One day several years ago, I was called to an Engineering
meeting in progress where a telephone conference call was
being conducted. An excited voice on the far end was describing
a problem they were having that shut down the assembly line.
An assembly that had not given problems before had suddenly
started breaking bolts during the tightening sequence. I was
assured that the fastening equipment had been checked and
was performing correctly. They were also using the correct
part numbers in the assembly. I returned to the Fastening
Lab and proceeded to order the parts and get the test equipment
together to try to duplicate the failures. When the test fixtures
were finished I was ready to try to duplicate what the assembly
plant had experienced. I ran several samples with no failures
and called the test Engineer and informed him of my results.
We talked for a while and agreed that something had changed
in the assembly or in the tooling to cause such a drastic
We set up another conference call and asked to have the equipment
operators present. We asked if there was anything they could
think of that may have gotten changed to cause the breakage.
One operator said that he thought the bolts looked different
than they had the last time they ran the assembly. When asked
what they looked like he replied they were grey instead of
black. He had checked the part number to print and the part
number matched the print part number. The required part was
M12 x 1.5 x 3.5” long with a phosphate/oil finish for
retarding corrosion during service.
Somebody had changed the finish to zinc for better corrosion
resistance and did not change the part number to indicate
the change. The area in which the assembly was being made
was subject to a FINE OIL MIST FROM THE AIR DRIVEN TOOLING
The phosphate/oil finish was not affected by the oil mist
during rundown and tightening and no one thought to test the
new finish under the same conditions with the oil mist present
and with the part number being the same there was no red flag
that caused concern. What happened to the finish on the new
bolt was a disaster. The oil mist, however fine, got onto
the zinc finish and the result was a slimy coat that affected
the friction adversely to the point of consistent bolt breakage
at the same tightening torque applied. With this new information
it was immediately apparent that the tightening strategy needed
to be changed. The reason my testing did not show a problem
was that the parts I ordered from our stock was the phosphate/oil
finish and resulted in the proper result at the specified
tightening strategy. We requested that they send some of the
new bolts to our Lab so we could retest and come up with a
new tightening strategy until they could get the older style
bolts ordered. The bolts showed up the next day and I was
ready to begin testing.
First I ran a failure mode test to determine the maximum
torque that could be used with an oil mist on the zinc finish.
Surprisingly the maximum torque required to break the bolts
was less than half of the specified torque used in the original
system. No wonder they were in a frenzy!!
The testing that followed uncovered another problem that
resulted from various amounts of oil mist on the zinc. The
friction range was so great that the clampload varied from
low to breakage of the bolt!
It was difficult to make a determination of an absolute torque
+/- a variance, due to the wide range of friction. I then
decided to try to determine a tightening strategy that would
hold the joint together and not break the bolt. The joint
was one that needed a minimum clampload to eliminate slippage,
so to get the clampload it was necessary to get the torque
to a narrow range for repeatability. This would require the
same amount of oil mist to be on each bolt (haha) to get the
friction to be consistant!! But because friction is so non
repeatable this appeared a conundrum! I decided to try a torque/angle
answer but this could also be a problem because the assembly
plant was not equiped with state of the art tooling.
There were approximately 2000 joints worth of bolts (4000
bolts) that were affected before the proper bolts could be
delivered and manufacturing was not going to shut down for
even 2 days to wait for the correct bolts.
I got the OK to go ahead and develop the torque/angle strategy
and they would implement it by hand for the remainder of the
bad batch of finish. To make a long and drawn out story shorter
due to the various amounts of oil to be sprayed on the test
bolts I finally determined that 15 Nm + 90 degrees of rotation
would give the amount of clampload required to keep the joint
together and not allow slippage! The workers quickly adapted
and with a torque wrench set to click at 15 Nm and eyeballing
90 degrees +/- roughly 5 degrees the joints were sucessfuly
assembled and production resumed, a little slower than before
but enough to get the product out safely. This was a plus
derived from a minus that helped to sell manufacturing production
on the idea of torque plus an angle of rotation to assemble
certain joints that required a minimum clampload to survive
heavy service!! It is very difficult to change from just torque
to a more certain strategy due to the type of tooling. The
better computer controlled torque + angle tightening tooling
is expensive and a time sensitive program. The added cost
is really offset by the improved reliability and less returns
for retightening the joints.
The following figures show what happens when certain finishes
are subject to contamination by an external influence. The
first figure shows the original joint with the phosphate/oil
finish, torque control and the resultant clampload and variation.
The second figure shows the wide variation in clampload at
the same torque control, with various amounts of oil mist
on the zinc plated bolts. The third figure shows the relationship
of angle turn and the resultant clampload.
George Lorimer, Retired.
G.M. Powertrain Fastener Lab.