Shear Pins

Last updated 08/19/2004

The test fixture is made of a carbon fiber coupler and a length of 38mm body tube. 3/4" plywood bulkheads were epoxied into each section and a welded eyebolts were secured with nuts washers. Several holes were drilled in the assembly to test various shear pin sizes and quantities. The picture on the left shows the test fixture. The picture on the right shows it in action, testing four #4 nylon screws. Click on them to see a larger image.

The first thing you'll notice is the high number of tests. I was not comfortable with the variation in data. I was using my small 500 lb load cell that was calibrated from 0 to 500 lbs with a 0.00005 lbs resolution. The accuracy is +/- 0.0001 lbs. The column below "Peak Load Each Pin" should all be the same for the same size pin. The rate of loading for this test was 20 IPM.

Discussion

It can be seen that when using only two shear pins: 1) the average peak load is higher for each pin and, 2) the spread within the average is greater. (The spread within an average is known as sigma.) This was true for both the #2 screws and the #4 screws. It was determined that only using two pins allowed the assembly to cock to one side causing a binding effect, thereby increasing the load variance (sigma) and average.

Using three or more pins eliminated this binding effect, producing a much lower sigma and much more consistent results.

How to use this data

Using shear pins is a tried and true method of keeping a recovery compartment secure prior to deployment. Friction fitting a coupler works, but is not consistent. We can see that using nylon screws create a consistent and predictable way of securing your coupler assembly. Knowing the load to overcome during ejection is a helpful way to determine how much ejection powder you need to both shear the pins and eject your recovery system. Remember that you still need to ground test your recovery system but using nylon screws creates a predictable and consistent recovery system.

Below is the same test as described above. The fixture was made the same way, but is made from 2.25" USR white glassine cardboard tubing. The holes were drilled and stiffened with liberal amounts of Cyanoacrylate. The #4 screws began to tear up the tubing rather quickly. The tubing held up rather nicely to the #2 screws.

There is not enough data to draw any conclusions.

Below are tests using polystyrene fork tines. These were tested in the carbon fiber fixture. The notes column on the right describes the hole size in which they were tested. The results vary widely, most likely due to the taper in the tines and probable inconsistencies in the material.

Although many people use them, this test shows that using fork tines is a poor idea. There is absolutely no consistency or predictability.