MW Industries, Inc. Frequently Asked Questions
Our staff of engineers will answer any questions you might have regarding product design, material selection or application, and will assist you with developing the right part for your project. For more information, contact us.
In addition to the questions and answers below (for a plain text version, click here), also see the "Glossary of Terms" for common spring and fastener terminology.
- How is a spring's fatigue life predicted?
- Does MW Industries provide product design support?
- What features need to be toleranced when developing a spring design?
- What spring materials are available?
- What are acceptable design stress levels?
- How do you analyze complex spring geometries?
- Can MW Industries provide metallurgical analysis and support?
- What types of corrosion preventive options are available?
- How much energy is stored in a compressed spring?
- What are the common terms used in describing 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
- Should spring ends be ground or unground?
- Can MW Industries develop manufacturing techniques or processes to address specific product needs?
- How do you assure product consistency from run to run?
- 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 of a wire diameter can be coiled and how small of 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" in diameter.
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.
- What are the advantages and properties of stainless steel springs?
- What are residual stresses?
- How is square wire used to increase the force from torsion springs?
- 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 MW Industries 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?
- What is the development process for wire forms?