FAQs

What are the common terms used in describing a spring?
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

How is a spring’s fatigue life predicted?
Paragon Medical utilizes analysis methods consistent with SAE and SMI for spring life prediction. Essentially, the minimum and maximum operating stresses are calculated and compared to a family of reference Modified Goodman Diagrams. In cases where unusual spring configuration or non-standard materials are employed, it is best to demonstrate fatigue life capability through testing. Under these cases, Paragon Medical employs Reliability Engineering techniques to develop test methods and analyze test results in an effort to ensure that our products meet customer expectations.

What features need to be toleranced when developing a spring design?
Each application often has its own needs, but there are some general rules of thumb. 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 (e.g., 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 spring materials are available?
The divisions of MW Industries, including Paragon Medical and MW Components are familiar with all of the common spring and fastener materials. In addition, select divisions 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. Please 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 are acceptable design stress levels?
Unfortunately, there is not a simple answer to this question. The appropriate stress limits depend on the material type, operating environment and whether the loading condition is static or cyclic. Please contact us with your application and our engineers will answer any questions you might have regarding spring design, material selection or application and with developing the right spring for your project.

How do you analyze complex spring geometries?
In addition to handbook calculations, Paragon Medical has developed a variety of proprietary models that enable us to accurately model complex geometries. These can include variable wire diameter, spring diameter and pitch.

How much energy is stored in a compressed spring?
The stored energy is the integral of the load versus deflection curve. For a spring with a constant rate k deflected x from its free length, the stored energy will equal (kx^2)/2.

Should spring ends be ground or unground?
The purpose of grinding spring ends is to distribute the force applied at the spring end across as large a surface area as possible. This is typical when the spring is to be compressed between flat end plates.

End grinding is one of the most expensive processes in spring manufacturing. If the production volume of your assembly is high enough, it may be more cost effective to design mating components that mate with unground spring ends in a way that the load is still distributed across a large surface area. 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?
There are three basic spring manufacturing methods. 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 strength, this process is typically limited in how large a wire diameter can be coiled and how small an index can be achieved. Depending on the equipment and process that is available and strength of the material being coiled, 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 0.875 inches in diameter.

The final process is hot winding. In this case, bars are heated to approximately 1,700°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 inches 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.

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.

What are the advantages of stainless steel springs compared to carbon steel?
Corrosion resistance is probably the most common reason for choosing stainless steel. In addition to being resistant to rust, the austenitic stainless springs are resistant to a wide variety of chemical media and suitable for use in the food processing industry. A secondary reason is stainless steel is not as magnetic as carbon steel, but magnetism in stainless steel increases with cold working.

How is square wire used to increase the force from torsion springs?
Often, customers have a spring application that requires a lot of force in a little space — usually too little space. Paragon Medical 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.

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 which can affect your selection.

The type of spring you are using 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.

The most important thing, however, is to contact Paragon Medical for more information about the material options available, and to discuss the pros and cons of each.

Where should load points be specified in a compression spring?
Load points should be specified between 15 and 85 percent of the possible deflection in a compression spring. Load points outside these ranges are typically inconsistent with expected/calculated values. The values are not linear outside this range and are often unpredictable. Stock compression springs are typically less flexible and manufactured to different standards than custom medical compression springs.