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Overview
The Rocket Team uses bolts as its standard method of fastening the motor. This page will provide information on bolt torque, bolt shear stress, and bolt thread engagement calculations.
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Motor Case Bolt Torque Specifications
Grade 8 Steel Fasteners
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 table provides more general information, this section will provide specifics to get a more optimal value of bolt torquemore specific calculations.
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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30.5 ft/lbs is now our final value for the optimal torque. This is necessary to guarantee that the bolt is not too loose (which would cause our seal to not be as strong), or too tight (which would deform our bolts and potentially damage the material being clamped).
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Motor Case Bolt Shear Stress Calculation
When building a motor with a radially bolted case, you must make sure that your bolts will be able to stand up to the forces applied to them. To demonstrate this, we will look at the Phoenix booster as an example.
Calculated Booster Max Booster Pressure = 826 psi
Forward Closure Inner Area = 10 in^2
Number of Bolts = 24
First, we must calculate the force on the forward closure due to this pressure.
F = P*A
F = 826*10 = 8260 lbs
This force is assumed to be perfectly distributed among each bolt, so the force on each bolt is:
8260/24 = 344.16 lbs
Our bolt is under a single shear, which looks like the reference diagram below. In order to evaluate the shear stress, we need to use the equation on the right. Our bolts have a diameter of 1/4 of an inch.
F = 344.16 lbs
D = 0.25 in
T = (4*344.16)/(pi*0.25^2) = 7,011.17 psi
That is the tensile load that each bolt experiences, so now we must compare it to the max shear rating of the bolt. The bolts we use are here: https://www.mcmaster.com/91253A537/. Using the specs here, we can calculate:
Actual Shear = 7011.17 psi
Max Shear = 120000 psi
Factor of Safety = 120000/7011.17 = 17.12