"Tiny Tears" Teardrop Camping Trailers
Technical Tips
Epoxy Tips
17 August 2002
The following technical tips come from "Epoxy Works" Number 13,Spring 1999. This is a magazine published by the Gougeon Brothers, makers of WEST System epoxies. They can be found at http://www.westsystem.com
Springback, useful for laminating kitchen hatch
frames.
Story: "When a strip of wood, a ply, is bent to a curve and released, it springs back to its orininal shape. A stack of loose plies will do the same. When these plies are glued together while clamped to a curve, they tend to stay curved, but will straighten out or springback a little. When building the mold or form for the laminate it's common to make the curve a little tighter to compensate for the springback. The amount of springback depends on the number of plies in the laminate, not in the ply material or the ply thicknes. A simple formula can help predict the amount of springback you can expect: y=x/(n*n) where y=the amount of springback, x=the amount of deflection, and n=the number plies. Thus with four plies, the springback (y) will be 1/16" if the amount of deflection (x) is 1". By using plies half the thickness you double the number of plies for a given thickness of laminate and reduce springback four times - in this case 1/64".
Story: "Hole size: For most applications wherethe shear strength is greater than 800 psi, we recommend a hole sized 0.25"larger in diameter than the major diameter of the bolt. This givesan 0.125" epoxy annulus around the bolt. We vary the length of the bolt and depth of the hole to balance the breaking strength of the bolt versus the withdrawal resistance of the epoxy plug. The 800 psi epoxy shear strength is the limiting value. Example: A 3/8"diameter 18-8 stainless-steel bolt with an UTS of 85 ksi and effective cross-sectional working area of o.0775 square inches has a breaking strength of 6,587 lb. The hole depth should be the breaking strength (6,600 lb) divided by the epoxy shear strenth (800 psi), divided by the circumference of the hole (0.625" x 3.1416 = 1.9635") to equal 4.2". In actual practicewe used a 5/8" diameter hole with a dpeth of 4.5" for a 3/8" diameterbolt with a 4.25" of bonded length"
"Wood Screws - Tests done here on side-grain withdrawal of wood screws show a 100% increase in the holding power of a fastener inbedded in a 105/205/404 mix over the same fastener dry hole."