Metals and Materials Consultants
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Only a dumbell comes apart like this ...
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Oops. This thirty-five pound dumbbell came apart, causing serious annoyance and minor injury. |
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The threads had stripped out of the handle. |
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The threads that stripped came out of the inside of the
handle. That happened in spite of more than two diameters worth
of engagement of the cap screw threads in the wrought steel handle's
tapped threads. The rest of the stripped threads have been lost; that's how the cap screw seems to have come out of an impossibly threaded hole. |
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Here's one reason why: The handle was made from a large bar
of free-machining steel. The lines at the smallest diameter of
the handle (close to the inside diameter of the tapped hole) are traces
of the former manganese sulfide stringers that give the steel its free
machinability (chips break instead of curling). The steel was not
sufficiently ductile in the direction in which the threads were sheared. |
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The "good" end is also stripped; it simply hasn't come apart
yet. |
| SUMMARY: The
graded weights in the set from which this dumbbell was selected had
been handled roughly by being repeatedly dropped on the floor.
Each time the weight was dropped, the threads yielded a bit because of
the stiffness of the assembly. The longer bars of the heavier
weights gave more spring and absorbed the impact energy without
loosening as readily. Therefore, this dumbbell would not stay
tightly together. The Allen wrench was
not always handy to tighten the cap screws. Eventually, the users
simply resorted to hammering the weights back on with another one of
the weights. Finally, there was nothing but friction to hold the
dumbbell together, and it came apart while the lifter was doing curls,
holding the handle vertically with both hands. When the one
weight came loose, the remaining portion banged the weightlifter in his
forehead. The injury was not life threatening. Just
annoying. |