Industry:
Medical Equipment
Linear Motion Pushers
Problem:
A large medical equipment producer conceived a method of patient
transport
through a scanner, designed to be smoother than anything before. Using
this method, they anticipated better image resolution, but, on the initial
assembly line run the hex recesses of the stainless steel flat countersunk
head socket screws rounded out.
Injuries:
None.
Damages:
There was no damage to the equipment, but the long anticipated new
machine
couldn't be assembled, and there was lost production time and potential
sales.
Observations:
The stainless screws were #10-32 x 1/2". In an
unused condition,
they appeared entirely normal. There was also ample evidence of screws
that had their hew recesses grossly rounded out. There were inch pound
torque wrenches available by the machine. They had short sections of hex
keys held into the sockets on the wrenches.
Investigation:
Samples of the fasteners were taken to a lab for review. A test
with a
magnet showed the parts non-magnetic. As the fasteners were made of a
stainless steel that couldn't be heat treated, the sole source of
any added strength was from cold working. While forming a hex recess imparts
added toughness to the socket, this is still a relatively
weak material, and the screws can't be installed
with a great amount of force.
We decided to conduct a test to evaluate these screws for assembly. The
test consisted of screwing the fasteners into a hardened test block by
hand until the heads bottomed out and stopped turning. Next, a new hex
key in a calibrated torque wrench was used to assess the ability of the
hex recess to hold a prescribed torque amount. It's not a test required
by ASTM F 879, the specification covering this product. Our intent was
to determine if the hex would cam out, that is strip or round out, prior
to achieving the manufacturer's recommended installation torque
of 34 in-lb (3.8N-m), which was 11 in-lb (1.2N-m) less than what was used
at the plant. Three specimens from the lot were tested. Each of the screws
easily held the assembly torque without damage. We then tried increasing
the torque to see what value the hex would round out. Surprisingly, we
took the torque up to 68 in-lb (7.7N-m) on new specimens. That's
twice the installation torque! Still there was no damage. We next checked
the cut-off hex keys being used. The dimensions were right, but the hardness
at the ends where force was exerted was low. After asking
the workers, we found that the keys were shortened by
grinding without coolant; therefore,
the excess applied heat annealed and softened the keys.
Conclusion:
The screw's recesses were very strong, but hex keys were
poorly
modified making the screws appear to be the problem. Additionally, the
torque used was too high.
Recommendations:
Hex keys are high hardness items that must be cut with an abrasive
cut-off
disk that is water cooled. The cooling will preserve the hardness. Further,
manufacturer's torque recommendations normally take into consideration
a number of variables; therefore, unless you have truly proven your assembly
method, it's a good idea to follow their advice.
As appeared in Fastener Technology
International magazine. |
