SPRING 2011

10 THE DISTRIBUTOR’S

10 THE DISTRIBUTOR’S LINK Guy Avellon Guy Avellon has been in the MRO and fastener distribution industry for over 30 years. He began his metallurgical engineering career at Republic Steel Research Center in metal coatings and has since held positions as sales engineer; Chief Engineer; Manager of Product Marketing, Product Engineering and Quality and Director of Quality and Engineering. He founded GT Technical Consultants where he performs failure analysis, presents lectures on fastener safety, worked for law firms and designs and audits Quality systems. He is a member of SAE and is Vice Chairman of the ASTM F16 Fastener Committee and Chairman of the F16.01 Test Methods Committee, since 1988. He also received the ASTM Award of Merit in 2005. Guy and his wife, Linda currently reside in Lake Zurich, IL and may be reached at 847-477-5057 or visit his website at http://www.BoltDoc.com. HOW TORQUE VALUES VARY As we have seen before in prior articles, friction and the lack of friction, or rather the presence of any type of lubricant, greatly affects the applied torque and clamp load the fastener produces, as well as the integrity of the joint. What we are now going to show is how torque relates to clamp load and how the ‘k’, a representative factor for the coefficient of friction, influences the torquetension relationship. Going back to our simple formula for calculating torque, we have: Torque = k x D x W Using the above formula will provide torque in pound-inches. To compute our familiar pound-feet values we must divide this product by twelve (12), hence: Torque = k x D x W ÷ 12 = pound-feet k= the “coefficient of friction”, determined experimentally. D= the nominal diameter of the fastener (1/4”, 3/8”, etc.) W= the preload with a 25% safety factor. The value for W is derived from taking the proof load and multiplying it by 75% to provide for the fastener’s safety factor. For example: proof loads for Grade 5 fasteners are: 85,000 psi from 1/2 - 1”, 74,000 psi from 1 1/8 - 1 1/2”, and 55,000 psi from 1 1/2 - 3”; Grade 8 fasteners are 120,000 psi from 1/2 - 1 1/2” and 180,000 psi fasteners are around 135,000 psi 1/2 - 1 1/2”. Some examples of common ‘k’ factors are: k = 0.30 Black, non plated fasteners. k = 0.235 Socket Head Cap Screws, as received. k = 0.20 Zinc plated k = 0.17 Phosphate and oil k = 0.15 Zinc plated locknut with wax (also depends upon the wax) k = 0.12 Metallic Anti-Seize (copper, nickel, aluminum with wax) k = 0.09 Cadmium plated locknut with wax Again, these values can change slightly by viscosities, chemistry and even temperature. The ‘k’ factor can be determined by using a small fastener into a test plate. Simply apply the unknown lubricant to the test fastener and assemble it into the test plate. Then tighten the nut using a beam or dial indicating torque wrench. As the nut is tightened, the indicator will increase in value proportionately to the tightening effort, and then will begin to slow and stop altogether even though you can still turn the nut. At this point the fastener has experienced yield. Take the highest reading and subtract between 25 - 30% for a safety factor. Calculate the per cent difference and use this for any size or grade fastener as the ‘k’ factor. Let’s now apply the formula to a few conditions: - For a non-plated 3/8”-16 Grade 8 cap screw: T = k x D x W ÷ 12 W = 9300 x 75% = 6,975 lbs. T = 0.30 x 0.375” x 6975 lbs ÷ 12 T = 65 lb-ft. - For zinc plated 3/8”-16 Grade 8 cap screw: T = 0.20 x 0.375” x 6975 lbs ÷ 12 T = 44 lb-ft. - For zinc plated with anti-seize 3/8’-16 Grade 8 cap screw: T = 0.12 x 0.375” x 6975 lbs ÷ 12 T = 26 lb-ft. - Using a non-plated 1/2”-13 Grade 8 cap screw: T = 0.30 x 0.50” x 12,788 lbs ÷ 12 T = 160 lb-ft. - A zinc plated 1/2”-16 Grade 8 cap screw: T = 0.20 x 0.50” x 12,788 lbs ÷ 12 T = 107 lb-ft - For zinc plated with anti-seize, 1/2"-16 Grade 8 cap screw: T = 0.12 x .050” x 12,788 lbs ÷ 12 T = 64 lb-ft. Here are two examples of three different surface finishes commonly used with fasteners. The same torque cannot be used in all cases. This is also true with socket head cap screws because they are tightened by the head and have a slightly oiled surface. Always use the manufacturer’s recommended torque for these products. However, these values will change if a lubricant is added. It can clearly be seen how using the incorrect torque value can easily place a lubricated fastener into yield. It is especially difficult to gauge the correct torque when not using a torque wrench but by merely going by feel. please turn to page 71