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Case Study #22 — Fastener Failure Workshop

Robot Clamps


Automation Systems
Robot Clamping Mechanisms


A large manufacturer of robots designed a new-style clamping arm to be used in a host of assembly lines around the world. The advanced design was going to be the envy of the industry, because it was smaller, lighter, and could still lift large loads. When all of the bugs were worked out of the prototypes, purchasing then built up the parts inventory to begin production. Production was assembled with duplicates of the parts used for the successful prototypes. All 10,000 new clamping arms were then boxed and shipped to their distributors. Sales took off at a brisk pace, until the units developed problems. Reportedly, when the arms lifted heavier loads, but within their limits, the bolts holding the arms together were failing and the arms dropped a variety of expensive industrial components.


There were no injuries reported.


Many different items of varying costs were damaged by the arm failures. The expense totals would be substantial. However, the biggest drop was to the bottom line of the robotics firm, because all the promotional activities building up to launch created great new sales, but the return of the products were even greater.


The fasteners breaking were M8 x 50mm Property Class 12.9 Hex Cap Screws. They connected the hinge points at the "elbow" of the robot's arm. The fasteners were elongated in the threaded area. There were flat washers under the screw heads and under the all-metal locknuts they used. There was no apparent damage to either the heads of the screws or to the nuts. That is, marks that may have been made by over tightening and thus deforming the corners of the hex. Manufacturer's I.D. marks were present and linked with sources that are generally believed to be of reputable supply.


Laboratory testing showed the products were of normal hardness. The flat washers were through hardened, as they should be. Scanning Electron Microscopy on the fracture surfaces showed a very ductile break. This is normal appearance for a fastener pulled apart — as in a tensile test. We asked if the fasteners used in production were different from those for the prototypes. We were assured there were the same, but we tested for confirmation. We next reviewed the fastener installation practice at the plant. The productions line supervisor said he followed the torque value from the screw source, and exactly as printed. He also had his people put a little grease under the head of the screws to make a smoother installation, and they also put some liquid anaerobic adhesive on the threads so that the screws wouldn't come loose.


The supervisor caused the failures by using a torque value for "as received" screws, and then added two sources of lubrication. This excessively slippery combination was enough to yield the screws so that the arm's lifting action pulled the screws to final failure.


Anything that touches a fastener bearing surface or threads must be taken into consideration when torque is used. As you can see, torque charts can easily be misunderstood. If you are not sure, consult with a source that can provide you with expert advice.


As appeared in Fastener Technology International magazine.

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