When Round Wire is NOT the Best “Fit” for the Application

Sponsored content

The most common problem in spring design is obtaining as much force as possible in a small area. The area is usually a round cylinder with a minimum inside diameter, the shaft, and a maximum outside diameter, the hole. The least expensive and highest quality wire is always round in shape. However, the round wire is the least efficient use of a round cylindrical shape when it comes to getting the most metal in a cylinder. The most metal is directly proportional to the highest load.

The most efficient shape to fill a cylinder is rectangular shaped wire rolled into a circle just big enough to slide into the cylinder and slide over the rod. Ideally, if you could roll the wire onto itself, you could fill the cylinder completely thereby placing most metal in the cylinder. This is not possible due to the high strength and high hardness of spring wire.

The spring index is the term used to define the value that describes how tightly you can wrap the wire. This is the ratio of the mean diameter of the spring (the mean diameter is equal to the outer diameter minus one wire size or the inside diameter plus one wire size) divided by the wire width for rectangular wire or the wire diameter for round wire. The smallest recommended index is 4.0. (If trying to push the limit to get the most force, indexes of 3.0 are possible.)

There are many grades of spring steel, but the one that will produce the most force is of course the hardest and strongest. For steel this is chrome silicon, for the stainless group this is 17-7 ph, and for the exotics this is maraging steel.

There are two ways to form springs into a tight index using rectangular wire. The first is with pre-tempered wire. This method uses wire that has already been drawn and single strand tempered to a high hardness in the low rc 50’s. For the second method the wire is annealed and formed into a spring, and then the spring is heat treated to a high hardness. With this process the hardness can be made higher into the rc 55’s. The first process is less expensive, but the second process will allow a tighter index and have a fully hardened spring throughout its cross section. This will allow for higher hardness which generates more force.

Rectangular wire can be produced in a number of configurations: square corner, round corner, fully rounded sides and partial rounded sides. Sharper edges mean more force can be developed, but this is more susceptible to failure due to the stress risers caused by the sharp corners. The most efficient configuration for rectangular wire is to use a trapezoidal shape.

All spring wire tries to rotate when it is coiled into a spring. This rotation takes the rectangular wire and makes it trapezoidal in shape. We can calculate the upset, or amount of trapezoidal shape. This upset is put back into the solid height calculation. This is the one drawback to rectangular wire for compression springs. The upset reduces the amount of travel you have and the force that is obtainable. If you make the wire in the trapezoid shape at the beginning it will become fully rectangular as you coil it. The rotation in the process moves it back into the desired exact rectangular shape and it eliminates the upset. This keeps the solid height to the number of coils multiplied times the nominal thickness of the wire. This difference in solid height between regular rectangular wire and the trapezoid wire can decrease the solid height substantially and allow for additional travel which provides additional force.

The three main configurations of springs are compression, torsion and extension. The advantages of rectangular wire over round for compression springs are measurable for non-trapezoidal designs, but they are substantial for trapezoidal wire compression springs.

The same is true for extension springs but rectangular wire is generally not used for extension springs due to the difficulty in forming hooks.

Rectangular or square shaped wire is superior for torsion springs. The most common use for rectangular wire is die springs. These high force springs are used to re-set dies for most forming presses used in all types of metal forming and stamping. The most common rectangular wire used for die sprigs was chrome vanadium from the 1920s up to the early 1980s. With the advent of chrome silicon as the primary grade of spring wire used for engine valve springs in the automotive industry, the quality of chrome silicon wire was drastically increased due to industry pressure. This improvement caused die springs to be made exclusively from chrome silicon wire. Again the very high strength of the chrome silicon is what allows for the most force.

Stainless has been similarly improved over the last two decades. Type 302 was used for many years as the most common grade for torsion springs made from rectangular wire. With the improvement of the 17-7 ph heat treatable stainless over the last 20 years it has replaced type 302 as the best stainless to obtain the most force with rectangular wire springs.

For exotic materials such as the high nickel grades, bronze and copper grades three materials stand out as superior for rectangular shape wire springs. The first is the maraging steels. Developed for liquefied natural gas (LNG) tankers it has a very unique molecular cross section that prevents cracks from propagating. This can be made to a very high hardness and strength. There are 4 grades 200, 250, 300 and 350. These numbers stand for the yield strength of the material. It can be formed hot or cold and its only drawback is that it is not as corrosion resistant as 302 stainless.

The second material is beryllium copper. This has a very low modulus which allows for more force in the same area and is also very corrosion resistant and heat treatable. This is used extensively in marine applications for hatch cover springs on submarines and air craft carriers. The last material and the most expensive is titanium beta c. This not only has high strength, but a low modulus and of course it is very light.

The bottom line: if you need more force go to a non-round shaped wire, keeping in mind that it is always more expensive. If you can adjust your space enough to go back to round wire it will be much more cost effective.