October 2009<\/p>\n
by Newman<\/p>\n
Shear Pins<\/strong><\/p>\n <\/strong><\/p>\n Most rocketeers realize the need for shear pins through first-hand experiences:<\/p>\n In each case, the result is unintended and can result in either extreme damage or a long recovery walk.<\/p>\n 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.<\/p>\n Shear Pin Choices<\/strong><\/p>\n <\/strong><\/p>\n The most common shear pin materials used are styrene rods\/square tubing and nylon screws.\u00a0 Aluminum tape has a smaller following but I\u2019ll leave any discussion on that material up to people more knowledgeable.<\/p>\n In choosing a shear pin, the shear strength of the material is critically important.\u00a0 Most distributors will list tensile strength for a material according to a given standard of measure on their websites.\u00a0 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\u00a0 number.<\/p>\n 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.\u00a0 Styrene has a much broader range and can be anywhere from 1200 to7500 lbs.\u00a0 Keep in mind that shear strength is defined as the perpendicular force required to break the pin material.\u00a0 It is not tensile strength or the strength required to pull a material apart or deform it permanently.\u00a0 Therefore, again, the strength rating or tensile strength must be multiplied by .60 to arrive at shear strength.<\/p>\n The Calculations<\/strong><\/p>\n <\/strong><\/p>\n Shear strength of the pin is defined by<\/p>\n Shear lbs = (cross sectional area of the pin)(tensile strength * .6)<\/p>\n Example #1:<\/strong><\/em><\/p>\n For a Styrene rod, begin by calculating the area of the cross section:<\/p>\n Area = pi * (radius)^2<\/p>\n For a 1\/16\u201dstyrene rod,<\/p>\n Area = 3.1416 * (.03125)^2 = .00307 sq in<\/p>\n Shear lbs = (.00307)(5000 * .6) = 9.21 lbs per styrene shear pin<\/p>\n Assuming 3 pins are used, the \u201cholding\u201d force of the pins to be overcome will be 27.63 or 28 lbs .\u00a0 Coupler or nosecone shoulder friction will add to that, of course.\u00a0 Consider adding 5% or so just to be safe.<\/p>\n <\/em><\/p>\n Example #2:<\/strong><\/em><\/p>\n Consider a #4 nylon screw.\u00a0 Calculations are a bit different in this case due to the screw threads.\u00a0 Most source sites will list the screw dimensions using 3 metrics:<\/p>\n Most people use either the minor or pitch thread diameters for calculating shear force.\u00a0 Let\u2019s use the minor diameter in the example with 6\/6 #4 nylon screws:<\/p>\n Shear lbs = (cross sectional area of the screw)(tensile strength * .6)<\/p>\n #4-40 screw has a minor diameter of .0813\u201d<\/p>\n Tensile strength is 12,400 lbs<\/p>\n Shear is approximately 60%<\/p>\n Shear lbs = [(.0813\/2)^2*3.1416]*12,400*.6 = 38.62 or 39 lbs<\/p>\n 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.<\/p>\n With this information, it\u2019s time to determine deployment charge size.<\/p>\n Other Considerations<\/strong><\/p>\n 1.\u00a0 Equally space shear pins around the circumference of the rocket and near the bottom (within .25\u201d – .50\u201d or so, if possible) of the NC shoulder or airframe coupler.\u00a0\u00a0 The placement will lessen the rare possibility of a pin \u201cwedging\u201d in a sloppy fit between a coupler\/shoulder and the airframe after breaking.<\/p>\n 2.\u00a0 Before drilling, try to mate the airframe\/nosecone to adjoining pieces of components to minimize any gaps.\u00a0 Twist to fit and sand as necessary to reduce large gaps.\u00a0 Slightly separate the pieces and use a fine tip Sharpie to mark a \u201ckey\u201d on the edge or the airframe and on the coupler wall for future assembly.\u00a0 Put it together again and use blue masking tape to keep the pieces together in place.\u00a0 Use a seamstress tape measure to mark where to drill the holes.\u00a0 I prefer to drill shear pin holes after painting (it avoids clogging the holes with paint and having to redrill).\u00a0 I like to put a piece of masking tape at the drill point, make the mark and then drill.\u00a0 The tape helps to avoid chipping the paint and protects it from damage caused by hitting it with the chuck.<\/p>\n 3.\u00a0 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.\u00a0 An extra step that can prevent elongation in non-composite components and especially in plastic nosecones is to use a \u00bd inch square piece of 24 ga or so brass sheet to reinforce where the shear pin holes are drilled.<\/p>\n Use a Dremel to remove material equal to or a bit greater than the thickness of the brass on the coupler\/shoulder.\u00a0 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.\u00a0 Remove the tape, sand the excess epoxy and\/or brass edges to conform to the coupler\/shoulder. \u00a0\u00a0Now, after lining up\u00a0 and assembling sections, drill through the existing shear pin holes to provide holes through the brass.\u00a0 You\u2019re good to go!<\/p>\n 4.\u00a0 Some fliers prefer to \u201ctap\u201d shear pin holes when using screws.\u00a0 It\u2019s not necessary but certainly can\u2019t hurt.\u00a0 The important thing is to avoid a loose fit and there\u2019s even some leeway in that instance<\/p>\n 5.\u00a0 It is prudent to take care of the shear pin material.\u00a0 Plastic rods and screws are subject to deterioration as a result of age, exposure to the elements and ultraviolet light.\u00a0 As with just about anything involved in rocketry, repeatability is a desired trait.\u00a0\u00a0 Keep shear pin materials \u201cfresh\u201d.<\/p>\n 6.\u00a0 Ground test, ground test, ground test.\u00a0 Can\u2019t say it enough.<\/p>\n\n
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