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Engineering > Meetings

A meeting was held on February 2 to discuss bolted joint issues.

Review of design methodology and criteria for bolted joints. HM Fan did an excellent job documenting a design methodology and criteria for bolted joints.  The document was reviewed and accepted w/o any changes.  Williamson will develop a spreadsheet consistent with the proposed methodology and criteria.  Zatz noted that the NCSX Structural and Cryogenic Design Criteria document is basically silent on friction coefficients for metallic structures.  Until we get this settled, please use the following provisional guidelines.

The allowable coefficient of friction (a) must always be determined in a conservative manner. Unlike stress, in some cases it is conservative to permit a coefficient of friction higher than the average measured value and, in some cases, lower than the measured value. The guidelines are:

amin = agm - 0.15 but ≥ 0.02

amax = agm +0.15

For coefficients of friction above 0.45, use

amin = 2/3 agm

amx= 4/3 agm

where agm is the minimum coefficient of friction determined by testing under representative conditions.

Following the meeting, Zatz provided an excerpt from "Steel Structures" by William McGuire friction type and bearing type joints.  Based on that reference, please use the following provisional guidance.

For friction type joints, the shear load transmitted through the joint shall not exceed

Smax = Pmin x amin

where Pmin is the minimum bolt preload.  For bearing type joints, the performance of the joint shall be evaluated assuming a shear load through the bolt (Sbolt) of

Sbolt = S -  Pmin x amin

where S is the total shear load transmitted through the joint.  The joint shall satisfy the NCSX Structural Design Criteria as amplified by HM Fan.

Comments on these guidelines are welcome.

Adding bolts to the inner leg.  Brooks discussed the impacts of adding bolts to the inner leg.  Specifically, he discussed the option of adding 3 bolts at the B-C flange and 3 bolts at the A-B flange.  The B-C flange bolts have the greatest impact,  reducing the required COF from 0.75 to 0.45. The bolts have the added benefit of improving the uniformity of the clamping force around the perimeter of the coils during field period assembly.  For this benefit, the spacing of the bolts is important.

There was consensus that adding the bolts improves the robustness of the modular coil interface design.  We should go ahead and [1] add the 3 that can fit on the B-C flange [2] add at least 3 to the A-B flange (to minimize the maximum spacing between bolts and bring the minimum preload below 0.45) and [3] add at least 2 to the A-A flange (again to minimize the maximum spacing between bolts and bring the minimum preload below 0.45).  Brooks will propose appropriate hole locations to Williamson.

The holes could be bored and tapped at PPPL by PPPL technicians or by a contractor, e.g. Hydratight-Sweeney.  Dudek will determine the best route to go.

Bolted joint layout.  Williamson discussed the reference design of the bolted joint assembly.  Part drawings were provided in the presentation.  The bolted joint assembly features an A286 stud with a hex nut, a spherical washer assembly, a 718 load washer, a G11 insulating washer, a G11CR bushing around the stud, and a single hole shim which is captured on the bushing.  The single hole shims are all a standard size (but a few might have to be trimmed in some places).

The shims in the inboard region which are not captured by bolts have not yet been defined.  Williamson presented a concept which relies on shear pucks to transmit the shear between shims mounted to mating flanges.  This is the inverse of the shear plate design (sort of).  Myatt analyzed the stresses in this concept and found them to be high enough to require the use of steel pins.  Ellis discussed how metallic pins coated with alumina were successfully used in the design of the JET last alpha detector to provide a reliable electrical break.

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