Technical information of springs
Since the movement is in most cases connected to the primary function of the mechanical system, the spring is a vital part of the whole function. “It won’t do a thing, if it ain’t got that spring”, if we paraphrase one of Duke Ellingtons hits.
A spring can have many geometrical shapes. A spring is a component that deforms elastically under a mechanical load and where the relation between elastic deformation and load is important.
The relation between load and deflection is called the spring characteristic or the spring rate. When mechanical systems are developed and a need for a spring function arises, we define the requirement on the relation between load and deformation.
The type of spring and its geometry is chosen based on the geometrical space that is available for the spring function, together with considering reliability and cost. There are some spring types that have proven to be cost and space effective ways of realizing certain spring functions. Helical coil springs, disc springs and wave springs are normally used for axial compression loads and each of them has their own range of force-deflection relation where they give the most space effective solution.
It is similar for rotating deflection, where helical torsion springs, clock springs, torsion bars or power springs each has their own optimum range of torque-deflection relation. But a component that has a built-in spring function can have practically any shape. Wire and sheet can be formed to complicated shapes and many such components have more than one function, of which only one is a spring function.
(Helical) Compression springs
There are a few different types of springs where force increases when axial length decreases, but helical compression springs are the most common spring type of all and they are therefore often referred to as just “compression springs”. Also within the group helical compression springs, there is a variety of geometrical shapes. Common for all is that a wire is formed to a helical shape where coiling radius and pitch can vary with the coil position. The load is introduced into the spring at the end coils, which normally are closed, which means that the pitch angle is reduced as much as possible at the two ends. The ends of the spring are in most cases grinded, which centers the load in the spring and gives a symmetric deflection of the coils.
Most common is the linear, cylindrical compression springs that has a constant coil diameter. The pitch is also, except for the closed end coils, constant. A cylindrical, progressive compression spring is designed with different pitch in different parts of the spring. Conical compression springs are often designed to enable compression to a height equal to the wire diameter. The load-deflection characteristic is normally progressive, but the pitch can be varied to give a close to linear load-deflection relationship. The coil diameter can also be such as giving a barrel or hour-glass shape, which gives properties that can be desired in certain applications.
Spring nests is a way of minimizing the required space for the spring function. Two or three compression springs are designed to fit inside each other, meaning that the otherwise empty space inside the spring inner diameter is also used. The coil direction is alternated right-hand and left-hand between the springs.
Circular wire cross-section dominate, but it can also be square, rectangular, elliptic… The axial deflection of a compression spring gives torsional stresses in the cross section of the wire. All types of helical compression springs can be theoretically treated with the use of semi-analytical method as long as the load is axial. Detailed analysis of other deflection modes requires non-linear FE-analysis.
Extension springs (also called tension springs) are helical coil springs where force increases with increasing length. From a stress analysis point of view, they are very similar to helical compression springs. The opposite loading direction does however necessitate some distinguishing features.
The ends of an extension spring need to transmit an extension force. The most common solution of the ends is to bend the wire at the spring ends to a shape suitable shape. This can be an open hook or a closed loop, it can be placed to center the force along the spring axis or have it out-of-center, the height and diameter of the loop can be varied within the limits of manufacturing.
This kind of integrated end in extension spring often becomes the weakest link in the chain if the spring is used in a dynamic application with many load cycles.
This is due to a stress concentration and an unfavorable residual stress distribution in the bend needed for the hook. For highly dynamic applications, this need to be considered in the design, but instead of designing the complete spring with the low stress levels needed for the hook to survive, solutions with separate end fittings is often used.
The coils are normally wound tight and with an initial tension force. The spring starts to deflect first when the external force is larger than the initial tension. The initial tension reduces installation length for the spring, compared to a spring without initial tension. Initial tension is not possible in extension springs that are hardened and tempered after coiling. There are also spring requirements that favors an extension spring without initial tension and gap between the coils.
Helical torsion springs are used for rotational movements and produce a torque when deflected. In many applications, this torque is rather used as a force which then equals the torque divided with the lever arm, which is the perpendicular distance between the acting line of the force to the center of the spring. Some springs that have the shape of a helical torsion might be theoretically better treated as wire forms.
The load is introduced into the spring body through legs, which can have a large variety of shapes. The basic types of the legs are however tangential, radial or axial. Tangential legs are the simplest, where the legs simply follow the tangent at the point where the coiled spring body ends. Radial legs are either radial inwards or radial outwards. Any direction between tangential and radial legs is of course also possible, as are legs bent more than 90°. Multiple bends on the legs are also possible and common.
As for all torsion springs, the load is ideally introduced into the spring as a torque rather than by a point force. This means that the deflection and stresses are distributed more evenly in the spring material if the legs are fixed. A force couple on the legs gives better function and lifetime of the spring compared to a single force.
Cold coiled torsion springs have residual stress from the coiling and this gives them a higher elastic limit if they operate in the up-winding direction compared to the un-winding direction. They are often assembled on a mandrel (although not necessary if the legs are properly fixed) and it must be secured that the diameter decrease during loading in up-winding direction is not prevented by the mandrel. If that happens, the legs will be the only part of the spring that can deflect and the over-loading that results will lead to spring failure. The same will happen if the axial space is not enough for the increase in length that is a result of load in up-winding direction.
The material in a helical torsion spring is stressed in bending. Analytical methods implemented into computer programs are normally used to predict torque-deflection characteristic and stresses. These methods are based on the assumption that loads are introduced as torque or couple and that the bending stress is symmetrically distributed in the spring. The legs contribute with a large part of the deflection for springs with few coils and/or long legs and this must then be accounted for in the calculations.
Wire parts, or wire forms, can have virtually any type of geometry as long as it is possible to manufacture. They often have an integrated spring function, but other functions – such as locking other components during assembly – are integrated into the same wire part. The spring function can be relatively simple and not seldom connected to the assembly and disassembly of other components.
The stress mode in the material from its spring function is normally bending. Since the geometry and boundary conditions are often complicated, the load-deflection characteristic often requires a FE-analysis to be determined. In most cases though, the design of wire parts is a result of discussions between the customer and our technicians, followed by samples in different wire sizes to determine which size gives the right load-deflection-characteristics, alternatively the desired “feeling”.
A torsion bar is in its simplest form the geometrically simplest of all spring types. The active part of the torsion bar spring is then a straight wire and the ends can be bent 90 to enable transmitting a torque to the torsion bar. The spring rate depends only on the length and the material grade and cross section of the wire.
This type of torsion springs is well suited if the radial dimension needs to be minimized, but there is space available in the axial direction.
Disc springs – also called Belleville springs – belong to the type of compression springs. They have the shape of an axially symmetric, holed coned disc whose cone angle is reduced when loaded with an axial force. The stresses in the material will be normal stresses in the circumferential direction of the disc; compressive stresses on the convex side and tensile stresses on the concave side.
Disc springs is often a better alternative than helical compression springs in applications where forces are high and deflections relatively small. If the radial space available is small, this also speaks for disc springs. Disc springs can be used as single disc, but it is more common that they are stacked. Stacking can be either in series, which increases deflection, or in parallel to increase force. Combined parallel and serial stacking is also possible.
Disc springs and stacks of disc springs have a slightly degressive force-deflection characteristic, which means that spring rate decreases with deflection. How pronounced this effect is depends mainly on the ratio between cone height and thickness.
Disc spring dimensions are standardized in EN 16983, and it is often possible to find a solution with the desired spring characteristic by stacking standardized discs. Customer specific dimensions are also possible. In EN 16983, discs are divided into three dimensional series where Serie A has a low ratio between cone height and thickness and have therefore an almost linear force-deflection characteristic. Serie C are clearly degressive discs and Serie B between those two. Serie A are also stiffer discs (high force and low deflection), Serie C are the opposite.
EN 16983 discs are also divided into three groups depending on the material thickness and corresponding requirements on the manufacturing process. Group 1 are discs with thickness less than 1.25 mm and group 2 from 1.25 to 6 mm thickness. Group 3 are discs with thickness above 6 mm. Group 1 and 2 are quite similar from a user perspective. Group 3 discs have flat contact surfaces at the point where force is transmitted. This is to increase contact area and decrease the contact pressure between discs and between discs and the components that transmit the force to the disc spring stack. This flattened contact area has a width of only about 1/150 of the outer diameter. Nevertheless, it gives an increased spring rate, which is compensated for by manufacturing the discs from a material with reduced thickness.
Wave springs belong to the group compression springs, where force increases with decreasing length. Wave springs are made from flat rolled wire, which means a close-to rectangular cross section but with naturally rounded edges. The material is coiled to a helical shape with a specified diameter and number of coils, but in addition the wire is given a wave shape along its length. The waves are close-to sinusoidal in their shape and the number of waves per coil is typically 3.5 or 4.5, but can have other values depending on the coil diameter. The decimal part of number of waves per coil is always 0.5, because a wave tip shall meet and be in contact with the wave valley of the next coil.
A wave spring is the best choice of compression spring type in application where the space available for the spring function has a torus shape and is very narrow in radial direction, and relatively high forces are required for small installation lengths. Wave springs can be manufactured with waves also in the end coils, which gives no dead coils and a low installation length. A flat end coil (shimmed end) decreases contact pressure but adds a dead coil per end and hence increases installation length.
Wave washers can be either closed or open. Closed wave washer are manufactured by punching from sheet or strip material.
Open wave washers are normally coiled from flat rolled wire in the same fashion as wave springs.
The stresses in the material in wave springs and wave washers are bending stresses. Each wave works like a beam, supported at the contact points. The number of waves per coil therefore has a strong influence on the spring rate, which increases with the fourth power of the number of coils per wave.
Volute springs are used in compression and made from material with rectangular cross section. The material is coiled to a conical shape with the coils overlapping each other. During compression of the volute spring, considerable friction can occur between the coils and volute springs are often used when energy needs to be absorbed.
Large volute springs were previously used in railway buffers, but have been replaced by ring springs in many similar applications.
Double volute springs are found in garden secateurs and are coiled from a V-shaped blank to form a spring that is symmetric around its axial center.
The material is stressed primarily in torsion, but the stress distribution and theoretical treatment of volute springs is rather complicated.
Clock springs are also torsion springs and are the direct opposite to torsion bars if we consider the shape of the available space for the spring function, as we increase deflection in a clock spring by increasing number of coils, meaning that radial space needs to be available. They are made from material with rectangular cross section, which is either flat rolled wire or cold or hot rolled strip.
The load is introduced into the spring via legs, typically a 90 inwards bend on the inner radius that is fitted into a slit in a shaft. For the outer leg, more variations in leg shape are possible.
As for helical torsion springs, a load introduction through a force couple acting on the legs gives a much better behavior and dynamic lifetime for the spring compared to load introduction as a point force only.
Clock springs are supposed to operate without any contact between the coils and hence without any internal friction. This is only possible if the load is introduced into the spring in a proper way, i.e. via force couples rather than point forces. When the deflection and the number of coils increases, it is difficult to avoid contact between the coils and we gradually move towards a design that have more similarities with power springs.
Power springs is the name used for flat spiral springs with a large number of working turns. The typical application is for winding up electrical cables or safety belts. They have similarities with clock springs, but the strip length and the number of coils is much larger and internal friction is natural in power springs since the coils are in radial contact with each other.
The load introduction in power springs is similar to clock springs, with a radial inwards leg at the innermost coil that fits in a shaft as the most common design. The outer leg can be designed with larger freedom, but a fixed outer end and load introduction through a force couple gives a symmetrical load distribution in the coils and the best behavior and dynamic life of the spring.
Power springs can be either conventionally coiled or prestressed. Prestressing increases torque output and enables design with up to 50 working turns, whereas conventionally coiled springs have a limit at around 20 working turns. Dynamic life is however less for prestressed springs.
We often assemble power springs into their final casing during manufacturing, alternatively deliver them with a temporary casing that is released during assembly into the final casing. A power spring shall always be preloaded a couple of turns and the number of working turns is counted from this preload position. The amount of preloading depends on the design. Starting at the preload position, the torque-turns characteristic is close to linear up to a loading point that leaves a couple of turns before all of the strip is tight wounded around the inner diameter and the spring is at its solid position.
Power springs can also be used as motor springs, where the outer end of the strip is wound up around a second shaft. If the strip is wound up in the opposite direction from its free shape, it is called a B-motor. Less common is A-motor springs, where the strip is wind up in the same direction as its natural shape.
Constant force springs
Constant force springs are extension springs, in the meaning that force increases with length. The increase in force is however very small, therefore the name constant force springs. They consist of a tightly wound spool of strip material, which have been given a bending radius that is constant throughout the length of the strip. The spring is assembled so that the spool can rotate freely – either on a shaft or in a slot – and the outer end of the spool is pulled out.
The force required to pull the outer end out, results from a torque balance with the bending moment required to straighten the strip from its natural bending radius. There is a maximum limitation for the maximum force of a constant force springs, but they can be arranged in series and in parallel to increase both force and reach. Assemblies can also be designed to use the spring as a compression spring.
Circlips are rings used for locking components in axial direction. They have an extension in circumferential direction of between 270° and 360° and are used for either inside assembly in a hole or outside assembly on a shaft. The spring requirement that they need to fulfil is often limited to be radially expanded or contracted to the maximum or minimum diameter required during assembly. This expansion or contraction need to be within the elastic working range of the circlip.
Burrs on the ends of a circlip can be detrimental for the surrounding components in assemblies secured by circlips. Burrs can be minimized or completely removed by either special cutting techniques or deburring. Stress relieving after coiling is important for circlips that are expanded during assembly. Any plastic deformation during assembly will lead to a loss of locking force compared to the expected.