Industry:
Automation Systems
Robot Clamping Mechanisms
Problem:
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.
Injuries:
There were no injuries reported.
Damages:
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.
Observations:
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.
Investigation:
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.
Conclusion:
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.
Recommendations:
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. |
