October 2009

by Newman

Shear Pins

Most rocketeers realize the need for shear pins through first-hand experiences:

  • A single deployment project unexpectedly puts out the chute as the recovery package slides hard against the nosecone at burnout.  Typically a result of fast-burning motors and light rockets.
  • Somewhat similarly, a rocket separates between the fin can and the payload bay at motor burnout (drag separation).  Hard to fly right after “max Q” with your rocket in two pieces.
  • A nosecone comes off and spills the laundry at apogee due either to a “crack the whip” or a horizontal deployment at high speed in a dual deployment project.   Get out the hiking boots – time to see the countryside.
  • A rocket with an unvented payload bay “pops” a nosecone due to the difference between the trapped internal air and the external air pressure.  “Pop” goes the Velociraptor!

In each case, the result is unintended and can result in either extreme damage or a long recovery walk.

This article will discuss common materials used for shear pins and will detail the calculations necessary to determine the force required to break the shear pins consistently.

Shear Pin Choices

The most common shear pin materials used are styrene rods/square tubing and nylon screws.  Aluminum tape has a smaller following but I’ll leave any discussion on that material up to people more knowledgeable.

In choosing a shear pin, the shear strength of the material is critically important.  Most distributors will list tensile strength for a material according to a given standard of measure on their websites.  Specifications can vary between distributors so try to be consistent in sourcing material. If tensile strength only is listed, assume shear is 60% of the stated  number.

For example, the nylon material most often used for shear pins is Nylon 6/6 with a tensile strength rating of between 10,500 and 12,400 lbs.  Styrene has a much broader range and can be anywhere from 1200 to7500 lbs.  Keep in mind that shear strength is defined as the perpendicular force required to break the pin material.  It is not tensile strength or the strength required to pull a material apart or deform it permanently.  Therefore, again, the strength rating or tensile strength must be multiplied by .60 to arrive at shear strength.

The Calculations

Shear strength of the pin is defined by

Shear lbs = (cross sectional area of the pin)(tensile strength * .6)

Example #1:

For a Styrene rod, begin by calculating the area of the cross section:

Area = pi * (radius)^2

For a 1/16”styrene rod,

Area = 3.1416 * (.03125)^2 = .00307 sq in

Shear lbs = (.00307)(5000 * .6) = 9.21 lbs per styrene shear pin

Assuming 3 pins are used, the “holding” force of the pins to be overcome will be 27.63 or 28 lbs .  Coupler or nosecone shoulder friction will add to that, of course.  Consider adding 5% or so just to be safe.

Example #2:

Consider a #4 nylon screw.  Calculations are a bit different in this case due to the screw threads.  Most source sites will list the screw dimensions using 3 metrics:

  • The narrowest diameter or “minor diameter”
  • The largest diameter or “major diameter”
  • The “pitch diameter” which is basically the diameter midway between the width of the major thread and the space threads.

Most people use either the minor or pitch thread diameters for calculating shear force.  Let’s use the minor diameter in the example with 6/6 #4 nylon screws:

Shear lbs = (cross sectional area of the screw)(tensile strength * .6)

#4-40 screw has a minor diameter of .0813”

Tensile strength is 12,400 lbs

Shear is approximately 60%

Shear lbs = [(.0813/2)^2*3.1416]*12,400*.6 = 38.62 or 39 lbs

Again, using 3 equally spaced pins, the force required to shear all 3 pins would be 3 * 39 = 117 lbs plus 5-10% for the friction between couplers or NC shoulders and the airframe.

With this information, it’s time to determine deployment charge size.

Other Considerations

1.  Equally space shear pins around the circumference of the rocket and near the bottom (within .25” – .50” or so, if possible) of the NC shoulder or airframe coupler.   The placement will lessen the rare possibility of a pin “wedging” in a sloppy fit between a coupler/shoulder and the airframe after breaking.

2.  Before drilling, try to mate the airframe/nosecone to adjoining pieces of components to minimize any gaps.  Twist to fit and sand as necessary to reduce large gaps.  Slightly separate the pieces and use a fine tip Sharpie to mark a “key” on the edge or the airframe and on the coupler wall for future assembly.  Put it together again and use blue masking tape to keep the pieces together in place.  Use a seamstress tape measure to mark where to drill the holes.  I prefer to drill shear pin holes after painting (it avoids clogging the holes with paint and having to redrill).  I like to put a piece of masking tape at the drill point, make the mark and then drill.  The tape helps to avoid chipping the paint and protects it from damage caused by hitting it with the chuck.

3.  Composite airframes and NC shoulders will hold a shear pin hole very well; phenolic and cardboard material less so but can be reinforced with CA if the holes begin to enlarge through use.  An extra step that can prevent elongation in non-composite components and especially in plastic nosecones is to use a ½ inch square piece of 24 ga or so brass sheet to reinforce where the shear pin holes are drilled.

Use a Dremel to remove material equal to or a bit greater than the thickness of the brass on the coupler/shoulder.  Roll the brass to fit to the shape of the shoulder/coupler, rough up the epoxy side of the brass with 60 grit sandpaper, epoxy it into place, tape it securely and let the epoxy cure.  Remove the tape, sand the excess epoxy and/or brass edges to conform to the coupler/shoulder.   Now, after lining up  and assembling sections, drill through the existing shear pin holes to provide holes through the brass.  You’re good to go!

4.  Some fliers prefer to “tap” shear pin holes when using screws.  It’s not necessary but certainly can’t hurt.  The important thing is to avoid a loose fit and there’s even some leeway in that instance

5.  It is prudent to take care of the shear pin material.  Plastic rods and screws are subject to deterioration as a result of age, exposure to the elements and ultraviolet light.  As with just about anything involved in rocketry, repeatability is a desired trait.   Keep shear pin materials “fresh”.

6.  Ground test, ground test, ground test.  Can’t say it enough.