Machine Design

Advanced Manufacturing: When will that padlock break? Simulation tells all.

Hercules Industries,
Scientific Forming Technologies Corp.

Simulation results helped padlock manufacturer Hercules Industries, Prospect, Ohio, develop its new “breakaway” aluminum padlock. Unlike a typical keyed lock, the device was intended for one-time use only. Users would torque-down the bolt on the front of the lock until the head sheared off at a recessed diameter, resulting in a permanent closure. Engineers from Scientific Forming Technologies Corp., Columbus, Ohio, analyzed the lock’s pull strength using the company’s Deform 3D simulation software, in collaboration with the development staff at Hercules.

After building the padlock model, engineers rotated the virtual bolt inward until its tip fully engaged with the shackle. They then performed virtual pull tests to determine the device’s mode of failure. Initially, engineers thought the bolt tip could deform and let the shackle dislodge. Or, the shackle might unbend and let its free end pull out of the lock body. Because each possibility seemed feasible, engineers modeled the bolt and the shackle as 3D plastic objects. A fixed boundary condition was applied to the bottom of the lock body.

Engineers analyzed pull strength by assigning a constant upward velocity to a rigid object placed inside the shackle. They also accounted for the contact between the bolt and the shackle, as well as between the shackle and body. Engineers performed a multitude of tests, simulating locks made of common aluminum alloys with shackles, bolts, and bodies that had undergone different heat treatments. They modeled 5 and 6-mm shackle diameters to determine the effect of diameter on pull strength.Results showed that both the bolt and the shackle deformed. At the start of the virtual pull test, the tip of the bolt plastically deformed. As the test continued, the bolt then remained structurally sound while the shackle started to unbend. This process continued until the shackle pulled free from the lock base.

The bending of the shackle was the primary mode of failure in the simulated pull tests, which matched prototype tests at Hercules. When using the same diameter shackle, the lock made from 6061 aluminum had a higher pull strength than the lock made from 6062. Of course, excessive strength in the shackle prevents the bolt from dimpling the shackle, resulting in a lower pull strength with a failure mode where the bolt tip shears off.  While these results might seem intuitive, Deform provided solid guidance throughout an accelerated development cycle. The final design decision was based on cost, simplicity, and pull strength.

© 2010 Penton Media, Inc.

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