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 |
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 more specifics to get an optimal amount of torque.
Max Tensile Load
First we must calculate the maximum tensile load of the bolts being used. In the case of Hermes 3, we are using 5/16-18 by 5/8" Grade 8 Steel Bolts.
The equation for this is:
P = St x As
P = tensile load (lbs,. N)
St = tensile strength (psi, MPA)
As = tensile stress area (sq. in, sq. mm)
Using the following spreadsheet,