# Designing and making springs with music wire

1. May 22, 2009

### elect_eng

I would like to make some custom made extension springs out of music wire. Can anyone provide the formula's to determine the wire thickness, coil diameter and coil length to achieve a desired spring constant?

The motivation is that I need some small springs in my research and it is difficult to find existing springs that match what I need. I've been told that it is relatively easy to wind your own springs with music wire. I'm sure I could derive the formula's, or just experiment, but this will take time. If some ready made formulas and material properties for music wire are available, I wouldn't mind saving some time.

2. May 22, 2009

### Staff: Mentor

I googled spring design, and got lots of good hits. Here's one of the first hits, and it appears to have a spring calculator link at the left:

http://www.efunda.com/DesignStandards/springs/spring_introduction.cfm

.

3. May 22, 2009

### Cyrus

Get a copy of Shingleys Mechanical Engineering Design. Its in there.

4. May 22, 2009

### Q_Goest

5. May 22, 2009

### elect_eng

Thank you all for your very good help! This puts me in good shape.

I still will welcome any additional suggestions. In particular, if anyone has experience winding small diameter (1 cm) but long length (50 cm) extension springs with music wire, I'm sure I could benefit from a quick roadmap of the tricks and pitfalls.

I also, have a question about something I learned today about extension springs. Apparently, many extension springs are made preloaded in a way that a positive offset force (i.e. greater than zero) must be applied before the spring will start to open it's coils. I can't help but wonder how these are made. Does anyone know the basic principle involved with forming an extension spring, having constant diameter, with a preloaded compression force built in?

6. May 23, 2009

### Cyrus

They twist the wire as they coil it. This gives it a preload.

Note: You really don't want to try and make these yourself. I was told they are very, very hard to make.

7. May 23, 2009

### elect_eng

Thanks for the explanation. That one had me scratching my head, but that makes sense now.

Do you know what the issues were in making the springs? I'm not looking for high quality and I don't need to have a pre-load built in, but my requirements are unusual and I haven't found any commercial springs that come close. If I don't succeed in making my own, I'll have to have them custom made by a spring company. This creates a long delay and considerable expense. The work is important enough to me so that I will go down that road if I'm forced to.

Oddly enough, I can get the spring constant I want with elastic rubber bands, but the rubber has too much damping for my application. So if anyone knows a material similar to rubber, but with low damping, I can consider that too.

8. May 23, 2009

### Q_Goest

Hi elect_eng. What can you tell us about the spring you want designed? I'm assuming a conventional wound compression spring is all you're looking for. If you provide the following I'll tell you if it's possible, how many coils, etc...
Must pass over diameter:
Must fit into diameter:
Free Length:
Spring constant needed:
Is this a cyclic load (yes/no) explain:
If you have a specific wire you want to use, what is it (material type and diameter):
Any unusual temperature range? Fluids it may be in contact with?
If it's not a conventional compression spring, I probably won't be able to help.

9. May 23, 2009

### elect_eng

Thank you so much. I appreciate your very kind offer. Actually, I need extension springs in my application. I think I can provide the information you listed above, but I will understand if extension springs are different enough from compression springs to generate too much additional time investment for you to justify (for a stranger ). I will list the things I know off the top of my head just in case it helps you (or anyone else) provide some basic pointers for me.

No passover diameter required
Must fit in 1 cm diameter
Range of motion: zero to 0.7 meters of extension beyond unloaded length
Unloaded length: ideally < 0.3 meters (but any length necessary can be tolerated)
No pretension necessary
Spring Constant (at extension distance 0.35 m): 3.1 Newtons/meter
Linearity of spring constant is not too critical, but hard to quantify
Static Load conditions: 0.11 kg mass hanging vertically at 0.35 m in equilibrium
Operates oscillating continuously at 1.2 sec period with max amplitude 0.35 m
Room temperature operation in air
No material requirements but low damping needed (assume music wire is appropriate)

10. May 23, 2009

### Q_Goest

I generally use inches and pounds so I'm converting here. My understanding, you want to hang a .11 kg weight (1.08 N) on this spring, and it will oscilate such that it will extend the spring to 0.7 meters. Spring rate is 3.1 N/m, so load at an extension of 0.7 m is 1.24 N. Free length is 0.3 meters.

Assuming a spring OD of 9 mm (to stay inside 10 mm) and a wire diameter of 0.018 inches (0.457 mm) requires a total of 224 turns assuming music wire. Solid height is about 4.06 inches (0.103 meters). Free length can therefore be made at the 0.3 m length you requested. Stress should be low enough (44 ksi) so it should have infinite fatigue life.

If you get a spring manufacturer to do this I'd guess the cost will be a few hundred $. That's just a ROM cost (rough order of magnitude). Or you can buy a roll of wire from McMaster Carr for about$6 (part number: 9666K18).

Last edited: May 24, 2009
11. May 23, 2009

### elect_eng

Wow, thank you so much for your help. This saves me so much time. I can basically get in the ballpark and start doing experiments to see how effectively I can make my own. If this doesn't work out, the expense of a few hundred dollars is not too bad and can be justified.

By the way, I play guitar and the 0.018 inch wire corresponds to the G string (third string) on a typical steel string guitar. So I can actually play around with this while the wire spool is on order. Clearly, the length of a guitar string is too short, but I can experiment with the winding methods and tools that are needed.

I hope a day comes when I can return the favor in some way!

12. May 25, 2009

### Cyrus

Let's see it working first... I think its going to be harder than you think. (Hopefully not though!)

13. May 26, 2009

### elect_eng

Yes, things are always harder than you think, and I never thought it would be easy, so it could be really hard . However, it seems cheap enough to try. I'd like to say, it can't hurt to try, but actually it could be dangerous if I'm not careful! (I will be careful though) The good news is that, if I fail, the cost of custom springs from a professional may be cheaper than I originally thought.

14. May 29, 2009

### elect_eng

I thought I'd report back on the progress here. Thanks to the good advice given to me, I was able to successfully make springs today. I received the music wire this morning and by afternoon I had made springs suitable for my application.

One key to quick success was being put in the right ballpark with Q_Goest's initial design. The choice of 0.018 wire seems to be right on. I experimented with 0.014 wire too, but it was too difficult to work with those springs. The slightest over-stress and the spring deformed.

Another key was the information from Cyrus about using twisting to create pretension in the spring. This turns out to be important because if I just wind the spring without twist, the coils end up spaced out too far and the free-length of the spring is too long. A little twist produces nice tight coils and it was even possible to achieve pretension.

It was amazing how easy this was once I got the hang of it. I made a winding tool out of a long screwdriver and a tap-holder. Then I clamped the wire in a vise - pulled the wire straight - put some twist and tension and then started spooling the wire on the screwdriver (0.25 inch diameter shaft). The trick is to get the right number of twists which is quickly discovered by trial and error. Another trick is to figure out the spooling diameter needed to end up with the final diameter needed. The coil needed to be wound at 6 mm diameter to end up at about 9 mm diameter. Again, just trial and error.

I'm now planning to improve the quality of the springs with better tools to maintain constant tension as I form the spring. This should result in a professional looking spring but even today's springs work.

Thanks again for the good advice. Perhaps next week I'll post a picture of a spring.

15. May 29, 2009

### Cyrus

Cool, post a picture of it! My hats off to you sir.

16. May 29, 2009

### Staff: Mentor

Yes, very cool. I learn something new every day here on the PF.

17. May 31, 2009

### elect_eng

I'm posting two pictures of a spring which is very close to what I need to make. With a 0.1 kg mass, it has 1.1 s period, which is close to the 1.2 s period that I need. I just need to make another that is little longer and I'm there.

This spring was made with about 25 feet of wire with 2 full twists per foot and 10 pounds of line tension. I made this one using a controlled tension, but I really didn't see much better results compared to when I just put a little tension by hand. However, pre-twisting the wire is critical to get nice tightly spaced coils.

It works fine for my application, but I find that it is very difficult to control the pretension over the length of the long coil. The beginning of the coil ends up with a lot of pretension and the end has almost none. However, since I don't need pretension, I was able to slide the spring over a 3/8 inch rod and stretch out the coils with pretension. You can visually see the non-uniformity over the length, but this is just a visual thing that does not affect the overall spring performance. You end up with a spring with low-damping, high natural frequency and an effective spring rate that can be matched to the specification.

One picture is the free spring lying on the table with a centimeter scale ruler in view. The other shows the spring extended while holding the 0.1 kg mass in equilibrium.

Thank you all again !!!

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18. May 31, 2009

### Cyrus

Good god that a long spring. I wouldn't want to coil all that wire! Hahah, good job though.

19. May 31, 2009

### Q_Goest

Thanks for the pics. Maybe you should go into business making hand wound springs!

20. May 31, 2009

### elect_eng

Now I can't resist showing a picture of his big brother. He has a period of 1.6 s when attached to a 0.1 kg mass.

This next spring came out a little better and I didn't use the 10 pound constant tension this time. I also used less pre-twist (1 twist per foot), which seems to be about right. I figured that if I made an extra long spring, I could cut out the section that looks best.

However, once again, there was pretension in the first part of the spring, and almost none at the end of the spring. Does anyone know the physical explanation for this effect? I figured a pre-twist would distribute itself evenly over the whole length, but it seems that the beginning of the coiling process wants to take up more of the existing pre-twist. I'm wondering if this is expected, or maybe I have to look and see if a step in my process is inducing this effect.

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