By Gary L. Boehm
Wire springs rank among those technological marvels whose value is immediately recognizable. We find compression, extension, or torsion springs just about everywhere, from screen doors to keyboards. Advances in materials and manufacturing technology have improved springs in the centuries since they were introduced, early in the Industrial Revolution, but the basic idea is the same: spring wire is coiled hot or cold to create an elastic device.
Not all springs are coiled wire, though. There is an alternative in machined springs. They cost more than wire wound springs, but where the application calls for it, machined springs can put unique properties to work.
Although any machinable material including plastics can be used, metal in the form of bar stock is most common the starting point for machined springs. The bar stock is first machined into a thick wall tube form, and then a helical slot is cut revealing multiple coils. When deflected, these coils provide the desired elasticity.
The cost to manufacture machined springs greatly exceeds that of winding wire springs. Wire wound springs can be created with just a few seconds of process time, where a machined spring requires minutes at a minimum. The machines used to create both forms are highly specialized and benefit from computer numerical controls.
Examples of wire springs and machined springs.
The cross-section of coils found on wire wound springs is typically round. Sometimes the cross-section is rectangular or rectangular with rounded OD and ID surfaces. The rectangular forms, which are less common because they cost more, provide increased stiffness and compactness of design. The orientation is usually with the long side of the rectangle radial, although it is also possible to orient the long side longitudinally.
Rectangular wire comes in set sizes. To venture away from those sizes will usually increase cost and lead time.
The coils of machined springs can be made in forms that cost more in wire springs: square, rectangular, or trapezoidal. Trapezoidal coils are common to springs used in lateral bending and lateral translation. The shape allows for additional lateral motion without coil contact.
Orientation of the rectangular coils can be radial or longitudinal. There are no standard sizes to the coils.
On wire wound springs the slots—that is, the space between the coils—is typically uniform for torsional springs. Coils of compression springs are uniform too, but the end slots usually taper to zero. This feature is created by an additional forming process known as “closing” the ends.
Extension springs can have a uniform slot width from zero to almost any size. If desired, the coils can be prestressed so that an extension spring exhibits a zero slot and the coils do not start to separate until a force threshold is reached.
Currently, machined springs come with minimum slot of about 0.020 inch (0.51 mm). Wider slots, but generally not exceeding 0.250 inch (6.35mm), are possible. The slot width can be closed to near zero using a stress relieving process, but no prestressing is currently available.
Coiled wire vs. machined: These springs have the same length, outer diameter, and compression rate
If a compression spring application requires the absolutely best repeatability to support calibration or high precision uses, it is best that the coils never touch. Even better, the minimum slot width needs to be wide enough to not permit any interference between the coils from restricting or changing the compression motion. Machined springs are ideal for calibration and precision usages from this standpoint.
The sizes of wire wound springs range from very small delicate springs made from cold forming fine wire to very large, hot-rolled ones that originate as bar stock. This range is quite impressive.
Machined spring sizes are limited by machining practicality. The smallest are about 0.100 inch (2.54 mm) in diameter, and the largest are 6.0 inches (152 mm) across. Maximum length is about 24 inches (610 mm), but this applies to 1.0-inch (25.4mm) to 3.0-inch (76 mm) diameter springs. Smaller or larger diameter springs will need to be shorter.
Wire wound springs can be made very long, as in a garter spring. Length is limited only by the quantity of continuous wire available on the feed spool.
Machined springs, on the other hand, are limited to about 30 coils. Indeed, machined springs with more coils than that are rare.
Comparing Features: Wire Wound and Machined Springs
In a coiled spring, the entire length of the wire contributes to elasticity because the forces and moments are distributed end to end.
Machined springs are different. The flexure, the section providing the desired elasticity, is captive between the end sections, which provide structure and attachment features but contribute no elasticity. The slots on machined springs do not taper to zero at the ends. As a result, to accomplish the same elastic performance, machined springs likely need to be longer than wire ones.
Precision is another question to consider. The reality is that precise dimensions are easier to accomplish with machined springs than with wire springs, and precise dimensions are an important part of the foundation for precision performance.
Precision: Wire Wound vs. Machined Springs
Production time is the major influence in cost, and machined springs cannot approach the low cost of the wire wound product. It would be very surprising to find a very simple and inexpensive machined spring produced in high quantities and costing less than $1 each.
However, there are a half-dozen features of machined springs that may be important considerations in some designs. They are:
1 Integrated attachments.
2 Enhanced performance or functionality.
3 Higher precision.
4 Reduced assembly and acquisition efforts.
5 No sound created by coil contacts.
6 No debris created by coil contacts.
If an application does not gain value by including a spring with one or more of these attributes, there is little justification of specifying a machined spring.
Wire wound springs are typically made from medium and high strength steels, nickel alloys, titanium, and stainless steels that gain their strength predominantly from heat treating and cold reduction. Spring wire and malleable bar are common to wound springs, but cannot be used for machined springs. A completed wound spring will retain various amounts of residual stress. While stress-relieving efforts are made to reduce residuals in a wound product, the sum is not zero.
A machined spring exhibiting residual stress in the free-state will be subject to free-state deformation. Such deformation is always undesirable. To reduce the chance of residual stress in machined springs, materials are selected that have been subjected to residual stress eradication such as solution annealing, and the heat treatment (with no cold reduction) is limited to lower temperatures without quenching. Quenching will, by itself, induce residual stress. Martensitic steels are preferred.
Typical machined spring materials include corrosion resistant steels (CRES) such as 17-4PH per AMS 5643, 15-5PH per AMS 5659, or CC455 per AMS 5617; very high strength steel, C300 per AMS 6514; high strength or very high strength aluminum, 7075-T6 or 7068-T6511; 38644 Beta C titanium, which has very high strength and is corrosion-resistant; or machinable plastics, such as Delrin 100 or Ultem 2300. Any machinable material that can be made free of residual stresses is a valid candidate for being use for a machined spring.
Wire springs are often shot peened for enhanced fatigue resistance. This process is possible because the gap between the coils is typically wide enough to permit passage of shot that can condition the inside of the opposite coil, as well as the outside of the coils.
Machined springs typically have coil slots that are too small for the passage of shot. To insure fatigue resistance, features such as stress relief holes and slots can be added to the slot ends. Selecting high strength, fatigue resistant, materials is also a significant benefit.
Wire springs can be plated with materials such as zinc and nickel for corrosion protection.
Plating machined springs is uncommon because of sharp edge corners that typically receive insufficient coverage. Corrosion resistance of machined springs arises from the material itself, CRES and titanium, for example. Springs machined from aluminum are typically anodized or coated to prevent corrosion.
One advantage of machined springs is that they can possess any feature that can be machined.
Machined extension springs, for example, can contain machined studs, threaded holes, or flanges, and many other features are available to machined springs.
Some of the many machined spring attachments.
More Than One Start
While wire wound springs have a single coil, machined springs have the potential to have one coil or more than one, a design described as “multiple starts.”
In nature, a DNA molecule is effectively a double start. This form provides balance and functionality which is essential to life. In the world of springs, the double start machined spring makes the transition from basic functionality to concepts that are intrinsic to the function of mechanical devices.
In regards to compression and extension springs, multiple start springs allow for a pure force reaction. A moment is created by compression or extension forces occurring at the spring coil, radial, width center which is a distance from the spring centerline. In multiple start springs these moments resolve to zero within the body of the spring. Hence, compression and extension springs configured with multiple starts provide elastic motion without the need of corrective moments. In single start springs, wound or machined, these moments must be resolved at the interface between the spring and the components providing the force and deflection.
Multiple start spring configurations significantly unify the lateral bending and lateral translation forces and moments around the spring’s circumference given a lateral deflection. Multiple start configurations as high as five have been employed to unify the lateral reaction of machined springs. Another advantage of a multiple start design is that if a failure occurs, the remaining coils will let the spring continue to function, albeit with a degraded performance because of the missing coil.
Multiple starts will add to the length of a machined springs.
Multiple start flexures in machined springs.
Compression and Extension
Stresses in both machined and wire wound springs used in compression and extension are dominantly torsional shear. In machined springs subject to compression and extension, the maximum stresses are located on the spring ID and on the coil sides. It is very rare to find the maximum stresses on the spring OD. Stresses at the sharp corners are functionally very low.
Examples of machined compression springs.
Machined springs used in compression may benefit from stress relief holes (SRH) or elongated holes at the slot ends. Machined extension springs nearly always require a stress relief hole or an equivalent feature at the slot ends to mitigate the effects of harmful tensile stresses. Without an SRH the spring’s performance must be greatly reduced to avoid a failure caused by the tensile stress riser at the coil ends.
Linearity of compression and extension springs is influenced by five factors:
1 Geometric changes in the spring during elastic deformation from free length.
2 Residual stresses in the material.
3 Increasing coil contact during deflection (compression springs only).
4 Boundary condition fixation.
5 Spring rotation during deflection.
When helical springs are compressed and extended, end to end twisting occurs. Three remedies exist with Machined Spring to do away with the torsional deformation.
1 Fix the end of the spring using the myriad of attachment techniques available. Constraining the spring end will increase the elastic rate.
2 Use two concentric springs one with an RH flexure and the other LH. When properly designed, the twisting of the inner spring counter acts that of the outer one.
3 Place two flexures on a single spring blank. Make one flexure RH and the other LH. This configuration allows the interface between the RH and LH flexures to twist, but the ends do not.
Machined spring flexure configurations for compression and extension springs that resist torsional deformation.
Since machined springs have a constant slot dimension, there exists a slot width at both of the spring’s ends that does not close. When that dimension is added to the solid structural end of a machined spring, one finds that a machined spring used in compression is longer than an equivalent wire spring. Given the envelope of a fully optimized wire spring used in compression, a machined spring cannot be configured to provide equal performance. Hence, when it comes to compression springs, equivalent machined springs are always longer than wire ones.
Compression and extension machined springs with multiple starts have proven to be very successful in systems that operate at resonance. Multiple start springs exhibit a combination of low tolerance elasticity, continuous slot dimension (no touching at coil ends at any time guarantees no noise or debris generation), internally resolved moments and uniform cross axis stiffness.
Compression springs can exhibit tipping, bowing, and buckling. While buckling is often blamed for tipping and bowing, it is a separate phenomenon which needs engineering review when springs are in the design stage. Tipping and bowing may precede buckling, and encourage buckling.
Tipping: As springs are compressed, an unresolved moment is created at the coil ends. If insufficient restraint exists (typical with single start springs both wire wound and machined) the spring tends to lift off the compressing platen at one point on each end. The spring ends are said to be tipping when they lift off. Tipping does not occur in multiple start machined springs because the reaction moments created at the coil ends are resolved to zero.
Bowing: All coils bow somewhat when compressed. This phenomenon applies to both wire wound and machined springs. Multiple starts don’t help either. Double start springs can exhibit coils bowing opposite to each other. When coils bow, the coil ends stay in the same parallel planes, end to end, as when the spring was at free length. It is only the coils that displace laterally to a small amount.
Buckling: Buckling comes from static instability in columns. Static instability is a function of load, geometry, and elasticity (a material property). The issue of buckling becomes more prevalent as the number of coils increases.
Springs in Torsion
Besides the wide range of attachments that can be designed for them, machined torsion springs differ from wire torsion springs in that the coil ID can be reduced to add stiffness to the torsional rate. Adding stiffness to wire torsional springs requires a change in the basic configuration (OD, ID, wire size, number of coils, etc.) or a change in the wire shape. Wire is available in rectangular and trapezoidal sections. Trapezoidal sections are generally selected for smaller ID springs so that post wound sections will approach being rectangular.
Since the stress in torsion springs is that of simple bending stress applied to a curved beam, there are no benefits from using multi start configurations. Said differently, if one wanted to double the stiffness of a torsional machined spring, it would be less costly to double the coil thickness. The alternative, using a double start of the original coil thickness, would give a similar result but would be more expensive, longer, and possibly a candidate for buckling.
The maximum tensile beam stresses are on the OD when the spring is subject to wind-up deflections, and the stresses on the ID are at maximum compression. When the spring is subject to unwind, the stresses are the opposite. Machined torsional springs should most often contain stress relief holes (or slots) to help mitigate stress risers as the coil section makes the transition into the solid, structural section of the spring end.
Typically, torsional machined springs are a little longer than equivalent wire ones.
With torsion springs, buckling must be considered. This issue becomes more important as the number of coils increases and as the coil width decreases. Both wind-up and unwind directions should be analyzed for buckling.
Torsional machined springs.
Bending and Translation
If a spring is anchored at one end and the other is subject to a moment, the load case is labeled as lateral bending. While it is possible to subject a wire spring to lateral bending, machined springs are more commonly used in this load case because of the availability of attachments.
Lateral translation occurs when one end of a spring is anchored and the other end is laterally displaced by a force plus a moment to insure the end faces of the spring remain parallel. Such deflections are again better suited to machined springs because of attachment availability.
Buckling is rarely a concern for lateral bending or lateral translation springs.
Take Care With Calculations
Much analytical work has been accomplished by spring experts such as A.M. Whal, S.P. Timoshenko, and J.N. Goodier. Design guides have been published by SAE, the Spring Manufacturers Institute, and other sources. For the most part, all this work has been accomplished to provide closed form solutions for wire products. Since the geometries can be similar between wire and machined springs, it is not uncommon for one to attempt to use the equations sets for both.
There is a concern particularly with machined springs users that closed form solutions may not fully apply. Many options exist in regards to boundary conditions and actual geometries that may exceed that researched for wire springs. Since geometries also change with displacement, it is unwise not to include these effects in computations. Simply said, closed form solutions for machined springs are sometimes correct, sometimes a little in error, and sometimes very much in error, and it is difficult to determine which is what. As a general rule, closed form equations wane on accuracy when the number of coils is less than three and/or when the ratio of coil sides exceeds two, height to width. For these reasons, quality FEA is nearly always needed for elastic and stress computations related to machined springs.
Gary L. Boehm is senior research engineer at Helical Products Co. in Santa Maria, Calif.
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