How is a spring's fatigue life predicted?
We analyze our spring’s fatigue life predictions using methods consistent with SAE and SMI guidelines. Essentially, the minimum and maximum operating stresses are calculated and compared to a family of reference Modified Goodman Diagrams. It is best to demonstrate fatigue life capability through testing. This testing is especially important in special cases with unusual spring configurations or the use of non-standard, unique materials. In these cases, MW Components uses reliability engineering techniques to develop test methods. We carefully analyze these test results to ensure our products will meet and exceed customer expectations.
How much energy is stored in a compression spring?
The stored energy is the integral of the load vs. deflection curve. For a spring with a constant rate of k deflected x from its free length, the stored energy will equal (kx^2)/2.
What are the common terms used to describe springs?
d – wire diameter
D – mean diameter, the diameter of the spring as measured at the wire centerline
ID – inside diameter, D-d
OD – outside diameter, D+d
Na – number of active coils
Nt – total coils, active coils plus any inactive coils. For a spring with closed ends, Nt=Na+2
FL – free length, the spring length with no load applied
P, F – load or force, the force exerted by the spring under a given deflection
l – instantaneous spring length, the spring length corresponding to a given applied load
x, s – instantaneous deflection, the amount the spring is compressed from free length to length l. x=FL-l
k – spring rate, the derivative of the load-deflection curve. k=P/x=(P2-P1)/(l1-l2)=(P2-P1)/(x2-x1)
C – spring index, the ratio of the mean diameter to the wire diameter. C=D/d
What are residual stresses?
Residual stress forms when a product is welded, cut, cast, or undergoes some other manufacturing processes involving heat or deformation. Residual stress may be beneficial or not, depending on the application.
MWI engineers will answer any questions you might have regarding product design, material selection, or application.
What are machined springs?
A machined spring is a single piece of material machined into a spring configuration and offering the traditional load cases of Compression/Extension, Torsion, Lateral Bending, and Lateral Offset. Key to the versatility of the machined spring is the Helical® Flexure, a flexible helix beam concept. Because our springs are “machined” to specific design requirements, they provide more precise performance, attachment features, and functions than more traditional types of springs.
Why choose machined springs over traditional wire-wound springs?
With machined springs, desired features or functions can be machined integral to the spring, therefore eliminating complex or expensive wire-wound spring assemblies. Features and functions can include custom attachments, precise spring rates, multiple start coils, and other special characteristics. These aspects are generally not possible with traditional springs. There are big differences in performance, reliability, versatility, integrity, and cost-effectiveness.
What spring materials are available?
All MW Components locations are familiar with common spring and fastener materials. In addition, select locations have in-depth experience with more advanced application-specific materials. These include titanium alloys, nickel-based alloys, beryllium copper and other special high-temperature alloys. Contact us with your needs and our engineers will work with you to develop a solution with the right material and processing for your application.
What types of corrosion prevention options are available?
Our facilities have a variety of in-house finishes available. These include electrostatic powder epoxy or polyester (GM Type III approved) and an assortment of wet coating processes for painting or color-coding. We also maintain relationships with a supply base that can provide a full range of plating and coating systems. Simple oil-based and water-based rust preventives are always available for short-term protection.
What are the advantages and properties of stainless steel springs?
Stainless steel springs offer better appearance and corrosion resistance. They also offer some unique properties not obtainable in carbon steel springs.
Why is stainless steel better for industrial use than carbon steel?
Stainless steel is resistant to corrosion and chemical media, making it ideal for use in industrial manufacturing. Carbon steel, while stronger and more naturally magnetic than stainless steel, may rust and corrode when exposed to moisture. If you need a magnetic metal, cold working increases magnetism in stainless steel.
Is Inconel 718 the best material for springs operating at high temperatures?
Inconel 718 has material properties that make it very well suited for high temperature springs. It maintains its strength at temperatures in excess of 1,000°F as well or better than any high nickel material on the market. However, there are several factors to consider when choosing an Inconel material for your spring application.
The type of spring you use will help with your decision. Inconel 718 is typically available only in annealed sheet or bar form. If you are using a flat spring at these elevated temperatures, Inconel 718 is generally going to be the right choice. If your spring is a compression spring or extension spring using round wire, Inconel X750 might be the best choice because it is much more available as a wire product than 718. It can be used at temperatures in excess of 1,200°F if it is properly sized and designed for the application. There is even a special heat-treat cycle for the X750 which can help with these applications, though it is costly and time consuming to perform.
Another thing to consider is the volume of springs required in your production. If you are designing for a large production volume and have the time and budget for having spring wire custom-drawn, then Inconel 718 can be made into spring-tempered wire without a large impact. If your needs are for short-run, small-batch orders of round wire springs, then the readily available sizes of Inconel X750 should be considered since there are a number of wire vendors that stock it.
Spring Design Support
What features need to be toleranced when developing a spring design?
While each application has its own unique needs, there are some general guidelines to follow. The various wire specifications typically include diameter tolerances. So, citing a wire type and specification along with a wire diameter tolerance can be either conflicting or redundant. The application may place some dimensional constraints (i.e. minimum or maximum free length, maximum solid height, maximum OD or minimum ID, etc.). Those significant to your application should be cited on the spring requirements. Spring force output at reference heights are often significant and can be toleranced. In general, spring rate and total coil count are referenced. Flexibility on these items provides the spring maker sufficient freedom to assure that the true key characteristics meet your needs.
What are acceptable design stress levels?
Appropriate stress limits on springs and other components depend on a number of factors. These include material type, operating environment, and whether the loading condition is static or cyclic. When you contact us with your application needs, our engineers are prepared to answer any questions you might have regarding spring design, material selection, or application. Let us help you develop the right spring for your product.
How do you analyze complex spring geometries?
In addition to handbook calculations, MW Components has developed a variety of proprietary models that enable us to accurately model complex geometries. These include variable wire diameter, spring diameter, and pitch.
Should spring ends be ground or unground?
The purpose of grinding spring ends is to distribute the force applied at the spring end across the largest possible surface area. This is typically done when the spring will be compressed between flat end plates.
However, end grinding is one of the most expensive processes in spring manufacturing. If your assembly’s production volume is high enough, it may be more cost-effective to design components that mate with unground spring ends. This way, the load is still distributed across a large surface area without the high cost of end grinding. This is typically the case in automotive McPherson strut assemblies. Another case where grinding might be avoided is large index springs, particularly with very small wire diameter.
What is the difference between "cold winding" and "hot winding" and when is one chosen over the other?
The most common is cold winding. In this case, wire that has already been heat-treated or worked to its final strength level is coiled into a spring. Because the material is already at peak strength, large wire diameters and small indexes are difficult to achieve. The typical maximum wire diameter for this process is 0.625 inches.
The next process is less common, but still falls under cold winding. In this case, wire is coiled in a soft state and then heat-treated to its final strength condition after coiling. For a given piece of coiling equipment, larger wire diameter and/or smaller indexes can be coiled with this method. This process is used for wire sizes up to .875″ in diameter.
The final process is hot winding. In this case, bars are heated to approximately 1700°F and coiled. Usually, the red-hot spring is quenched in oil and tempered to complete the heat treatment. Coiling at such a high temperature enables spring manufacturers to work with far larger bar sizes than could be coiled at room temperature. This process is generally used for bars up to 1.75″ in diameter.
Which process to use is determined first by the size of wire that must be coiled. Once that is determined, the type of material, final wire strength level, and spring index will drive manufacturing toward a process that is most compatible with the available equipment.
How is square wire used to increase the force from torsion spring?
Often customers have a spring application that requires a lot of force in a little space — usually too little space. MW Components believes springs should be designed to fit your product and application, and not the other way around. One way of maximizing this force is to use square wire.
Where should load points be specified in a compression spring?
Load points should be specified between 15% and 85% of the possible deflection in a compression spring. Load points outside of these ranges are typically inconsistent with expected/calculated values. The values are not linear outside of this range and are often unpredictable. The illustration below represents calculated vs. measured values for a load specified outside of the 85% range. The values are as expected until we exceed 85% of the deflection.
What is a bellows?
A metal bellows is a flexible, lightweight, spring-like component with ribbed or corrugated tubing which gives it an accordion-like quality. Metal bellows are precision components designed to perform a variety of functions in various machine components and assemblies.
What is a bellows function?
Metal bellows convert pressure, mechanical, vacuum, and temperature changes into linear or rotational motion. They can also be used in flexible electronic contact applications. Bellows provide a specifically defined dynamic response as part of a larger machine component or assembly often providing a more precise, more reliable, or less costly alternative to an existing engineering solution.
How are bellows made?
Our metal bellows are produced one of three ways; electrodepositing, hydroforming, or edge-welding technology. More information on each of these three processes and a breakdown of specifics between these three processes, including which process is preferred for which application, can be found in our Metal Bellows Comparison Guide.
What materials are recommended for metal bellows?
Nickel, Copper, Silver, and Gold. Materials used are generally the types of metals that can be formed through electroplating.
How long do bellows last?
Bellows can be designed to have an infinite cycle life. Our design engineers will design custom metal bellows to meet your application specification and will work with you to optimize performance.
What is a bellows maximum pressure allowance?
This depends on the process used. Electroformed bellows can accommodate up to 3000 PSI. Other methods (hydroforming and edge-welding will have different results). The specific application requirements will help us calculate the actual maximum pressure. More information on each process including maximum pressure can be found in our Metal Bellows Comparison Guide.
What are recommended joining methods for bellows assemblies?
Besides soldering, our bellows can be welded and various adhesives including epoxies onto mating components.
What end types are required for welding bellows to a hub?
The preferred method of welding bellows is with an electron beam welder. Since this method does not use filler material, it is important that there is no gap between the bellows and hub at the weld joint. Ends such as Type A, D, or I will result in bad, incomplete, porous welds since it is difficult to eliminate gaps between the bellows and hub and there is limited material available for fusion. An end such as Type E will provide a good weld joint because the flange can be pressed against the hub, eliminating any gap, and the beam can be focused perpendicular to the axis of the bellows ensuring that there is plenty of material from the bellows and hub that can be fused together.
What can be done to protect metal bellows from corrosion or make it biocompatible?
Depending on the application and media the bellows or electroform will be exposed to, it can be plated with a thin layer of gold or coated with parylene. Gold adds protection against corrosion in all types of climates and environments and is also an excellent conductor of electricity. Parylene is a polymer coating that provides moisture, chemical, and dielectric barrier properties, as well as dry-film lubricity. Both gold and parylene are biocompatible.
What is reverse bending?
From the free length, the bellows may be operated in compression and anywhere therein to max compression and up back to the free length, OR, from the free length, the bellows may be operated in extension and anywhere therein up to max extension and back to the free length. Reverse bending is when the bellows is operated in compression and is moved through the free length into the extension and vice versa. Reverse bending will reduce the cycle life of the bellows.
What metals can be used to make edge-welded bellows?
Any metal that can be easily formed, blanked, and welded. The common materials used at MW are:
- Stainless Metals: 304L, 316L, 321, 347, AM350
- Nickel Metals: Inconel 600, Inconel 625, Inconel 718, Inconel X750, Hastelloy C276, Haynes 242
- Titanium Grade 5
How many cycles can an edge-welded bellows endure?
This all depends on the customer’s design criteria. Axial stroke, lateral and angular offsets, pressures, media, temperature, cycle rate, and environment are just some of the variables that affect cycle life, so it is important to know all of these variables before predicting cycle life.
A bellows made from 304L, 316L, 321, 347, or titanium, with vacuum on the inside of the bellows, will typically be designed for cycle life less than 1,000,000.
A bellows made from AM350, Inconel 600, Inconel 625, Inconel 718, Inconel X750, Hastelloy C276, or Haynes 242, with vacuum on the inside of the bellows, will typically be designed for greater than 1,000,000 cycles.
How can I get the bellows force for a given deflection to be lower?
Here are three ways to reduce force. First, the easiest way is to reduce the bellows material thickness but depending on bellows performance requirements it may reduce the cycle life. The second, is to make the bellows longer by adding convolutions. The more convolutions a bellows has the lower the force will be if axial stroke length is unchanged. Third, is by changing the bellows material. The stainless steel we use all have a similar force but Titanium bellows will have about half the force of an AM350 or 316L stainless steel bellows.
Atherm Metal Bellows Assemblies FAQs
What are athermalization bellows assemblies?
Athermalization (atherm) bellows assemblies are filled and sealed flexible bellows assemblies that translate changes in temperature into precise linear mechanical motion.
Atherm bellows assemblies take advantage of the flexible, expandable nature of precision electroformed metal bellows, and the steady volumetric expansion of incompressible fluids with changes in temperature. These bellows assemblies have known temperature-dependent rates of length change depending upon the fluid used in the assembly.
Each end of the bellows connects to system components using stock fittings or custom fittings designed to meet customer requirements. These fittings allow the dimensional change of the bellows to translate into mechanical axial motion that effects the desired change in the system.
What is athermalization?
The term athermalization is most commonly heard in optics applications. Optical engineers use atherm assemblies to give their systems optothermal stability, meaning that the systems’ optical properties are immune to changes in temperature. Athermalization applies to nonoptical applications, too. More generally, then, an atherm is a device that uses components which undergo linear, temperature-dependent changes in length in order to affect or counteract temperature dependent changes in the system.
In athermalization systems, the rate of length change is determined by the assembly’s makeup. It is precise enough that engineers can use it in a variety of ways from adjusting optical focus to triggering electro-mechanical systems at specified temperatures.
Where is athermalization helpful?
Atherms are useful in a variety of applications. They are commonly used in optical applications because of the precision required to maintain the properties of sensitive optics over the range of operating temperatures.
Other applications put temperature-dependent length change to mechanical use, similar to the operation of a thermostat’s bimetallic strip. And atherm assemblies can shield delicate systems from the effects of temperature change.
Where can atherm bellows assemblies be used?
Common athermalization applications like adjustments to the focal lengths of optical lenses or mirrors to prevent temperature-related drift are ideal for atherm bellows assemblies. In infrared applications they control the flow of liquid nitrogen or other fluids used to cool the black body that is used for IR reference.
Atherm bellows assemblies also work well in precision electromechanical applications that are sensitive to temperature. They can be used like thermostats to trigger heat or cooling system valves or other components when temperature rises or falls. In precise metering applications, they can adjust orifice openings so that the mass flow of gas or liquid is normalized independent of temperature.
These assemblies provide temperature-dependent mechanical actuation without the need for programming or electricity. Their
simple, robust design lets them operate through hundreds of millions of repeatable cycles without drawing power or requiring recalibration.
Thus, engineers use atherm bellows assemblies as back-ups to electrical controls that can continue to operate in the event of power loss. These assemblies can also entirely displace electrical systems with improved reliability and energy efficiency.
How does the bellows assembly translate temperature change into mechanical motion?
The bellows assembly is filled with an incompressible fluid. The volume of the fluid sealed within the bellows assembly will expand or contract in response to changes in temperature.
The bellows in the assembly has a constant effective area. This constant effective area combined with the volumetric thermal characteristic of the fluid causes linear, axial movement of the bellows assembly in response to the temperature change.
For example, a bellows might couple to a rod that controls an orifice, a valve stem, or a lens. In each case, a different connector is needed. In fact, the majority of bellows assembly connectors are custom designed based on the needs of the application.
How do the atherm bellows assemblies operate with respect temperature?
Atherm bellows assemblies can be designed to operate at temperatures ranging from -130° to 300°F. Each assembly is designed for a specific temperature range which corresponds to range of movement, expressed in length per degree of temperature (i.e. in/°F). The specific temperature range and deflection rate depends on the fluid selected.
For example, a stock atherm bellows assembly from Servometer is designed for a temperature range of 10° to 300° F. Within that range, it provides movements of 0.0002 in/°F. This makes it ideal for use in applications requiring reliable precision motion or adjustment in response to the temperature.
How big is an atherm bellows assembly?
Precision electroformed bellows come in a wide variety of sizes. Their diameters range from 0.250 to 9 in, and they can be as long as 9 in. Atherm assemblies can be designed using any diameter within this range.
Suppliers such as our MW Components - Servometer location stock components in the most common sizes, diameters ranging from 0.313 to 1.625 in. These sizes are readily available for prototype designs and proof-of-concept testing. Custom components can be designed for applications with needs outside the above range.
How can I get the right atherm bellows assembly for my application?
The design of an atherm bellows assembly is typically application specific. Consequently, the most effective design will result from a collaboration between the application specialist and an engineer experienced in bellows design and athermalization. Engineers at suppliers such as MW Components can work with you to recommend the right assembly given your application’s temperature range and the movement required.
Are custom couplings more expensive than standard couplings?
No, the engineering process required to create your custom flexure is completely free and is part of our committed service to you. Pricing is a function of the complexity of the design and the material specified.
Do specialty or non-standard couplings take longer to manufacture?
No, just because they are referred to as “non-standard” does not mean that we may not already have these products in our inventory.
What is meant by “single start” and “multiple starts”?
A single start spring is a single continuous coil element that starts at one end and terminates at the other end. This configuration is common to most springs. A “double start” spring has two intertwined continuous coil elements phased 180 degrees apart. In effect, this puts two independent helixes in the same cylindrical plane. Multiple start flexures such as triple start etc. are similar extensions of the concept.
What are some of the benefits of multiple start flexures?
Multiple start flexures are beneficial because they not only provide redundant elastic elements should a failure occur, but a failed element (coil) will be physically trapped by the remaining ones.
A multiple start compression or extension spring is balanced when a load is applied, which prevents tipping. Traditional wire springs and single start machined springs have an un-resolved moment when a load is applied. A coupling with a multiple start flexure will have a stiffer torsional rate and can transmit more torque compared to a single start flexure.
What does “relief” mean for a coupling?
A coupling with relief allows both shafts to enter the flexible region. The diameter through the flexible part of the coupling is larger than the diameter of the two shafts. The ends of the two shafts can operate very close together when a relieved coupling is used. For a non-relieved coupling, each shaft must not go into the flexible region. In some cases, there will be a step to prevent the shaft from entering the flexible region.
What are the pros and cons of selecting a fastener with a fine vs. a coarse thread?
Fine threads are easier to tap into harder materials and thin-walled structures. They have bigger tensile stress areas which can make them more resistant to tension than coarse threads. Finer threads also have greater minor diameters, providing higher shear strength.
Coarse threads are recommended for more brittle materials and less likely than fine threads to cross thread. Coarse threads are better suited for thicker coatings and plating and last longer without thread adjustments needing to be made. Coarse threads are also more tolerant of harsher work environments and disassemble quickly and easily.
What does "Thread class" refer to?
Thread classes are used to determine what type of thread is best for a particular application. Other considerations include thread form and thread series.
For unified inch threads, there are 3 thread classes describing external threads (1A, 2A, and 3A) and three for internal threads (1B, 2B, and 3B). All of these are “clearance fits” which indicates that they are assembled without interference.
Generally speaking, the higher the thread class, the tighter the fit between mating threads. For example, an assembly which mates class 1A and 1B threads will have a looser fit than an assembly that uses class 3A and 3B threads.
Classes 1A and 1B are used least often but are suited for applications that require quick assembly and disassembly. 2A and 2B are the classes used most often due to their cost, consistent performance, and the ease at which they can be manufactured. Classes 3A and 3B are used when safety, strength, and extremely close tolerances are necessary. This makes them ideal for socket set screws and similar applications.
How can I avoid issues with separating or loosening parts?
This problem is often due to thread “galling” or seizing, which happens when the surfaces of mated parts are more abrasive. Galling only occurs with metal fasteners and is more likely when external threading is cut as opposed to rolled because when thread cutting is used, the machining process creates a rougher surface. Surface oxidation may also lead to galling when particular materials are used. Galling refers to when parts are joined and small particles break free from the surface and lodge between the connected parts which result in the parts sticking together. Thread galling can become so severe that the mating parts completely seize up, making it virtually impossible to disconnect them.
Galling should be considered when fasteners are designed and can be avoided by producing mating parts of different material compositions and hardnesses. Lubricants can also be added to the threads to avoid galling as well.
What is the best Stainless Steel or Stainless process to prevent corrosion?
Stainless 303 is a free-machining austenitic stainless material that is not as resistant to rust as other material grades. This is due to chemical additives used during the free machining process which attract corrosion. 303 stainless also requires different bath chemistry than other grades to achieve passivation. Types 302, 304, and 316, which are also austenitic and are more highly recommended for marine applications and other saline environments. Choosing a smooth surface finish, ensuring thorough cleaning of that finish, and added passivation treatments may also be recommended to achieve optimal corrosion resistance. Passivation is when parts are submerged in a 30% nitric acid solution, removing iron contamination from the surface which could potentially lead to rusting.
How do I calculate shank length with countersinks?
If you wanted to figure out the shank length knowing the countersink will be 82 degrees, first we would assume you're countersinking to around 1/2 panel thickness (the recommended depth). Panel thickness in the below drawing is labeled (T).
Once this is information is collected, we would apply the below formula:
- 10 GA = .1345 Nominal thickness (T)
- Shank length is therefore: .067 + .088 which yields .155. To ensure that shank is flush to under-flush, reduce the .155 by .010, resulting in .145. You would use this information to select the appropriate part.
Does MW Components provide product design support?
Yes. Along with our corporate engineering staff, each of our divisions retains their own engineers. These divisional engineers routinely review existing customer designs and work to develop new designs based on customer input.
Does MW Components provide metallurgical analysis and support?
We have a metallurgist on staff ready to address your specific needs. In addition to our in-house lab capabilities, we’ve contracted with strategic labs in the area to ensure that we have immediate access to the latest technology in electron microscopy and electron dispersion spectrography.
Can MW Components develop manufacturing techniques or processes to address specific product needs?
Our skilled engineers will work with you to develop a method to address your specific needs. For example, when faced with a continuing fatigue failure issue on a snap ring used in an automatic transmission, we developed a technique to increase fatigue life. This process development solved the customer’s failure issues without requiring a re-design.
How do you assure product consistency from run to run?
We select the appropriate process control tools for each product based on production quantity and customer requirements. Our facilities are certified in various quality system standards, such as ISO 9000/9001, AS-9100, and TS 16949. For more information on these and other certifications, visit our certifications page.
What is the development process for wire forms?
Customers should involve our engineers during the development process for a wire form product. A slight change in an angle or radius can make a big difference in production times. It can even enable the part to be manufactured in a single operation, rather than in a process that includes costly and time-consuming secondary steps.