SPRING DESIGN

If you're trying to make a spring to replace a broken one, you don't need to know a whole lot about design. On the other hand, if you're making a prototype of a machine, for instance, and you don't know exactly what you want, then this page is for you. Here you'll learn some basic data about spring design, which is what you'll need to know to make exactly the spring you want.

General Principles

There are three basic principles in spring design:

The heavier the wire, the stronger the spring.
The smaller the coil, the stronger the spring.
The more active coils, the less load you will have to apply in order to get it to move a certain distance.

Based on these general principles, you now know what to do to change the properties of a spring you already have. For instance, if you want to make automotive valve springs a little stronger than stock, you can a) go to a slightly heavier wire and keep the dimensions and coil count the same, b) decrease the diameter of the spring, keeping the wire size and coil count the same, or c) decrease the number of active coils, keeping the wire size and spring diameter the same. Naturally, you can also go to a stronger material to achieve the same result.

Now, what if you're making a spring from scratch, with nothing to go on in the way of a sample? You can engineer your own design (see the next section of this page for the math), coil a spring, and then test it. If it's what you want, fine. If it's, let's say, a skosh too strong, then you can a) go to a lighter wire, b) open up the coil diameter, or c) increase the number of active coils to get a slightly weaker spring.

Or, if you want to make things really simple, go to the Addendum, where you'll find a few websites that offer online design!

Mathematics

Naturally, spring design software is available - you can find out where to get it in the Addendum. For the purists (or those who don't want to pay for a program), here's a very short summary of the mathematics of spring design. These equations, by the way, are taken from The New American Machinist's Handbook, published by McGraw-Hill Book Company, Inc.in 1955. I don't pretend to understand them.

There's a lot more in the way of engineering that goes into spring design: these are only the basic equations. If you're interested, you can contact someone who makes spring design software or (gasp!) find it in the library under Dewey classification number 621.824.

You can also contact the Spring Manufacturers' Institute: they make a handy-dandy spring calculator, suitable for simple design work, that anyone can learn to use. They also have spring design software, training classes, and a bunch more stuff. Dave sez, "Check it out."

Design Limitations

Depending on what kind of spring you want to design, and depending on where it will be used, your design will be limited:

For all springs:

A spring under load is stressed. If you put too much stress on a spring, its shape will deform and it will not return to its original dimensions.
The material from which the spring is made will have an effect on the strength of the spring: it will also have an effect on how much stress the spring will withstand. The section on spring materials will tell you more about this.
When you heat spring wire (which you always do), it may change its dimensions. Again, the section on materials will tell you more about this.

For compression springs:

If the spring will set solid (compress all the way, so that all the coils touch each other) at the limit of its travel, the diameter of the wire times the number of coils cannot be greater than the space allowed, unless you want the spring itself to act as a mechanical stop to the motion.
Springs that operate in a high-temperature environment (like for instance inside an engine) will need to be made slightly longer to compensate for the fact that the heat may have an effect on the length of the spring. The section on finishing will tell you more about this.
As a compression spring assumes a load and shortens, the diameter of the active coils will increase. This is only a problem when the spring has to work in a confined space.

For extension springs:

There should be some mechanical limit on how far the spring will extend, or the spring will lose its shape and not return to its initial condition with all coils closed.
Extension springs operating in a high-temperature environment may have to be coiled extra-tight, as the heat will tend to weaken the spring. The section on extension springs will tell you more about this.

For torsion springs:

When a torsion springs assumes a load, the diameter of the coil body will decrease. If the spring has something inside the coil, it will act as a mechanical stop to the action of the spring.

Buying Design

If you want to have a mechanical engineer design your spring, your best bet is to call a spring shop. You can find spring shops in the phone book. If your phone book doesn't list any, go to the library: they should have phone books for major cities where spring factories are -- try Detroit or Los Angeles if there are none in your area.

A spring shop will generally do the design work for you for a small charge. They will also try to get you to let them make the spring for you, which you may or may not want.

The section on spring shops will tell you more about how their business operates. The addendum will give you links to spring shops, suppliers, people who make spring design software, and a whole slew of other stuff.

Forward to Materials