Stud Preload Maintaining the preload on the stud is critical for maintaining contact pressure and contact resistance Using a long narrow bolt results in a much higher bolt elasticity than that of the Flag (10X). Applied cyclical loading adds relatively small additional loading to the stud. With higher elasticity, loss in preload due to deflection is minimized.
Preload Continued Belleville washers are used to account for any unexpected yielding of bolt or copper –While the bolt length provides adequate elasticity to accommodate design load scenarios the addition of Belleville washers prevents relief of the preload in the event of unanticipated strain or creep –The washer has a stiffness equivalent to that of the bolt, for every.001 inch strain 220 lbf preload is lost (total washer deflection =.026”) –With a strain as high as.010” washers will prevent preload from dropping below 3,900 lbf. (The washer and the bolt each relax.005”) –Testing of prototype will verify that preload is maintained. A washer plate is added to spread out the compressive forces under the nut and minimize local yielding of the copper. Bolts to be pre-tensioned with a hydraulic puller to eliminate stored torque along the length of the bolt.
Flag Stud Characteristics Fastener is a 3/8”-16 stud threaded at both ends To increase elasticity the bolt shank diameter is just slightly larger than the root diameter of the threads (a creep of.001” results in a loss of 210 lbf of preload) Loading: –A preload of 5,000 lbf is applied with an equivalent root diameter stress of 64,900 psi –Thermal loading after ratcheting applies an enforced deflection of.0043 inches corresponding to a stress adder of 6,300 psi with compression washers –As a result of stiffening the hub structure additional mechanical loading is minimal so that almost all fatigue loading is the result of thermal stress With the 5,000 lbf preload and the thermal stresses applied the bolt sees a mean tensile stress of 69.4 ksi and a mean amplitude of 4.5 ksi The ultimate tensile strength for the Inconel 718 bolt is 210 ksi and the yield is 185 ksi
TapLok Threaded Inserts A “TapLok” 3/8-16 “Medium Length” insert is used (OD into copper is.50”) Loading: –The bolt preload of 5,000 lbf results in 11,800 psi in shear at the outer threads of the insert into the copper. –Thermal + Mechanical loading (with washers) adds a cyclical load of 1,600 psi Thermal loading is due to ratcheting which occurs at a much less frequent rate (once per day = 1,000 cycles total) than the mechanical loading. Per the inspection certification the Cu Tensile strength = 38 kpsi and Yield strength = 36 kpsi. Values of 34 kpsi used for yield to account for the observation of slight degradation to hardness after thermal cycling With the 5,000 lbf preload and the mechanical + thermal stresses applied the copper threads see a mean shear stress of 13.2 kpsi and a mean cyclical amplitude of 1.4 kpsi
Threaded Inserts (cont.) Testing shows margins may be greater than the numbers indicate. The lowest pull out force measured = 11,500 lbf equivalent to 27 kpsi ultimate shear strength in the copper (as compared to 22 kpsi) The insert was tested for both pull out strength and pull out strength after cycling Cyclical testing did not result in any degradation to pull out strength for the test samples Further testing is planned. A mechanical prototype will test for maintenance of preload after application of a cyclical load.
Modified Goodman Diagram for Insert in Copper Conductor
Shear Key, Bolts Shear Key added as most effective way to react vertical load Custom compression washers are used to maintain preload. (Provides.007” deflection) Use of Inconel Bolts ensures large safety margins even with loss of preload and without friction
Shear Key, Threaded Copper Unlike the Flag studs the Shear Key bolts have more depth of copper available (and less width) so no inserts are used Testing indicated similar pullout strength for the deeper tapped holes without inserts Analysis indicates adequate shear area for both cyclical and static loading Analysis indicates strength is adequate to survive off normal conditions Testing included monitoring of hardness and subsequent pull out testing of threads on soldered test samples
Modified Goodman Diagram for Shear Key Bolt in Copper Conductor
Summary, Acceptance Criteria As plotted against the 20x life fatigue curve nominal design points fall within acceptable limits for inserts and copper threads. Copper threads and Inserts were tested at 2x nominal design cyclical stress values at 50,000 cycles or greater with no failure Static stress values fall within 2/3 yield for stress in the copper threads Analysis and testing also predict that failure of preload and or friction will not lead to failure in the copper threads (albeit with smaller margins of safety) Stress values for the Inconel Shear Key Bolts and Flag Studs have larger margins of safety for all of the above criteria