its fittings; the wire diameter; the spring diameter and the cost of the end product. If it is important to maintain a force within close limits throughout the life of the springs, the amount of relaxation that will take place should be quantified, especially if the temperature is to be elevated. There are tables available for all materials to assist the spring designer with these calculations. The working environment must be taken into account when designing a spring. Any environmental factors that may affect the spring’s performance or the performance of the product are important. Corrosive environments, elevated temperatures, the ability to conduct electricity and magnetic fields will all affect the choice of material. It is best to consult the spring designer / manufacturer when applying tolerances to a spring component. Standard drawing tolerances can increase the cost of the component.
Nomenclature and Units
Spring Index = 4 The Spring Index gives an
indication of how tightly a spring is wound. A low spring index indicates the spring has a high spring rate. British standards for coiled spring tolerances only apply to springs with indices between 3.5 and 16. Springs outside this range can prove more difficult to manufacture.
The spring rate is the increase in load for a given deflection. If the loads and deflection of a spring are known, spring rate can be easily calculated using the following equation:
For a compression spring of known dimensions the following formula can be used:
It should be noted that in the above formula the wire size is to the fourth power and the mean diameter is cubed, therefore small changes in wire diameter and mean diameter can lead to large changes in spring rate. This is an important consideration in calculating spring tolerances. Therefore the load at any deflection can be calculated from:
The theoretical solid load can be calculated from:
Residual Range It is important to remember that a compression spring should not exceed 85% of the total available deflection. Therefore, the minimum working length and maximum load can be calculated from:
Stress Calculations If the dimensions of the spring are known, the stress can be calculated using the following formula:
The factor K is the curvature correction factor, used to correct for the uneven stress distribution across the section stemming from the curvature of the wire. The formula below is the Sopwith curvature correction factor.
It should be noted that the greatest stress is at the inside face of the spring. This is why when the spring is operating over a shaft, great care must be taken to give the correct clearances. The MATERIAL REFERENCE TABLE on page 6 gives the maximum allowable static stresses as a percentage of the ultimate tensile strength of different materials. Values of ultimate tensile strength can be found in the relevant British Standards. If the spring is operating dynamically, more care needs to be taken with the design (see section on Factors affecting the Fatigue Performance of Helically Wound Springs, page 21).
Conical Springs are used when the application requires a non-linear spring rate and/or where space is limited. The non-linear spring rate is created when the spring is coiled so that when the spring is deflected, coils begin to contact. The larger coils move farther as they have the lowest spring rate and will contact sooner. This reduces the number of active coils thus increasing the spring rate (see diagram 2).
Conical springs can also be coiled so that when the spring is compressed the coils lie inside each other. The spring will then have a solid length of one wire diameter. This is very useful when the space is restricted (see diagram 3).
The design of conical compression springs is much more complex than that of parallelsided springs. The calculations can only give an approximation of the spring’s behaviour as small changes in the pitch of the spring can produce large changes in the load/ deflection characteristics.
Nesting springs means to have one or more springs sitting inside a larger spring. Nested springs enable the spring designer to get more loadbearing material into a fixed space. By so doing, the springs are able to support a greater load than one spring alone could withstand. By reducing the working stresses within each nested spring, the probable working life of the springs is increased. Nested springs also enable the designer to reduce the length of the spring, thus reducing the chance of buckling. When designing nested springs, it must be remembered that the springs adjacent to each other must be coiled in different directions, otherwise tangling is likely to occur in operation. These springs are used widely in demanding applications where high loads and long fatigue life are required, particularly within a small space.
The difference between helical compression and helical extension springs is in the direction of load application and the method by which it is applied. In order to apply the force, special end forms generally have to be used, either utilising the formed end coils, or special screwed-in inserts. Examples of end form inserts are shown below.
The more complex the end formation, the greater the manufacturing tolerances and the greater the likely manufacturing cost. The formulae used to calculate extension springs are very similar to those of compression springs except for an extra property called initial tension.
Initial tension is the force which holds together the coils of an extension spring: as a force is applied to an extension spring, the force must exceed the initial tension before the spring deflects. This will give a load/deflection curve as shown below.
The initial part of the curve shows where the end loops deflect and where the initial tension is not constant throughout the spring as some coils become active before the rest. The amount of initial tension that can be coiled into a spring depends on the relationship between the mean diameter and the wire diameter (spring index), the strength of the material and the manufacturing process used. There are preferable values of initial tension, falling outside this range can lead to greater manufacturing tolerances. Using initial tension enables the designer to produce springs with large initial tension but with a low spring rate. The spring will then give a nearly constant load/ deflection characteristic. Examples of this are electrical switchgear, tensioning devices and counter balances. Sometimes initial tension is not desired. In this case the spring needs to be coiled with a slight pitch. The spring will then give a linear spring rate. An example of this is a governor spring in a diesel engine.
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When designing extension springs is recommended that the maximum working position is at most 85% of the total possible deflection. When defining the free length Lo, tolerances are generally not specified when there are two or more working positions, as they are determined by the tolerances on the working loads. This is unless required for assembly purposes. If it is important to maintain a force with close limits throughout the life of the springs, it is important to quantify the amount of relaxation that will take place, especially if the temperature is elevated. There are tables for all materials that assist the spring designer in undertaking these calculations. Extension springs are sometimes not heat treated as this process reduces the amount of initial tension in the spring. If an extension spring has not been heat treated, the maximum allowable stresses must be reduced. The working environment must be taken into account when designing a spring. Any environmental factors that may affect the spring’s performance or the performance of the product are important. Corrosive environments, elevated temperatures, the ability to conduct electricity and magnetic fields will all affect the choice of material. It is best to consult the spring designer/manufacturer when applying tolerances to a spring component. Standard drawing tolerances can increase the cost of the component.
Initial tension can be calculated by taking measured loads at lengths and using the formula below:
The spring rate of an extension springs is calculated using the same formula as for calculating compression springs:
It should be noted that in the above formula the wire size is to the fourth power and the mean diameter is cubed, therefore small changes in wire diameter and mean diameter can lead to large changes in spring rate, an important factor in calculating spring tolerances. Therefore the load at any deflection can be calculated from:
There are a number of complex formulae to calculate the end loop stress, if you wish further information on this, please contact the author.
The stress for a given load can be calculated using the following equation:
As stated previously, extension springs should not be stressed as highly as compression springs. The MATERIAL REFERENCE TABLE on page 6 gives the maximum allowable static stresses as a percentage of the ultimate tensile strength of different materials. Values of ultimate tensile strength can be found in the relevant British Standards.
If the spring is operating dynamically, more care needs to be taken with the spring’s design (see section on Factors affecting the Fatigue Performance of Helically Wound Springs).
The mode of operation of torsion springs is different from compression springs and extension springs. Compression and extension springs are stressed in torsion, whereas torsion springs are stressed in bending. A torsion spring is, in effect, a woundup cantilever. Torsion springs supply or withstand torque, to’supply this torque torsion springs require some form of spring leg. The type of spring leg is dictated by the application and can be as simple as a tangential straight leg or much more complex. It should be noted that it is best to keep the legs as simple as possible to reduce manufacturing tolerances and manufacturing difficulties. A number of leg forms can be seen below.
Torsion Springs should always be operated in the wind up condition. If a torsion spring is operated in the opposite direction, the spring is unable to withstand as great a deflection.
Generally, the spring requirements are specified to the spring designer as a load applied to the legs. It is necessary to convert this into a torque. The torque can be calculated by: Torque T = Applied load X Distance to Spring Axis
The distance is from the applied load to the spring axis at right angles to the applied load.
It should be remembered that when torque is applied to a torsion spring, the following happens: l
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The number of coils increases hence the body length increases. The body length increases by one wire size for every 360º of deflection. This can mean that, if not enough space has been allowed within the application, the spring will bind, and this will probably lead to spring failure. The mean diameter of the spring decreases. This is important to remember as most torsion springs work over a shaft. If not enough clearance is allowed between the shaft and the spring, the spring will bind onto the shaft. The legs will then take all of the torque and will consequently take a permanent set. It is best to consult the spring designer/manufacturer when applying tolerances to a spring component. Standard drawing tolerances can increase the cost of the component.
If there are known torque deflection requirements, the spring rate can be calculated using the following:
If the springs dimensions are known, the following formula can be used:
The above formula takes into account the deflection due to the applied torque. The torque/deflection curve for a torsion spring is generally not a straight line. It is more like the diagram shown below.
The torque when unloading is less than the torque when winding up for the same position. This is due to friction within the spring and the mechanism. One way to reduce this is to coil the torsion spring with a slight pitch, but due to how torsion springs are operated it is impossible to remove all friction.
The deflection of a torsion spring under an applied torque can be calculated as follows
The body length of a close coiled torsion spring unloaded is expressed as:
In the spring’s working position the body length is:
The bending stress for torsion springs can be calculated using the following:
Where K, the stress correction factor, =
C
c - 0.75
With torsion springs, the applied stress is in bending. Because of this torsion springs can be operated to higher stress levels than compression and extension springs. Unprestressed torsion springs can be stressed up to 70% of the ultimate tensile strength of the material, and prestressed torsion springs can be stressed up to 100% of the ultimate tensile strength. The spring designer should reduce these figures by 15% to ensure that the spring is never overstressed either in operation or during installation. If the spring is operating dynamically, more care needs to be taken with its design (see section on Factors affecting the Fatigue Performance of Helically Wound Springs below).
Reliability is of great importance to many spring users. Without the spring many applications will either cease to work or work less efficiently. Therefore much study has been made of the behaviour of springs under fluctuating loads. If a spring is operated less than 10,000 cycles during its operating life then it is deemed to be working statically and fatigue does not play a part. Over this, fatigue will affect the performance of the spring and should be taken into account during the design process.
The factors that affect the fatigue performance of springs in the main are: l l l
Working Stress Material Surface Quality Wear
In operation springs generally work between two fixed positions. The working stress at these positions can simply be calculated, the results can then be used to predict the working life of the spring. To do this, Goodman diagrams need to be used, these are based on data that has been obtained through many years of experimentation at centres such as The Institute of Spring Technology. Goodman diagrams are available for the many different grades and types of material used. An example of a Goodman diagram is shown below. To calculate the expected life of -the spring the working stresses are plotted against the relevant axis. If the intersection of the plotted stresses falls within the shaded area, the spring can be expected to work for the number of cycles the graph represents. Generally the graphs represent 95% surety, i.e. 95% of the springs can be expected to achieve the number of cycles. The majority of Goodman diagrams only apply to compression springs.
Extension springs suffer a number of problems when operating in a dynamic environment, they are: l
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Breakage near the loop The most common cause of failure in extension springs is when the loop of the spring breaks off in the area where the hook meets the body of the spring. This point of transition between the spring body and the loop is generally the point of highest stress. The loops are subjected to a bending stress and torsion stress and the majority of Goodman diagrams for spring materials are for materials stressed in torsion. Tooling marks creating stress When loops are formed in extension springs small tooling marks are unavoidably created. Such marks are stress raisers which increase the likelihood of a failure at this point. Loop bends too small Another reason is that sometimes loops are formed using bends that are too small. A small radius is a stress raiser.
Different types of end loop will lead to different fatigue performances. There are two available solutions: l
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Use a loop with a transition radius between the spring body and the end loop of approximately the body radius. Use coned ends with swivel loops. This is very successful in reducing fatigue, providing the swivel loops only contact the coned section (see diagram 10).
If an extension spring is required to work dynamically, it must be remembered that extension springs have approximately 20% lower performance with regard to fatigue than compression springs. 4 The lower the working stresses the greater the expected life of the spring.
Material surface quality is important when seeking to avoid risk of spring failure. Fatigue cracks generally propagate from the surface of the material, therefore the greater the surface quality, the better the fatigue performance. It is possible to improve the surface quality by a number of methods. Most popular is Shot-peening. Shot-peening involves firing small rounded beads of material at the surface of the spring. This will lead to a small residual compressive stress on the material surface which lowers the chance of a fatigue crack propagating and increases the working stresses possible. Shot-peening is generally carried out only on compression springs and large leaf springs as the shot would get trapped in the coils of close wound torsion and extension springs. Also, the inside face of the coils would not be peened and this would eliminate the benefits of the process.
* The better the surface quality of the material, the better the fatigue performance.
Wear can be caused in a number of ways. When a spring is operating dynamically it is important that the maximum deflection should not exceed 85% of the available deflection. The reason for this is that when a spring is working close to its solid (coil bound) length, the number of active coils will reduce due to coils coming into contact with each other. When this happens there is a chance that the contacting faces of the coils will wear. This can lead to a reduction in material cross section, increasing the stress at this position. Another cause of wear is when a spring works over a shaft or in a bore. If a spring is allowed to contact either the shaft or the wall of the bore when operating, the wear can lead to premature failure especially if the inside diameter of the spring is worn as this is where the working stresses are the greatest. Wear, therefore, should be avoided at all costs. Torsion springs have a lower fatigue performance than compression and extension springs.
This is mainly due to the friction and wear between the spring and the shaft that it is working over and the leg fixings. This can be reduced by good design, but is unlikely to be eliminated. Other factors that affect the fatigue performance include corrosion, material cleanliness and speed of operation. Any questions regarding these or any of the above should be directed to the author. * Removing the possibility of wear in a spring application will improve the spring’s fatigue performance.
When a spring is prestressed there are dimensional changes. This means that the springmaker must allow for this during manufacture. The prestressing operation for compression springs is relatively simple. Once the spring has been coiled, stress relieved and ground, the spring is placed on a press or similar and compressed to a solid or fixed position which is greater than its maximum working position. This is then repeated a number of times, generally no less than three. The spring will then be shorter than the coiled spring but with the correct initial set up, it will be possible to achieve the required final length. Prestressing can also be carried out for tension and torsion springs. Unfortunately, when an extension spring is prestressed the amount of initial tension is reduced and is therefore not often carried out. Torsion springs require special jigs to successfully prestress them. When prestressing is carried out the leg relationship changes. ie the number of coils slightly increases. As prestressing is an additional operation in the manufacture of a spring, this will increase its unit cost. The benefits, however, generally outweigh the additional cost.
As with wire, there is a wide range of strip materials available to the spring manufacturer. As many parts produced in strip are not primarily used as a spring, many low strength alloys are used, generally for their formability and electrical conductivity. Strip materials can be obtained in different grades of hardness, and some spring materials are able to be heat treated to increase their strength and hardness. Due to the vast range of materials available this section will deal with carbon steels, stainless steels and copper alloys only. If you require more information on materials such as nickel alloys, please contact the author.
There are a number of grades of carbon steel strip. These grades are classified according to the carbon content, the method of manufacture and whether a heat treatment is used. Annealed carbon steel strip is used where formability is required, a heat treatment after forming will increase the materials strength and hardness. Where formability is not an issue there are heat treated grades of spring steel and texture rolled materials. These materials are obtained in the hard condition and are used in applications such as clock springs and seat belt retaining springs. British Standards for annealed spring steels include BS5770 Pt 2 CS50, CS70 and CS95 annealed. In the hard condition which is covered in the British Standards by BS5770 pt 3. there is available CS70HT, CS80HT, CS95HT. The number in the grade designates the percentage of carbon in the alloy e.g. CS70 has 0.7% carbon. Texture rolled materials can be obtained under a proprietary name Leetex 80. The material has good surface finish, uniformity of mechanical properties and precision thickness tolerances. It is a high strength material and is used widely in seat belt retainers. If operating in a corrosive environment, carbon steel springs require some form of protection as these materials will corrode readily.
These materials are widely used for their corrosion resistance, their ability to withstand elevated temperatures and their resistance to relaxation. Stainless steels are generally obtained in the hard rolled condition, strip components designed to be manufactured from stainless steels should take the effect of spring hardness into account. Stainless steels are about 20% weaker than heat treated springs steels of the same size. As the hardness of stainless steel is generated during the cold rolling process, the work hardening will cause the stainless steel to be slightly magnetic. British Standards covering stainless steel strip materials include BS5770 Pt 4 302525, 301521, 316516. All these grades can be obtained in varying levels of spring hardness, depending on the thickness of the material.
Copper-based alloys are used where high electrical and thermal conductivity and or where being non-magnetic is a priority. Copper-alloys also exhibit good atmospheric corrosion resistance, but as the majority of copper-alloy strip components are used as electrical contacts many copper parts are electro-plated. There are three alloys that find a place in spring manufacture covered by EN 1654. They are CuZn36, a spring brass strip, CuSn5, CuSn6 phosphorus bronze and CuBe2 beryllium copper. Spring brass strip CuZn36 and phosphorus bronze CuSn5, CuSn6 get their material strength from the work performed in cold working. Phosphorus bronze, with its high tin content, has the higher tensile strength, and due to this it is the most widely used copper alloy. Beryllium copper CuBe2 is a precipitation hardening material. It can be purchased in a variety of hardnesses depending the amount of heat treatment carried out at the mill. It is the most expensive copper alloy, but as it can be precipitation hardened it can be used to greater working stresses than the other copper alloys.
The fatigue performance of strip materials is greatly affected by the edge and surface condition. It is possible to purchase some strip materials with a dressed or rounded edge which greatly improves the fatigue performance, but if the components are punched out of the material, the edge finish will depend of the performance of the tooling.
Flat strip parts can be very complicated in their form. Inside many products such as mobile phones, computers and medical equipment there are a wide variety of shapes all formed from a simple coil or sheet of flat material. Many flat strip parts are designed to perform more than one mechanical function thereby reducing the number of components. The number of different variations of strip parts is virtually infinite. The only obstacle to strip design is the imagination of the designer, and the practical limitations of manufacture. The simplest strip spring is probably a leaf spring operating as a cantilever, with simpleto-calculate loads and deflections. Many strip parts are, in effect, made up of a number of sections operating as cantilevers. Strip springs are not limited to just simple cantilevers. There are spring washers such as disc springs which are able to provide a high spring rates over a small movement, and constant force springs, used in seat belt retention, devices that are able to provide an almost constant force over a large deflection. Due to the wide variety of strip parts it is difficult to discuss them in any great detail, also the complexities of many of the equations fall outside the bounds of this Guide. When designing a strip component it is good practice to ask the advice of a spring designer. The simplest part to produce is the most economical to produce in small quantities. But even complex parts, when produced on production tooling, can also be produced at low cost.
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Material hardness is very important when designing a flat strip component. The hardness of the material affects the minimum bend radius. Below is a table of ‘minimum bend radius’ for a number of materials.
t = thickness The direction of rolling is along the strip. The bend radius refers to the inside bend radius.
Going below the above figures would prove to be difficult, and may lead to cracking of the material on the outside bend radius of the material. As can be seen, the orientation of the bend on the strip affects the minimum bend radius. If a component requires bends perpendicular to each other with radii close the minimum bend radius, it is good design practice to orientate the component by 45º relative to the rolling direction. Avoid punched holes or slots too close to the edge of the component or another hole. This can cause the hole to deform the edge or the other hole. Avoid punched holes or slots on a bend or too close to a bend. This may cause the hole to stretch and affect the smoothness of the bend. When forming a bend in a spring material it is important to remember ‘Spring Back’. Depending on the hardness of the material, all spring materials will exhibit some form of ‘Spring Back’. For instance, when forming a bend of 90º, the material will return to an angle greater that 90º. The spring back will also affect the radius of the bend. This must be taken into account when designing the tooling and consequently when designing the part. It is best to consult the spring designer/manufacturer when applying tolerances to a spring component. Standard drawing tolerances can increase the cost of the component.
Due to the complexity of strip parts, the calculations of force and stress are much more complex than those for helical compression, extension and torsion springs. Depending on the shape and loading of the component a number of standard equations exists (see diagram 71).
If the part does not correspond to any of these parts, more complex solutions must be sought. A more accurate calculation tool is Custigliano’s 2nd theorem. The component is broken down into a number of sections comprising of beams and curved sections. The sections can be of varying width ie. tapered. Equations are set up for each section to calculate the deflection using the bending moment. The deflections for each section can then be summed up to produce the deflection of the whole component. The maximum working stress can be calculated at the furthest distance from the applied load, using the bending moment at this point. Due to the complexities of the mathematics involved the calculations are best carried out on software such as MathCad. The most accurate form of calculation for strip components is by using Finite Element Analysis. This is generally only used where the component is complex and has a number of loads applied. Finite Element Analysis is carried out using specialist software packages. Due to the nature of flat strip design, it is best to validate the spring design by manufacturing a number of samples. These can then be tested to verify the performance.
There are a number of ways flat strip components can be produced, generally depending on the volume required. When a small number of components are required e.g. prototype samples, it is possible to produce most parts without tooling. Wire-eroding or chemical milling can produce development blanks where required, and standard tools can be used to form the parts to the required dimensions. This process is time consuming but allows the customers to have parts without investing in production tooling. If the volume is larger, components can be blanked out on tooling, and formed in subsequent operations on separate equipment. The tooling cost is relatively small and increases the production speed considerably over the previous process. For medium to high volume production, the flat strip component is manufactured complete on one piece of equipment. The two main ways on achieving this is by using progression tooling or multislide form tools. When producing parts on progression and multi-slide tooling, the developed components are not completely blanked out. A small section of material is left to carry the part forwards to the subsequent forming stages. In progression tools the material is indexed forward to each forming stage. As the part progresses through the tool the component undergoes a sequence of forming operations, until the part is fully formed. The last stage cuts out the section of material that has carried the component forward. The tool has to be designed carefully to form the component in the correct sequence (see diagram 72).
In multi-slide tools there is an initial blanking stage, but then the material is indexed forward to where a number of forming slides operate. These slides are able to move forward and backwards along their axis, controlled by either cams or servomotors. On the ends of the slides there are forming tools designed specifically for the component. During the forming operation the slides move inwards in a predetermined pattern, bending the material as desired, and parting the component from the strip. The number of slides employed in this procedure is determined by the complexity of the finished component (see diagram 13). These tools are complex to design but are able to produce finished parts at very high speed allowing very low unit prices. Using new CAD technology it is possible to design tooling for strip components precisely and very quickly, allowing us to design the tooling as efficiently as possible.
Where the spring operates in a corrosive environment some form of surface protection is required. Depending on the application, this can take a number of forms. Obviously, the spring can be designed in a material that will not readily corrode in the application’s working environment, but this may not be possible where costs need to be kept down or where the material needs to be of a required strength. Nickel alloys in. particular, are excellent for corrosion resistance but the cost of the materials can be prohibitive.
The simplest method is to simply oil or grease the springs. This should give sufficient corrosion protection for springs in transit, or in storage providing the conditions are not too testing. Another method of protecting the springs from corrosion is by either plastic coating or painting. The problem with this method is that the protection is only effective until it is damaged. The spring material will then be liable to corrosion underneath the finish. A metallic finish is more generally used. The easiest method is as stated earlier, to manufacture the spring from carbon steel, wire drawn with a galvanised coating. This may be sufficient in some circumstances, if not, a better protection is required. A popular method of obtaining a metallic finish is to electroplate the springs. It is important to use the correct electroplated metal as this is the key to good corrosion resistance. Zinc plate and cadmium (rarely used due to its toxicity) corrode in preference to steel and so will protect even when the surface coating is damaged. Nickel, copper and chromium plate, when damaged, will lead to the steel corroding in preference to. the surface coating and so is not recommended. Nickel plate is only generally used when the component will undergo soldering, and so is used widely in the electronics industry. It is important to note that with electroplating there is a risk of hydrogen embrittlement. This will lead to component failure when it is loaded. To minimise the risk a deembrittlement process is carried out. The de-embrittlement process is where the components are held at an elevated temperature of 190-200°C for up to 24 hours to drive out the hydrogen. Low alloy spring steels such as BS2083 685A55 should not be electroplated under any circumstances due to the high risk of hydrogen embrittlement. A mechanical zinc or zinc alloy plate will give zero risk of hyd rogen embrittlement, and an equally effective corrosion resistance. Other methods include coating the spring with a resin impregnated with zinc flakes. These are proprietary processes and can either be obtained under the name Deltatone or Dacromet, and can generally be obtained in either a black or silver finish. These processes give a superior protection to mechanical or electroplating, and avoid the risk of hydrogen embrittlement.
“Springs and pressings technology is a complicated business and I hope that this booklet serves as a lasting reference tool. There are, of course, many areas that space limitations have prevented us from exploring, but we are always available to discuss any technical challenges you face.” Matt Drew B.Eng(Hons) Technical Director
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