b'c oMMone xtensions PrinG Hooks and Loops Extension springs make use of hooks or loops.Loops are closed while t erMinoLoGy hooks have an open side or section.Symbol Units Description FormulaA L0 mm, in Free length toleranceD mm, in Mean diameter D = De - dDe mm, in External diameter De = D + dDi mm, in Internal diameter Di = D - dd mm, in Wire diameter d = De - DE,Mpa, PSI Modulus of elasticity E = /F0 N, Lbf Initial tension Pi = 2P1 - P2L1, L2 N, Lbf Target loads erminology(related to target heights/lengths) No HooksMachine Hook and LoopFc Th N, Lbf Theoretical load/force at set solid These types of ends have oMachine hooks are a common stress or fatigue on the ends oftype of extension spring hook. Fn N, Lbf Load/force related to Ln (smalledthe extension spring. The amountThese hooks are stronger than length) of pulling force and distancecross over center hooks because fe Hz Natural frequency increases and the life cycles rethe radius of the bend to make k - Stress correction factor longer. No hooks have the abilitythe hook is not as pronounced. TL0, FL mm, in Free length/height to use a bolt to thread into the inner diameter of the extension or D1, D2 mm, in Target Length/distancetension spring thus securing the (related to target loads)ends of the spring Lc mm, in Solid length Lc = d(n+ni+nm)Ld mm, in Length of wire Ld = p D [ 2 + nm + n / Cos(z)]LK mm, in Buckling lengthLn mm, in Smallest allowed operatingLn = d (n + ni +nm) + Salength (gemometric)Lr mm, in Smalled allowed operating length (stress)M g, lb Mass M =Ld pd2 10-3 / 4m mm, in Spring pitch m = [ L0 - d (ni + nm) ]/n Crossover Center HooksExtended Hooks N Number of cycles Crossover center type extensionExtended hook extension springs n Number of active coils n = G d4 / (8 R D3) springs are very common.Thisare very useful when you need type of hooks is made by liftinga long length inside hooks but ni Coils related to the ends the last coil and twisting the coila short body length to get more nm Number of dead coils towards the middle thereforeforce out of the spring through nt Total number of coils nt = n + nm + 2 crossing the center. less coils. It is the most expensive R, KSR N/mm,Spring Rate R = G d4 / (8 n D3) orhook type. Lbf/in (L1-L2) / (D1-D2)Rm Mpa, PSI Ultimate tensile strengthSh mm, inSpring travel Sh = D1 - D2W Nmm,Stored energy W = 0.5(L1+L2)(D1-D2)Joulew - Spring index w = D / dSide Hooks This hook type is used when the body of the spring must not interfere with the components of a mechanism. This is due to the fact that the hooks are on one side of your spring therefore the other side of the spring is offset. They are made by simply bending the last coil so they are more economical than cross over center hooksstarrett.com 197'