# Metal spring thermal expansion

I am attempting to figure out why a laser layout tool I have is drifting over time. It was designed with a small adjustment mechanism that relies on a spring to apply pressure to a fulcrum that rotates the entire laser. A screw is used to apply and release pressure on the spring thus moving the laser very small amounts left and right.

I am getting a drift in the lasers horizontal position of about 1/2inch at 100 feet over the course of several hours. I am doing this test at night and there is about a 15 degree Fahrenheit temperature from when I start the test to when it is complete.

I made a Sketch-Up model of the device and determined that 5 micron change in position (at the end of the fulcrum) is all it would take to move the laser 1/2 inch at 100ft.

My question is it possible that my spring is expanding and contracting 5 microns with a 15 degree temperature change?

I am not an engineer and am looking for some speculation just to rule in or rule out the general possibility.

The spring looks similar to this:

The diameter of the entire spring is about 1/4 inch. The diameter of the coiled metal wire is about 1/6 inch. The spring length is about 1/2" uncompressed. It consists of about 8 loops.

I do not know the metal it is made of, speculation on different scenarios would be helpful as, again, I'm just trying to rule in or out the possibility.

• diameter of the coiled metal wire is about 1/6 inch - did you mean 1/16"? Also, a picture or diagram of your setup would be helpful to point out possible sources of drift. Right now we're only guessing at what your setup looks like. Aug 28 '15 at 14:15
• It would be very helpful if you could post an image of the SketchUp that you made. Are you sure that the alignment device depends on the spring for alignment? Most laser alignment devices (adjustable mirrors, stages, etc.) only use the spring to maintain contact between the movable part and an adjustable screw drive. At any rate, 0.5 mrad isn't an exceptionally large thermal drift. Aug 28 '15 at 14:38
• Could you artificially heat the spring (and surrounding area) in a shorter time period to ascertain whether it's thermal expansion of that section causing the issue? Apr 18 '18 at 21:05

## 2 Answers

Assuming the spring is not fully compressed/bottomed out, it doesn't really matter if the spring material is expanding or not because it's a spring - it will deform to fill the space between the coils.

Instead of providing an increasing distance, it will increase the force the spring exerts on your fulcrum. This shouldn't be an issue if you have the adjustment locked in with a set screw or similar.

What could be happening is that the drift isn't due to the spring but some other part. This table gives some linear expansion coefficients. The metric side's units can be read as "microns of expansion per meter of original length, per degree Kelvin."

So, if you have an aluminum screw 1 meter long and you heat it 1K, it expands 22 microns. 0.5cm in original length, it expands 0.11 microns. If the temperature rise were 8.3K, about 15F, then a 0.5 cm screw would expand about 0.9 microns.

I would say the effect you are seeing might be thermal expansion, but I would check other places for drift too, such as not locking down the setting after the adjustment is made.

Yes, it is the spring that twists as its contracts and expands due to temperature. Change the setup, no other solution. There may be one or two reason, one is the shape of the spring on an isotropic expansion the second is because the spring is made out of a straight wire, it may not be to isotropic anymore. Actually, both effects are acting together.