Spring Essentials – The Conical Compression Spring
One of the oddities of spring designs is the conical spring. This cone-shaped helix can have as many different geometries as the mind can create.
But what really is the reason behind conical designs? What are the advantages or disadvantages?
A conical spring is characterized by having one end larger than the other. Some conical springs have a very slight difference in end diameters and can barely be distinguished, while others are quite obvious. Figure 1 and Figure 2, below, show a comparison.
One common reason to use conical springs is to get a lot of travel in spaces where there isn’t that much room to move. If a conical spring is designed to totally collapse, the spring can be deflected almost all the way to solid height. This means there is no typical solid height to contend with. This is called “nesting,” when the coils collapse around themselves and there is no contact of coils throughout the whole movement of the spring.
Another reason to use a conical design is for stability. If a spring is somewhat long and slender, it may tend to buckle during its travel. One way of preventing that scenario is to make one end of the spring a larger diameter than the other. The large end will serve as a base, like a stable foot or pod.
Sometimes, springs must purposely have a rate increase during their movement. This means the application must carry more force as it travels. A standard spring with consistent diameter and pitch will have a very linear rate. This means the load increase per inch of travel is constant. However, if the rate needs to increase, a constant-pitch conical spring will do this. The reason is that the larger coils on a conical spring have a weaker rate than the smaller ones and collapse first. Then, as the spring moves, the body diameter gets smaller and smaller, which makes the rate increase. The more the spring travels, the higher the rate. (A standard compression spring can also have variable rate, but the pitch needs to be varied, which is just the opposite approach of the conical design.)
Conical springs that require ground ends tend to cost more. They need special fixturing or require only one end ground at a time. This can double the cost of the grinding operation. Production-style spring grinders require that springs be placed in a hole or tube. If the spring does not stand up straight when forced into the grinder, the spring will tilt, and this will greatly affect the squareness of the grind. Conical springs do not have a consistent diameter and must be ground with different, slower techniques.
Conical designs do not have a predictable stress distribution during movement. A typical compression spring has a constant diameter and a constant pitch. The twisting of the material during travel creates stress that is somewhat predictable with typical compression spring design calculations. Conversely, conical designs create stresses that are not consistent; therefore their behavior, especially concerning cycle life, will not be easy to predict. This means they should not be used for applications where safety is a major concern. If used for that end, the deflection should be short, the material must be of premium grade and real-world testing is a must to verify high life.
Spring design calculations, which are based on theory, are very accurate concerning linear stresses. However, any time a spring has a shape that is not a consistent diameter and pitch, the stress placed on the spring material will be unpredictable. This means any shape that varies from a standard compression design requires verification of the design. Consequently, conical springs are used where the function is best served by the shape of the spring, and safety is built into the part by over-designing and making sure the calculated stresses are low enough to ensure a robust design that will not fail within the life of the application. ◆