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Case Study #21 — Fastener Failure Workshop

Satellite Rockets
 

Industry:

Aerospace Vehicle Fabrication
Satellite Launching Rockets

Problem:

A major aerospace manufacturer was in the process of making the first prototypes of a new series of rockets. There rockets would be tasked to launch larger and heavier satellites. The computer controlled press brake used to bend the heavy, flat sheets of material into conical nose cones malfunctioned. Instead of coming straight down, one side of the press was going much faster. This action caused some heads of the large fasteners holding the machine together to violently shoot more than 30' (9m) across the room.

Injuries:

There were none, although two people from the plant that were working near the machine were almost hit.

Damages:

The equipment was heavily damaged, as enormous pressure was involved in the operation to bend the plates. The machine was repairable, but valuable time was lost towards meeting the target launch dates for the rockets.

Observations:

There was a 2-1/2" (65mm) diameter dowel pin used as a shear pin between the main plates. It was cleanly sheared. The fasteners were 2" (50mm) x 7-3/4" (197mm) specially made socket cap screws. The screws appeared to have the smaller head diameter of the 1936 Series standards, and the fillet radius was undercut into the bottom of the head. The screws also had a reduced body diameter adjacent to the fillet. Tooling marks made it obvious that these fasteners were machined from a bar, and that the hex socket was made via the "drill & broach" method, rather than forged. Additionally, the clearance holes for the screws seemed very large.

Investigation:

Laboratory testing of the dowel pin showed the material to be 1117 steel that had not been heat treated. The screws did have the head diameter of the small 1936 series, and we learned that the undercutting of the fillet radius was intended to maximize the head contact area. Oddly, there was ample room available for the current, larger 1960 series head.

 

The diameter of the reduced body under the head was smaller than the minor diameter of the threads. Lastly, a normal clearance hole size is 1/16" (1.6mm) greater than the body diameter. However, in this application the through hole was 1/8" (3.2mm) larger for the first 2" (50mm) under the head, and then enlarged yet an additional 1/8" (3.2mm) for the next 3" (75mm).

Conclusion:

This collection of linking faults could have seriously hurt someone. First, the computer malfunctioned and skewed the load. Then a soft dowel pin allowed a load change breaking the screws — made much easier by having the cross-sectional area under the head less then the stress area of the threads. This shifted the loads to the weak fillet. The oversized holes then created a fulcrum point, and that facilitated the prying action that broke the heads off.

Recommendations:

First, get the computer fixed. Next, the dowel pin should have been made from a better material and through hardened. A standard 1960 series socket screw design would have worked fine — and with a normal fillet radius. A chamfer at the top of the hole would protect the fillet, and the 1/16" (1.6mm) normal clearance hole could have been used.

 

As appeared in Fastener Technology International magazine.

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