Smalley has been in the business of manufacturing rings and springs for over 50 years. During that time, the company has been asked every question you can think of about their products and how they make them.
ENGINEERING.com had the opportunity to speak with Ben Moskalik, senior research and development engineer at Smalley, to discuss 10 of their most frequently asked questions.
1. When fully compressed, does a wave spring lay flat or does it still have some height?
Ben Moskalik (BM): When fully compressed, a wave spring is flat. It won’t have any waves to it. However, we would strongly suggest that it’s never compressed to solid height because the stresses ramp up exponentially at the last portion of its deflection. The spring may become damaged if it’s ever fully compressed to solid height.
A heavy enough load could permanently deform wave springs. Very stiff springs can take a lot of load, but very light springs wouldn’t take nearly as much.
2. What rates can be designed into a wave spring?
BM: There are numerous variables that go into any spring design. We have the ability to change all of them.
One of the variables that goes into changing spring rate would be the thickness of the wire that we use. Even the width will slightly effect the spring rate.
The number of waves in a spring also makes a really big impact. We can change the number of waves per turn on a wave spring or change the number of turns. There are also variables like gap size, spring diameter, and so on.
If someone needs a really light duty spring, we can use thin wire or reduce the number of waves, among many other techniques, to achieve a wide variety of spring rates.
3. How important is working height?
BM: Working height is typically associated with load. For example, if a spring is put between two parallel plates, the working height would be the distance between those plates when they are compressed to exhibit a certain load. This is a measured load at a specific work height.
Anything between uncompressed free height and fully compressed solid height would be considered a working height of some measurement.
4. Can Smalley build a rectangular cross section spring with a wider cross section?
BM: Yes. The flat wire that we use has a slightly rounded edge to it, so it forms a rectangle with small radii on either edge. Any cross section can be used if it can be coiled.
It is possible to make a perfect rectangle cross section, but it’s actually easier for us to have those rounded edges based on how the machines operate.
The stress riser on something with a sharper corner isn’t as good, so those radius corners actually do us better service in spring design.
5. Does consistency in the wire cross section affect spring rate?
BM: The wire holds a fairly tight tolerance, but the width is not typically the most critical dimension for either the rings or the springs. However, a consistent cross section is critical to manufacturing a quality spring.
The tolerance will change as the size gets bigger and it will obviously be a tighter tolerance when the spring size gets smaller. The wire we use, which we roll flat, has pretty tight tolerances.
There’s a second phenomenon that happens when we coil the wire, which is called a keystone.
The wire section changes from a rectangle into a very subtle keystone shape, which means it is thicker on the inside diameter of the part and thinner on the outside diameter. The wire section becomes more dramatic in the thickness, but also in the width – it actually gets slightly smaller.
What the wire size is and into what diameter you’re coiling it will effect the degree to which that keystone phenomenon happens. We know what sort of tolerances and dimensions customers need, and so we’ll be looking out for that.
It’s usually the final dimensions that are critical to us, we make sure that the wire size meets a customer’s final dimensions. Once we’ve got that size and dimensions locked in, it’s not going to change.
6. What alloys are the best for use in high temperature environments?
BM: There are a variety of answers to that question, depending on which definition of high temperature we’re working with. If we’re talking marginally higher, like 250 degrees (F), than we would go with our 300 series.
When we get into even higher temperatures, such as if someone was operating at a maximum of 1000 degrees (F) we would use an alloy called A-286, which is commonly used in the aerospace or oil & gas applications. Inconel x-750 and Inconel 718 are two alloys that can be thermally processed in such a way that they can handle temperatures up to 1300 degrees (F).
To go one step higher than that, at 1350 or 1400 degrees (F) we can use an alloy called Waspaloy and this alloy is probably about our top limit. Once we get anything higher than 1350 or 1400 degrees (F), no alloy we have found can hold its strength and maintain its spring properties.
The creep phenomenon is greatly accelerated when you’re talking high temperatures, so even though we use those exotic alloys that can resist high temperatures, we still need to keep the stresses low.
7. Do wave springs allow for the same f=-kx relationship as a flat spring?
BM: Yes. The springs still possess similar properties if you have a load deflection curve, because you still have a linear relationship that means the further you compress it, the more load you get, which relates to the spring rate, your K value.
Those sorts of characteristics still exist with the wave spring. We may have different formulas that get us that spring rate, but ultimately it behaves very similarly.
8. Is it possible to build crest-to-crest wave springs capable of sustaining loads up to 40,000 lbs?
BM: Yes, but the spring would have to be really big. We can make springs that handle a lot of load, but generally they have to be made in a larger magnitude of size and using a larger magnitude of wire cross section.
The oil and gas industry would have the type of application that would use something with that high of a load. And, it’s generally going to require a very big diameter.
9. What tolerances can you hold on spring rate and spring length for crest-to-crest wave springs?
BM: We generally don’t want to tolerance spring rate at all. There are so many variables that go into what affects the spring rate, like wire size, diameter of the spring itself, and et cetera.
If you have any sort of variation in the diameter of those measurements, then you’re going to start getting a slight variation of spring rate, and all those variations can add up.
What we do is tolerance a specific load at a specific work height, then we would give that load a plus or minus 10 percent. A higher spring rate may require a wider tolerance. Any other working height that a customer might need, we would use as a reference to load. We would want to get close to that working height, but we wouldn’t necessarily tolerance it.
We can hold tolerances for diameter and wire size, similar to what we would hold for our retaining rings, and all those tolerances are listed in our catalog. We can hold for thickness, diameter itself and so on.
Free height is a tricky one because free height generally isn’t as important to people as the load or the spring. We generally tolerance the load, but if the load isn’t as important and we have to tolerance the free height instead, the answer is going to vary depending on what kind of spring it is.
If the spring is just a single turn spring, it’s much easier to control the tolerance, which is going to depend on the size of the spring.
If the spring is a multiple turn spring, like a crest-to-crest, all the tolerances per turn become cumulative. It might not be as tight a tolerance as something you had with fewer turns. It’s a difficult question to answer with just a general rule of thumb, because there are so many variables that go into that.
10. If you’re designing for high stiffness and load capacity, do you turn to heavier wires or would you use a different material?
BM: If someone needs a lot of stiffness but not a lot of deflection, we would go to something called a nested spring, where the turns are in parallel rather than in series like with crest-to-crest springs. When you put multiple turns in parallel, you will get a much higher spring rate.
If we’re just talking crest-to-crest, we can change the thickness of the wire and increase the number of waves, which gives you more spring rate. But whenever you increase spring rate, stresses go up with it. We always have to be careful about increasing spring rate.
For your own copy of Smalley’s latest Engineering & Parts Catalog, visit smalley.com. If you have questions that weren’t answered here, ask a Smalley expert.
Smalley Steel Ring Company has sponsored this post. It had no editorial input into this post. All opinions are mine. --James Anderton