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Dan, et al,
The purpose of torqueing down a bolt to a "specified" value is to provide a preload in excess of the actual tension loads to be placed on the bolt. The bolt is rated and sized to meet or exceed either the tension loads or shear loads to be placed on it. Let's discuss three different situations and the importance of torque.
1. Engine cylinder head bolts - sized and torqued to exceed the worse case tension load placed on them by the Newtonian reaction of the regular explosions occurring in the cylinder. We would not want them to be under stressed since each explosion would be "working" the stretch of the bolt. Also, the bolted together metal should not react by compressing under the bolt tension preload.
2. The enormous diameter Lancair 320/360 wing-spar main-spar attach bolts - These bolts main function is to act in shear so as not to allow the wings to fold up (or down). They also function in tension to add some rigidity to the main spar, but they are not torqued to the bolt standard since it would crush the aluminum bushings, plate and surely the pre-preg glass structure. I don't remember (and I don't have a build manual handy) on the suggested torque. This value would be sufficient for the required rigidity and the size would be apprpriate for the shear load. I think someday that I will clip the safety wires and re-check the torque of these bolts that I have not twiddled since 1995.
3. The four AN4 bolts used in shear to hold the forward main gear mounting plate to the 1/2 inch (?) phenolic block Hysoled in as an aft main spar web (and also covered with x layers of glass. We know we could tighten these bolts to their book torque value, but we don't know what value would cause the phenolic (or glass) to compress. We don't know whether their size was chosen merely for their shear resistance value or for their tension value. We know their shear value is improved by the friction between the plate and the block. This is known as the creak-torque value. We just don't know the number. The same is true for the rear spar. One thing I know from experience - Glass moves.
This is fun, but I gotta go take care of the dogs - in response to the leak-torque (arm twisting).
Scott Krueger
N92EX
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