Overview
In order for a fastener to perform its job, it must be appropriately pre-loaded. All fasteners shall have an installation torque called out on their assembly document. To determine the appropriate torque for a fastener, a specific value may be determined or a value my be used from the reference tables below.
Grade 8 Steel Fasteners
All calculations are for non-lubricated, non-galvanized fasteners.
| Size | Major Diameter [in] | Minimum | Nominal | Do Not Exceed | Notes |
|---|---|---|---|---|---|
| #2-56 | .086 | 2.1 | 2.5 in-lbs | 4 in-lbs | |
| #4-40 | .112 | 4.4 | 5.2 in-lbs | 8.7 in-lbs | |
| #6-32 | .138 | 8.2 | 9.6 in-lbs | 16.3 in-lbs | |
| #8-32 | .164 | 16.8 | 19.8 in-lbs | 33.7 in-lbs | |
| #10-24 | .190 | 19.4 | 22.8 in-lbs | 41 in-lbs | |
| 1/4-20 | .25 | 63.9 | 75.2 in-lbs | 143 in-lbs | |
| 5/16-18 | .313 | 112 | 132 in-lbs | 295 in-lbs | |
| 3/8-16 | .375 | 201 | 236 in-lbs | 528 in-lbs |
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Motor Case Bolt Torque Specifications
This will provide an example on how to calculate the optimal bolt torque, using calculations for the bolt torque specification of the Hermes 3 motor case.
While the upper section provides more general information, this will provide specifics to get a more optimal value of bolt torque.
Max Tensile Load
First we must calculate the maximum tensile load of the bolts being used. This is the measurement of the maximum amount of tension force the bolt can withstand before it fractures. In the case of Hermes 3, we are using 5/16-18 by 5/8" Grade 8 Steel Bolts.
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F (optimal clamp load) = 0.75 x 7800 == 5850 lbs
K Value, or the Nut Factor
In the equation for calculating torque, which is T = K x d x F, the value K is the most variable of the values. While d (the nominal diameter) and F (the clamp force) have very simple calculations to acquire their values, K is much more complex.
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Side Note: One reason we do not used zinc-plated bolts is because zinc becomes a gas at 300 degrees C, and our rocket burns at around 2800 degrees C.
Torque Calculation
Now that we have our tensile load value, as well as our K factor value, the optimal torque can be calculated.
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